truck was running fine['78 350 hei] then went to crap like running on one cylinder.changed ign. module.cap and rotor have a few hundred miles.getting backfire threw exh.chaged fuel filter.should i try new cap and rotor?coils do not go bad very often i've heard.help.thanks
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chevy 350 problems
- Thread starter rebelss
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shane416ex
Well-Known Member
Sounds like a spark problem more than air or fuel. Sounds like a timing issue with the back fire issue but if it was running great without and adjustment then started acting up that wouldnt make much sense. I have had coils randomly go out on those hei's so coil could definatley be bad but if it was you shouldnt have spark at all, even it one cylinder. I would check cap and rotor and wires then my next check would be the distributor drive gears and the cam gear.
TreeClimber
Well-Known Member
Both these are good.Start with a compression check, just to eliminate if the head is blown or not. Then spray the manifold down to make sure your not getting an intake leak.
start there, report back.
Check first the cap/rotor. You may have carbon-tracking inside, which is causing a mis-fire / back-fire issue. Any time you have a random misfire on a simple system like the old chevys, it's best to eliminate the tune-up items. It's doubtful that a low-compression would create a backfire. More likely, it just won't run. While you've got the cap/rotor off, use a wrench and rotate the engine to TDC, and check for a jumped timing chain. The crank should be at the timing mark, and the rotor should be point exactly at number one plug wire, or 180* off. Then look for a vacuum leak. Old engines have old vacuum hoses. Perhaps one fell off, cracked, etc. (And make sure the plug wires are not grounding out internally. An old wire could have a crack, grounding out to the block creating a problem. Good luck
Poohbair
Pabst O'Holic
Sounds like its a Chevy... :flipoff:
Thegreasemonkey
Well-Known Member
If you check the basics and can't figure it out I hade a 79 1ton that broke a pushrod because it couldn't push oil through the litters anymore, it started backfiring through the carb. Check cap+rotor first, run the engine at night and look at it you might be able to see the plug wires arching. Good luck!!
CHOP SHOP
Well-Known Member
Just use a spray bottle of water and spray each exhaust outlet on the manifold or header and see if they all sizzle off instantly or if one or more will stay wet and not sizzle off quickly like the ones that are firing. If one stays wet its not firing on that hole.
But I agree you probably have something dumb like a loose rotor or bent contact or cracked cap or ??
I dont understand you post either? did you change the cap and rotor recently or were thining of doing that?
If you changed it recently check the ground tab inside the cap also.
Check battery voltage at the HEI unput lead and make sure it has a full 12 volts. it will run like **** with less than 11 volts too.
did you fool with anything at the same time it went crappy?
But I agree you probably have something dumb like a loose rotor or bent contact or cracked cap or ??
I dont understand you post either? did you change the cap and rotor recently or were thining of doing that?
If you changed it recently check the ground tab inside the cap also.
Check battery voltage at the HEI unput lead and make sure it has a full 12 volts. it will run like **** with less than 11 volts too.
did you fool with anything at the same time it went crappy?
zukkev
Cornfuzzled yet again...
These caps/rotors aren't nearly as problematic as the later model one...but it is possible...coil's usually either gonna work, or not---I have owned plenty of these rigs, and have seen the carbon button burned completely away, and it would still actually run not too bad, until I really got on it...:redneck:
you test the pick-up coil? This is the piece inside the dist that has two wires and plugs into the module---scope's best, but can be easily tested with a DVOM on a/c volts scale---
Magnetized distributor???Seen a few cause very similar symptoms you're describing...
