BF90 carbs rebuilt & refitted - one cylinder not running.

Hi guys


I've just assembled my BF90 carbs/manifold and fitted them after quite some time removed from the engine. The engine starts and runs very easily, and idles happily, but appears that the third cylinder down from the top is not fuelling somehow.

The carbs were stripped and thoroughly cleaned before I fitted them, and one carb was replaced with a brand new one (top carb). Jets are clean, and passageways all 'seemed' to be ok when blowing air/carb cleaner through. It's possible I didn't check those quite correctly I guess.


I've determined that's the case by removal of the plug caps one at a time while running the engine, and each cylinder drops revs when I do so, except the third down. There is a spark cracking away when I try this, so it appears to be lack of fuel that's causing the dead cylinder as far as I can tell.


I can't check compression as my tester doesn't have a small enough adaptor for these plugs, but I've no real reason to think the cylinder itself should have such a problem.



So, I have done/tried/checked the following things to help try to figure out the problem:

• Fuel pump - the feed from the problem carb was removed while running and fuel pumps out of the hose just fine.
• Floats - these were correctly set while removed, and the one on the bad carb must be allowing fuel into the carb ok as fuel pours from the bowl drain screw if it's loosened. They all moved up & down freely when set/inspected, although there's notable wear to the seating of the float pins from high mileage.
• Idle mix screw - these are all set to 2.25 turns out from closed, as per the honda carb manual for the BF90 carbs. I have tried adjusting the bad carb's screw fully closed then further and further out while running, but it did not affect the running at all, whereas it does affect it when doing the same to the other carb mix screws.
• I did completely remove the bad carb mix screw while running, and could hear a slight hissing, presumably indicating suction through at least the idle circuit, if not the carb in general.
• The top carb is brand new, but this shouldn't make any difference to the other ones other than perhaps affecting the synchronisation of them.
• Testing by removing the plug cap while running has been done at idle, high idle, and higher revs. At no point did the removal of the third plug cap down appear to affect the running of the engine.

Can't think of anything else offhand that I have tested on it but I might have done and forgotten possibly.


I'm confused as I know fuel is going into the carb ok, the mixture screw is set right, and I know the jets are clean or were when assembled. Even if one jet got a bit clogged then I'd expect it would flow at either idle or high revs depending what circuit was clogged.


There is one other thing I noticed but perhaps foolishly dismissed while attaching them to the manifold today. One of the plastic spacers between the carbs and the manifold had a very subtle crack through the thin part of it.

When I put a little pressure on the spacer to try and open it up a bit, sure enough the crack opened a little. In theory, I guess it's possible this has opened a bit when the manifold was torqued down, and that it's sucking external air in through that, instead of sucking fuel through the carb. Is that likely at all?

The more I think about this, the more it makes me think it could be the cause, since no suction (or inadequate suction) through the carb presumably would mean the fuel fails to be drawn through, and could explain how the carb itself appears to be in fine fettle but is apparently not flowing any fuel to the cylinder....

With that in mind I will remove them again and check which spacer is the damaged one.

I have a spare engine the same, so I'll remove a good spacer from that, and try reassembling.
I have reused all the gaskets involved when reassembling as they were all intact, but if I have to order any parts I'll get new gaskets at the same time to rule them out as contributing to the fault.


Anyway, I'd appreciate any suggestions from people who are very carb savvy, or who might have encountered similar problems either through fixing them or having the same on their engines. In particular if anyone has had trouble like this and found a cracked spacer to be the cause then that would be great to hear!

Many thanks
Jim

To check on your insulator crack, spray it with some carb cleaner or brake cleaner as the engine is running. If the engine picks up speed, you have a leak and the insulator and/or gaskets need to be changed for sure. That is a problem, but probably not "the" problem.

You have to isolate the firing problem for sure.....is it the cylinder or is it the carburetor?

A few of things to try....

