Back firing into exhaust and carb 354 poly 4bbl

BIRD

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Car was running fine.. started a short journey and engine performed nicely... after about 25 minutes of easy driving the engine stated to "buck" and then miss .. I managed to keep it running for a short time by pumping the accelerator.. may be cause of this action it started to backfire though in the exhaust system and through the carb.
Shut engine down and coasted to a stop... it would start again after cranking and pumping accelerator but only "run" in a matter of fashion with immediate backfiring a couple times and then die..
When I got it brought home, removed the carb and noticed in take was wet with fuel .. (maybe because of my pumping accelerator?) anyway had the carb rebuilt just in case.
Engine has complete new ignition parts in place ( car was not run for 25 years)
Heads were refreshed and reinstalled with new gaskets .. compression was checked after got running and had used car for a number of test runs. Compression readings were even across the board. generally either side of 125.. 4 months later checked compression again( engine cold and had not been running in the 4 months time) .. readings were down around the 90 mark but varies as low as 70 and high as 130 ... second test after cranking engine for a bit brought readings up by 15 but still erratic across the board.
I have checked the condenser and coil ... I do get spark.. if I have a helper manually adjust the distributor while turning engine over it tries to fire ( as if it were flooded.. and it is not)
Wondering if timing chain could have jumped a tooth... might give the new compression readings as the valves may not be closed completely when they should be?? I removed the valve covers and the valve train moves properly, push rods rotate .. timing mark on crank pulley looks right when #1 is TDC .
Any ideas any advice would be helpful.
Was thinking of upgrade to electronic pick up for the distributor.
 

Big_John

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IMHO, if the timing chain "jumped", it probably wouldn't run at all. IIRC, those cars don't have the plastic gears, but do have a "silent" chain rather than a roller chain.

I would check point gap first. A sudden change after new ignition parts could be the issue. Next would be ignition timing... maybe the lock bolt wasn't tight.

From there, I would start looking at fuel delivery. My suggestion here usually gets ignored, but I used to see good professional mechanics do this back in the carb days... Disconnect the fuel line and run a hose to a soda bottle. Crank the engine over a few times and see if the fuel just trickles out or is a healthy squirt, filling the bottle at a fast rate. That will tell you about everything you need to know about fuel pump condition or clogged lines or filter.
 

73Coupe

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I agree with Big John that it would not run if the chain jumped a tooth. Speaking from experience.

Perhaps the valves need adjusting or you have a sticking valve? This is what comes to mind with the backfiring. Not out of the question to have sticking valves after having work done, if there was any contamination that was left in the guides upon reassembly. If the valves are not seating all the way or seating with enough duration they can overheat which can also lead to sticking. Note that the best opportunity for a valve to dissipate heat is when they are in contact with the seats. Did they install new guides with the "cylinder head refresh"? What exact work was done?
 

BIRD

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Heads were magna fluxed to check for cracks.. When the car was first started after 25 years, ran well with one tick ( one lifter needed to be replace as it was broken) and white smoke was present from right hand side exhaust..
New head gaskets solve that problem... wanted heads checked to see they were not part of that problem, heads were cleaned as well, valve seats a valves cleaned up and new valve seals installed.. After put back together engine ran smoothly, no smoke and sounded good. Temperature and oil pressure good .. (engine itself does pump off a lot of heat but not enough to burn the paint off. lol) I am running a new fuel pump from a 318 with the Hot Head's adaptor plate .. proper fuel pressure rating and easily fills the gascolater.. no issue with not getting the correct amount of fuel.
checked the condenser and it checks ok.
 

BIRD

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Gas was/is fresh...
will go out tomorrow and remove clean plugs and check gap again.
do compression check.. check and adjust points .. check timing with the marks and ensure distributer is properly in the right place. keep my fingers crossed.
 

