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Discussion Starter #1
I have a 72 EC with the 402 BB. It has 65K miles on it. I have not been able to get this thing to idle smoothly since I bought it last September. I can see in the receipts from the previous owner that he had the pushrods and rocker arms replaced a few thousand miles ago. Has headers, and the original Q-jet and the TCS solenoid are all gone. Chased down a couple vacuum leaks and I have replaced the ignition wires, plugs, and cleaned and rebuilt the carb. It contines to run very rough when cold, backfires on light acceleration, and sometimes when you really get into it. I'm down to the cam. Wanted some other opinions before aI go for a new cam. I have been told these big blocks are subject to premature cam wear.
 

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What is the timing set on? Late timing can cause those issues
 

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x2 check the timing first..

is it only when its cold?
maybe the choke is a little lean??

cant remember when the BBC had the cam problems,,
but I think they blamed it on the machining, mis-aligned lifter bores on some blocks, at one time??
:dontknow:
you could measure the cam lobe/lift wear with a dial indicator..
 

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x3 on timing check first. And where is the vacuum advance getting it's supply from? Manifold or ported?
 

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Discussion Starter #5
Advance is using the. Port on the carb. Checked the timing awhile back, but I'll try it again. Thanks.

Tom
 

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Advance is using the. Port on the carb. Checked the timing awhile back, but I'll try it again. Thanks.

Tom
My 71 402 is a much happier animal with manifold vacuum. Gas mileage improved too. JMHO(just my humble observation)
 

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Maybe an intake lobe on the cam is worn (?) Less mixture volume on one cylinder may possibly cause a lean condition & back fire (?)
Checking lift by measuring rocker arm travel is a good idea if you can do it.
When you change oil, cut the filter & inspect the filter media for metal particles. If it's OK , one less thing to wonder about. You didn't mention the distributor cap....did you inspect it for cracks or carbon tracks ?
Just some ideas for you......
 

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Turn dist. 1/4 inch counterclockwise, Idle should go up. If you go to far the starter will buck when hot. Or just put a timing light on it if it is at say 6* move it to 12*,14*.....
My little blown 383 likes 18* and 32* total.

Don
 

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Hook a vacuum gauge to a manifold vacuum source, and start the motor. See if the needle has a "tick" of several inches (example: it is MOSTLY reading 17", but has a rhythmical jump/drop of a few inches). If so, you probably have at least on cam lobe going flat.

Aggressive cam lobe designs combined with the removal of zinc from modern oils has made flat tappet cam failure a common occurrence for ALL makes of motors.

Best fix is to convert to a roller cam. Next choice is to use less aggressive cam lobe profiles AND make sure to use an oil designed for flat tappet cams or use a good zinc additive.
 

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Discussion Starter #10
I have been looking at the suggestions and now I am wondering about the carb that is on the car being the root of my problems. The PO put a Holley Street Avenger 770 CFM carb on a while back. I used the calculator on the Holley site and it says that I need a smaller 570 CFM carb. Does anyone know what the CFM of the 7042220 carb that was supplied with the car was from a CFM spec? The thing spits black, and backfires a lot when cold - and the choke is not even closed. Any ideas?
 

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The thing spits black, and backfires a lot when cold - and the choke is not even closed. Any ideas?
THAT is da problem. It should be closed when cold.
 

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The thing spits black, and backfires a lot when cold - and the choke is not even closed. Any ideas?
THAT is da problem. It should be closed(NOT all da way) when cold.
 

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7042220,, most all Qjets were 750-800 cfm..
both carbs are vacuum secondarys, which means they kinda size them selves to the motor,
and with some tuning both should work ok.. the Qjet would probly work better, with the small primarys..
http://www.carburetion.com/CarbNumber.asp?Number=7042220

as for the choke, it has to be set or tuned also..
you cant really expect the carb to run right when the motor is cold, it would need to be real rich..
how does it run when warm? get it running right warm then adjust the choke later..

what kind of choke does the holley have,, is it hooked up??
put some pics up..

as said the timing has a lot to do with it too..
how is the distributor curve setup??
initial, total, vacuum, centrifugal??

:texas:
 
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