F100B stalls in RPM range 2500 - 3500

[One Ocean]

Hi everyone,

We have some trouble with our F100B (6OC X 1014050), which we have as a dual engine setup.

It will run absolutely perfect between idle and 2500 RPM, as well as 3500 – WOT, but anything between 2500 and 3500 it will stall rather quick, within seconds. It sounds like it is not getting enough fuel or too much air, as the engine sounds very low and “hollow”.

What I have done so far:

• Swapped TPS between the engines and verified correct voltage.
• Completely disassembled and cleaned the carburetors and all their internals.
• Swapped spark plugs between the engines.
• Swapped coils between the engines.
• Verified a good continues rhythmic spark is present by the use of a timing light.

None of the above have changed anything at all or let the problem travel to the second engine.

My last thoughts would be a chipped reed (although I would assume that this would impact idle more than mid RPM), the stator (Although I would have expected an unusual spark pattern) or the CDI itself.

Any thoughts are greatly appreciated.

[yam911]

What reed?

At about the point where it normally will stall I would start misting some gasoline into the air intake system and see what happens.

[Brianmoore25]

Try swapping the carbs between the engines if you can. These are very finicky carbs on these engines.

[One Ocean]

What reed?

You are of course right, there are no reed valves. I was just too used of having them after carburetors that I was dumb enough to not check the parts catalog properly.

I will try to spray some fuel into the air intake and also swap the carburetors later on and report back.

These are very finicky carbs on these engines.

Could you elaborate on that please? Having them apart and putting them back, as per the Yamaha service manual, seemed really easy and straightforward to me. Nevertheless the problem remained exactly the same, so maybe I missed something and swapping the carbs will show this.

[One Ocean]

Thank you two for the input, the problem is fixed now.

Spraying fuel into the air intake "fixed" the problem, which points of course to the carburetors.

We swapped them between the engines and the problem indeed traveled. After taking the carburetors completely apart and cleaning them once again the problem disappeared. I cannot say what was actually wrong. They looked nice and shiny already before the first time they have been taken apart, and even more so before the second time around.

We assembled them exactly the same way as the first time, but now its running great. Whatever the problem was I must have missed it the first time and truth be told even the second time, but I'm glad its working.

I wish I could leave more details for whoever finds himself in a similar situation, but unfortunately I cannot.

Thanks again for your input, I would have chased ghosts by now.

[yam911]

Thanks for the feedback. Glad the problem is resolved even if you don't know exactly what fixed it.

There are nanosized orifices and passageways within some of those carburetors. Hard to get to and harder still to remove any debris that might be dinging within any particular carburetor.

[Brianmoore25]

Thanks for the feedback. Glad the problem is resolved even if you don't know exactly what fixed it.

There are nanosized orifices and passageways within some of those carburetors. Hard to get to and harder still to remove any debris that might be dinging within any particular carburetor.

Very true on the small orifices. Also to the OP be sure and sync the carbs using vacuum. Also if since the carbs have been removed from the other engine it also needs to be vacuum synced.

[One Ocean]

Very true on the small orifices. Also to the OP be sure and sync the carbs using vacuum. Also if since the carbs have been removed from the other engine it also needs to be vacuum synced.

Thanks everyone again for the input. The first time around I soaked the things in carburetor cleaner, the second time around I "just" used a carburetor cleaner spray. I guess the compressed gas has dislodged whatever devilish nano sized grain was stuck somewhere.

Just to be sure, I have swapped the carbs back to the original engine, after the problem became obvious. Both engines have their original carburetors on them now. If its still necessary to vacuum sync them, I will get someone in, as I'm lacking the tools and knowledge to do so.

[Brianmoore25]

If they have been removed it is necessary to set the vacuum to get the best idle.