backfire thru the carb.

Started to happen all the time so I pulled the carbs, sprayed them down with carb clean. Seemed to help but one carb still has seldom backfire.

timming is wrong
could be advancer
could be timming chain
its the advancer always has been

I was having this problem, and it turned out one od my idle jets was clogged. All my other jets were clean. The bike still ran pretty well when warmed up, was just a bit hard to warm up (had to keep fiddling with the choke or it would die).

Or the timing...like vrdash says. When does it normally backfire? Idle, constant load, acceleration, all of the above?

That can be caused by the carb holders leaking. Check them for leaks like this:

Make sure the vacuum caps are tight on the carb holder nipples.

Temporarily set the idle as low as possible without stalling the engine. It should be down below 1000 rpm. The lower the better because when it is set low the carb slides are closed (or nearly closed) and this raises the vacuum level in the carb holders. The combination of the increased carb holder vacuum level and the very low idle amplifies the effect of the leak test.

Once the idle is set low, spray carb cleaner around each of the carb holders where they bolt to the cylinder head and where the carbs attach to them. If there are leaks, the vacuum in the carb holders will suck the carb cleaner in and this will affect the idle. It may cause the engine to stall. If the leaks are at the carb holder / cylinder head mating surface you need to replace the holders. If the leak is where the carbs attach to the holders you may be able to tighten the clamps enough to seal the leak. Some folks use starter fluid or other substances for the test, but carb cleaner works best as it doesn’t evaporate too quickly like ether and leaves no residue on the engine like WD40.

If you do need new holders just bite the bullet and buy them. Trying to seal things up with some kind of sealant or goop will only lead to frustration as it will at best only last a short time.

Ed

DivideOverflow wrote: I was having this problem, and it turned out one od my idle jets was clogged. All my other jets were clean. The bike still ran pretty well when warmed up, was just a bit hard to warm up (had to keep fiddling with the choke or it would die).

Or the timing...like vrdash says. When does it normally backfire? Idle, constant load, acceleration, all of the above?

it backfires at idle and constant load

A too tight intake valve (insufficient clearance) -- or other condition that prevents the valve from fully sealing when closed against its seat (burned, broken, etc.) -- can result in backfiring through the carb, as part of the combustion escapes past the leaking "closed" valve.

During the power stroke, the combustion is supposed to be fully contained within the cylinder. But if some of the combustion is escaping past the closed intake valve, it goes on out through the intake port and backward through the carb.

Would check the valve clearances (the simple, relatively easy first step), and if necessary, adjust the clearances to within specs (the more complicated follow-up).

Valve clearance is needed to allow the valves to continue fully seating at normal engine operating temperatures.

Good Fortune!