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Vexxing BF40 problem above 2k RPM

T

taylormade

Member
First, allow me to say "first time poster, long time lurker" and I've learned and tried a lot from this forum, so THANK YOU in advance, even if this post gets exactly zero replies.

Ok, so I have a late 90s BF40a that ran great when I bought it three years ago and then sat for a year until about 3 months ago. Never had ethanol gas in it and did have Stabil (just getting it out of the way). I knew the carbs were going to need to be cleaned so I didn't bother even launching it without doing so first. I also dumped the approximately 8 gallons of gas into my tractor. When I launched it, it would start right up, idle perfectly and cruise around with no problems until I got to about 2500 or so RPM where it would bog down and run rough.

I had a trusted mechanic come over, pull the carbs off and take them home with him where he put them in the ultrasonic bath and cleaned them thoroughly before installing and synching them. While it ran better, it ultimately didn't change all that much. Problem continued.

Replaced all the grey fuel hoses with new, high quality hoses as well as the fuel bulb, which is firm. Ran it off a 2G external tank, bypassing all the filters... no change. Did some more research and saw that the fuel pump could be the culprit. Threw $100 at it... didn't fix it.

Got the SeaFoam out. Nada. Yamaha Ring Free shock treatment. Nothing. Changed the plugs (which were actually pretty clean), nothing changed. Pulled one plug off at a time and there's definitely a drop in performance on each cylinder. Good compression in each cylinder and good spark (both on an inline tester and grounded to the block).

It's not making oil... oil is clean, normal level and doesn't smell of gas.

I tried spraying WD40 around the carbs while under way to see if there were any air leaks... nope.

Tried shooting carb cleaner in the carbs (removed the silencer) and sometimes it revvs a bit, others it boggs (assuming it's because they're full of fuel then).

Today I tried pulling the manual choke rod out when it's running rough and when I do so, just a tiny bit, it cleans it up a bit. It's still nowhere near where it's supposed to be, but it runs a little bit better with about an 1/8th choke.

All this brings me back to the carbs but before I pull them off AGAIN for the third time, is there anything else I'm missing?
 
Can only be carbs, were they green with varnish? If so I would consider ordering 3 new main jets, they can be very hard to clean properly, even with a sonic cleaner. You can try cleaning them by spinning them on bristle from a wire brush but for a few bucks I would replace.
 
I agree it is probably the carbs but you should also verify that the timing belt is tight and marks are lined up properly. I would do a check of the valve lash and make sure it's within specs.
Hopefully you are working with the Honda shop manual but the cheap books will likely get you by for this.
Then.....
....I highly recommend getting the Honda Marine Carburetion Manual BEFORE revisiting the carbs. It's really an essential tool for doing your own work on these. Lots of GREAT info and will help you get them cleaned and put back together correctly. I was succesfully cleaning and rebuilding these carbs but when I got that manual it really helped me to better understand how they work and what to look for to get it right first time every time.

Good luck with your outboard.
 
iang6766 said:
Can only be carbs, were they green with varnish? If so I would consider ordering 3 new main jets, they can be very hard to clean properly, even with a sonic cleaner. You can try cleaning them by spinning them on bristle from a wire brush but for a few bucks I would replace.
Click to expand...

Nope, they weren't too bad the time I cleaned them... just a little crust here and there, certainly not green. Agree that I'll buy new jets and float needles along with gaskets fo the carbs and intake manifold. I bought a ultrasonic cleaner since I have a handful of machines with carbs and I find myself cleaning them a lot. Thanks for the response!
 
jgmo said:
I agree it is probably the carbs but you should also verify that the timing belt is tight and marks are lined up properly. I would do a check of the valve lash and make sure it's within specs.
Hopefully you are working with the Honda shop manual but the cheap books will likely get you by for this.
Then.....
....I highly recommend getting the Honda Marine Carburetion Manual BEFORE revisiting the carbs. It's really an essential tool for doing your own work on these. Lots of GREAT info and will help you get them cleaned and put back together correctly. I was succesfully cleaning and rebuilding these carbs but when I got that manual it really helped me to better understand how they work and what to look for to get it right first time every time.

Good luck with your outboard.
Click to expand...

I forgot to mention that I had someone check the timing along the way and it appeared in spec (although admittedly I wouldn't be able to tell the difference).

Dangar Marine has a great video on YT on tuning and synching carbs and he just so happens to be working on a BF40 so I'll follow it after I soak and clean the carbs. Thanks very much for the advice!
 
As an update, I went ahead and purchased all the components needed to rebuild the carbs and they should be in tomorrow. I'll update this thread with the results once I can get them all done.
 
Pilot jets are notorious for clogging with scale. No amount of chemical or sonic vibration will clear them Mechanical cleaning with a wire like from a wire brush is necessary. Also check your valve clearances.
 
yeah I actually bought all new pilot jets, jet sets, main nozzles, main jets and gaskets. just need to find time to tear into it now! I'm hoping to do so this week. I'll have to dig into the manual to learn how to check the valve clearances as I'm not familiar. Thanks (all) for your advice!
 
welp, I finally got around to installing all the parts into the carbs and sure enough, she runs a lot better. Not perfect however, and I think I'll be tearing it all down again soon to look at the top carb. It's running much better but I think it's only running on two cylinders. When I was tuning the carbs, I was able to bottom out the top carb's pilot screw and it had zero effect on the engine's running performance. The middle and bottom ones changed dramatically but the top did nothing. I pulled the spark plug on the top cylinder and it didn't change at all either.

The good news is that it's still running better than it was before, ha.
 
Well... kinda. I didn't honestly but I did hook up the gauges to them and they were both within one inch of the bottom carb. One thing that WAS weird though is that they were all really low on vacuum. I feel like the last time I cleaned and synched them, they were around 11" each and this time it's around 6.
 
