Dear Ole Mechanic;
I am just beginning to restore a 1969 Chevy pickup. People that I know and trust say that I should do all mechanical work that needs to be done before I start the body work. Do you agree with what they say?
There is an engine problem that has me stumped. At idle, the engine runs nice and smooth. When I put any load on the engine, however, it starts running rough and doesn’t want to pull smoothly at all. A fairly knowledgeable friend says that it shakes like a one cylinder isn't firing. Once I get up to speed and cruise on the highway, it seems to smooth out, but if I give it any gas at all it, acts up again. The engine is a late-model 1978 350 cubic-inch V8 with an Edelbrock carburetor and manifold. Other than that, the engine is stock as far as I can tell. I will be rebuilding it, so do I need to find the miss before I do the body work? What could be causing the miss under load and not at idle?
The best way to get rid of that ‘Novice’ first name is to do a restoration. Your friends are absolutely correct. Do all the mechanical work before starting the final body work. Why? Because it is really hard to pull an engine back out without scratching new paint. Trying to do a brake job after painting will result in getting brake fluid on the new paint and it will wrinkle. It is all part of Murphy’s Law “If anything can go wrong, it will--and at the worst possible time.” You will never drop a wrench and chip the paint unless it is new paint. Now don’t get me wrong, there are some body repairs like welding in patch panels and beating out dents that can be done before or during the mechanical work. Just get the mechanical work done before the final paint and polish. I would even do the engine rebuild and test drives before starting the final paint--even though that means pulling the engine out again before painting the engine compartment.
By all means find the miss-fire so that you don’t need to spend a lot of time leaning over the fender and scratching the new paint with your belt buckle. The Edelbrock parts are good equipment and unless they were installed incorrectly, should not cause a miss under load. Problems with the carburetor or manifold are usually vacuum related and show up at idle, not under load. As for your miss, you didn’t say if you had run a cylinder compression check, or replaced the spark plugs, plug wires or distributor cap. Usually, a miss due to low compression will show up at idle. While that does not seem to be your problem, I would still do a compression check on all cylinders. That would let you know if there are any cylinders that need close inspection and attention. Since you will be, or should be, replacing the plugs, plug wires, distributor cap and rotor, do it now. Getting things fixed now will not keep the components from looking new and shiny when you get the restoration done.
What is causing the miss-fire under load? Usually it is one or more of the components that I just named breaking down under load. That was the worn spark plugs, a bad plug wire or a cracked or tracked distributor cap. Even if you have replaced the spark plugs, look at them closely. I once found a plug that looked just fine, but when I turned the plug so that the electrodes were down the broken porcelain dropped down covering the electrodes spark gap. Yep, a bad one will slip through once in a while.
I strongly recommend doing all of the mechanical work before doing the final body restoration.
Herr Professor Nuzanbolts