Before getting to the carb issue, redo the cylinder drop test and listen for the rate of ticking on each cylinder when you take the plug wire off. If the ticking doubles or triples on the bad cylinder, then you have a spark issue. All cylinders should tick at the same rate.

Disconnect the plastic choke linkage from the carburetors and as the engine is running, manually operate the choke of the #3 cylinder carb. Try holding it at various durations. If the engine picks up in rpm, then most likely the carb is starving for fuel.

If that is not definitive, swap the #1 and #3 carbs and see if the trouble follows the carb.

Once you isolate the issue then you can attack the problem.

If it is the carb, most likely the idle mixture passage, primary jet path, and/or transition ports (the small pin sized holes at the top of the carburator bore) are clogged.

You may have gotten them clean originally (maybe not), but many times on older engines, particles from the inside of the fuel lines dislodge and get caught in the small passages in the carb....especially the jet set.

Mike

Thanks for the suggestions Mike - much appreciated.

I was planning on trying spraying around the manifold spacers as you suggested but had forgotten about trying the choke thing. Stupid since I've read that several times while looking stuff up and forgot all about doing that!

Swapping the carbs around is a bit of a faff, but if it's necessary then I'll have to try that. Either way, they'll be coming off to replace the cracked spacer, so I guess when I reassemble it'd make sense to stick them in a different order and see if the fault remains on 3 (is 3rd from top 3 or 2?!) or moves with the carb.

While removed, I'll also strip and examine/re-test the carb passages/jets as best I can.
I need to remove the carbs off the other engine as well so I can nick one of its spacers to use for now. I will thoroughly inspect the other ones too.
I'm tempted to just buy new spacers/gaskets, plus the big 4 carb gasket on my credit card so that when I get them I can at least rule those out as a problem source. Runs me at about $124 all in, or about £80.

I have ordered a step adaptor for my compression tester to take it from the 14mm plug size down to 12mm size that the honda uses. At least once I have that, I can get (hopefully!!) peace of mind that the engine itself is still fine and this is just an irritating fuelling issue as expected.

Re' the fuel lines, I've replaced last summer the fuel lines all the way from tank pickup at the front of the boat, the bulb, small filter, fuel line down the length of the boat, and the connectors at the outboard, but I guess it could conceivably have crud in pipes on the engine or in the fuel pumps etc.

If the problem persists and follows the bad carb despite further cleaning etc, I'll be tempted to order a second new carb even though I really can't afford it right now. I've spent all last summer rebuilding the transom on this boat after buying it, so I want it good to go this year well in advance, without problems I hadn't anticipated!

Ok, this is probably going to make me sound like a clueless idiot that doesn't know one end of an engine from another (assuming I don't already), but I think I need a complete rethink on this problem after what I just tested/found.


Went outside, and replaced the plug on the 'bad' cylinder with one I took from my other engine.
Started it up, and it ran just fine. Pulling the plug cap on the bad cylinder caused it to slow down this time, like it should. Bingo! I thought, it's just the bloody plug.
But what I did next was to pull the other caps and check they did the same, and they didn't.


This time, the top cylinder failed to drop revs. Remember, this is the one with the brand new carb that's factory set. Sorry, but I can't imagine for a second that this suddenly stopped fuelling and the other suddenly began! And this was on the same plug that it's used all day. Suspicious.
So with no plug in the cap I held the cap (with pliers) near an engine bolt. It sparked, but not as much as I thought it would. Hard to tell with being really windy etc but I think despite it moving around a bit that the spark was stopping periodically, and when I held it near the plug, I could see the spark tracking along the back/sides of the cap, and arcing weakly across to the head from there. Now surely the spark shouldn't be appearing on the outside surface of the cap like that?


Further testing needed methinks, but I'm wondering now if this is either dodgy plug leads/caps, causing the spark to drop out or to leak to earth when it feels like it. Worse though, my real concern is that it could be a faulty coil/CDI unit. Not sure how to test that accurately to know if it's faulty, but I suppose what I'll need to do is to halt carb meddling forthwith, and try swapping over first the leads and caps from the spare engine, then potentially a coil/CDI unit as well?