You need to get them exactly the same or it will not idle smoothly, amount of vacuum is irrelevant, as long as they are exactly the same as the bottom cylinder.Idle mixture screws should all be 2 and quarter turns out. How is the high end power now?
 
So it idles very well and the mixture screws started at 2 turns out. The bottom ones ended up being turned out a little bit more but it's idling well. It's running significantly better than it was prior. The throttle is smooth and the RPMs are higher than before but it's not exactly right still. TWO sounds good but it's not hitting close to it's "usual" high speed... it's still about 60%. It's honestly like the top cylinder isn't firing. I can't imagine being able to bottom out the mixture needle without it affecting idling. Maybe I'll take an IR gun to the exhaust ports and see what's going on.
 
So a bit of an update. After letting it sit for a few hours, I went back out and fired her up. She purred right away so I let it idle a little higher than normal while I did some troubleshooting. I loosened the drain screw from the bowl of the top carb and fuel started flowing out so it's getting to the carb. I turned the engine off and pulled the plug and it's bone dry. So I'm thinking it has to be a stuck float pin (but wouldn't that flood rather than starve it?) or maybe somehow I screwed up the installation of the jets on the last carb.

I can pull the third carb off without taking the entire manifold off so I think I'll try that tomorrow unless anyone else has a suggestion of what else I can try? Thanks for all the help thus far!

edit: sorry, I realize I just tossed out TWO in the above post without clarifying it. I meant throttle wide open. It runs well at throttle wide open (or at least sounds good, it isn't bogging) but it's not getting even close to it's normal full speed.
 
Before tearing into the carb, make sure you have spark at the spark plug. And it should be a strong spark. If it has that, then redo your carb.
 
Well more confusion I'm afraid. I pulled the top carb off, tore it apart, cleaned it again and reassembled it all. Fired it up... no change. Pulled the plug, no difference, bottomed the mixture screw, no difference. So I started backing off the screw and lo and behold, she responded! The idle picked up the more I backed off. I got to about 3.5 full turns before it didn't make a difference so I turned it back in half a turn. I then pulled the plug and immediately it responded. Plugged it back in and we're good again. Broke out the synch kit and they were back to 11 again. I synched them and revved a few times in idle. It sounded great so I took it out for a test spin.

It did the same thing before I replaced all the jets... just kind of bogged down and didn't sound as good as it did running on only two cylinders. I screwed the mixture screw in on the top carb and it sounded better and ran better (although still not right). I loosened the mixture screw up top again and then went throttle wide open and pulled the choke. It took off.

What am I missing here? Could it be the mixture screws themselves? Something else?
 
Air screws only affect the idle circuit.It sounds like it's leaning out,were all the gaskets replaced? Are the compressions okay, valve clearances checked?
 
Yeah, I replaced all eight gaskets. I haven't checked compression lately but when I started with this problem, I checked and they were good. I'll yank the plugs and check compression tonight. I haven't checked valve clearance but did find the procedure in the service manual and have feeler gauges on hand. It's weird that it runs better (smoother) on 2 cylinders than all three, presumably because it's getting the fuel it needs?
 
This is a long shot but worth checking too, make sure the check valve for the dash pot is shutting off one way and check the dash pot diaphragm has not ruptured, it has to be sucking air somewhere because opening the mixture screw enriches the mixture and you say you had to screw No 1 out an extra turn more than the others.
 
iang6766 said:
Sorry,only one pump on that engine ��
Click to expand...

Ha, no worries. I will say I was looking high and low for another fuel pump lol. My Mercury on my bay boat has a HP and LP pump so I didn't question it. Yeah I replaced the fuel pump already early in the troubleshooting.

iang6766 said:
This is a long shot but worth checking too, make sure the check valve for the dash pot is shutting off one way and check the dash pot diaphragm has not ruptured, it has to be sucking air somewhere because opening the mixture screw enriches the mixture and you say you had to screw No 1 out an extra turn more than the others.
Click to expand...

How would I go about checking those? I know where it is and it's easy to get to, but I'm not familiar with what it should look like.
 
Another update. Pulled the plugs out and checked the compression. Top cylinder is 135, 2 and 3 are 140. Checked valve clearances. Top was at the low end of spec, 2 and 3 were a little too tight. I loosened them all to within spec, put a new valve cover gasket on and sealed it back up. No change in performance. I can pull the choke out a bit (1/3 to maybe 1/2) and it cleans up. Note, even with the choke out, it's not full speed, but it's a lot better than with it open.
 
No, those were taken in neutral and cold. In reading, I see that I'm an idiot lol. I can redo in the am with it warm, open and with the fuel line disco'd.
 
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Been enjoying this thread. I been hanging "in the weeds". Here is my take. If it runs better with a partial choke on #1 carb, it of course means an overly lean mixture being delivered to the combustion chamber. With everything you have done and chased around, it has to be a vacuum leak. What would cause a vacuum leak.....especially from storage? Good question. Perhaps the original symptoms were a result of partially blocked jet......whatever was done from then until now may have created a vacuum leak....leaning out that cylinder, and creating the power loss. I will bet that the associated spark plug would become a lighter color if it was run long enough.
 
'appreciate the response and the take! I've replaced the plugs and I remember thinking that they all looked the same (and not bad to boot). I've got about 6 hours on the current plugs in it's current condition and they all look the same (new). I've been looking for an excuse to buy a leak down tester. Would this be a good time to pull the trigger?
 
Still may not help diagnose your problem, however. We have to keep in mind that the problem began after a prolonged storage period. With such excellent compression numbers we have to assume the motors integrity is fine.
 
I agree with Tim, did you replace the intake to head manifold gasket and if so did you get all the old gasket material off properly because they take a bit of effort to remove?
 
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