Engine Spitting Back Through Carbs?

[stever]

Good day, everyone! Question, why would an engine spit back (backfire?) through the carbs when it is first started up and rpms increased? My engine has started doing this recently. I have topped up the carbs with damper oil (from Moss) and the timing appears correct - but it still does it. It only does it until the engine is fully warmed up, then it is ok. Otherwise runs really great, no misses, lots of power, smooth acceleration, etc. Appreciate your help! STeve

 

[RF Thom]

Symptom of lean mixture with choke on. Check choke mechanism to ensure that it is moving freely when the cable is being pulled, also watch the mechanism between the carbs again to ensure that it is attached at both ends.

Regards, Bob

 

[aeronca65t]

I'll assume that using the choke minimizes this problem (does it?). Sounds like slight lean conditon.

Check fuel filter.

Look for any vacuum leaks (a good test of possible leaks is to spray WD-40 near suspect areas....the engine will ingest the spray and slow down a bit).

Check float level...could be set "low" and you may have to bend float tab.

Dirt in main jet (quick fix: pull piston up and blow back through hole in bottom of float chamber with *low* air pressure or WD-40 spray straw).

Loose or "dropped" needle.

Is you main jet adjusted down the correct amount of "flats"?

Also, never hurts to check plugs and points (just in case it's a bit of "cold" misfire)...and be sure air filters are clean.

 

[ThomP]

My BJ8 had this problem from the day I bought it, what I discovered was the one of the needles was not properly located in the piston. The old style needle has no shoulder on it to indicate proper placement in the piston, this needle doesn't even have the groove that some needles do. What I discovered was that one carb would be RICH at idle, and quite lean at normal operating speeds while the other carb was within limits at both speeds. One might think a different needle was required. What I discovered was that by lowering the sholderless needle, I could get the same operating balance as the other carb.

I purchased a Gunson Gastester to discover this problem. Since reseating the needle and adjusting the CO level to about 2.75% at idle, the car has become a dream to start, I can remove the choke sooner in the warm-up cycle and seldom (very cold days choke off too soon) do I have that nasty backfire.

The car came with numerous other little carburetter features that have also been sorted, like the wrong springs, and idle speed adjustment via use of the Fast Idle adjusting screws. These were very easy to find compared to the mis-aligned meetering needle.

Hope you have luck in finding you gremlin.