72 454 misfire

[John Mac Mahon]

Help! I have a '72 454 4spd. I can't get cyl. #8 to fire. It has goood compression (150 lbs), a new cam (stock replacement), properly adjusted valves (1 full turn down after lash is removed) and a new Edelbrock carb. It's got an old Accell electronic distributor. With new wires and plugs it shows decent spark, not that bright blue crackling spark I'm looking for but orange and capable of jumping about 1/2 to 3/4 inch to ground. I reset the distributor 180 out and reset the firing order to try and isolate the problem but the miss stayed at cyl #8. I also checked voltage at the coil, shows 12v at crank and run. Last resort, dropped in an old HEI same problem. I did notice though that with an HEI spark tester (looks like plug with ground clamp and electrode buried deep in ceramic) I dont have good enough spark to register until I bring up the revs to about 1500 or 2000 rpm. I am considering running a hot wire directly from the batt to the coil but otherwise I'm out of ideas. Sorry about the long post but I'm just trying to give all of the facts. Thanks in advance.

[Jim T.]

Re: 72 454 misfire

John your 72 originally had a resistor wire that connected to the fuse block side in the engine compartment and ran through the wiring harness and connected to the coil. It functions the same way as the earlier Corvettes using the ballast resistor. Both reduced 12 volts going to the coil after start up. To get a full 12 volts to the HEI consistantly run an appropiate guage electrical wire from the "IGN" terminal on the fuse block inside your 72 through the firewall to the BAT terminal on the HEI. Your ignition switch will turn this connection on and off.

[Chuck R.]

Re: 72 454 misfire

When you say that you dropped in an old HEI, do you mean a complete distributor?

If your talking only a coil, I would be suspect of the distributor cap at tha #8 post.

Could also be that you have a lobe issue with your new cam.

Chuck

[Tom D.]

Or a bad spark plug? *NM*

[Terry M.]

Could be a bad spark plug wire *NM*

[Michael W.]

How do know it's #8 not firing? *NM*

[John Mac Mahon]

Re: How do know it's #8 not firing?

Thanks for all of the responses. I'm going to try to address all of the questions so far. First off it seems that whoever put the accell distributor in the car replaced the resistor wire because I do have 12v at the coil with the key in the run position. I think I'll run a new one just to be sure though. I have swapped out the plugs and wires to eliminate them as possible causes. I also reset the firing order on the old distributor so that if it were the cap the miss would have moved to another cyl. It stayed at #8. I dropped in another distributor (HEI) still has the same miss. I rechecked the cam when I rechecked the valve adjustment, all seems well there. I used an infrared thermometer to check header temp. All cylinders go way above 500 degrees right away, cyl 8 stays at around 125 degrees. It is also the only cyl that gives no idle response when pulling the plug wire. The temp starts to come up when I rev the motor but I cant tell if it's firing or is it just heat from commpresion. Did I answer any questions or just confuse everyone? Thanks again.

[Wayne W.]

Re: How do know it's #8 not firing?

Check #5 also. Vacuum leaks will affect 5 and 8. The vacuum fitting is on that runner. Pull the hose and stop the hole and see if it changes.

[Dave P.]

Re: #8 not firing?

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[Robert M.]

Possibly too much #8 lifter lash ?

I wonder if maybe you have excess lash on either the intake or exhaust valve, and causing one valve or the other not to seat, so as not to allow the air to compress. I had a similiar problem once. I've read recently something Duke wrote, and he suggested a valve lash of 1/4 turn preload.

Readjusting my valves to 1/4 preload, as he suggested, solved my problems.

[Denny J.]

Re: 72 454 misfire

John
Make shure you have a good tight connection from the wire to the spark plus,You should hear a click when the wire is pressed on too the spark plug. You may have the spark plug to the side rubber and the wire connector.Also when you removed the wire from the plug you may have wiggled it to much and may need to take a pare of plyier's and squeez the plug wire connector together for that tight fit.Same with the distributor cap push them all in tight.#35141

[John Mac Mahon]

Thanks guys

Vac leak. I'm embarassed to admit how long I've been looking for an ign. problem. It also seems to run better with the lash at a 1/4 turn. Thanks for the help.