Thoughts on this appreciated, but I can't believe that in the space of swapping out a plug that a fuelling fault could 'jump' from one cylinder to the next, which just happens to have a brand new carb.


I think it's actually an electrical despite all my previous surmising about fuel.

Oh, and I did spray around the spacer and no change to the idle speed at all for what it's worth.

You are not nuts....this does happen. Sometimes it is dirt in the lines, sometimes there are minute cracks in the jet set (skinny tube going up through the carburetor).

I think you are correct to focus on the spark for now.

1 and 4 fire at the same time....2 and 3 fire at the same time. You can measure the resistance from cap 1 to cap 4. It should be the same as the resistance from cap 2 to cap 3. So a problem (like a bad plug in #1) can affect either 1 or 4, etc.

The spark is very small on the 90's and it will be hard to see holding the cap to a bolt.

If you have an actual spark tester to put in the line, that will tell you how the spark is for sure .

Just hold it slightly off of the spark plug and listen for the ticks, liked I mentioned before. The sound of the ticks on all cylinders should be the same and the same number/sec etc. I have seen some where the CDI crossfires and sends spark to one cylinder that should be going to another, doubling up on spark.

If it is a spark issue, it could be as simple as a bad ground. So check all the connections, especially the ground wire to the ECM.


Mike

Thanks Mike

I do have a spark tester, although I'm not sure what condition it's in. It's the type that is inline so it goes between plug cap and plug, although it needs the screw on bit adding to the plug, and I think part of it comes off each time I try to use it! Will check anyway.

Interesting to know these fire two at a time though. I'd thought that in the past when pondering on a different issue, but someone suggested they might well be individually, like 1342 etc, but I wasn't ultimately sure.

Will try to monitor the frequency of the ticks if I can, but on a fast idle earlier it was sparking so fast that I'm not 100% sure how I'd manage that.
Possible that it's doubling up like you describe I guess. I did once make a sound recording of something like that and then analyse the waveform graph & count the peaks on it from each spark (or whatever it was I was recording/counting - I forget what it was but remember doing it). I guess I could tell from that and a given RPM whether the number of sparks are correct, although it's a bit fiddly & time consuming to do!

Will have to check the honda manual I've got in terms of how easily the electrics detach, but it might just be easiest to swap over the leads & coils from the other engine then see if that cures it, then swap the CDI if it doesn't?

Good point re' connections too, thanks.

The plug wires and coil are all one piece.

It is good that you have some others to switch.

As for the ticking. If you can slow the idle down to a normal rpm - 950 or so.

Just move quickly from plug to plug and you should be able to tell the difference in ticking pretty easily. It sounds like you are into recording and audio. It will be like listening to middle C then going one octave higher and listening to that note. They sound the same but different.

I may be focusing on the ticking too much, it is just a quick test.

Mike

Cheers Mike

The coil wires have the appearance that maybe they come apart but I couldn't see any pics in the manual that showed them separated so I had my doubts.

I can try recording a wav if need be, to compare cylinders, but I'll try the spark tester first as it's easier to do a quick check I think.
If there's an obvious inconsistency then I'll swap the corresponding coil and try again. The fact it was number 3 bad first and then number 1 is a bit disconcerting though, since they're not on the same coil, so potentially it's both coils are dodgy, or something further up the line I guess.

Will start with the simple (cheap) bits though!

Hmmm, I was just looking at the parts diagram online and notice that the two coils are actually different parts.
The top coil is 30500-ZW1-004, and the bottom one is 30550-ZW1-004. The top one is vaguely around $20 more expensive.

Any idea why it doesn't just have two identical ones? Sort of a shame they're not the same part since it means there's effectively one spare on my other engine rather than two of them.

Seems a bit odd but that's only because I can't think of a good reason for it, unless it's just because of the plug leads being different to reach the plugs in question?

