C3 misfire?
Thread Starter
5th Gear
Joined: Jan 2018
Posts: 5
Likes: 0
C3 misfire?
Dear all,
we have purchased, a c3 350ci 78 vette,
the car is as stock as I could find, based in the middle of the Mediterranean. its a matching numbers, the only non stock items are a summit intake manifold and an edelbrock 600cfm carb.
we are however encountering a strange issue, when you are driving along and you hit WOT, you sometimes (not always) get a weak misfire (doesn't bang or pop loudly) but you get a series of numb sounding misfires...at which point I immediately lift off throttle.
you take the car back into the shop, on neutral, and rev the hell out of it, and this misfire does not occur again.
Go back on the road, under load, and they come back.
I bought a modern HEI distributor, new spark pulg wires - since from this forum I read that they are the possible culprits, and yet here we are. it's still doing the same thing.
At this point I need your help. Cause I am still baffled why this happens under load (i.e. driving around, and when stationary in garage it revs like a champ).
Thanks in advance
we have purchased, a c3 350ci 78 vette,
the car is as stock as I could find, based in the middle of the Mediterranean. its a matching numbers, the only non stock items are a summit intake manifold and an edelbrock 600cfm carb.
we are however encountering a strange issue, when you are driving along and you hit WOT, you sometimes (not always) get a weak misfire (doesn't bang or pop loudly) but you get a series of numb sounding misfires...at which point I immediately lift off throttle.
you take the car back into the shop, on neutral, and rev the hell out of it, and this misfire does not occur again.
Go back on the road, under load, and they come back.
I bought a modern HEI distributor, new spark pulg wires - since from this forum I read that they are the possible culprits, and yet here we are. it's still doing the same thing.
At this point I need your help. Cause I am still baffled why this happens under load (i.e. driving around, and when stationary in garage it revs like a champ).
Thanks in advance
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
H-m-m-m. New plug wires? Very common error is to allow cylinder # 5 & #7 plug wires to come in contact with each other. They are side by side on the distributor cap. Because of the firing order of a SBC, those two cyl can actually cross-fire. Follow the routing of those two wires and seperate them as far as possible in their wire looms.
Race Director
Joined: May 2006
Posts: 18,734
Likes: 2,582
are you sure the secondary air flap is opening correctly, you can not test it by revving the engine in the shop. There is a diaphragm on the passenger side which is designed to prevent it from opening too soon.
Thread Starter
5th Gear
Joined: Jan 2018
Posts: 5
Likes: 0
H-m-m-m. New plug wires? Very common error is to allow cylinder # 5 & #7 plug wires to come in contact with each other. They are side by side on the distributor cap. Because of the firing order of a SBC, those two cyl can actually cross-fire. Follow the routing of those two wires and seperate them as far as possible in their wire looms.
Thread Starter
5th Gear
Joined: Jan 2018
Posts: 5
Likes: 0
how do i test this?
Thread Starter
5th Gear
Joined: Jan 2018
Posts: 5
Likes: 0
After more driving today and testing we have more feedback.
this misfire occurs only when we are driving slowly around 2500rpm then suddenly go to WOT. At around 4000-5000rpm.
when driving at higher engine speeds on the motorway...the misfire does not happen and she revs freely to 6k.
we are well baffled.
this misfire occurs only when we are driving slowly around 2500rpm then suddenly go to WOT. At around 4000-5000rpm.
when driving at higher engine speeds on the motorway...the misfire does not happen and she revs freely to 6k.
we are well baffled.
Drifting
Joined: Dec 2004
Posts: 1,714
Likes: 42
From: Austin Texas
Hi,
To me, it sounds like an accelerator pump issue. That is the circuit I would look into.
It may be failing but before I replaced any parts I would try to adjust it and see if I could increase the pump shot.
Post the part number and maybe some pics of the carb and others more familiar with Edelbrocks should be able to comment on the adjustment if needed.
Best ,
John
To me, it sounds like an accelerator pump issue. That is the circuit I would look into.
It may be failing but before I replaced any parts I would try to adjust it and see if I could increase the pump shot.
Post the part number and maybe some pics of the carb and others more familiar with Edelbrocks should be able to comment on the adjustment if needed.
Best ,
John
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