AMC 360, 50 miles after rebuild, popping when cold

jrchaffee

I recently had my AMC 360 rebuilt. The mechanic did all the initial run-in and cam break in. I had it and seated the rings right after and put about 50 miles on it. I had to store it for a couple months. Now I have it again and it is much colder now, mid 40s, and when I drive it it makes a popping sound, almost like a backfire, through the exhaust, but only for about 10 min. Then it goes away. I originally thought it was just old gas but I took it out again today with fresh gas and it did it again but went away after about 10 min. I'm still in the initial break-in of the engine so I didn't know if this was normal or not. I thought maybe I need to adjust my carb now that it is colder out or maybe the exhaust headers were leaking. Any ideas? I haven't changed anything with the engine since I got it back.

tufcj

You're on the right track. I'd check all the header bolts, they're notorious for coming loose. If the gaskets are bad, use Remflex gaskets, they're expensive, but I've never had one leak. Bob tufcj

jrchaffee

tufcj;133504 wrote
You're on the right track. I'd check all the header bolts, they're notorious for coming loose. If the gaskets are bad, use Remflex gaskets, they're expensive, but I've never had one leak.

Bob
tufcj

I think I found the culprit...I went to tighten all of the header bolts and some were a little loose but not really all that bad. I tightened them down and then I noticed that on the drive side there was a piece of the header flange missing. It was between two bolts where you wouldn't think it would have much of an effect but after I tightened them down it still made the popping/backfire noise. Would a big straight pipe exhaust amplify the noise when I'm driving?

tufcj

The exhaust can echo down a pipe. A missing piece of flange would surely cause a leak. Headers (especially inexpensive ones) usually have thinner flanges and small sealing surfaces. I had a set of $99 headers on my Jeep that never would stay sealed. When they rusted out, I replaced them with a $250 set of coated headers and they never leaked. It's all in the quality of the flange and the weld that creates the seal. Bob tufcj

jrchaffee

These were definitely inexpensive and I guess I'm paying for it now. I guess it's about time to get a new set. Any brand recommendations? I've heard Hedman are pretty good. Is it ok to drive with the leak? It's a brand new engine and I don't want to damage anything so early in it's life.

tufcj

It depends on the size of the leak. A large leak can let enough cold air in to warp a valve. If it's small enough that you can't feel a puff of air with your finger, then you can probably get away with it for a while. I run Hooker super comps on my AMX. I haven't had any trouble with sealing or leaks in about 8 years since I built it. Bob tufcj