Honda BF40 Idle issues- Please help me if you can.

[TD_Maker]

I just inherited my Dad's old VERY VERY low hour 1999 Honda BF40. The motor was used very little the past four years, and it did sit quite a while. I now have the boat and motor in Florida going through it.

I have a aggravating miss at idle. So far I have:

1. Changed Impeller
2. Changed Plugs
3. All Fluids
4. Pulled the Carbs and cleaned them thoroughly. They were gummed up a little.

Compression is great on all three cylinders. I have good spark on all three cylinders. I am using brand new fresh gas from a clean tank.

I am wondering how many turns out should the carb screws be after a cleaning and rebuild? Can someone guide me on how to sinc the carbs? This Honda 4 stroke Honda stuff is different than the 2 stroke stuff I am used to.

 

[Sea Rider]

Re: Honda BF40 Idle issues- Please help me if you can.

Better get a Honda Service Manual, that info should be stated there. The best thing you can do before moving any idle screw, mixture screw is with a vernier tool measure the distance from upper tip to base, specially if engine is new and before you disassemble complete carbs to clean. Usually maintain all my engines calibrated same as when taken out of the box. New engines are factory adjusted and tested at factory, so adjustments should be quite right.

Happy Boating

 

[TD_Maker]

Re: Honda BF40 Idle issues- Please help me if you can.

I do have an update on the motor.

I took the boat out today for a shakedown run. The little Honda ran great all day... accelerated smoothly and topped out at 5300 RPMs. However, when I pulled back the throttle to idle...the RPMs were very eratic.... 1300- 1200- 1500- back down to 1000 eventually. It did have a sneeze at idle like it was missing a little.

I ran a can of Seafoam through it today. It still has that little miss though.

 

[wrench 3]

Re: Honda BF40 Idle issues- Please help me if you can.

A common method of returning a mixture screw back to its previous position is to count the number of turns that it takes to lightly seat the screw. Then on re-assembly, lightly seat the screw and back it out the same amount.
It's also normal to have to re-adjust the idle mixture to compensate for wear and other changes in operating conditions.
I'll check my service manual tomorrow and get back to you with specific adjustments for your engine.

 

[TD_Maker]

Re: Honda BF40 Idle issues- Please help me if you can.

Thanks Mr. Wrench.

I did count the turns like you suggested; HOWEVER, a few years ago, my Dad had some jack-of-all-trades mountain mechanic clean his carbs out while the boat was parked dockside. The motor never quite ran right since that day...but it ran good enough for the old man he said. The problem is that I think the mechanic never seated the screws correctly, and I just returned them to his incorrect settings.

I would appreciate any help you can give me sir. I would just like to start with a clean slate so I can maintain from there. The little Honda runs really nice and very quiet. The sneeze is bugging me though.

Thanks Again.

 

[Sea Rider]

Re: Honda BF40 Idle issues- Please help me if you can.

Did you just disassembled the carbs and cleaned them throughfully with generic cabrb spray ? If so probably idle low passage circuits are still gummed a bit. In that case will need to disasseembly them again and soak all carbs for some hours in proper dip cleaning solvent to have them into inmaculate clean condition to work evenly. Remove all plastics and rubber parts from carbs before dipping.

If adjusting each carb idle with needle turs as facoty stated doesn't give 100% results. You can adjust manually, will need to have a near musician ear to do so, if wanting syc testing perfection a vacuum tester pluged on each carb would be ideal. There are on the market lead vacuum testers, very easy to read and adjust.

Happy Boating

 

[wrench 3]

Re: Honda BF40 Idle issues- Please help me if you can.

They show the base adjustment for your idle mixture screws as 2 1/8 -2 1/4 turns out from lightly seated. Then adjust 1/8 turn at a time to get the best idle on a warmed up engine. They also say to start with the bottom cylinder, then go to the top one and finally adjust the center one, revving the engine and returning to idle periodically during the adjustment procedure.
Idle speed should be 850-950 RPM and should be adjusted before and after the mixture adjustments. The idle speed adjustment is on the bottom carb.
If the carburetors are out of synchronization (not all opening the same amount at the same time), That's a whole new ball game. I would have to scan and post a couple of pages out of the manual. Hopefully you won't have to go that far.
And "don't call me Sir, I work for a living".

 

[TD_Maker]

Re: Honda BF40 Idle issues- Please help me if you can.

Thanks so much. This really helps me. I think the screws are only turned out 1 1/4 turns from lightly seated. I will give this a shot.

Sorry about the "Sir" thing. Its a "Southern Thing" and how I was raised. When my son was in basic training, his Army Drill Sgt smoked him quite a bit for the same thing. LOL

 

[wrench 3]

Re: Honda BF40 Idle issues- Please help me if you can.

1 1/2 turns is the "generic" setting for mixture screws. Maybe that's what the last guy set them at.