I have a 75 xlch, .20 over , and 10:1, has edelbrock quicksilver 40mm carb ( i know they suck), cant afford a new one right now. So first 3 gears seem ok, get into 4th and it starts hesitating, and kinda sputtering if I roll on the throttle, now if I do some quick bursts on the throttle it seems to keep picking up speed, after I do this sometimes it will back fire. I have been doing the plug checks as I had to do a lot of tuning when I got the bike. The ceramic is just very,very light tan ( pretty close to white) the outer rim is black and the piece thats over the ceremic is discolored. plugs tell me that im pretty much on the money as far as tuning. anyone have any suggestions. I even tried to richen the carb a little and it ran pretty poor when I did that.

I actually had a very similar problem not that long ago, but I run a mikuni carb. But I think your problem might actually be the same as mine.

Just for S&G's make sure that your intake manifold is matched up properly. This is where my problem lied. Here's why, the harley design here in this one particular area is little to be desired. They used those very small and not very reliable o-ring gaskets that match the head to the manifold. If when your putting those on and they get screwed up any way or the manifold gap is uneven it will lead to problems. Thats exactly where my problem came from. One of the rings had come up slightly while I was putting the carb back on.

Like I said check that first (I know it sounds like it's impossible but just look).

Another thing it could be is your intermediate and/or your high speed jet (I am not familiar with how the quicksilver is set up) if one of those are fouled or clogged any with debris it will do exactly what you say. If the above doesn't work try removing your carb, dissaembeling it and cleaning all the jets paths with carb cleaner. Make sure to note where everything is set before cleaning and be careful as you go as parts are probably limited like parts are for my carb.

Good luck.

Supr2nr.......
I agree with you regarding the possible manifold leak in that he should check that out.
The main reason I have found to cause backfiring is an air leak.
On the other hand, if the manifold leaks air it almost always makes the engine run and idle roughly at low end.........pg

Same thoughts... leak on manifold make sure that your airbox is tightly in is't place all bolts tight
thighten O-ring bands after that.