honda 40 hp idle rough under load

Hi from Australia
great forum heaps of good advice.I looking for help with my 40 hp 2004 model honda tiller steer.The motor idle nice and smooth out of gear in the water but in gear dead slow trolling it has a annoying sort of rough idle,if I put it out of gear give the throttle a rev and put back into gear it will run really smooth for about 10 seconds then start its rough idle again.after reading though the old posts on the site I have done all the checks reccommended plugs,valve clearances,cleaned the carbs,done gaskets,synced carbs all ok.I have ordered a new set of idle jets the long thin ones that go up the centre of the carby as reccommended so i will change them when they turn up,any other ideas?also read in a old post about a honda outboard carbureter manual that shows the fuel air passages etc wondered if anyone had a pic for the 40 hp? I have had a look on ebay one site sells the manuals but shows no postage to australia? thanks for any tips/advice Yamp

You might see is helminc will ship to australia. http://www.helminc.com/helm/product...=&from=result&Style=helm&Sku=TM044&itemtype=N

Some basic items. If your model is a BF40A4, the idle mixture screws should be 3 1/2 turns out from lightly seated.

You said that you synced the carbs. Before you take them apart again, make sure that you clamped off the hoses going from carburetor to carburetor while you were syncing them. The hoses are about 12.7mm (outside diameter) and run vertically on the outside of the carbs. Two hoses #24 http://www.boats.net/parts/search/Honda/Outboard%20Engine/2004/BF40A4%20LRTA%20/CARBURETOR/parts.html
and #29 http://www.boats.net/parts/search/H...2004/BF40A4 LRTA /CARBURETOR ASSY./parts.html

You may have to increase the idle a little to keep the motor running by adjusting the idle stop screw on the bottom carburetor. Once the carbs are sync'd, remove the clamps and readjust the idle to the proper rpm 950 plus/minus 50 . I usually go on the plus side.

One other thing to check.....be sure the engine is coming to temperature. If the thermostat is stuck open, it will cause a poor idle.

Mike

Thanks for feedback Mike,I changed the thermostat and crimped the hoses when I synced the carbs.I went though the order process on the hem site $40 for the manual,then $30 handling and $100 shipping? So I decided I might give the manual a miss.I mainly only wanted the manual to see the fuel air flow passages so I could get a better understanding of the carbies to make sure there is no passages in the carby body that I haven't cleaned properly. thanks Anthony

update to my ongoing honda 40 hp idle issue,I just replaced the slow jet tubes all the carbys blew though the idle passages with air and carb cleaner,reassemble the carbs set the idle mix screws and synced the carbs.I did all this with the motor running in a drum of water.I even put the motor in gear in the drum and the idle seems to be nice and smooth nearly as good as the out of gear idle.I took the boat out for its first run on the water yesterday and found that the in gear idle was rough as guts,it sounds like its staving for fuel in gear idle but its fine out of gear nice and smooth.when i adjusted the idle mix screw it said to adjust the mix screws to achieve the highest idle speed possible then reset the bottom carb idle speed screw to the spec 950 plus or minus 50.I checked the position of the mix screws on water they where 2 1/2 out not the spec for my motor 3 1/2 out,thinking that must have been the starving issue I adjusted them all to 3 1/2 out but the rough idle and vibration in gear was still the same maybe slightly better.If I give the motor a squirt of throttle out of gear and then put back in gear it will idle nice and smooth for about 10 seconds then run rough again.looking for help please thanks Yamp

Sounds like one cylinder may not be firing correctly at idle.

Did you do a cylinder drop test? Using an insulated plyers, pull spark plugs wires off one at a time to see if the motor changes rpm. The motor should slow down each time you pull one off. If it does not, then that is the cylinder that has an issue.

I just had two 40's in that kept doing what yours is doing. On the hose, they idled nicely, but when taken to the water and run under load and run up, drawing in more fuel, the nice smooth idle went away.

After cleaning several times, we did what we should have done in the first place....replaced all the hoses in the fuel system, including those hoses from the fuel pump to the carbs and the other small hoses on the carbs. Turns out, we kept finding small pieces of black debris ( hose pieces). It ran fine after that. I guess we had our "dumb hat" on that day.

Mike

Just did the cylinder drop test seemed to be fine revs dropped for each plug,I think my fuel lines are ok as I didnt notice anything wrong with them when i pulled the carbs down last time.I remember reading somewhere about checking the carbs for leaks by spraying something around the carbys and seeing if the revs picked up not sure what the spray was,I will see if I can find it on an old post. the other thing with my set up is I have a cartridge type filter after the tank before the motor and the clear bowl in it always seems to be full and I have checked it for moisture and it all seems ok.I suppose i could look into the fuel lines again.the other thing I noticed was the gasket that goes between the air box and the carbys doesnt line up that well with the face of the carbys and the old one was the same anyway I will keep trying thanks for any help yamp

Sorry just noticed that I must have erased my reply when I composed it. My senility seems to be kicking in faster that I expected.

