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69 AMX - new motor problem

1969 American Motors AMX

I purchased a brand new motor and had it installed in my AMX w/ a 650 demon carb. This motor has as approximately 20-30 miles on it after engine was installed. Engine had some blue smoke coming primarily out of passenger side exhaust at first. The smoke was light to non-existant when car was started, and got progressively worse the longer the car was running - after driving for 3-5 minutes, the smoke would envelop the whole road. Smoke also got worse as a few more miles were put on the engine. Engine started fine and idled fine - no problems when stopped at a light for a few minutes. Oil pressure fluctuated between maybe 35 psi to 90 psi while driving (to the best of my memory).

Brought the engine in to be looked at. I’ve attached pictures - the ones in the engine were taken w/ a scope.

Found the following:
Oil extremely fouled and smells like gasoline.
#8 spark plug gunked up and black
#5 spark plug black (across from 8)
Ceramic burned of #5 header runner
no oil leaking past intake manifold gaskets
Fuel (and maybe oil?) on top of intake valve, and no carbon deposits
Fuel (and maybe oil?) on top of piston
vertical scoring on top of cylinder wall
PCV valve connected to intake manifold, going directly to #5 and #8 - not sure if this is a problem or not.

What I’m trying to figure out, is this a problem with the motor, or a problem with the installation?

What do you think?

How was the breather tube for the PCV system routed? If the right valve cover is identical to the left one I don’t even see a breather.

I would prefer the PCV to be sourced from the base plate of the carb rather than just 2 cylinders.

At this point I would want compression and leakdown tests on the engine.

I also think the carb may be running too rich.

I don’t know, is your engine a 290 or a 390 and is the intake manifold stock? If it is a 290 witha a stock cam and manifold, i would fault whoever decided to stick a 650 Demon atop it especially if the linkage isn’t vacuum controlled. Where can you get a brand new 1969 AMX engine anyway.

It’s a 390. I supplied the block and had it custom built. Neither cam or manifold are stock.

I absolutely agree with oldtimer, but I think you have bigger problems.
#5 and #8 are badly oil fouled, and 2&3 look like they’re starting to carbon up. The 650 CFM carb might be responsible for carboning, but the oil fouling is most likely coming from the cylinder walls. Whoever rebuilt this engine… well, what can I say. Rings & walls, rings & walls. IMHO the motor needs another rebuild.

When looking at the pictures, the first thing I question was whether the lifters, and these should be hydraulic lifters, were adjusted properly. On some engines, the stud for the rocker is threaded all the way down so the lifters do require an adjustment. If not properly installed and adjusted, you would get two cylinders fouled with excessive fuel.

My second thought is that the rings were not properly gapped before they were installed. That is putting the ring in to the cylinder before putting it on the piston to check the end gap of the ring. If this is not done, the ring could get shattered on installation. That is just a possibility, but it could also come from lack of cleaning the cylinder bore after honing or improperly adjusted lifters.

The burned header could also be attributed to the lifters, only by now, I suspect you will find the exhaust valve on that cylinder has a burned face and will need to be replaced.

You also might have the float level in the carburetor set too high and are flooding the engine.

If the lifters were not adjusted properly, this could damage the cam as well.

Document everything. I bought a Corvette 327 engine years ago with 4 bolt mains. It seized up after about 50 miles in the car. I watched them tear down the engine and it looked like it was assembled by monkeys. The thing that did the engine in was the main bearings which were installed upside down…no oil hole. They were all installed exactly the same way so the guy (monkey) really did not know what he was doing.

Did you run the engine @ 2,000 RPM’s for at least twenty minutes before driving the vehicle?


The engine was run @ 2,000 RPMs for 20 min. If the carburetor is flooding the engine, would the car have started easily and idled ok?

I am going to try to get a compression and leak-down test done next week, my question is, could the damage to the engine have been caused by gas flooding? Both the mechanic installing the engine and the engine builder have excellent reputations.

Just curious, but how much money did this engine set you back?

Was the block vatted, cylinders bored (not meaning honed…), new oil pump, valve lifters, etc, etc.

“vertical scoring on top of cylinder wall” … This means the engine gets pulled and torn down. There is a major problem…

Don’t want to say exactly how much it cost… but it wasn’t cheap. Block was vatted, I believe cylinders were bored, new valve lifters. For the most part new or re-machined everything.

There’s no reason to put off tearing the engine down if there are scored cylinder walls. And if the carburetor were flooding badly enough to score cylinder walls in a few minutes of operation the black smoke should have been obvious.

@Keith’s thought on lack of proper ring end gap seems like a good possibility.

Yeah there was definitely no black smoke…

Could they have failed to pre-lube a few cylinders and maybe not cranked with the sparkplugs out for several second to prime the oil system?

The engine builder said he primed it, and the installer also primed it before installing.

Yeah, we would hope. My Dad always said, “Look for something simple.” it’s amazing how well that advice has done me over the years. Everything points to destroyed oil rings on at least two cylinders. How many ways can you do that?

Good, experienced engine builders will lube everything with a special assembly lube (looks like STP) to eliminate any dry start problems…Can we assume the heads were rebuilt too and they are the correct ones for this engine? These 390 AMX cars were VERY rare, special order only, in '69 they only made 2100 390, 4-speed cars…Over the 3 year model run, they produced less than 20,000 vehicles
total…Check this video out…This is not your fathers Rambler…

The heads were rebuilt, and were from the original engine. It is extremely rare… getting parts for this thing and getting it together have been extremely difficult and pricey… I’ll post a picture at some point.

My first thought was it was running so rich it washed down the cylinders. That’s why you have cylinder scoring and oil that smells (and looks) gas contaminated.