PDA

View Full Version : Backfiring thru the carb.



ron3865
01-30-2011, 07:46 PM
I've got a Loadstar 1600 with either a 304 or a 345 with a backfiring problem.
It starts without to much of a problem and idles ok, hot or cold. The backfiring happens when a load is applied. Come up easy on the power and there isn't too much of a problem but give it the gas and it pops thru the carb in what seems like a pretty regular beat, as maybe only one cylinder is the problem.
The plugs, wires, points cond., etc., etc. are new. The carb is clean and the idle air is set the beat I can by ear. No pushrods are bent and the plug wires are all on the right plugs. I've seperated the wires thinking there might be a short between touching wires but that doesn't seem to be a problem.
I can't see a timing mark on the bottom pully so I just used #4 as it was easy to get at and timed it off of that one using the thumb in the hole method to get tdc.
I'm at a bit of a loss as I would think an intake valve has to be open for a carb backfire but why does it run much better when the throttle is applied slowly or there is no load on the engine?

Allan E.
01-31-2011, 12:12 PM
I've got a Loadstar 1600 with either a 304 or a 345 with a backfiring problem.
It starts without to much of a problem and idles ok, hot or cold. The backfiring happens when a load is applied. Come up easy on the power and there isn't too much of a problem but give it the gas and it pops thru the carb in what seems like a pretty regular beat, as maybe only one cylinder is the problem.
The plugs, wires, points cond., etc., etc. are new. The carb is clean and the idle air is set the beat I can by ear. No pushrods are bent and the plug wires are all on the right plugs. I've seperated the wires thinking there might be a short between touching wires but that doesn't seem to be a problem.
I can't see a timing mark on the bottom pully so I just used #4 as it was easy to get at and timed it off of that one using the thumb in the hole method to get tdc.
I'm at a bit of a loss as I would think an intake valve has to be open for a carb backfire but why does it run much better when the throttle is applied slowly or there is no load on the engine?

You can time off of number 5 or 8, but not 4. Are you sure you have an SV series engine? Anyway, if it's running, and the wires are in the correct order, you can time off of number 5 instead of 8.

The symptoms you describe are common for crossed plug wires. 304/345 firing order is 8-4-3-6-5-7-2-1. Make sure 5 and 7 are well separated.

The other causes for the backfire that come to mind right away are timing too far advanced, vacuum leak, and valves not closing all the way. My first thought would be vacuum. Try plugging off the vacuum advance and check it with a gauge.

Viatanovite
04-25-2011, 02:08 AM
has anyone had a Acty not start and fouling the plugs with fuel and backfiring intermittently thru the carb while cranking? I have replaced the entire carb. with a know good one,replaced plugs,fuel pressure is within specs and has great spark at the plugs and I still have the same problem-this all started after i washed the engine several weeks ago,engine was running and at operating temp-it ran for 10-15 min, lost power and died and now it floods/backfires while trying to start

Allan E.
04-25-2011, 10:58 AM
has anyone had a Acty not start and fouling the plugs with fuel and backfiring intermittently thru the carb while cranking? I have replaced the entire carb. with a know good one,replaced plugs,fuel pressure is within specs and has great spark at the plugs and I still have the same problem-this all started after i washed the engine several weeks ago,engine was running and at operating temp-it ran for 10-15 min, lost power and died and now it floods/backfires while trying to start

You have the symptoms of a bad distributor.