Missing

CarlG

CarlG

Well-known member
This problem crept up earlier this week.
My engine developed a distinct miss-fire. Traced it down and the #4 cylinder was not firing. All the proper tests done; grounding out one cylinder at a time, swapping plugs between cylinders, checking all the electrical connections, point gap, plug gap, etc. Still no fire (or not much anyway) on #4. Grounding out 1,2,or 3 produced a drop in rpm, grounding out #4 didn't.

Did a compression test and #4 was 10+# higher than the other 3. I didn't have a baseline to compare it to, so don't know if this was a change or what. The #4 spark plug was all black, whereas the electrodes on the other 3 were grey. And like I mentioned above, swapping the plugs didn't change the location of the "miss".

Changed out the rotor, dist body, cap, plug & coil wires.

All amounted to little or no change.

Even my "mentor" was baffled!

Any suggestions????
 
Ya know what i have this same problem, im useing a fs distributer and the stock before it. If i change out the spark plug it fixes it for a while.
 
Warning! Long answer, but that's just the way I am :o

I am sure there are other things (electrical, fuel, manifold leak, etc.) that can cause the misfire, but this seemed to work for me.

Your results may vary.

I did four things that helped my misfire on #4.

  1. Bushings in the distributor. I had a lot of slop in my distributor shaft. I replaced the bushings and the upper shaft (drilled for oil) and this helped to eliminate some of the misfiring.
  2. Upper plate: The upper plate was sloppy in the housing, so I used a small nail set to peen the plate to the housing. This took out the slop. This helped eliminate some additional misfiring - and it helped during the timing as I was able to set the point gap more 'consistently'.
  3. I still had a misfire and found that the cam was drilled off center. It was a reproduction part (came in a box-o-parts with the car and I know not the source, nor the age.) I had a friend check several other cams that I had obtained (various swap meets, auctions, websites, etc.) and found one that was properly drilled. That eliminated misfires almost completely, almost but not entirely.
  4. The last thing I did that seemed to end the misfiring (especially while cold) was to remove the paint from the plug holes on the head to assure a good ground for the plug.

    During my excitement waiting for my replacement engine I had the head surfaced and cleaned. I then proceeded to put down a layer of primer and paint (if a little is good, a lot is GREAT - just kidding!) I DID not cover the plug holes and there was a LOT of paint in there. I'm not sure if this was causing any problems; however, I did work some paint stripper into the threads, used an old plug to work it in, wiped off all the stuff I could see and replaced the plug - good ground!
Now it runs great and when I ground-out any plug the engine responds - a distinct rpm reduction and a noticeable 'miss' from the grounded cylinder.
 
Almost sounds like you might have a bad cam lobe on the #4 exhaust valve. This would cause higher cranking pressure, natural EGR effect (sooty plug), and low power in that cyl. Hope that's not the problem.
 
I had the same problem. I just went out to the shop and fixed it. The car had been sitting for a while. I have several model A's and because of the miserable hot weather here I haven't been driving them for a while. When I cranked the car it acted almost like it was flooded, it was skipping on two or more cylinders and wouldn't really clear up. I managed to get it out of the garage and over to the shop, put,put,put. I first removed the plugs and they were all nice and tan, a couple were damp with gas I dried them off cleaned and checked the gap, all seemed good there. I checked filed and re set the points, no real problem there. Checked the timing and it was just the way that I had set it previously. Drained and checked the gas in the cast iron sediment bowl, no problem there. The car had one of the burn out proof condensers and was tight on both ends. Cranked it back up and it was still skipping. Pulled the dist cap and tinkered a bit more. Got back in and tried cranking it and it wouldn't crank. Tried a couple more times with no luck but noticed that the engine was turning way too freely. Looked under the hood and saw that in my desperation, I had left the top cap off the dist. I put the cap back on and it cranked but was running ruff. I decided maybe if I took it out on the road and really blew the soot out of it maybe it would clear up. It ran so bad that I didn't trust it out of the driveway. I went back in the house to cool my head and puzzle over what to do next. I kept thinking about how fast that the engine was turning with the starter. I decided that maybe the new stainless steel valves that I had installed when I rebuilt the engine may be moveing a little slow, not actually sticking but just slow enough to cause a miss. I went back out and cranked it back up and sprayed a little PB blaster in both of the carbs and it seemed to speed up and get a little smoother. I removed the vacume line and connected one end to the intake manifold and stuck the other end into a bottle of Marvel Mystery oil I stood at the side working the throttle as needed to keep it running, It was really putting out a fog of smoke. I would remove the vacume line from the bottle, hold my finger over the vacume line and rev it until it cleared and then repeat. After a little bit it was running good enough to reconnect the vacume line. While it was idleing I added more gas to the tank with a liberal amount of MMO and added some to the crankcase. It was running good, the throttle response was good and no skipping. I pushed up the spark and listened to the chicka lunka chicka lunka idle. Took it out on the road ran it through the gears and all seemed good opened it up enough for the high speed circuit to cut in on both B carbs, My heart is thrilled. As far as i'm concerned, it is fixed. I think that the 10% ethanol fuel and the stainless steel valves don't get along and caused slow valve action. Slow valve action will cause a miss. I had added Stabil to the fuel but it wasn't marine Stabil. I think that I will start adding MMO to the fuel and crankcase on a regular basis. Hope that this helps, it may not be your problem but it sure solved mine.
 
