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Allis Chalmers C tactor misses backfires at high rpm

R

radios

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I found this older discussion and it seems very similar or same to my problem: http://www.yesterdaystractors.com/cgi-bin/viewit.cgi?bd=ac&th=83007

I have a 47 and a 49 C. The first one smoked badly so I pulled the head and had it redone. New valves, guides, rockers polished, etc. Now it runs smoothly at idle or when not under a load, but add power and it sputters and backfires. I decided to sell it as I could not fix it. So, I did the head on the good C just to make sure it was OK (since the other one had broken rocker shaft etc). Now that tractor does the exact same thing. No power, coughs and backfires when pulling, no power.

No matter what is wrong, what bothers me is what I did that caused the exact problem on both tractors? And how did these other people have the same problem when they changed the head? What could we have done? What on earth is the problem?
(I tried gas, plugs, wires,...nothing helps)
 
If you didn't change the ignition timing from before working on the head, your valve lash is probably too tight.
 
I have a B that I changed plug wires and it ran great until it went under load. Had my grand son driving pulling a spring tooth drag and it missed and sputtered I walked a long side and the plug above the oil filter was jumping to the oil filter we put some rubber tape on the pug connecter and it ran great. Randy
 
Have you set to valve tappet gap as it should be and then gone back and rechecked it???? Firing order correct?? 1,2,4,3 with #1 being by the radiator??
 
Thank you for the suggestions. However, the first thing I did was change plugs. The wires were changed not long ago and the tractor ran fine before I messed with it. I also set the valve clearance to .011 (the book called for .010 and I added .001 just for measure).

The thing is that nothing changed except the head and the tractor ran fine before I touched it. Since I set the valve clearance as prescribed I am wondering what on earth I could have done? I did not mess with anything in the distributor or timing.

The tractor runs smoothly at low rpms or if I creep it up. It seems to run ok. Just under load it backfires and misses.

The funny part is that other people have had the same problem when changing the head and I have two of these that do the same thing after replacing the head. Oh well.
 
I just ran the tractor again and I think it runs ok at all speeds EXCEPT when it has to work or pull. If it just sits idle or medium rpm's, no trouble. It is just when it has a load. I am wondering if the spark advance is somehow changed? I cannot think of why changing a cylinder head would change the spark advance but I have to admit, I do not know how the spark advance works on the C, or if that indeed is the cause. I will have to look into the spark advance but I am still wondering why both tractors have the same symptom after a cylinder head change.
 
On both tractors, you removed the manifold's to do the head work! A leaking manifold gasket could be suspect, on both engines. While running the engine, use an UNLIT propane torch, around the intake gaskets, and listen for the engine rev's to change, or you could use spray carb cleaner. You didn't say if they are magneto, or distributer engines, but, worn, brass bushings on the distributer drive, will also cause this'
 
You might want to swallow the pride and go back and double check the valve lash again. I worked until early morning getting one ready for a pull. Started it up the next morning after a couple hours sleep and the dang thing was missing. I could hear the hiss in the exhaust. Pulled the valve cover and had two tight valves. I was sure I had every thing done correctly. It is either that or you are not getting enough fuel under load. If you have an adjustable power jet open it up a turn. You might have knocked some crap off in the gas line when having it apart causing a slight restriction. I have put a lot of heads on and any time I had a problem it was because I made a mistake.
 

Now is the time to question if the "New" Plugs and Points are faulty.. it happens a lot now..

Put in another set and check how it runs with those parts..

It also may just be that since you now have better compression, a weak or marginal ignition system ( for what-ever reason) can cause your problem..

If your Compression measures up to snuff, the Ignition is operating (and Timed) correctly, the only other thing you need is a Proper Fuel Mixture...

Ron..
 
removing the head would have nothing to do with the timing advance. If it has a magneto it doesn't have any timing advance.

I'm guessing a valve problem or an intake gasket not sealing.
 
You were all correct. Check what you have done.

What did I do? When I replaced the head in the good C I just looked at the bad C for the firing order. Why waste time looking it up in the book or asking here first? It was easy to just copy the placement of the plug wires as they were in the one that did not run right.

The problem was the firing order and now runs super smooth. Thanks so much to everyone.
 
Thank you to everyone who helped me. The problem was the firing order. The first tractor never did run correctly and I did not take the time to really figure out what was wrong with it. When I did the head on the second tractor I just looked at the plug wires on the bad running tractor so I copied the same problem over to the both tractors. I had 2 and 3 wires reversed.

Thank you again.
 
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