I have a 1966 with a sb. Engine not org. Did compression check and got these numbers. 1-130,3-165,5-155,7-140 2-152, 4-148, 6-152, 8-155. Pulls good through the gears. Get up in the 4,000 + range and can get an occasion back fire. Mech. thinks maybe bad cam lobe. ???

Planning on running it for now. Would prob. replace with a 350/390 hp Blueprint motor, if needed. Just a driver.

 

have you pulled the valve cover to make sure everything is on up and up?

 

What should I be looking for with the valve covers off ?

 

i would focus on the cylinder 1 and look at the valve springs and rockers for failure point

 

OK. Will check.

 

Compression should be within 10% or so on all cylinders. #1 and #7 are low. Squirt a teaspoon of oil in each of these two cylinders and repeat the test. If compression comes up, you have weak compression rings. If it does not come up, you may have a valve not seating or burnt seat or poor adjustment. The backfire could be a valve, but could be a loose exhaust manifold, and other issues. A leakdown test will tell you more, but doesn't look like it will get any better. A burnt valve should show as a fluctuation on a vacuum gauge at idle.
I have driven worse for many miles.

 

Good idea about cylinder #1 and #7. I will try that today or tomorrow.

I am quite sure the exhaust manifolds are tight, but will check each bolt. Factory exhaust manifolds.

I don't think a bad cam would leave me sitting by the side of the road, will it ? Reliability is also my concern.

 

your know when the cam goes flat.

 

It may not leave you by the side of the road, but the metal particles from the cam and lifters of a bad cam will wreck the rod and main bearings as well as other components. If you have to readjust a rocker after putting in a new cam, it is time to measure the lift on that lobe and see if you have lost a lobe. My son put a used cam and lifters in a perfectly good engine and lost the crankshaft, rod and main bearings, cam bearings, rockers, and more due to one lobe grinding apart. The particles become grinding particles.
IF you have loose rockers and readjust them and they become loose again, you have lost a lobe or have bad lock nuts. Check the lift on the cam.
A leaking exhaust manifold donut gasket can also cause a backfire, but not as likely.
Ron

 

Galen - How did the plugs look when you pulled them for the compression test?
Mike T - Prescott AZ

 

A leak down test will show where the compression is leaking. Intake Valve, Exhaust Valve, Piston Rings. Does not take any longer than a compression test.

It would be nice to learn where the problem is before getting out wrenches to replace an engine.

 

plugs only have 30 miles on them so they looked normal.

 

When I was fighting my oil consumption problem I could foul a set of plugs in under 30 miles--to the point of misfiring.
The solution to my problem tured out to be NO thread sealant on the rocker arm studs. Once I applied the sealer all problems suddenly disappeared---clean plugs AND my static compression jumped from 140 to 180 AND my leakdown test improved from-30% to -3% and best of all my #1 and #7 cyls which were the worst became the best.
Eddie

 

Wow. Good you could fix that problem. I have read about the "NO thread sealant" issue some place before.

I think the plugs looked normal. No heavy carbon or black on the plugs. A little from just being used, but I did not think any of them looked bad enough to misfire. I am going to take the car for a longer drive today. 50-100 miles and check things over again.

Interesting comment by R66 about, "a leaking exhaust manifold donut gasket can also cause a backfire". I actually did or do have some exhaust leak sounds coming from the right side. I remember cleaning the donut before installing it again. I found small pieces of carbon or dirt around the donut, probably should have left that on, but was thinking the donut will not be in the exact same place so maybe the deposits would make the fit worse instead of better so I removed those pieces.. I figured it was particles accumulated over time filling small air leaks. Never figured it would or could cause backfire, if it was not completely sealed. The leak was worse when first assembled, but less or gone now. I did tighten the 3 nuts a little more that hold the donuts. Maybe over time, hopefully shortly there will be no more exhaust leak and the backfiring will be gone.

Thanks for the information.

I will look into

 

Question. Never have done valve work. Was the lack of sealant on the threads screwing into the head or the threads the on top that the nuts screw on to ? Or both ? What type or kind of thread sealers should be used ? How tight are the studs screwed into the heads ? Is the tightness of the studs screwed into the head measured in foot pounds ?

When doing or checking rockers and pushrods I am thinking that I need to have a piston at TDC on compression stroke when checking pushrod play and tightening of the rocker arm stud nuts. Just like when putting the motor together for the first time. (1) bring piston to TDC on compression stroke (2) tighten down rocker arm stud nuts until no play is felt of the pushrods (3) give nut an additional 1/2 turn. Done. Is this the correct procedure after applying thread sealer ?

 

The leak Eddie is describing was on his Rocker Arm Studs and I believe it was with aluminum heads. If your heads are factory stock the rocker arm studs are pressed in and not threaded.
So before pursuing you need to see what heads are on your engine, It is common for studs to be installed when the factory heads are rebuilt so maybe your heads are threaded studs but you would need to check. Easy to see with valve cover off. Threaded studs all have a bolt face right where the stud bottoms in the head. Press in studs do not have a place to use a socket or wrench to remove them.

 

Got it.

Nice dry day. Took the car on a 80 mile drive. Steers nice, no vibrations, if it don't misfire I would trust this engine. Towards the end of the drive I down shifted to 3rd and slow brought up the rpm. Once you get in the 4000 to 4500 you get some backfiring. 4500-5000 you get a lot of backfiring. Hmmmm.

If, bad cam, hard to tell how long it has been bad. I figured it would prob need a timing chain and gear if I do the cam.

Also, don't I want to drop the pan and use Plastigauge to check bearing clearance ?

Heads are not alu. Heads don't have any tapped holes in the front side of the heads. Prob. Pontiac heads like I think the block is. Block # F0531HCH.

 

Yes, I would start with the selected piston at TDC Compression strolke and go at it from there.
MY issue was with Edelbrock E Street ALUMINUM heads. The holes for the oushrods are NOT blind holes as they are with the original cast iron heads. Edelbrock drilled them all the way through. The studs are threaded and Edelbrock makes a point of tightening the studs to 35lb/ft but makes absokutely no mention that these holes are drilled through INTO THE INTAKE STREAM and that sealant on those threads is VITAL. Shame on them!!!!

I used Permatex white hi temp thread sealant. and systematically went thorugh the entire engine. There was absolutely NO TRACE or evidence that anyone had ever applied sealant to the heads when they were assembled. So far, after better than 1000 miles I have no more issues. The compression has increased dramatically as noted above and I use virtually NO oil.

Eddie

 

Right on. Thanks for clarifying this for him.
Eddie

 

I’ve had spark plug wires break down at higher rpm .