Intermitent mis fire on AMC 304

mbalbritton

mbalbritton

#@$%!
Joined
Mar 22, 2005
Location
Lakeland, FL
I've been dealing with a misfire that comes and goes on my 304. It doesn't feel like a full on misfire, but more of a very weak ignition. If I had to verbalize it in text, it sounds like...

bum...bum bum..... bum...... bumbumbumbum.......... bubum...

It may or may not happen each time the engine runs. but it is most noticeable while running 50-60 mph and under load when going up a hill or even when accelerating at lower speeds. I can hear and feel it.

A little about my set up:
MC2100 carbed 304 that's never been rebuilt as far as I know or can tell. And an HEI Distributor that's about a year old.

Some times the miss fire is there, sometimes it's not. when it happens it may happen every 2-5 seconds. sometimes it's one misfire, sometimes its 2 misses, a few times it has been a rapid 5-8 "bumps".

It doesn't seem to do it at idle, or at least it's not noticeable. So pulling plug wires isn't really telling me anything. Plugs look fine, but from the sound and the fact that my exhaust come out just in front of the rear wheels and my passenger rear wheel has more exhaust stain on it than the driver side, I'm going to guess that it's a passenger side misfire.

What are some causes to look at, and maybe somethings to check. I don't have a compression gauge, but I'll get/rent/borrow one soon.

I'm kinda leaning toward compression/worn out engine.
ajeepstercommandoclub.com_forum_images_smilies_icon_rolleyes.gif
but want to check the simpler and easier fixes first of course.
 
Just to cover the basics, the HEI needs 12 volts so the factory supply wire's resistor wire should be removed, and some of the HEI's had hard gears that will quickly wear out the factory drive gear on the cam and start to cause misfire before it fails. If the carb hasn't been off it may be time for new gaskets and check the plastic isolater, I have seen some that would no longer give a good seal causing a lean condition under load.
 
tknopp said:
You can also use an ir thermometer. Put the laser dot on each exhaust tube and see which one has the lowest temp. That will be the one that is miss firing. You can the start checking that particular plug and wire and so on.
Click to expand...
Funny I just ordered one yesterday for my beer brewing. I bought and IR so I could use it for this as well.
 
amcjeepman said:
Just to cover the basics, the HEI needs 12 volts so the factory supply wire's resistor wire should be removed, and some of the HEI's had hard gears that will quickly wear out the factory drive gear on the cam and start to cause misfire before it fails. If the carb hasn't been off it may be time for new gaskets and check the plastic isolater, I have seen some that would no longer give a good seal causing a lean condition under load.
Click to expand...
I'm trying to recall if I swapped my gear for the old one or not. I had heard this before, but had forgotten, thanks for the reminder!
 
Well, distributor gear looks good.

ImageUploadedByTapatalk1384611899.129453.jpg


ImageUploadedByTapatalk1384611921.385829.jpg


And my passenger bank spark plugs all look nice and tan.

ImageUploadedByTapatalk1384611955.638121.jpg


But my driver side plugs...

ImageUploadedByTapatalk1384611990.165836.jpg


ImageUploadedByTapatalk1384612006.128744.jpg


Ash deposit. Hoping it's just the oil additives I've put in and not leaky valve stem seals. But since it's just on one side I'm guessing I'm not that lucky.
 
mbalbritton said:
Ash deposit. Hoping it's just the oil additives I've put in and not leaky valve stem seals. But since it's just on one side I'm guessing I'm not that lucky.
Click to expand...

Were the ashed plugs in any particular cyls. (would bet the worst is #7) or all of them?
Valve stem seals on one head is more plausible than thrashed rings on that side only... :D
 
Caver Dave said:
Were the ashed plugs in any particular cyls. (would bet the worst is #7) or all of them?
Valve stem seals on one head is more plausible than thrashed rings on that side only... :D
Click to expand...
Worst was probably 3 then 5 then 1&7 about the same. Valve stem seals is what I'm thinking. I'm also thinking.... I have a set of 360 heads sitting in the garage. Kinda wondering if they are the correct ones to put on a 304!
 
Getting any smoke on cold start ups? Me and my pops done valve stem seals on an old 318 mopar. That was the major culprit. I was amazed at how hard and brittle they were, you could crush them into powder.
 
Ya, cold start smoke followed by fouled plug misfire until I bring the rpms up enough to clean them out. Guess someone needs a valve spring compressor for Christmas. :D
 
That is a hardened distributer gear so I bet your drive gear is not doing very well right now.
Do yourself a favor and install a matched set asap and I dont know that I would tear into the heads just yet unless your getting a lot of smoke I run denso plugs gapped at .045 in my 360 and it cleared up a lot of those problems.
Also dump a can of seafoam in the tank to clean out the carb.
Then if things are not better start taking things apart.

Buckeye Performance Inc.
828-779-2242
 
If it were a cam gear, your timing would be all over the place and it wouldn't be a miss one clyd like you are describing...
If all the simple stuff doesn't take care of it, you probably have a carboned up or burnt valve. The first rule of engine diagnosis is run a compression test. They don't lie, and they give you a great view of the health of your engine. Pull ALL the plugs out. If your battery is weakish, put a 50 amp battery charger on it so you get the same cranking power each time. Have a friend hit the switch for 5 seconds. Record each clyd's reading on paper. If one cyld is 20-30 psi or more lower than the rest, squirt motor oil in that plug hole and test it again. If the psi goes up, its the rings, if it doesn't, its a valve. If it is a valve, run the engine until warm, then hold it at 2000 rpms and dribble some water out of a drink bottle into the carb. This will shock the carbon off of all the valves and clean the combustion chamber clean as a pin. ( Ever seen a head that had a blown head gasket? Clean as hell!) If that doesn't take care of it, you need to pull the head and fix it. Or live with it.:D
 
Since I cleaned the plugs and put them back in its been running fine. But I don't expect that to last forever. I fully expect the driver side plugs to get fouled again. I'll see if I can rent a compression tester sometime soon to check that. But the next several weeks I'm not gonna have time.

Thanks for the help, I'll let you know.
 
After the Seafoam/water cocktail, you may find that it isn't the valves, but the induction hardened seats. When the Moss' major miss began, my #2 exhaust seat was gone... literally! Vacuum leak caused by stoopid "gutter flashing" intake gasket leaned her out and vaporized the seat :(
 
Lack of wheeling has been known to cause the same symptoms...

Sent while I should have been doing something constructive...
 
It was late and I was getting gas in the excursion, had to find some way to dull the pain. What better way than to relive middle school and pick on someone else?
Also, make sure you're getting at least 12 volts, maybe swap the hei module. But it's probably the valve seals as stated.

Sent while I should have been doing something constructive...
 
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