carb adjustment and backfire

[cabz]

Can anyone give me a hint as to why my gt750k is backfiring through the
right carb? It only does it if I crack the throttle.
I have set the timing , set the points , new plugs , very clean carbs.
and I have the carbs synced( as best I can that is )

also when syncing the carbs the unisyn float was perfect at idle , all
three carbs floating the ball the same height.
At around 2,000 rpm i rechecked and the center cyl has the float ball
on the unisyn to the next line up ? I have rechecked the cables and the
match marks and all are dead on.

any ideas?

[rngdng]

Interesting, you may be seeing the reason that Suzuki jetting the center cylinder differently.


In my experience, most backfiring is due to timing. If that's correct (and not drifting), I'm not sure what could cause it. Have you done compression checks on the cylinders? Crankcase pressure/vacuum would also be a good thing to check.


Lane

[TLRam1]

Could your points (not firing each time) be an issue or condenser. Are they new also? As stated backfiring is tied to timing unless you have an ignition problem causing it to misfire at times.

[cabz]

Interesting, you may be seeing the reason that Suzuki jetting the center cylinder differently.


In my experience, most backfiring is due to timing. If that's correct (and not drifting), I'm not sure what could cause it. Have you done compression checks on the cylinders? Crankcase pressure/vacuum would also be a good thing to check.


Lane

when you say drifting? what would cause that?
The compression test showed 120 per cyl as even as can be
the timing was way off and now it is set dead on and I still have this issue
I havent done a psi and vac test yet, I was hopeing to ride the old girl before I had an issue with crank seals

[cabz]

Could your points (not firing each time) be an issue or condenser. Are they new also? As stated backfiring is tied to timing unless you have an ignition problem causing it to misfire at times.

the points and codensor are old they where on the bike when i got it .
I cleaned and set them and they seem to be ok.
I am gonna get a new set though and replace them and recheck the timing .
right now everything is close enough for me to log some miles as long as I dont try and race the old girl I think it will be fine .

[rngdng]

I had one drifting around because of a failing starter drive gear. The screws were loose, and it allowed the timing to shift around randomly.


Lane

[Arne]

I'd ride it for a bit (watching the plugs closely and listening for detonation) and see if it clears out.

500 miles will fix some problems and reveal ones you hadn't even thought about!

[cabz]

I'd ride it for a bit (watching the plugs closely and listening for detonation) and see if it clears out.

500 miles will fix some problems and reveal ones you hadn't even thought about!

thats kind of where I am heading , I figure a tank or two of good gas and some road miles are just what the doc ordered .
I have everything close enough to not worry about melting a piston


How do you hear detonation on one of these old two strokes? I usualy cant hear myself think on one Thats kinda why the are such a stress reliver

[water cooled]

Cabz,

Here is a links to the Mikuni site that might help you a little. I suspect you have 32mm rounds on your bike. This information is for a different type of Mikuni carb but some of the information is general enough that it might help you zero in a bit more. It was helpful to me as I was trying to learn and understand carb problems I was having stepping up to 36mm and then 38mm for my bike.

Good luck...

http://www.mikuni.com/fs-tuning_guide.html[url]

[TLRam1]

Try this one.

http://www.mikuni.com/fs-tuning_guide.html

[water cooled]

Thanks Terry...

I had a lot of trouble sending that post for some reason. Thanks for fixing the link.

Hope it helps him.

[cabz]

Thanks Terry...

I had a lot of trouble sending that post for some reason. Thanks for fixing the link.

Hope it helps him.

thanks guys I'll give it a whirl

[water cooled]

Cabz,

I just finished up my sons YZ250F. He was having the same problems you were. Backfiring etc.

I found the rubber intake was cracked but wouldnt have seen it if I didnt remove both hose clamps.

Take a good close look at the rubber inlets and make sure they are good and in full contact around the spigot.

Good luck...

[bikegeezer]

also when syncing the carbs the unisyn float was perfect at idle , all
three carbs floating the ball the same height.
At around 2,000 rpm i rechecked and the center cyl has the float ball
on the unisyn to the next line up ? I have rechecked the cables and the
match marks and all are dead on.

any ideas?

This probably won't address the backfiring (or is it missing?), but you'll at least remove carb synch from the equation. You might want to pinch the petcock vacuum line closed and run the engine in the "Pri" position before messing with your adsjustments, just to be sure a vacuum leak in the petcock isn't causing your problem.

At correct idle rpm, there should be a small amount of slack in each throttle cable - around 3mm measured at the carb top adjusters, so the bike is idling on the throttle stop screws. The match marks aren't accurate enough for off-idle setting, though they should get you close. A better way is to raise the idle rpm up to 1500 using the throttle cable adjuster at the throttle grip. Then use the Uni-Syn to synch the carbs on the cables by adjusting each at the adjuster on the carb tops.



Stu

[tz375]

In my experience, the most likely causes are a slow jet flowing much less that the other 2, or a partially blocked pilot outlet passageway.

Next is a crack or leak in the intake manifold.