Still trying to figure out why it's missing

[indyblue]

'84 FJ1100 39K miles, Supertrapp 4-2, K-N pods, (supposedly properly jetted).

Blip test is perfect, no hesitation and revs fine then drops right down to idle properly. 

Plugs look pretty good (my first suspect was a fowled plug or two).  Maybe play with the gaps?  They are all at .035.

Runs perfectly smooth under moderate-high throttle, pulls like a freight train.

But under light throttle, just enough to keep constant speed, it misses - sometimes badly, and only between 2700-3400 rpm.  It runs properly under all other conditions.

What to look for?  If it were kinking fuel line it should cut out under WOT, but it will scream all the way to 9K rpm just fine.

Worn needles or jets?  Is there something in the carbs where there is a transition between low flow and high flow?

[racerman_27410]

the inside of your carbs are dirty.

partially clogged pilot jets and partially clogged holes (the small ones) in the emulsion tubes will cause this exact behavior.

kookaloo!

[indyblue]

Thanks, good observation and I hope that is it (though  I don't relish the thought of have to remove and break down the carbs).

Why does it run so well when cold though?  Wouldn't clogged passages affect cold and hot operation?  It gets worse the hotter it gets.

[FJmonkey]

Your idle circuits (the smallest ones) are clogged. Using the choke pulls fuel in from other circuits to make cold starts really rich. I had the same problem after sitting for a while. Took the idle jets out and soaked them over night, punched them out with a nylon bristle (not metal, easy to damage the jets), squirted cleaner through them. Reassembled and WOW, it worked proper. Now I use fuel stabilizer for the short cold rainy season and make sure I run the engine once a month if I don't ride. The occasional carb cleaner in the tank just for good measure. No need to winterize in SoCal...Good luck mate.

[indyblue]

I believe it is carb/dirt related, but it is really non-intuitive.  The choke doesn't seem explain it since I only use the choke to get it started.  After 1-2 minutes of running with choke it runs without choke just fine until it gets to full temp when it begins to miss.

[Flynt]

I've seen this sort of thing with electrical systems in cars.  Could be as simple as a cracked spark plug, but usually things that develop as the engine heats up are electrical in nature somehow...  my experience FWIW.

Frank

[indyblue]

Normally I would agree with the hot electrical possibility, but since it runs well at all other RPM ranges (even when hot) that doesn't fit.  A weak spark would probably show up when at WOT.

I checked the plugs, they had some soot on them indicating a rich condition and the Supertrapp baffles on the right hand pipe have become significantly covered in soot.  Left side has some soot, but seems to  be an acceptable amount.

If there is a blockage in the carbs, wouldn't that make it lean?

I am thinking either the jets/needles are worn or the carbs were not properly jetted as the PO claims (though he owned this cycle since new and is a racer).

Any other possibilities?

[SlowOldGuy]

It's beginning to sound like there's too much fuel in the float bowls.  Check your fuel level.  Under light throttle conditions, too much fuel can cause stumbling which clears up when you accelerate.  It also get worse when the engine heats up because the overly rich condition get even richer, which appears to be what the plugs and pipes are telling you.

DavidR.

[andyb]

In a taller gear (say, fourth), at a steady cruise around 2500, snap the gas to WOT.  What happens?  That's usually where fuel level issues show up for me.

[indyblue]

In a taller gear (say, fourth), at a steady cruise around 2500, snap the gas to WOT.  What happens?  That's usually where fuel level issues show up for me.

I goes!  Very fast.

[andyb]

Then i'd imagine fuel level is near okay.  I'd agree on checking for dirt in unusual spots, like on the emulsion tubes.  There's a chance of worn emulsion tubes as well, as that's just about where you're starting to transition onto the needles.  Depending on how much futzing you're willing to put up with, I'd probably shim the needles in either direction and see if you can shift it around.

[indyblue]

I found an interesting article at http://www.factorypro.com/tech_tuning_procedures/tuning_float_height_adj_procedure.html

Why adjust the float height?

Changing the float height changes the level of the fuel in the float bowl. The fuel height adjusts the full throttle, 2k-3k rpm and part throttle cruise, as in cruising around town, trying to be quiet... To give a scale of change, if the bike runs well when cold, but gets a bit sloppy when fully warmed up, lower the fuel level 1mm (i.e. go from 15mm to 16mm float height - remember the float measurement is "backwards").

So I will try lowering my float by 1mm and see what happens.

Any opinions if this will work?

[indyblue]

What does the pilot adjustment do?  According to the cutaway view in the service manual, fuel/air mix flows through the pilot screw and some of it constantly bypasses it through some passages in front of the screw.  I am guessing that turning the screw out makes it richer and in makes it leaner - is this correct?  The #3 & 4 cylinders are fouling plugs and I am having a hard time getting them re-adjusted correctly.  Too far in and it misses, too far out and it fouls ( or vice versa, I got tired of pulling/reading plugs by now).

I have not yet lowered the fuel level by raising the float level and that will be my next experiment.  I have been assured by the PO it is correctly jetted. and I tend to believe them since they owned this bike since new and has 2 R1's that are raced now.

It still runs perfectly when cool/warm, but starts missing when full temp at cruising throttle @ ~2700-3400 RPM

[RichBaker]

Turning the pilot screw out richens, in is leaner.  Sounds to me like float level is a problem, when you're in there be sure to check/replace the inlet needle seat o-rings. They could be old and leaky, which would contribute to this problem.