As far as the air box gasket is concerned, the best way to make sure it aligns properly is to make sure the small round holes fit over the little round pegs. Part # 15 holes to the pegs in part # 30 http://www.boats.net/parts/search/Honda/Outboard%20Engine/2005/BF40A5%20LHA%20/CARBURETOR%20ASSY./parts.html

If you think the filter is causing an issue, bypass it temporarily. They sometimes will accumulate air, especially is they are located much above the tank and/or the motor. Then is causes a fuel flow issue. If it continues to run good in the tank, what is different when you put it on the boat? Are you using a different fuel hookup? If so, use the hookup (tank, hose etc) that you used in the tank test. If it still acts up, then at least you know it probably is not the fuel system on the boat causing the problem and you can look elsewhere.

Try turning the mixture screws out another 1/8 to 1/4 turn and see if it smooths things out.

Generally, if there is a leak in the gaskets, the engine will idle high. There is always an exception. You can try spraying, in small spurts, carb cleaner or brake clean. I have tried using starting fluid and sometimes it works ok. If vaporizes so fast that the vapor tends to get sucked into the intake and causes the motor to rev up, making you think you found the problem...but you did not.

Mike

Thanks for the tips mike I was testing the motor again today and spray carb cleaner around the carbs were I thought it could have air leaks but didn't seem to change anything.in regards to the fuel tank set up I have the tank below the filter which is also below the carbs on the motor.i have been using this set up for all the testing.I have the motor still on my boat during the testing,I have it in a large drum so I can still knock it into gear in idle it just slashes around a bit.i noted today that if I set the idle out of gear then put in gear the idle revs drop back by around 150rpm so if its set to 1000rpm in gear it will drop to 850rpm or less.I also pulled the plugs to check the gap which was in spec,I also gave them a clean up they had a sort of black soot around the end and electrode not sure if that's a good or bad sign. i put the vacum gauges on and resynced the carbs and had all carbs near identical but the motor still not purring and it had a far bit of vibration which was alot wourse in gear.not sure which way to go next ,pull the carbs again reclean?or is the problem ignition side of things? Thanks for any help hopefully I will get it sorted cheers Yamp

If your plugs are blackened, your motor is running rich. A few of things could be the cause.

It could be possible that your thermostat is stuck open and the engine is not coming to temperature. If your model is BF40A4 (which makes it a 2004) in the USA, the temperature of the motor at the thermostat should be between 52 and 62 deg C. If it is not getting that warm, then check the thermostat. The motor needs to get to temperature to idle correctly.
This would cause all the plugs to be somewhat black. BF40's previous to the A4 run hotter.

The other possiblity is that the idle mixture screws are set too far out. As I look at the spec book, there are three different settings, based on the serial number or model.
Since you are in Australia, the models and serial numbers might not be consistent with those is the USA. So, I am going to list all the settings and their respective motors.

BF40A 2 1/4 turns out (engine BAYE-2000001-2007300
BF40A 1 5/8 turns out (engine BAYE-2007400 and later
BF40A4 and later 3 1/2 turns out
The serial numbers are actually the numbers on the block not the frame. Most likely you will not be able to see it, due to parts of the motor being in the way.

If you think your motor fits a different category, try that screw setting.

Another possiblity is your bistarter on top of the top carburetor is not closing down all the way thus allowing extra fuel to go to all the carburetors. If it is the problem, the motor should run pretty good at higher speeds, where it could burn the little extra fuel without running problems.

The thing I am looking for is some common thing affecting all of the carburetors...if all plugs look the same.

You could have set all of the floats incorrectly, but the same.

Make sure the oil is below the full line. Too much oil can cause this too. Generally, the idle is not always affected.

One last thing....if the timing belt is off just one notch, it will cause a rough idle.

There! Just a few thoughts.

Mike

Thanks for the help mike , did a few tests today,I took some temps around the engine block with the infrared gun,the thermostat seems to work got temps around 60 to 70 c,I also cleaned the plugs again,run for a short time on idle then gave a few motor a few bursts then hit the kill switch and pulled the plugs out again to check them,they were fairly sooty looking again,I was fairly sure the spec for my idle mix screws was 3 1/2 out although I am going off the plate number at the front of the motor.i will try running the idle mix screws at a leaner mix next run I do.In regards to the se choke set up is there anyway to test this is fully shutting down? It's seems to be working as the revs drop back as the motor runs for a few minutes and warms up. I am thinking of pulling the carbs for cleaning again probably wasting my time,it really sounded like the motor was starving for fuel when I last did the on water test at the start the idle mix screws would have been about 2.1/2 out which sounded really rough which I adjusted on the water to to 3 1/2 out which improved slightly but still was not smooth.looking for any other test to troubleshoot problem,could there be issue with the fuel pump I could test?the motor runs mid to high revs just rough with vibration in gear idle with no throttle thanks for any help Yamp

If you haven't yet, check compression. Should be around 199 psi. Check in throttle only with full throttle.

Check the timing marks. I do not think you said that you did. One notch out and blacken plugs.

