Backfire on cold start

[rlong74]

Greetings wise men/women of the forum.
My 81 450t has been running like a champ since I cleaned the carbs; replaced o-rings, dipped, all the good stuff. I also recently replaced the coils, ignitor and signal generator. Also there is a bug somewhere in the fuel delivery that necessitates riding with the fuel cock in the PRI position then returning it to the on position when off. I say bug because the petcock is new and sometimes it will run in the on position.
Anyway i will get to the point. The other day i had left the petcock not fully in the ON position and i tried to start her and BAM! backfire. This was followed by an unusual smell. Yesterday she started fine. Today, backfire again. I pulled the plugs and the left side was wet right side dry.
Does no spark in one cylinder cause an overload in the other and cause a backfire?
Any thoughts?

[earlfor]

First thought is it is not an ignition problem. I'm inclined to suspect a leaking petcock which is allowing fuel to seep into one cylinder via the vacuum line. I avoided that problem area on my 450 by installing a manual Pingel petcock. No vacuum line, manual on/off/reserve.

Earl

[chef1366]

Unburnt fuel on the non-firing stroke. The plug still fires with valves open.
Sounds like you got a bunk petcock.

[mike-s]

I'd also check that the needle float isn't excessively worn and that the float is also at the right level, as the petcock is only one of three things that must malfunction in order for excessive fuel to be leaked through to the carby's.

[rlong74]

Thank you. I appreciate the feedback. I checked both of those when i had them apart two months ago. I dread doing that again so soon. Earlfor, what did you have to do to mount that petcock? Did it bolt right on?

[earlfor]

I'd also check that the needle float isn't excessively worn and that the float is also at the right level, as the petcock is only one of three things that must malfunction in order for excessive fuel to be leaked through to the carby's.

The fuel inlet valves may be leaking, but fuel will not flow when the engine is not running if the petcock is shutting of fuel flow properly. Its like sticking a straw in a glass of water. Both ends are open, but put your finger over the top end of the straw, lift it out of the glass and it will hold water even though the bottom end is open.

Earl

[trippivot]

get total control of your fuel.

the fuel tap is the first stop in the delivery chain. fluid side - vacuum side- liquid has weight and pressure from gravity. it will find a way closer to the earth even while we sleep. inside the bike or out it don't care.

next is the float needles same deal there

worry about spark after the fuel is under total control. and get a couple few new NGK plugs.