Backfiring after rebuild

[Guest]

I'm trying to put the finishing touches on my rebuild and was syncing the carbs yesterday. While I had the carb tune hooked up and was idling about 2000 RPM the number 1 and number 4 cylinders kept backfiring and blowing the vacuum lines off. I'm wondering if I missed one of the adjustments on the carburetor that could cause this. Any ideas or anything I should check?

[LAB3]

I'm new to Suzuki's but not bikes or turning wrenches in general. If it where me I think I'd be looking into an issue with timing or the intake valves not closing/sealing properly. If it where a timing issue my best guess is it would be happening on all the cylinders, guess my next bet would be checking intake valve clearances now that the engine has spun a few times since reassembly.

[gsrick]

+1 on checking valve lash, but it's odd that one a four are effected, but not two and three. The intake cam may be off a tooth, but why isn't two and three also doing the same thing, unless the coil for two and three is weak. Did you set the cam chain on the cams by counting the pins from 2 and 3 marking on the sprockets? IIRC it should be 20, but check the manual to be sure.

[jdion81]

Yeah, I made triple sure that the cam chain was set according to the GS manual. I even double checked it when I pulled the cam cover to check if there was any slippage when the knob fell off of the tensioner (has been replaced and locktight used on the nut). It's not a constant or regular backfire. I am wondering if it could be the idle mixture.

[gsrick]

Yeah, I made triple sure that the cam chain was set according to the GS manual. I even double checked it when I pulled the cam cover to check if there was any slippage when the knob fell off of the tensioner (has been replaced and locktight used on the nut). It's not a constant or regular backfire. I am wondering if it could be the idle mixture.

Try enrichening those carbs a little.

[Burque73]

Mine was doing the same exact thing. I got little hose clamps to keep the hoses on while I was synchronizing the carbs. I thought maybe the spark plug wires we're causing a misfire so I'm in the process of digging the old wires out of the ignition coils. I'll let you know if that solves the problem in the next couple of days.

[jdion81]

I've got brand new Dynatek 3 ohm coils installed.

[05Astro]

Arnt 1 and 4 on the same coil? Perhaps a glitch somewhere there?

[Burque73]

I've got brand new Dynatek 3 ohm coils installed.

Boy that sure rules out crappy old wires then doesn't it LOL. I'm going to pull my cover and check the timing like Rick suggested which will also give me a chance to change a couple of shims to get the clearances exactly where I wanted them. They're in spec but just barely. New shims shoild be here soon.

I'll be following this thread to see if you figure it out. Each little "backfire" if it's the same thing you experience, coincides with or makes a little "tick" sound in the carb which I assumed was due to pressure going the wrong direction.

[jdion81]

I got the backfire out by adjusting the idle mix. After I synchronized the carbs I took the bike out for a test ride and while it ran great sitting in the driveway it was a little rough on the road. Could my synch job have been done wrong? Anything to check?

I should say that it did still run great while accelerating, but there is a limit to how much you can keep doing that.