AMC 304 Engine Knock - Ugh

adman02

Well-Known Member
Joined
May 15, 2009
Location
Durham, NC
Swapping a 304 into a scrambler I bought recently. I finally got a chance to start the motor today and SH!T - sounds like someone dumped a bag of marbles into it when it idles. I'm not highly experienced with motor internals and would really appreciate help diagnosing it (I'm hoping that it's something like a stuck valve, but am prepared for the worst).
 
Exhaust leaks can sound bad on those. Lifters can sound horrible as well. How long has I been since it was last started
 
Probably a year since it has been run. The guy I bought it from sent me a video of it running and there was no noise. I didn't do anything but drain the coolant, replace some freeze plugs, install an HEI distributor and install it in my scrambler, so I don't know what could have happened. Maybe it's the timing? I'll put my timing light on it tomorrow.
 
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do you have oil pressure ?
if yes, do a compression check.

Then you start speculating, not before.
 
Sometimes when my 304 has sat for 2 weeks or more, I'll have one or two lifters that leaked down. Takes them maybe 10 seconds to smooth out though. You probably ran it longer than that.
 
Thanks for the suggestions. Now I can’t get the engine to start at all. It just turns and turns - not even a stumble. I have fuel for sure. Not sure about compression but will do a test in the next couple of days. I don’t have spark though, so I’m thinking the HEI is bad? Is there a way to test it? I bought the HEI unit from CRT Performance which seems to have a good reputation but I pulled the #1 plug and grounded it on the block and cranked the engine - nothing (and it was dark outside so you’d think I’d see something).
 
Pull the cap and make sure the rotor turns while cranking is step 1 on a no spark. If it's not turning its going to require some exploratory surgery to figure out, broke timing chain, stripped dist gear, etc.
 
Tim C said:
Pull the cap and make sure the rotor turns while cranking is step 1 on a no spark. If it's not turning its going to require some exploratory surgery to figure out, broke timing chain, stripped dist gear, etc.
Click to expand...
Well, sh!t. The distributor is not turning. I did look at the distributor gear, and it seems to be in good shape (no marks or wear; no metal pieces). I guess the next step is to pull the timing chain cover? If it's the timing chain, what kind of damage could I have done by turning the engine over with a broken chain?
 
If you're lucky nothing. Worst case is bent valves, which means the heads come off. Bent pushrods is another possibility.

Without hearing the noise you described earlier it could possibly even be a broken crank or cam, but those are super rare.

It's possible the noise you heard was because it had jumped time and bent a pushrod or valve and now it's either jumped worse, or broke altogether.

I'd go ahead and pull the cover for starters. If the chains broke get a new one and try to restart or just do a compression test. If the compression is low that cylinder has a bent valve or pushrod, or if you crank it and it's still rattling then the valve covers come off next to see if the pushrods look like cooked spaghetti noodles, LOL, and to see if all the valve tips are even with no rockers installed, a bent valve will sit lower than the others because the valve won't seat.
 
Other than the labor which sucks when you've got other stuff you'd rather be doing to it, bent valves aren't a huge deal. I had 77 f150 I bought for parts that had the 400 engine chain jump (stupid nylon cam gears Ford used in the 60s through the 70s, AMC may have too). A local machine shop replaced 8 valves total and resurfaced the heads for around $300 5 or 6 years ago. I used those heads when I built the 351m in my trail rig because my original heads had parts of the exhaust manifold bolt holes broke off. So that, maybe some pushrods plus a head gasket set will have it fixed up.

You can test the pushrods by rolling them along a straight workbench or table. If they're bent you'll see it then.
 
With the distributor in hand, if you hold the rotor, can you rotate the gear? I have seen a dizzy where the gear slips (either a sheared pin or bad press fit depending on your manufacturer), a cap being installed incorrectly could have also stripped the rotor.

It wouldn't be the first (or the last time) a part has been bad right out of the box. I suffer from the same disease you do, take something that works, and change a bunch of variables.....then try to start it! Next time, toss the engine in first, fire it up, then go to town one piece at a time so if something goes wrong, you have a definite change point.
 
