Q: I have a 402 big block motor in my Chevy truck. It runs great from 1,800-2,000 rpm, on up. It has a Crane RV cam, Edelbrock Performer intake, and Performer heads. The rest is stock Chevy and pretty new (10,000 miles).
At idle, it has a bad miss on cylinders 5 and 7 (we put the engine on a diagnostic machine to find out for sure). I have changed the lifters, carburetor, plugs and plug wires, distributor shaft, even new heads. I still have the miss.
Could it be a bent distributor shaft? The two dead cylinders are next to each other on the distributor cap.
A. We have a few questions as well. Do you have spark at low rpm? Did you do a compression comparison on all the cylinders? Did you do a leak-down test? Have you had the same intake manifold on the engine while this was happening? If all of this checks out, then we’d say you have a fuel distribution problem or a cracked manifold causing a vacuum leak. We don’t think it is a bent distributor shaft, because then all cylinders would be affected.
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I would be looking for vacuum leak on the carburetor base or intake runner/runners. If smooths out above idle,most likely cause
Spray carburetor cleaner around base of carb and intake while engine running and if engine rpm changes, most likely found area to look at closely.keep in mind if vacuum hose is off carb and or intake, check source it goes to as well as the hose.
Very wear cause it could be the distributor cap,if you don’t have a new one yet,or the worse a blown head gasket or Crack in the engine head between those 2 cyl.
Miguel R.
I had that same problem once & it turned out to be the intake gasket on the same two cylinders I had used stock intake gaskets but the intake manifold & heads had big intake runers & the gaskets were not sealing on them two cylinders its most likely a vacuum leak with them two cylinders I would say the manifold or the gaskets spray some carb cleaner were the manifold & heads meet when motor is running best when motor is cool carb cleaner is flammable good luck
Could be that it has PMS and a heavy dose of chocolate and some cuddle time and it will be ok….wait, this isn’t the “understanding your woman” forum? oops…sorry.
I had a bad rotor causing a similar problem. The plastic that holds the rotor contractor was broken and the tab was retarding the firing of the cylinders. And since 5 and 7 are the only two cylinders that are fired next to each other in the firing order.” All the others fire With a cylinder between or different banks and from one bank to the other. This was on a 402 also in a 1970 chevelle. It also caused number 7 to fire with the exhaust valve slightly open which turn the header tube to turn red about 2″ from the header flange
Same here, just changed the intake gasket on my 402 as well. Problem solved. BTW be sure to follow the torque sequence listed on Edlebrock’s website. Good luck!
I had a old Monte Carlo with a 350 small block. I had changed the cap, rotor, plugs, and wires as part of routine maintenance. When I started the car up, it idled fine, but anything above idle had a dead miss in cylinder 5. It wasn’t until it was almost dark that evening when I found the issue. The new #5 spark plug had a hairline crack in the ceramic and would arc through to the head above idle.
Thanks everyone for the input and sharing your experiences. Very good knowledge on this page!
Make sure the plug wires are not running parallel to each other, #5, #7. Good luck
Fab a propane vacuum tester from old torch head. Use a small diameter vacuum hose and a small metal line and flatten the end. That way you can pinpoint the leak. Gasket shifting during the intake installation. Gray rtv works better than glue and use 3/4 inch bolts and fender washers to hold the gasket to the head till the rtv sets. Oh and ditch the rubber end gaskets and use the gray rtv instead. A 3/8 bead should be plenty. Let the rtv setup for at least an hour before filling with coolant.
I’d put my money on wiped cam lobes, intake valves out of adjustment or a valve issue. If he replaced the lifters that means that the intake was off and the gasket was replaced. Vacuum leak will cause noticeable fluctuations and you’d have trouble getting a steady idle. I bet if he pulled the cam that at a minimum the intake lobes on 5 and 7 are wiped not letting the combustion gasses in causing the misses.
had a similar running issue on a 350 chevy, pulled the valve covers and killed spark to the engine. Then had a buddy turn it over so i could watch the rockers, found that two hardly moved. Cam lobes were wiped out.
Check vaccum advance pot..that will also feel like that..suprised me, never had one go bad in my life(I’m 71)..