Backfiring through carburetor

[jmink427]

This has happen a few times with my bike... This seems to happen when I'm starting or just cracking the throttle. I was thinking that I had to much timing but it looks good. So then I thinking it may be lean, but it actually smells a little on the fat side... I guess the next thing is to speed the idle speed up a bit and see where this takes me... Any suggestions?

 

[wherearewe]

My first guess would have been timing. What rpm have you got the idle set too?

 

[peanut]

if your timing is ok and the cam chain adjusted correctly ,
might be worth checking the inlet valve clearance and do a compression test to eliminate an inlet valve not seating properly due to sticky valve ,too little valve lash etc
inlet 0.05mm (0.002")

 

[jmink427]

I do have it idled down a bit I'm not sure of the rpm since I took the all the instruments of the bike... I may pull a little timing and see what happens...

 

[scabber]

I would set the timing 40 degrees total advance. Since you don't have a tachometer.

 

[jmink427]

How are you checking for 40 degrees? I have just been using the factory timing marks.....

 

[scabber]

The factory marks are what I use. The 40 deg is factory setting

 

[grizld1]

jmink, the automatic timing unit on points ignitions (retained with some electronic systems) is usually the source of the trouble you're describing. When wear occurs and there's slop in the unit, the interval between full retard and full advance increases. The result is that when you set the advance timing for 40* BTDC as per the book you blow the carbs out of the boots from time to time and find yourself with a bad off-idle flat spot, and if you set the idle timing to 15* BTDC as per the book the advance timing winds up in the piston-holing range; in fact every on XS650 with a holed piston that I've postmortemed (and it's been a few, over the years--none of them trashed on my watch), a defective ATU was responsible. There are plenty of threads on this and I'm not going to rewrite them. Look 'em up and read. There are several cheap fixes, but the best solution is a new ATU or an electronic ignition that does away with it.

 

[scabber]

Like grizld 1 said. the best you can do now is see where you are at low idle or put a tach on to check. Not the best way but you wont hole a piston

 

[jmink427]

At full advance mine is right on the factory mark.... What rpm should full timing come in?

 

[TwoManyXS1Bs]

Full advance should be at 3000rpm.

 

[scabber]

Around 3500 RPM if I remember right. My bike likes 1500 rpm idle 1200 Rpm doesn't really change it that I can tell.

 

[jmink427]

Thanks.... I'm gonna look at the advance unit tomorrow make sure it's moving freely and hook a tach up and check to see when full timing is achieved....

 

[scabber]

Memory not as good as it use to be. As long as you advance gradually increases with RPS sounds like your set right