Some distributor bodies (the big plastic thing) develop an internal short. My memory is that this diverts the #4 spark to the #3 plug. Try a different distributor body.
 
I had the same problem and was a vacuum leak at the inlet manifold

I have an intermittent slight miss in the last engine I had rebuilt and which I'm having trouble finding the cause of . I suspect the modern guides and valves we put in it . The engine rebuilder insists that it can't be valves as it is not regular . My feeling it is that it is probably "slow valves" as you suggest . Next rebuilt will be with standard valves and guides -Karl
 
The MMO and some marine stabil may solve the problem. We will continue to have problems with the ethanol enriched fuels that we now must use, that we never had with previous fuel. Hopefully the right mix of special additives will solve the problem. The guides in my engine are the original two piece style .The valves are the original mushroom style but are made of non magnetic stainless steel. The car had never had this problem in the five years since it was finished. I think that the ethanol fuel and the fact that the car had sit for about three months, brought on the episode.
 
hello everyone... have been gone from the barn for over a year...browsing through tonight and wanted to see if i still know how to use it... is this message getting through.... thanks and great to be back... rocky d
 
Well guys, I went thru most everything you suggested. I need to pick up some MMO tonight, and give it a good dose.

The one thing that did the most good and seemed to "almost" cure the problem was to replace the manifold gasket. I left out the exhaust gland rings as I have read elsewhere on this forum that tend to be a problem.

Another suggestion that my friend that was helping me work on it last night gave me this morning was that it may have a broken spring on #4 exhaust. If the drive tonight after adding MMO to the equation, that will be the next step. Just wondered if anyone here has had an experience with a broken spring and what the symptoms were?

Thanks to all that responded, suggestions are great!
 
Update:

Found the smoking gun: #4 Exhaust Valve stem broke just above where the keeper fits. This would allow the valve to seal enough to get a compression reading while cranking it, but when the engine was running, the valve just floated. No compression - no fire.

Looks like I will need to order new valves.
 
My modified deluxe delivery with an FS ignition never ran right. I replaced it with the stock distributor and it now runs better than ever. Points forever.
 
CarlG said:
Update:

Found the smoking gun: #4 Exhaust Valve stem broke just above where the keeper fits. This would allow the valve to seal enough to get a compression reading while cranking it, but when the engine was running, the valve just floated. No compression - no fire.

Looks like I will need to order new valves.
Click to expand...

Very unusual failure. Never heard of a valve failing like that. The "slow valve" guys were the winners on that call, I'd say.
 
Carl,

What type of valve failed, I agree that this is a highly unusual failure.
Are these Model A style valves or modern valves?

Snow/Rain here in Fairbanks since Saturday night, luckily it didn't freeze overnight. Hoping it dries out before winter really sets in.

Darryl
Fairbanks
 
darrylkmc said:
Carl,

What type of valve failed, I agree that this is a highly unusual failure.
Are these Model A style valves or modern valves?
Click to expand...
Got the head off and the offending valve out last night. They are indeed steel Model A style valves. I guess my dilema now is, do I just go ahead and replace all of them? And with what? --There's just too many choices!

Also, even though this is a diamond block engine, it does not have hardened valve seats. Another choice to make!
 
Carl,

A couple of questions.
Does this engine have the original Solid Tappets or Adjustable ones?

Is there any indication of why the Valve Stem failed, almost sounds like the spring may have been loose or keeper not seated in the spring properly.

Do any of the other valves show any of the same signs of eminent failure?

As long as the seats in the block are OK I would not put hardened seats in, most diamond block only had exhaust seats anyway.

I probably have a Valve if you are able to replace just the one, all things being equal and the rest of the valve train is still good.

Can you tell how much margin is left on the remaining valves and the condition of the seats?

Would be nice to see a photo of this valve failure, along with the keeper and spring.

Darryl in snowy Fairbanks
 
Darryl,
1) It has adjustable tappets.
2) I'll get a closer look and a second opinion as to why, but the spring and keeper were in good shape, and positioned correctly.
3) At first glance, the other valves look OK, again, once I get them all out it may be another story. I've had several offers of replacement valves. Just wanted to asess the whole situation and the condition of the other valves before deciding to replace just the one or go for the gold.
4) Photos are in the works, took some last night of just the valve. I have all the keepers out, so it may be hard to tell which one is which.
 
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