Backfiring at 4000 RPM

[Red Baron]

I have a 78E, I have re-built and synchronized my carburetors, my bike idles and runs well. The problem is once I reach 4000 rpm and even the bike out to cruise I get backfiring from the #1 or #2 cylinder, I have re-cleaned the carbs and resynched several times. I have cleaned and resealed the carb boots. I have streamlined the fuel delivery system, per recommendations on this board, and cleaned and reconditioned the inside of the tank. New fuel lines, filters and I cleaned the individual air filters. Bike runs great other than this aggravation.
The Condition can be reproduced by revving motor to 4000 rpm and setting cruise control. Changing screw adjustment effects the backfire, from sporradic to uniform and from machine gun type sound to cannon type sound. I think I have narrowed this down to carb #1. Any suggestions would be appreciated. The backfire is through the exhaust. I have not noticed any carburetor spit back, but at 4000 rpm down the road it is hard to tell. As long as the bike is in a pull no backfire is evident. I am sure I'm missing the obvious thanks for your help.

History:
Bike has 30 k miles.
I have tuned it up. New plugs, oil change, adjusted the timing chain. Rewired half the bike and repaired most of the original wiring. Including the coil pick-up fix.
I have replaced the stock mufflers with Harley Fat Boys, per directions listed on this web site, by the way, easy and I am very satisfied with the results. However this problem occured with both the old original mufflers, and the new. I changed because the old ones were broken and I thought this may be the problem.
I have tried running the engine with the petcocks on pri and the vaccum lines plugged to see if I had a vaccum leak, but can not find one. I have also removed, checked and coated the rubber boots with the napa product suggested here. Remounted them but this did not help this condition.
I have checked the coils with a multimeter, they read in line with the numbers listed in the Clymer's manual, for resistance, etc.
The bike runs great with this exception, it will idle as low as 500 rpm, and has no hesitation when pulling off. I really love this bike but this is very irritating. I have a Clymer's manual if anyone could direct me as to where to begin a new search for the cause of this problem, or which search to begin again. One other observation the choke operates fine and the bike runs smooth on full choke, but when setting it to the second position the bike sounds like a bowl of rice krispies, but full release of the choke makes this go away.

[skids]

Re: Backfiring at 4000 RPM

I don't know how you have the individual filters rigged for the breather hoses. Someone reported that the fuel levels will rise significantly in the main nozzles due to a lack of vacuum above the gas surfaces if the breather hoses are not hooked up. I don't know reall for sure, but it makes sense to me.

[sixtysix]

Have you adjusted the vavles? And double checked cam timing? Float levels are correct in carbs? Did you check for air leaks around carbs with starting fliud or propane or something (whatever you use-not sure)? Sometimes cracks in the exhaust can cause backfires.

[Dennyz]

The XS fires each spark plug twice, 180 degrees apart. If the fuel air mixture in the exhasut system is right, it can be ignited by the second firing.

I have seen both rich and lean conditions cause this. I have also seen exhaust leaks do the same thing.

Since it does not do it under 4,000 or under power, my guess would be that one of the carbs is running rich (sounds like #1). If I remember correctly, by 4,000 rpm, the carb is out of the pilot circuit and fully into the main jet. Its possible the the float bowl is set slightly high.

That's my $.02

[newmaac]

When you say "backfire", are you referring to an actual backfire through the exhaust?
A lean situation, either from being ajusted too lean, or from airleaks, can cause popping back through the carbs.
Are you certain of your mixture setting and float levels?
Do you have access to a colortune kit?
I would be curious to see what the colortune would show.
Let us know what you find.

[newmaac]

I just re-read your original post.
The first time through, I missed where you stated that it sounds like it is in the exhaust.
You might want to check with colortune anyway though, just to see exactly what your mixture is.

[Red Baron]

Thanks All,
Turned out to be simple fuel starvation. Rerouted hoses for better flow and viola! No more backfiring. Neighbors much more civil.