same 74' 302 that I have been rebuilding for nine months now. 302 rebuilt engine (bought from a friend of a friend of a friend, I know I know) got it running & found oil & trans leak so took it to shop to put on lift. The guy said that the rear oil seal was put in wrong & not rotated & the trans seal I put in wrong, when I was putting in the seal I struck more of the inner seal instead of the outer seal causing damage which lead to tranny leaking badly. They cranked it up & found the truck vibrating extremely badly. They looked at the fly wheel & found a huge weight on it & they found another one to compare it to & found it too had a huge weight on it, so they left mine in. Then they checked cylinder compression & found cylinder # 7 had no compression. The rocker arm had broken off & rod bent they replaced both of those & it had good compression & ran much much better. The new 4 barrel carb was adjusted to smooth it out too.

There is still a noticeable vibration at low rpms.

My fear is that the guy who put this together didn't put things in correctly & it is a ticking time bomb. I hope not & this issue can be corrected to finally have a GREAT running old truck.

Thanks Everyone
Garry

 

How big was the weight. 302's changed imbalance around 82-83 to a larger weight. Older ones are 28oz and newer ones are 50oz. So you'd need the correct matching flywheel (flexplate), harmonic balancer, and crankshaft.

Maybe he put an incorrect hamonic balancer or flywheel on there? Just my .02

 

the engine he sold me was around 1971 or 72 & my year truck is a 74'. It came with a harmonic balancer & I used my old fly wheel. I didn't check the weight on it before putting it in. As for the crankshaft, I have no idea on that one.

 

Contrary to popular belief an imbalanced engine causes no harm. The main detractor is a slight loss in horsepower for a mild engine.

Harley Davidson bases their entire structure on an imbalanced engine.

Ever see a 4 banger idle? It's rocking all over the place, the reason you don't feel it inside a Honda or Toyota is the butter soft engine mounts.

Now I am not saying you DO NOT have a problem, it sounds like you do. But if compression is even for all cylinders and all cylinders are firing you then only have:

Harmonic balancer
Flywheel
Clutch

But that is only for idling, if you are talking about LOW RPMS while Driving, then you have a bunch more worms in that can.

 

just at idling

 

You know what else can cause weird idling is a defective alternator that is constantly kicking on and off. I doubt that is your situation, but something to keep in mind.

Are you perfectly sure the engine and transmission are lined up?

Josh

 

it better be, I took it to a mechanic that works on old cars & he seemed competent.

 

could it be that it is due to a vacuum leak, I didn't put the smog back together or any of the thermal vacuums or those connecting up to the air breather except for the pcv going into back of carb

 

A vacuum leak most certainly could cause the rough idle, especially if the PCV is plugged.

Do you still have the EGR bolted onto the intake, but it isn't hooked up?

Josh

 

My truck has all the emission stuff taken off. This is a diagram of the very few vacuum lines on my engine. The dark blue on the vacuum tree is just a loop to block off two outlets on the tree, but I have a vacuum gauge mounted in my truck so that is T'd into that loop (lime green line). Also I don't have power brakes so there is no line running to a booster.