Engine backfires under load

farmerdick

farmerdick

Senior Member
Hey Guys, Here goes another backfiring problem. I`ve read all the forums on this subject and did all the fixes such as the instructions on Marco T`s web site. All the distributor parts are new. It idles with a ever so slight skip. It loses a lot of power and starts backfiring going up a hill. I can almost feel the accelerator pedal getting heavy and then multiple backfires if I continue to hold the pedal down. No backfire going down hill or after turning the ignition off. I have as yet to do the vaccum leak tests(whats the chances its that?). The engine is a new rebuild this past winter. I`am running a Holly 2bbl on an aluninum intake bolted to a new exhaust manafold. Help!!!

Richard/Maine
 
Richard, Man this sounds like a timing problem! Also, ignition points set too close together. If the engine is not getting very hot, then I'm going to suggest points too close with too much advance. Check it out. If it is getting very hot, then the ignition is too retarded. Where is it backfiring? Exhaust or intake?
 
Chris, It does go to and slightly into the hot temp zone, but only when under that load and going uphill. Other than that, the needle goes 3/4 way into the green zone. I`am not sure what stroke it is backfiring on. How would I know which one, intake or exhaust? I`ve gone through this timing procedure so many times I could do it blind folded.
 
might i suggest your check the flow rate of the main jet and the compensator jet in the carb and check the points...but seems like a flow problem as your describing
 
Bill, It isn`t coughing back through the carb. No smoke or any gassy smells. I have tried changing the jets in the carb from .49 to .51. I will check the valves and let everyone know what I find.
 
How does it run at high speeds on the flat ?? If it starts doing the same above 50 mph, I'd suspect lack of fuel delivery..
I recently ran into such a problem,, this car would start losing power [ like it was running out fuel] and start backfiring above 50mph.. I drained 3 gallons of fuel from the fuel line with no issues so I figured the fuel system was fine.. Problem remained until I decided to look at the fuel cap,, the vent was plugged.. Cleared it and all is well..
 
We had this same problem with my son-in-law's 28 sedan. We went through the carb. and set the timing and point gap, all to no avail. We were checking the rotor to cap gap and discovered that one contact in the cap had a gap that was alot wider than the other three. The only correction we could come up with was to file the three correct contacts down to match the wide one. I asked him if he had another cap, and he said he had an old one in a box of stuff from his grandpa. We got the old cap and put it on and checked the rotor to cap gap and all four contacts were the same gap. We bent the rotor tip to get the correct gap and it was right on for all four contacts. We knew it was fixed when it started right up and idled nice. we took it around the block and it ran great with no missfire or backfiring. Apparently the first cap was a cheap repo and not worth using.
 
I experienced a similar problem. My coupe would run fine up to 57 mph – but only for a mile or two (depending on the speed). Below 35-40 mph there was never any problem, but after a few minutes above about 40 mph it would start to lean out and backfire. When checked, there appeared to be adequate gas flow at the carburetor, the timing was within spec, the air filter not plugged, etc., etc. Turned out the problem (self-caused) was a plugged micron-filter I had added to the glass sediment bowl.

Sometimes better is the enemy of good enough…
 
Make sure the points are set to .020. If they are closer it will backfire...if they continue to get closer than .020 it will get worse.
 
The problem is not fuel related; it is a weak spark, which could be for any of numerous reasons. What changes under load on a hill (as opposed to trying to go 65 rather than just 50, for example) is with the throttle open and relatively low rpm, the pressure in the head goes up and it's then harder for the spark to jump, so any ignition issue will lead to misfire. The worst case I saw was a guy who had scratches in his distributor body and one of the plug strips almost touching it.

Steve
 
I talked with the guy who rebuilt the carb and he told me he put a 4.5 power valve in the Holly, which would make it run lean, instead of the 7.5 stock power valve. He will also change the .51 jets to .53s. I`ll take the carb off and he will redo this Monday morning. I`ll keep everyone posted.
Richard/Maine
 
Sure could be low firing voltage, or a high resistance in the cap causing it to cross fire under load... easy to see with an engine analizer... either way if you know anyone who has a scope they can find it fast.... Joel
 
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