Welp...TimC is the winner.

Pulled the front end off the engine this AM and found this:
IMG_9919.JPG IMG_3692.JPG IMG_5578.JPG
Cam gear is nylon!


Timing chain: broken
Fuel Pump Eccentric: destroyed
Fuel Pump: roasted

Now a bunch of questions...
• Any tips on how to get the metal pieces out of the engine (besides an oil change)?
• Does the cam drive gear (the one that drives the distributor) look like it needs replacing? It seems like it has some pronounced wear.
• The fuel pump arm hit the timing cover and gouged it (about dead center); it didn't break through, but should I still replace it?
• Any suggestions for a new timing gear set? Any brands you like for quality?
• How do I retime the engine since the crank has spun since the chain broke? Is it as simple as lining the timing marks on the timing gears up after finding TDC? Or is there more to it than that?
 
Before sinking a lot of money in an unknown engine, I'd replace the basic stuff and fire it back up to check it's in good condition.

Retiming the engine is as simple as lining the marks up. Just make sure you didn't flip the cam 180 when you do that.
 
If the cover isn't cracked or worn through don't worry about it. If the dist drive gear doesn't have missing pieces of teeth or nicks on the teeth you can skip it too unless they're dirt cheap. Definitely replace the chain, fuel pump and eccentric.

If it's not a high performance build then a stock replacement timing chain and gear set from any parts store will be fine. They should all have a steel gear for the cam. I don't see any missing teeth on yours though so that's good.

Only way to get metal out is to drop the pan unfortunately. You can try to fish a magnet around with the timing cover off but no guarantee you'll get it all. It's a gamble whether it'll hurt you down the road or not. The pieces may stay in the pan and cause no trouble forever or they could get sucked into the oil pump and lock it up. The filter should catch anything before they ruin bearings though so it's your call there.

You may want to go ahead and check for valve train damage before you go too deep. If you're going to have to pull heads and pan it will be easier to just pull the engine and do it on a stand.

The timing chain should line up dot to dot. You can't put it 180 out because d 2 d happens once every other crank revolution. Even if the cam gear dot is 180 off it'll line back up in one revolution. The pistons don't care if they are TDC compression or exhaust, the cam decides that. The distributor is what you could have out 180 but it's off with the cover so nothing to worry about. If the timing chain is off a tooth it'll be obvious because the number of degrees per tooth.

If you leave it d2d when you put the cover on that should be tdc1 (it is on most other engines, someone with more AMC experience can correct me if I'm wrong). As long as you don't move the crank reassembling it you can drop the dist right in with the rotor pointing at #1 post
 
^ He is correct. MSD (I think) makes a matching gear set you can install on your cam gear and distributor that are guaranteed to mate. BJ’s sells them.
 
adman02 said:
• Any tips on how to get the metal pieces out of the engine (besides an oil change)?
Click to expand...

Assuming the pieces are somewhat large in comparison to the oil pump pickup screen, I'd just remove the drain plug and pour soem random oil of some sort into the front of the open pan area and see what comes out. Rinse and repeat as needed. Then a fresh oil change with a couple oil filter swaps should cover it.
Like others said, big chunks do zero damage, it's the ground up stuff that does.
 
Quick update, though no pics. Got an eccentric (hat tip to AMCJeepMan). Installed a new timing set. Dropped the oil pan and found two small pieces from the chain, but otherwise clean. Pulled the valve covers and examined the pushrods. They seem straight/no excessive movement from the rockers. I'm going to button her up and then give it a whirl.
 
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Engine is back up and running! Took the jeep for a spin today. Then...steam started billowing out of the hood and antifreeze started pouring on the ground. I thought I'd blown a head gasket because the antifreeze seemed to be pouring out of the DS valve cover. A closer inspection revealed a freeze plug in the head rusted through. Anybody have any idea what size the head plug is? I can only find sets for block plugs.
 
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