I have an engine miss issue that I have not been able to diagnose so I am sure someone will have an idea. ZZ4 350 with a manual trans. Has run fine in the past but started to miss at any RPM over 3,500 and up. Under 3,500 it seems to idle and run normal. Here is what I have checked so far doing one thing at a time but with no positive results: new plug wires, new fuel filter, changed carbs with one that I know is tuned perfect, adjusted timing all over the place with different settings. What I have not done is change the fuel pump or touched the HEI ignition. I live in KC and finding anyone who has a distributor machine and knows how to use it has been interesting. I am not one to take my car in as I am sort of determined to figure it out myself but this one has me stumped. A little more about the miss, it is almost a popping from the carb as it won't pull over 3,500. It does this even when you excel slowly and worse if you hit it harder. It is not a "timed" miss like a plug wire misfire, it is inconsistent. ?????

 

Do you have a timing light? See if you can watch the timing advance with the rpm's.
Or, you could have a bad distributor cap, either corroded contacts or carbon-tracked, causing a cross fire.
Poor fuel pump delivery starving at higher rpm's is another possibility, but I would start with the other two things first.

 

Hi I am from KC as well, have you looked at any of the spark plugs or replaced them? You didn't specify in your post

 

Thanks,
69 Camaro... I have checked the timing and it does advance at the RPM increases, I have tried setting initial timing from 9-16 degrees but it does not seem to matter. The vacuum advance is plugged.


67camRSSS As for the plugs, they are new. I replaced the headers and at the same time changed plugs, I did put a different plug in it for header clearance issues and now have AC 41-602 plugs in it now, it also has alum heads. I can't remember what plug it had in it and should have kept them, seem to remember them being an Autolite.

 

I'd agree that you need to finish the tune up, by replacing the cap and rotor. With new plugs, and wires, you still have those two components old. It's also the cheapest, and easiest thing to try before going forward.

 

Thanks all for the suggestions! Here is what I have done so far and still have the issue. New plugs, wires, fuel filter, HEI cap and rotor. I have tried 2 different carbs that perform fine on another car, both holley 750's. I am ready to replace the fuel pump but not sure it is bad as at idle and to about 5,000 rpm the fuel gauge shows consistent fuel pressure at about 6 psi, now that might be different on the road under a load but I can't check that as the fuel gauge is inline on the fuel line. Timing being set at 8 to 16 degrees does not seem to matter. It idles fine, drives fine until you pull it to about 3,000 and then starts cutting out, full throttle is out of the question. There are times it won't cut out till 4,000 and then some when it happens at 2,500 so it is not a consistent miss always at the same rpm. I have not changed the HEI in cap coil or the fuel pump everything else is new.

 

A few things I would double check.

1. Recheck your firing order, actually trace each plug wire to each spark plug. 1,8,4,3,6,5,7,2. Sounds stupid but its worth doing at least once. And as you do it make sure each plug is clipped in.

2. Do you have a dial timing light or regular? If regular are you using Vacuum advance? Have you installed a curve kit in your distributor? I've had issues with a standard HEI and had to install a curve kit, read the instructions and make sure you install it in the correct order. Pieces have a directions and can be installed backwards. Use the lightest spring set. When your adjusting you want time to hit fast on your timing light. Which in using the lightest spring will accommodate that. Do you understand that you have initial time, centrifugal time, and total time?

Prime example: On my car when tuning I had to install a curve kit, the orginal HEI curve was too much and it would pre detonate at higher RPMs. Causing popping, back fire etc. When adjusting time you have initial which I dialed into 10, my centrifugal time totaled 32 when reved up, if I was to run total advance it raised the total time to 55, way too much! I plugged the hole on my carb and on the distributor. Left it with 10 and 32 no total advance.

Not sure if this helps hard to tell over the net but it was issues I resolved And the car runs flawless.

 

67CamRSSS,
Thanks for the note, I will recheck things as you outlined above but can confirm a couple of your questions. The HEI is stock, not curve kit installed and it does have about 26 advance on timing so I had set initial timing at 12 for a total of 38. I have a regular timing light and the distributor advance had already been capped off so I don't have any advance from the carb at idle. I have had several off the wall suggestions like having a broken valve spring but I can't see how any engine would run well at low speed but then pop and backfire at higher RPM, valve train looks fine to me with no obvious broken springs but I am not sure it would be that obvious. I may put in an advance kit in the HEI and see how it reacts, it has black springs in it now which I think are just stock springs.

 

Something else to think about.. How many miles are on your cam? Do you use an oil with zinc additive? I use Brad Penn it has zinc in it for your flat tappet cams. Do you have a rocker making noise like a flat cam? That could produce misses as well.

 

Just saw your reply from earlier. If your total advance is plugged off then your initial time is 12 and centrifugal is 26 by your statement. You wouldn't have 38 for total, your total would then be 26. I used a curve kit for my application just a suggestion because it was pinging a lot! If you decide to add the curve kit read the directions and use the lightest spring.