Most likely they are different numbers because the lengths of wire may be different and the plug wires are numbered.

Don't make the ticks more complicated that it is. Just listen to the ticks. Don't try to count them. Just listen for any differences.

Mike

Hi again - and sorry for the delayed update.
I have the problem solved now. It was neither a spark issue, a compression issue, nor really a fuel "fault" as it were, but a setup problem that I simply hadn't gotten to yet, but hadn't anticipated the condition that this had given rise to.

To cut a long story short I compression tested the engine after getting the correct 12mm adaptor for my tester.
It's giving more or less 165psi on each cylinder. Admittedly the throttle was not fully wide when I tested it, and the other 3 plugs were fitted, so not an accurate test, but it showed good consistency and that's enough for me, even if the book figure is more like 200-213psi. It is a 17 year old engine after all.

Managed to hook the a spark tester up even though it won't fit these plug caps, and it showed a strong steady spark was being created.

The problem was that I simply had not syncronised the carbs yet. My rationale was that there was no point trying to synchronise them when it wasn't even running on one cylinder, but that was the very reason that it wasn't.
So carbs 2, 3, 4 were allowing fuel through, but carb 1 (brand new one at the top) was way out against the other ones, meaning at idle it was fully closed. Thus pulling the plug cap while the engine was running made no difference. No air draw/vacuum through it means no fuel going through it and no combustion.
Open that specific throttle a little and it'd begin to fire.

I've sync'd them now as best I can and the engine runs soooo much better. Borrowed a 'carbtune' from my friend to do this.
I couldn't get it perfect, and with each throttle snap, the bars settle ever so slightly differently. Here's the result:



So I'm very pleased at this point, having finally done the upgrade path from BF75 to BF90 conversion. The engine runs very smoothly, albeit with an occasional cough just as it transitions from idle to opening throttle. I've set the throttle cam roller clearance, and the mixture screws, have rebuilt the carbs with spotless clean jets etc, so I'm not sure what causes this. You can hear it a few times in the video I took, here:

Honda bf75/90 carb sync - YouTube

I can live with that, but it would be nicer to fix this if possible. I have a vague suspicion that this could be caused by perhaps old gaskets/orings somewhere around the carbs/manifold/head setup. I know that there is still that tiny crack in one of the inlet spacers for instance, and I'm sure all the gaskets and orings have seen better days. Possibly this may account for the carb tuner bars having a slight ongoing variation, and that two of the four bars settle very slowly after a throttle snap.
I notice with the dashpot there is a newer version which is meant to improve hesitation off idle, so perhaps it's worth trying one of the newer (brown rather than grey) ones to see if it helps.

I think at some point I'll have to steel myself for the fact that I ought to stump up the cost of replacing those bits, without knowing if it will make a scrap of difference.

Ready for the second sea trial then as it's running so well. It will be interesting to record the performance in comparison to the 34mph at WOT 5200rpm that she achieved as a 75. I then have a stainless 17" prop to try compared to the 19" ally one on there now. Fun times.

Thanks very much for your input on this Mike. Much obliged.

Jim

Yep...sync being way out can do it. Missed that one. The two times I have seen that was in Honda School (when the instructor setup a motor that we had to find the problem....he cranked the adjusting screw all the way down) and one other time, when a customer 'who knew it all" worked on their motor but did not tell us they "tinkered" with it. Hope I didn't take you on too of a wild goose chase...

Regarding your compression....if you had removed all your spark plugs, the rotation of the engine would have been faster. Also, with full throttle, the compression would have been a bit higher.

The sneezing may have been caused because you were accelerating without the diaphragm/dashpot combo being activated. The diaphragm is activated when the throttle arm moves ... you were moving only the carburetor linkage with your finger. Which is fine to clear the carbs to check the sync but...
If you would accelerate from the shifter, in throttle only, that will activate the diaphragm and the sneezing may go away. Otherwise, turn your idle mixture screws out about 1/8th of a turn to see if it improves. This may all change once you get the back pressure of the water. Also, make sure that the engine is up to operating temperature. Hopefully, you did the syncing only after the engine was warmed up. (If you do not have a temperature gun, just grab the oil filter....if it is pretty warm, then the engine is probably warm enough.

