Backfiring through carb on starting

[Guest]

Thanks for the quick response. I haven't adjusted the valves I'm ashamed to say. I'll get on that.

[gsrick]

Started a profile because I'm having a problem I can't find an answer for in this forum. Bike is a 1980 GS 750 E.

Never had a problem until I got back from a 6 hour trip yesterday
Used to start right up no problem. But now it will backfire loudly or of the#1 carburetor. I still turning it over, then it starts up like normal. The vacuum petcock was leaking through the vacuum line and I plugged it and run the bike on prime. In 4000 miles that hadn't given me any issues but maybe it is now?

For it to backfire out of the carb an intake valve has to be open at the time of combustion.

Have you adjusted the valves since you've had it? It's more than likely one of the intake valves on that cylinder and possibly others has gotten so tight that the valve is being held open.

[Ryan J]

For it to backfire out of the carb an intake valve has to be open at the time of combustion.

Have you adjusted the valves since you've had it? It's more than likely one of the intake valves on that cylinder and possibly others has gotten so tight that the valve is being held open.

Thank you. I thought that would have to be the case but thought I'd check before taking the valve cover off. I'm ashamed I haven't checked them. 22k miles when I got it and I don't know if it's ever been done. I'll order a new valve cover gasket and get it done. Silver if there's a possibility I burned a valve?

[gsrick]

Thank you. I thought that would have to be the case but thought I'd check before taking the valve cover off. I'm ashamed I haven't checked them. 22k miles when I got it and I don't know if it's ever been done. I'll order a new valve cover gasket and get it done. Silver if there's a possibility I burned a valve?

I doubt it, but I guess it's possible. What you can do is a compression test and then a leak down test before and after adjusting the valves. Once you know the clearances are correct and you still have a valve leak then I would think you may have a burnt or bent valve, but I think thats rare especially if it has just started doing it.

[Tom R]

If it's only on start up, it's probably a lean condition caused by a partially clogged pilot jet or passages. Have you cleaned the carbs lately?

[Clc021583]

Did you ever figure out what the cause was?