Allis Chalmers C missing at more than 1/4 throttle

I started to slowly have problems with my A/C C where it would miss when I accelerated, and then later the same thing would happen when I gave it more than three notches of throttle, which is about 25%. I pulled the magneto and carb and cleaned both, and replaced the plugs with the same Autolite 295 type and gapped to 0.30. Same problem. Bought a rebuild magneto and put it on, and same problem. Tested compression and its about 75 lb on each cylinder. I'm getting spark to all four cylinders. I have spark at each plug. Pulling the spark plug wire while idling does show that each cylinder is firing as indicated by the lower performance.

I'm running a Zenith carb type 83 that was originally for a Wisconsin engine but its worked great until now, so I don't see how this can be causing these new problems with missing at anything about 1/4 throttle. The engine will die after about 50% power. It seems to start and idle best with the idle screw turned all the way in, so something tells me there is some kind of fuel/air issue. I checked the float on the carb and it did seem to bind, but after adjusting it, it still has the same problem. Considered the governor but I don't see how these symptoms could point there. The only thing I haven't done is undo the hose to the carb to see if its a blocked air intake. Help.

Getting enough gas flow ? Getting worse over time sounds like something slowly getting plugged and cutting off fuel. Just what I would check.

2x what 300jk said, I don?t know if these tractors have a drain plug in the bottom of the carb if so you should be able to pull that out and it should flow enough to fill a pint jar in three minutes.

Check your manifold for any leaks by holding your hand up near the gasket. My B had a miss which I worked on for years and finally discovered that the new Chinese manifold was low on one of the ports, and after I got it machined the miss disappeared.

Pull the valve cover and check the tappet clearance. Sounds like you have a valve that isn't closing all the way when it gets warm. That's assuming you're checking compression when it's cold.

Have you tried a partial choke?

If that improves it, it's likely a lean fuel condition.

Start with checking the fuel availability to the carb. With the engine off, fuel valve open, remove the drain plug in the bottom of the carb. Catch the flow in a clean glass. It should start with a full flow, then slow to a trickle. If the flow stops, or slows to a trickle, there is a fuel delivery problem. Could be the sediment screen, possibly a restriction in the tank valve, or the screen in the carb inlet fitting.

Look at what was caught. If cloudy, it has water contamination. If rusty or dirty, the tank is contaminated and will need to be cleaned. Whatever was caught will also be in the carb. It may need to be eased apart and cleaned out. Easy job, just don't loose anything!

Has the air cleaner been serviced? There is a wire mesh above the oil bowl that is often overlooked. Also check the entire pipe and canister for mud dobber nests and rodent nests.

Once everything has been checked and remedied, and you are happy with the rest of the components, as in properly tuned, valves set, etc, then the carb jets can be adjusted.

Adjust the idle jet first. The engine needs to be at slow idle, down around 400RPM. Turning the screw in richens the mix, out leans it. You are adjusting air bleed, not gas. Back it out to rough idle, then back in to best idle, then slightly more in.

Assuming it has an adjustable main jet, adjust it next. The main screw adjusts fuel flow, so backing it out richens the mix.

To adjust, idle the engine down, then suddenly open the throttle. If it dies, back the main jet out 1/2 turn, try again. Keep repeating until it will take sudden throttle without hesitation. A single puff of black smoke is the goal.

If the main is not adjustable, or it won't adjust rich enough to take throttle, something is clogged in the carb. It will need the main jet and distribution tube removed and thoroughly cleaned. Dried up ethanol gas tends to form a hard, sticky residue that must be scraped out of jets and fuel passages.

I am with all the others but will add valve springs issue. Use a can or either around the manifold to look for leaks. Also try to pin it down to, is all cylinders or one dead one? What ever you do, do one thing to a time.

Also check the timing per the book as I just went through this on one.

Sounds like fuel restriction to me also. Start at the tank by removing the sediment bowl and looking in the neck for rust fines. The threaded neck can be full of fines and still let some fuel thru. I have found washers in a gas tank as well as a rag.

I bought one of those from the school janitor when I was 9 years old. That one had a miss. I came home from school one day and Dad said he'd fixed it. He said,of all things,it had something to do with the oil pump. Don't ask me what or why,I have not idea.

Whenever this engine was rebuilt, the internal timing of the magneto drive gear wasn't set correctly. Its set at about 10:30 instead of 12:00. I had to use the timing method where you remove the caps,turn the magneto armature backwards, then get turn the fiber gear to align with the timing lug. It started right up, but I had I need to re-do it because it idles best with the magneto rotated on its mount all the way clockwise as you look at it from the back of the tractor looking forward. Once it get it, I'll put two drps of paint on the timing gears so I don't have to fiddle with this again with this magneto.

The idle screw along with the stop screw needs to be adjusted for idle only on this tractor. When the idle is set leave it alone. That screw adjust the air mixture with the fuel at idle at low a RPM. The power jet adjust the fuel mixed with the air being pulled thru the engine at higher RPM,s. The idle circuit is to adjust the fuel air mixture when the engine is drawing a small amount of air thru the engine. The engine is actually an air compressor that pumps air out the muffler that fuel at a proper mixture is ignited with spark that keeps it pumping air with left over gasses from the fuel being ignited pumped thru the muffler.

When properly gear timed at assembly this is how the magneto drive would be set. After reading you second post I changed my mind on fuel restriction. I would guess that the advance in the magneto is either stuck and not working or the magneto does not have the 30? advance. The advance degree is stamped on the flange inserted into the block. If it is snapping the impulse at 12 oclock it is either the wrong magneto or the stop pin is set for counter clock ways. If you figured out how to make it run in time this way would not cause you a problem. I had a B Allis that the magneto advance stuck and would start and idle but would not increase the RPM's as well and an Oliver 1850 that the advance stuck on the distributor that would only run about 1/3 throttle. I had the carburetor apart several time before I pulled the distributor out of the Oliver 1650 and solved the problem. It may not happen often but I had two.

I can achieve full throttle by selectively pulling back the choke until the choke is almost completely pulled back. I pulled the sediment bulb and it was clear. What is your guess about what to do next? I guess I'll pull the carb and check it once again.

Gas runs freely all the way to the carb. I just pulled the carb and will see what I missed. I use premium no ethanol gas and just rebuilt this carb a year ago.

Got it fixed. The gaskets were coming apart. Looks like it was sucking air at the intake manifold and there was a bit of the middle gasket caught in some of the ports inside the carb. Now to just finish timing it. This is a working tractor and I mow my acreage with its Woods belly mower, so I was starting to sweat with May 1 and grass mowing season coming on.

I have it running, but the gear is sitting at 10:30 when the flywheel is at "fire". I need to pick up a timing light at the store and get the timing better, but for now, after the carb rebuild i did today, its able to go from idle to full throttle without a hitch.

I must have been lucky somehow. I thought that 12 o'clock was correct not 9 and 3. I guess 10:30 is right between 9 and 12 and somehow I got it aligned, but not because I knew what I was doing.

All 4 cycle engines static time at TDC on a compression stroke. The Fire line on your tractor should be in the center of the inspection hole if checking with a timing light.
When assembling using the gear timing marks the engine is actually at the TDC on the compression stroke.

The three pictures were take to answer a timing question back in 2002.