The constant changing of the balance could be due to several things.

-Engine not warmed up
-Idle too high (sounded like the idle was high...of course my ears are not a tachometer....idle should be 950 plus/minus 50.
-Jet sets in carbs could have hairline cracks or bad o rings
-Not waiting long enough for a cylinders to settle down. It did not seem that you were waiting long enough....but that could have been due to you were trying to demo that for the video.

Overall, it sounds pretty good. Glad you worked things out.

Let us know how the water test goes.

Mike

Yep...sync being way out can do it. Missed that one. The two times I have seen that was in Honda School (when the instructor setup a motor that we had to find the problem....he cranked the adjusting screw all the way down) and one other time, when a customer 'who knew it all" worked on their motor but did not tell us they "tinkered" with it. Hope I didn't take you on too of a wild goose chase...

Click to expand...

That's quite interesting - I'd love to do courses like that. Aside from the possibility of missing out on time on the water (and having a lot less free time than I used to have) I quite enjoy troubleshooting and analysis. It's satisfying to resolve an issue like this without having to pay someone to do it for me. I couldn't afford my boat/bike/"toy" car etc if I didn't do this stuff myself.


No wild goose chase at all mate. All very useful suggestions and appreciated.

If you would accelerate from the shifter, in throttle only, that will activate the diaphragm and the sneezing may go away. Otherwise, turn your idle mixture screws out about 1/8th of a turn to see if it improves. This may all change once you get the back pressure of the water. Also, make sure that the engine is up to operating temperature. Hopefully, you did the syncing only after the engine was warmed up. (If you do not have a temperature gun, just grab the oil filter....if it is pretty warm, then the engine is probably warm enough.

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I'm certain the engine was well up to temp. It is running in a 50 gal barrel, and the water had warmed in this when I got to that point. I can usually tell once it's warm as the telltale water jet gets weaker. Not certain why this is (I've removed all the built up sediment that I can from the cooling system, and fitted a new thermostat, cleaned the valves etc), but my uneducated guess would be that once the thermostat opens the cooling circuit has a lot more volume which causes a drop in pressure (not sure why as such since you can't compress water, but I think that's the outcome).

The constant changing of the balance could be due to several things.


-Idle too high (sounded like the idle was high...of course my ears are not a tachometer....idle should be 950 plus/minus 50.
-Jet sets in carbs could have hairline cracks or bad o rings
-Not waiting long enough for a cylinders to settle down. It did not seem that you were waiting long enough....but that could have been due to you were trying to demo that for the video.

Click to expand...

Idle was a tiny sliver above 1000 on the tacho. I'd set it at the higher end of what the book says since I was going to be pulling plug caps off and didn't want it to be possibly stalling each time. That said, my son (age < 3) has become very good at starting and stopping the engine on demand for me and enjoys being involved and being able to sit onboard the boat. The extra pair of hands is a big help for this sort of thing!


Interesting that you mention o-rings though potentially. I can get the full gasket kit for 4x carbs, plus the big gaskets for the airbox, the head/manifold gasket, and the 8x ones for the spacers for about £75 delivered from the US. I'm tempted to bite the bullet and order it so at least I know those bits are all sealing properly, and you never know that it might just refine it a fraction more.

The wind looks promising for this Saturday but I think the wife has stuff she wants me to do. Need to get some brownie points first....

As they say, happy wife.......happy life.

Mike

Easier said than done Mike sometimes!

Anyway, here's an update on things relating to carb upgrade on a BF75 (effectively changing it to a BF90).
I finally got a much more successful run out yesterday on the water.


