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Question about Honda BF75 EFI Throttle body cleaning and adjusting

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Pikeman59

Member
I have a CC with twin Honda BF75EFI and the starboard engine has lost 400 RPM at WOT compared with the port engine. The starboard engine runs great but has a slight misfiring at idle but runs strong all the way up to WOT. I have done everything to find out what causes the difference, changed plugs, switched and cleaned injectors, all new filters and new high pressure pumps. Switched ECU:s, props and so on. But the rpm difference is still there. This spring I took off the air silencers and cleaned the throttle bodies and cleaned Map sensors. The Map sensors looked OK but cleaned them. The throttle bodies were a bit dirty with some oil in the dirt. Both throttle bodies had the same amount of dirt in them but the starboard butterfly was not fully horizióntal at WOT, not the port one either, but a little more open than the starboard one. Adjusted both butterflies to almost horizontal. My question is if it is possible that they change over time and should the butterflies be totally horizontal at all? If not then you not getting WOT or?? When starting the starboard engine it was running ok and it feels that the misfiring is almost gone so next step is to take a testrun on the lake to see what happens. Any input is appreciated.
Thanks//Pikeman59
 
I may get a load of crap over this, but regarding your horizontal butterfly question, you would be amazed at how little difference a slightly closed butterfly might make as compared to a fully open one.

This is typical of ANY butterfly type carb. Try it on your lawnmower or something to prove it to yourself. There is VERY little extra power available/being made past about 60 degrees of travel, so the difference in a throttle plate, one at 88 degrees vs. one at 90, is virtually nothing.

That said, I'm not sure if the throttle position sensor knows that. It may be adding more fuel to the mixture well past 60 degrees.

Slide type carbs, like those used on motorcycles and snowmobiles, are totally different, have their own set of rules. What I'm saying applies to BUTTERFLY carbs only....


Regarding the difference in rpm one engine vs. the other, I'm curious if you have done a compression check yet? If so, and everything is within specs, I'd just run it.
 
Alan Hicks said:
I may get a load of crap over this, but regarding your horizontal butterfly question, you would be amazed at how little difference a slightly closed butterfly might make as compared to a fully open one.

This is typical of ANY butterfly type carb. Try it on your lawnmower or something to prove it to yourself. There is VERY little extra power available/being made past about 60 degrees of travel, so the difference in a throttle plate, one at 88 degrees vs. one at 90, is virtually nothing.

That said, I'm not sure if the throttle position sensor knows that. It may be adding more fuel to the mixture well past 60 degrees.

Slide type carbs, like those used on motorcycles and snowmobiles, are totally different, have their own set of rules. What I'm saying applies to BUTTERFLY carbs only....


Regarding the difference in rpm one engine vs. the other, I'm curious if you have done a compression check yet? If so, and everything is within specs, I'd just run it.
Click to expand...



Compression is done and within specs and its a fuel injected engine, not a carburated one. I think the TPS will add more fuel up to horisontalposition, at least I hope so.
 
Throttle body vs butterfly carb, the point is the same...and will be as long as there is a butterfly controlling air volume.
 
What tacho's are you using. If faria , then you may be chasing nothing, they are crap and not accurate and can go out for no reason. I would use a reliable tacho to confirm before you chase a problem that probably doesn't exist
 
iang6766 said:
What tacho's are you using. If faria , then you may be chasing nothing, they are crap and not accurate and can go out for no reason. I would use a reliable tacho to confirm before you chase a problem that probably doesn't exist
Click to expand...

Maybe switch tachs (or tach wires) to see if the problem follows?
 
Now you need DrH so that ypu can monitor sensor values. For example, an O2 sensor can become slow to adjust values but not set a fault code/alarm.
 
Just a question here:

How did you go about cleaning the MAP sensors and what "solvent" did you use if any?
 
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