Cause of Backfiring Through Carbs

Since I rebuilt the carburetors on my '87 VFR700 I've noticed I get a 'PFFFT' sound out of the intake while cold when the choke is on. It goes away as soon as it's warmed up a bit and I open the choke. When I did the carbs I used the higher float level setting for bikes outside the U.S., which seems to have helped the performance all around. The jetting is stock otherwise. Any suggestions as to what's causing this?

Thank you,

Rick

 

Are you certain that all your intake valves are adjusted properly and the valve seats are completely clean?

If one of your intake valves is hung up by a tiny carbon deposit on the valve seat or is adjusted too tightly and held just slightly open it could cause a backfire through the carbs.

 

Yes, I'm pretty sure. It had never done it before, and now only does it when cold with the choke on. I think it's either too lean, or too rich...But there isn't much control with the choke...It's not supposed to do that...

 

I agree w/above. if possible do a compression check on that cylinder.
my 1985 did same thing and it was a valve . hope you get it figured out.

 

As mentioned above valve tight or leaking can cause spitting out the carb, vacume/intake boot leaks, a carb is too lean, carbs sync, all the diaphrams seated correctly, float levels all the same, plugs all good, idle mixture screews set correctly, I think mine are 2-1/2 turns out and I never need to choke it unless it's below about 55 degrees. There's a pic in my restoration pics of the clear tubes I added on my carb drains to check float bowl level while on the bike.

 

Thank you very much for the advice...Last night while on the way home from work it did it while warmed up for the first time. Checking the valves is a pain on this bike, but I suppose I have little choice. I last checked and adjusted them about 10K miles ago. There was one that gave me a bit of trouble if I recall, though it was an exhaust valve...I'll first try confirming the float levels and adjustment, of course

Thanks again,

Rick