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Problems are dieseling and backfires - blowing my mind

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zookagumby

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Discussion starter · #1 ·
I have a 1964 el camino with a slightly pumped 350...comp cam, roller rockers, 10:1 compression. It ran great for a year or so then I started to have problems with the engine running with ignition off, lots of knocking, etc. and some backfiring (acted like a miss at first). Since the valve seals were leaking and causing carbon build up I decided to pull the engine check everything out, install new valve seals, gaskets,timing chain and clear out any carbon buildup. It turns out the engine is low-time, no cylinder ridge, valve train is in very good condition. I used the plunger depth method to preload the lifters (about .050 to ensure they'd be right). So I reassemble. I set the ignition ( speedway motors HEI ignition #9109551) to 8 degrees advanced to start and the engine fired up. It ran well for about 10 minutes when I shut it off for the first heat cycle. Re-tightened manifold bolts the next day and went to start it again...then it backfired unmercifully and proceeded to run on after the ignition was shut off. What? There's not a hint of carbon build-up anymore, I'm using the standard 44's for plugs which I blasted clean before installing. I swapped out the ignition wires just to be sure and no difference. And remember, the engine ran good at first start-up go-round. I even swapped out the carb (an edelbrock) with a known good one just cause I'm starting to lose my cool...and reaching... no change.
So, today I replaced the distributor rotor and the ignition module. NO improvement. Tomorrow I plan to change the entire distributor so the pickup coil will be new.
Has anyone had this kind of experience? Any thoughts other than burning the car down? I think it might be possessed, LOL.
Thanks,
Tom
 
#2 ·
I think your timing is retarded at 8*. The distributor should have 21* of mechanical advance built in so the total without vacuum advance would be 29*. You want to be closer to 36* total. Secondly I've found that runon or dieseling is usually caused by too high of curb idle speed.
 
#3 ·
Did you re-use your old lifters? If so, are you sure you got them back in the same holes that they were in? If they are new lifters, did you use break in oil with a proper break in procedure?
 
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Discussion starter · #5 ·
oldcutlass said:
I think your timing is retarded at 8*. The distributor should have 21* of mechanical advance built in so the total without vacuum advance would be 29*. You want to be closer to 36* total. Secondly I've found that runon or dieseling is usually caused by too high of curb idle speed.
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Idle speed very low, timing is correct. This is an intermittent problem that is getting worse - remember problem existed before engine tear down then was gone for one run-up and is now worse.
 
Discussion starter · #6 ·
69SS454 said:
Did you re-use your old lifters? If so, are you sure you got them back in the same holes that they were in? If they are new lifters, did you use break in oil with a proper break in procedure?
Click to expand...
Old lifters were like new, and placed exactly where they were. Remember, the problem showed up before the tear-down.
 
Discussion starter · #7 ·
The engine was torn down and inspected and cleaned with only new parts being the valve seals and timing chain. Like I said before, the valve train was like new other than carbon build-up in combustion chambers and valve backsides due to shot valve seals. The cylinder compression varies only 5 lbs from 200 PSI. Since the problem is semi-intermittent (with the first cold start and running being fine after assembly), it sure seems like an electrical/ignition issue. This is my first time trouble shooting an HEI system, that's why I'm hoping someone might have some similar experience.
 
#9 ·
I hesitate to give tuning advice online since everybody is an expert. However, I have not seen this mentioned. In my experience, the type of run-on you're experiencing is caused by the throttle blades being open too far at idle. It's typically a band-aid for another unseen tuning issue.

Devin
 
#11 ·
FYI, i once did a quickie valve job ( mostly lapped them in) on a SBC ue to some spitting and popping. THis while in college. Well, we finished the work at night, and when i fired it up, i had a damned LIGHT SHOW going on under the hood. The wires were toast and the reason she was poppin' and a -spittin'. As you say you swapped wires.....

the other time it happened i forgot, or frankly missed, 1-8-6-5-4-3-7-2 . I had wires crossed. Do DOUBLE CHECK that. Happens alot.
 
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#12 ·
I am guessing your valve lash is too tight or the lifters are wiped. Back fires are always an exhaust valve open at the wrong time or the wrong firing order.
 
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Discussion starter · #13 ·
64SS427 said:
I hesitate to give tuning advice online since everybody is an expert. However, I have not seen this mentioned. In my experience, the type of run-on you're experiencing is caused by the throttle blades being open too far at idle. It's typically a band-aid for another unseen tuning issue.

Devin
Click to expand...
Yes, the dieseling aspect is baffling to me, as the throttle blades were so closed it wouldn't idle but would diesel with the key off - I'd have to kill the engine with the clutch while in gear. Huh? I read somewhere that it is possible that a failing ignition can provide mis-timed spark that can cause dieseling...but if the key is off, and there is no carbon build-up what is the process? Residual fuel due to incomplete combustion from a weak spark? Maybe I could get a priest to bless this car, it is just one thing after another.
 
#14 ·
You said you cleaned the spark plugs or replaced them? I’d try new spark plugs, btdt, YouTube Shovelrick1 and you can listen to my 454 back firing, I had Tom Mobley rebuild the Holley 3310 and no diff, changed plugs and it quit popping
 
#15 ·
I'd replace the distributor cap, like someone above mentioned. You said you replaced the rotor, but I didn't see anything about replacing the cap.
 
Discussion starter · #17 ·
69SS454 said:
I am guessing your valve lash is too tight or the lifters are wiped. Back fires are always an exhaust valve open at the wrong time or the wrong firing order.
Click to expand...
If that were the case, it wouldn't affect dieseling - and it does mimic firing order issues but will run smoothly sometimes, which indicates both lifters and secondary wiring are OK. I took special care to make sure the lifters were in good shape and properly preloaded (hydraulic).
 
Discussion starter · #19 ·
I've read that a failing ignition system can cause dieseling by providing incorrect spark timing. If that is the case, that would also be the cause of the increasing backfires. I don't know what the actual mechanism is behind this, as I replaced the module. The items I have no way to check would be the pickup coil and ignition coil. When I first fired up the engine after reassembly it had a fully charged battery, clean spark plugs, and had just been spun with no plugs installed to make sure the fuel mixture would be optimum. The engine ran fine. The second start up the problems came back. So, I suspect a weak spark coupled with some spark timing issues which would explain somewhat wet plugs( residual fuel for dieseling ?). I will provide an additional ground wire from the distributor housing to the firewall when I install a new HEI unit. I'll let you all know how that turns out.
Tom
 
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