To recap, the boat/engine details etc here are an 18 foot Coastworker console boat, with 2 people on board, running a standard Honda 19" pitched alloy prop.
Running as a BF75, she was hitting a maximum of 35mph on the GPS, at WOT rpm of 5200 - less revs than I'd like, and the engine felt like it was making an effort to do so.


After upgrading to BF90 carbs and syncronising them, the boat managed to hit 40mph when I got out for a test run yesterday with the same circumstances other than the carbs being swapped. WOT rpm varies from 5800rpm with the engine tilted down, to actually over-revving at 6200rpm with it tilted up as far as you would reasonably want to do. I didn't let it run at 6200 for more than a few seconds as the top of the recommended range is 6000 and I don't want to mistreat it. Tilting the engine back down a tiny bit corrected this to 6000rpm.


So upgrading from 75hp to 90hp has given another 5mph - not that important by itself but the engine revs out much easier now up to 6000 instead of 5200 like it was before, and it feels like it is having an easier time doing so. This is exactly the result I was hoping for.
The upshot being that I can drop back on the revs, still keep a good pace up, & use a lot less juice.


I had a stainless prop with me to try out but as it was 17" and the engine now revs out correctly with the 19" one there seemed no point in even trying it. If I can get a cheap enough 19" stainless one I'd be keen to try though to see if this adds anything noticeable to the top end. I'm wondering even if I could increase to 21" and drop the revs a tiny bit lower and gain a couple of extra mph on it as it stands. It's not important but I'd like to get the optimal setup for top end and economy if I can without lots of expense.


Quite happy though with this improvement though really.


There's a couple of issues to resolve with low speed running though:
1) After first firing the boat up and letting it warm for a couple of minutes it had a poor idle, like it was firing on 3 cylinders. After the first speed run this had gone away and the idle was perfect. No idea why this happened but I'm not going to worry unless it keeps happening.


2) If I had the boat in gear and running at idle speed, then jam the throttle fully open, the engine bogs and stalls. If I ease it open over a few seconds then it picks up very nicely and gets on plane fast, but you can't just ram the power on instantly. I don't intend to be doing this really when using the boat, but it suggests the carb setup is not right for transition from idle to WOT. Will have a look in the carb manual but is this likely too lean at idle perhaps?

If you change the 19 aluminum to a 19 stainless, that along should drop the rpms a 100 rpm or 2. If you go to a 21 stainless, the rpms may drop back to where they were when you first started.

You can use all the formulas etc to come up with the right prop....unfortunately, I have found that you just have to give them a try and see how they do.

The poor idle after about 5 minutes is not unusual if the motor is not fully up to operating temperature. This is especially true of the 75/90.

Lastly, the take off. I am going to assume that all the transition ports are clear in the carburetors. If they are and since the sync is now good, unscrewing the idle mixture screws about 1/8 out may help. You also should check to make sure the accelerator/diaphragm cam is adjusted correctly. The adjustment is in the maintenance section of the manual.

In any case, the carbureted 75/90 does not have the greatest response to rabbit starts even when everything is right. The fuel injected model is a different story....much better.

Mike

Thanks Mike.

Yes the prop thing seems to be a bit of a black art. I'd only consider trying other ones if I can either borrow to try out (I live in a small place so not that likely), or else pick one or two up really really cheap so I can always sell them again if they're no use. I'm happy enough in general so it's not a concern but I am also curious about the experimental factor of finding what's best if I can do so on the cheap.

Re' the idle thing, I'm wondering if you know a particular reason for why what I described would be common with these engines? Every other time I've run it since syncing the carbs it has idled really nicely, and was only once I dunked it in the sea (of course when you have a passenger about to climb on board is when any engine will choose to show up its faults!).

I'd assumed a low speed jet must have got a spec of dirt in somehow, although I've been very careful when cleaning out these carbs, their jets, and fitting new fuel line/filter etc etc. I increased the idle a bit back to 1000 as I didn't want the scenario of shoving off from the slipway, dropping in gear, then stalling. This was fine, except that after I'd gone a mile or two at speed then slowed to a halt, and dropped into neutral it was then idling beautifully but at 1200rpm as a result of me increasing it slightly. Obviously that's not good for putting it into gear cleanly.

