Motorcycle Repair: backfiring and spitting back, dyno jet kit, mile test drive

Question
QUESTION: I purchased a 2004 carbureted electra glide standard in 2008 with 3900 miles on it from an individual. The bike had screaming eagle slip-on mufflers and a screaming eagle air filter installed before it ever left the dealership. I am the second owner and the bike now has 9100 miles. While test driving it, I heard it spit back through the carb while sitting at a stop sign. This was only one spit and the motor recovered in a second. This was the only flaw that was discovered during the 15 mile test drive.I talked to an independant, well respected Harley mechanic and he advised me this was not a big issue. Ever since, this has been a constant problem and now it has started backfiring out the right muffler also. This occurs when cold and hot, slow speeds and highway speeds. It mostly occurs when making slow maneuvers.(Example- while making u-turns or slow left or right turns while pulling in the clutch and goosing the gas throttle on and off, the carb will spit back and cause the rear wheel to loose it's power and sometimes it has almost caused me to drop the bike.) Several times it has occurred just as I've pulled onto the highway and accelerating through the gears. Once it occurred while coasting down off a mountain in 5th gear at about 60 MPH. I first took the bike to the independant Harley mechanic and he put a larger jet in the carb. Don't know what size. This did not help. Next I took it to a Harley Dealership and they said it had a thunderslide kit in the carb and they replaced it with a whole new stock kit and rejetted everything back to stock settings. This did not help so I took it back and they enriched the fuel mixture. This helped a lot, but did not fix it. I asked them would they suggest enriching the fuel a little more and they said no and there's nothing else they can do. Next I took it to a different Harley Dealership and they replaced the stock kit for a dyno jet kit and tunned the carb on the dyno machine. This did not help and they said I just had to accept that the spitting back was just the nature of the beast and nothing else they could do. Next I took it to another Independant Harley Mechanic and he said he checked everything he could think of such as the lean/angle shut-off box, bad connections with the wiring and other things I can't remember. He found nothing wrong. The bike is still not fixed and now it has also started shutting off at highway speeds as if I had hit the kill switch. The check engine light does not come on. Any help GREATLEY appreciated. Thanks. Mark

ANSWER: it doesnt sound like you have a carb problem. I would start by check to see if you have a intake leak or a exhaust leak. You can feel a exhaust leak with yoru hand. Be careful and dont get burned. to see if you have a intake leak, start the bike and spray a little carb cleaner around the intake. If the RPMs go up, then you have a leak.
If the above is ok, then I would do a compression test and a leak down test to see if you have a valve that is not sealing. Since it pops through the carb, it would probably be a intake valve.
Good luck and happy riding
Mike

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QUESTION: Thanks for your quick response and expert opinion. I failed to tell you that this is not a problem every time I ride. It has happened anywhere from one time to as many as three times during one ride, and then it may not do it again for maybe 200 miles. Its like a cycle. It happens and then it goes away for away and then happens again. None of the mechanic's have been able to duplicate the problem. Also one of the mechanic's did check for leaks and did not find any. Knowing this, does your suggestion remain the same. Also, what about it shutting off at highway speeds. Is this related to the spitting back or a complete different problem. If it is not related, what could cause this? Thanks.

ANSWER: I would still do the tests on the compression and leakdown. Pull your plugs and let me know what color they are
Mike

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QUESTION: Front and rear plugs are of equal discoloration and are black, not just on the tip end but all over. They have 5100 miles on them. Also seems like there is a heavy build-up of carbon. I will have to take it to a mechanic for the compression and leakdown tests. Thanks.
Mark

Answer
Since your plugs are fouled, there are two reasons that can happen. First is your carb is jetted to rich and second is the coil or the plug wires are weak. You can do a OHMs test on the coil and plug wires to determine if they are bad. If they check out ok, then I would change the intermediate jet to the next size smaller and see if you still foul plugs.
Do the above before doing the compression and leakdown tests. you might not need them
Good luck and happy riding
Mike