Make sure that the bystarter is working properly. When you start the engine, when it is cold, the idle should be somewhat high, then slowly go down to the proper rpm. If this is not happening, then the bystarter is still feeding fuel even when it should not.

The black on the spark plugs indicates you are either running rich, too cold, or it is burning oil. You have ruled out the too cold. If it is burning oil(and the oil is not too high), the compression test should help confirm a cylinder problem.

Mike

I did a few checks today running the motor in the drum again, double checked the valve clearance were in spec all ok,double checked timing marks ok.run the the motor with diffferent idle mix settings which didn't really smooth things out at idle.double checked oil level ok.i did checks on the plugs after changing the idle mix screws cleaning between idle mix adjustments the plugs seemed to about the same level of soot regardless of mix screw setting maybe a bit less soot with less screw out.i also did notice the drum of water used to run motor had a ring of soot around the top exhaust gas I suppose is this normal? I usually run the motor with muffs. the se valve seems to be working as idle drops after warming up,is there a way to confirm se is shutting down properly,I thought maybe I could use a pinch tools to close off lines feeding from the top carb se lines. I haven't done compression test yet,could you please explain what's involved.The motor probably has about 150 hrs on it and runs great but is just rough at idle with a far amount of vibration thanks for any help Yamp

If you can find a compression gauge to screw in, the compression should be taken with the throttle wide open.
Remove all of the spark plugs...remove the safety landyard (so there is no spark). Crank the engine a few times. You should get a compression somewhere around 200 (give or take some). All cylinders should be very close in reading.

The SE valve can be tested. The resistance should be 15.8 - 24.2 ohms.

You will have to remove the carb for the next test... There is a hole inside the carb through on the side by the se valve. Take a vinyl tube and hold it up to it and gently blow into it. You should be able to blow easily through it.
Then connect a 12v battery positive to the brown/white lead and the negative to the black/green lead. Within about 5 minutes, try blowing through the hole with the tube again. Air should not pass through it. If it does, then the valve is probably bad.
They are pretty reliable, so be sure it is tested right before you just order a new one.

If you are going to remove the carbs again, check all the little hoses on the back of the assembly. Most of them are vent hoses. Hopefully, you have them all connected correctly.

Also, double check that your intake manifold gasket is in good shape and that you have cleaned the surfaces really good.

Also, double check to be sure that the little pin hole passage through all of the jet sets (small tube going up through the carbs) are clear. Also, make sure that the o rings on the jets sets and the idle mixture screws are in good shape and the ones on the idle mixture screws are in the right position.

I am scratching my head on this...starting to run out of ideas.



Mike

Just thought I would post up a pic of my plugs I took after testing in the tank, I would clean the plugs up in between making adjustments and remove them after running at that idle mix setting and it didn't seem to change the amount of soot?.i have done the resistance test to se plug which was ok.i have never checked the float height on the carbs or that the floats are seating properly,Could this cause the rough idle in gear yet run well mid to high range? I am still to pull the carbs off and do the tests on the se choke.should the plugs look tan coloured at any rev range or do they only show no soot if you have been running the motor at a higher rev range and then pull the plugs? Any thanks for any tips anthony

If these motors are not run a lot and sit with fuel left in the carburetors then the small jets and passage ways tend to plug up.----------I would make sure that carburetors are absolutely clean internally and it often takes folks 2 or 3 tries at that to get it right.

That motor should have very little soot on the spark plugs. If the floats are set incorrectly and cause the carbs to be too rich, it will cause running issues and the spark plugs to get black at idle while seeming to run ok at higher speeds.

Since you said the oil level was good, check the crankcase breather tube...the one that runs from the valve cover top to the upper part of the carburetor muffler.#9 http://www.boats.net/parts/search/Honda/Outboard Engine/2005/BF40A5 LHA /MUFFLER COVER/parts.html

Make sure it is not clogged

Also, recheck the other breather, vacuum, etc hoses for kinks, clogs or incorrect connections...starting with #29
http://www.boats.net/parts/search/Honda/Outboard Engine/2005/BF40A5 LHA /CARBURETOR ASSY./parts.html

The black looks too consistent for something as simple as wrong float height. Although, it is worth checking, if you are taking off the carbs again.

Mike

So I know it’s been a while but what did the outboard problem end up being? I have the same problem and have done alot of what you did. Please help!

Todd177
Sorry you're having problems. I know hondadude doesn't post here anymore and, since this post is almost a decade old, I doubt that the OP is still here as well.

No one will still be subscribed to this thread so you should really open a new one to get the maximum amount of attention to your problem. Still lots of good help available here but a new post gets the most readers.

Good luck.

jgmo said:

Todd177
Sorry you're having problems. I know hondadude doesn't post here anymore and, since this post is almost a decade old, I doubt that the OP is still here as well.

No one will still be subscribed to this thread so you should really open a new one to get the maximum amount of attention to your problem. Still lots of good help available here but a new post gets the most readers.

Good luck.

Click to expand...

thank you.