Is there a particular cause that you know of for this, or just that more warm up time is recommended? It'd had a good few minutes idling, then another couple of minutes underway at low speed until I cleared the harbour on the way out. At low revs while underway it would hiccup periodically as well. As soon as I opened up she ran just great.

Thanks for the tip about the mix screws etc. I've already set the accelerator cam to the correct setting as per the book but I will re-check the mixture screws. It's quite tricky I find to hit just the right spot as it's not so obvious to find the exact point where revs drop when adjusting the mixture screws, in order to get the mid point right.

I will meddle a little with it, but I certainly won't lose any sleep over this as I don't intend to be doing 'launches' like that. The idle is more of a concern.

I'm tempted to replace all carb gaskets & o-rings, including the ones to the manifold and the head, and see if this changes anything.

Very pleased though on the whole with the benefit of the carb swap.

It is possible some small debris got stuck in one of the jets, but most likely it just was not warmed up.

Even though you fine tuned the idle mixture, sometimes you need to back them out just a little more for good acceleration. You should not need to go more than 1/8 turn. You probably will not tell the difference in idle. If it does not help, you can always put them back where they were.

I would suggest....put some good fuel treatment in your fuel and go use the motor and enjoy your boat.

Mike

Cretster said:

Thanks Mike.

Yes the prop thing seems to be a bit of a black art. I'd only consider trying other ones if I can either borrow to try out (I live in a small place so not that likely), or else pick one or two up really really cheap so I can always sell them again if they're no use. I'm happy enough in general so it's not a concern but I am also curious about the experimental factor of finding what's best if I can do so on the cheap.

Re' the idle thing, I'm wondering if you know a particular reason for why what I described would be common with these engines? Every other time I've run it since syncing the carbs it has idled really nicely, and was only once I dunked it in the sea (of course when you have a passenger about to climb on board is when any engine will choose to show up its faults!).

I'd assumed a low speed jet must have got a spec of dirt in somehow, although I've been very careful when cleaning out these carbs, their jets, and fitting new fuel line/filter etc etc. I increased the idle a bit back to 1000 as I didn't want the scenario of shoving off from the slipway, dropping in gear, then stalling. This was fine, except that after I'd gone a mile or two at speed then slowed to a halt, and dropped into neutral it was then idling beautifully but at 1200rpm as a result of me increasing it slightly. Obviously that's not good for putting it into gear cleanly.

Is there a particular cause that you know of for this, or just that more warm up time is recommended? It'd had a good few minutes idling, then another couple of minutes underway at low speed until I cleared the harbour on the way out. At low revs while underway it would hiccup periodically as well. As soon as I opened up she ran just great.

Thanks for the tip about the mix screws etc. I've already set the accelerator cam to the correct setting as per the book but I will re-check the mixture screws. It's quite tricky I find to hit just the right spot as it's not so obvious to find the exact point where revs drop when adjusting the mixture screws, in order to get the mid point right.

I will meddle a little with it, but I certainly won't lose any sleep over this as I don't intend to be doing 'launches' like that. The idle is more of a concern.

I'm tempted to replace all carb gaskets & o-rings, including the ones to the manifold and the head, and see if this changes anything.

Very pleased though on the whole with the benefit of the carb swap.

Click to expand...

I've been through this drill a few times with my BF90. Sneezing, Bogging and hesitation is almost always a sure sign of a dirty/clogged low speed/idle system. The low speed side of the carb is the trickiest part to clean as there are very small passage ways that lead to small holes in the throttle housing. Clean her up good and she'll work like a champ!

Another note on sync'ing the carbs is that you need to get the air fuel mixture right along with syncing the carbs. If its too lean it will hesitate, bog, sputter, pop as well, although it shouldn't be as bad as if the carbs where dirty.

My 2 Cents...

Have fun