BIRD

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Will check.. but everything is new.. could be defective i suppose... have a spare older set which was on engine from 25 years ago
 

1970FuryConv

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Car was running fine.. started a short journey and engine performed nicely... after about 25 minutes of easy driving the engine stated to "buck" and then miss .. I managed to keep it running for a short time by pumping the accelerator.. may be cause of this action it started to backfire though in the exhaust system and through the carb.
Shut engine down and coasted to a stop... it would start again after cranking and pumping accelerator but only "run" in a matter of fashion with immediate backfiring a couple times and then die..
When I got it brought home, removed the carb and noticed in take was wet with fuel .. (maybe because of my pumping accelerator?) anyway had the carb rebuilt just in case.
Engine has complete new ignition parts in place ( car was not run for 25 years)
Heads were refreshed and reinstalled with new gaskets .. compression was checked after got running and had used car for a number of test runs. Compression readings were even across the board. generally either side of 125.. 4 months later checked compression again( engine cold and had not been running in the 4 months time) .. readings were down around the 90 mark but varies as low as 70 and high as 130 ... second test after cranking engine for a bit brought readings up by 15 but still erratic across the board.
I have checked the condenser and coil ... I do get spark.. if I have a helper manually adjust the distributor while turning engine over it tries to fire ( as if it were flooded.. and it is not)
Wondering if timing chain could have jumped a tooth... might give the new compression readings as the valves may not be closed completely when they should be?? I removed the valve covers and the valve train moves properly, push rods rotate .. timing mark on crank pulley looks right when #1 is TDC .
Any ideas any advice would be helpful.
Was thinking of upgrade to electronic pick up for the distributor.
I'm always in favor of electronic ignition. Removes a lot of potential problems. @halifaxhops can probably help. He's a super good seller!
My concern is the erratic compression after 4 months.
Have you replaced the timing chain?
Did you perform compression test with engine warm, all spark plugs removed, carb blocked open?
Worst case scenario: wiped cam lobes.
Good luck! Great car!
 

halifaxhops

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What ignition you have now? Also check the timing chain first.
 

70bigblockdodge

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Condenser in distributor. New ones are all crap. Making me debate going electronic. Not a fan of electronic ignition nowadays because of off shore parts and no on the shelf at auto parts, which means I have to carry more/bigger parts for on the road spares.
Good luck
 

halifaxhops

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Thinking that also or if electronic a pickup or ecu. Double check the chain first to eliminate that. I have parts if needed.
 

BIRD

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after cleaning and re-gapping plugs, ensuring timing was where it should be, checking the points gap, and the condenser.. actually had the original condenser saved on the shelf , tested it and replaced the new one I had installed last year when I did the major tune up.
Result was.... car started and runs.. think condenser was culprit and position of distributor was a little off.
right bank has a little lifter noise?? or maybe heat riser?? or slight bottom end noise??
only ran engine for about 2 mins with good oil pressure showing.
Will change oil and filter.. has really no miles on it but it has been in car for a year.
Thanks ever so much for the advice from all of you. Cheers Rudy
 

1970FuryConv

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Would be interesting to know if the condenser fixes the problem enough for driving.
At least 2 minutes is a step in the right direction.
Good luck Rudy!
 

BIRD

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Car is running well now.. I believe it was the condenser that was this issue ..
Has about 2 hours of run time with no obvious problems.. Accelerates briskly under full throttle with no hesitation.
Idles smoothly. Discovered a a vacuum leak once I had managed to get it running. After having carb rebuilt last year (thinking it might have been part of the problem) the base gasket provided turned out to be the wrong one and was quite a vacuum leak on the one side of the carb at the base.. New gasket and all is good.
Probably in the not too distant future will change over to electronic ignition.
Thanks again to all of you for all the information and advice
 

pomonamissel

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condenser will take a sh..t for no reason . get a couple extra or change out that points type dizzy for a electric type . i'm involved in building an adapter for the early hemi/poly headed motors to use the other guy (chevy) dizzy in those motors . but not retailing these yet . i designed it for magneto ignition use in my early hemi's 301 331 354 392 , it clamps the body of the ignition dizzy way better than the forks could ever do .

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