M stumbles and chugs until under load

wilamayb · May 14, 2011

I'm working on an M for a friend that said he was sick of looking at it. Since he was so disgusted with it not running he pushed it outside the barn about 2 years ago where it endured the elements during that time.

His problem was that it would sputter anytime the throttle was opened up or it was under load. I found that problem. It was mud dobber nests in the air cleaner.

The tractor had all the issues of sitting outside for 2 years....

I had the carburetor cleaned and regasketed by Popin John in Kerhonkson, NY who has always done EXCELLENT work on my Deere magnetos and carburetors so I trust that it is done correctly. He also rebushed the distributor and tested it afterwards.

It has electronic ignition and produces a very very hot spark all the way to the new Autolite plugs so I'm feeling like that is not an issue. Run timing is set to 20 degrees BTDC by timing light.

The problem.....It chugs while idleing and also under full speed if not under load. It actually chugs even under partial load. It really doesnt clear up until under full load.

I suspect tappet clearance but the rocker arms are so cupped from wear that feeler gauge adjustment is out of the question.

I've pondered adjusting the tappets by using the adjusting screws as the gauge but I dont know TPI's to be able to convert rounds of turning into a measurement. Does anyone know that info?

Any suggestions? I've tried adjusting the idle screw on the carb in both directions and it is effective in both directions but it does not ever allow it to run smooth.

Any thoughts or comments? I'm a two cylinder guy and am open to any IH suggestions..

Thanks

old · May 14, 2011

If it sat a couple years the first thing I would do is give it an ATF treatment to free up sticking parts and lube it up real good. I would also put some in the gas. As far as chugs does choking it help or hurt?? If it helps then it is running lean if it hurts it is running rich

Charlie M · May 14, 2011

Maybe a couple of years outside has rusted a hole through the intake manifold.

JWCarp · May 14, 2011

I have often found that not running smooth when not under load and then clearing up under load can be an airleak. A leaking gasket between the manifold and head or the carb and intake. Could also be in the carburator. When the engine is not under load the the govornor is closing the butterfly in the carb. This puts more vacuum on the manifold making leaks worse. Under load the carb opens up and puts less vacuum on the manifold making air leaking in less. The air leaking in leans out the mixture and causes the chugging.

This may not be it but I have found this to be the cause in the past. A quick easy way to check is to drip some oil where the gaskets are while it is idling. If it cleans up when you put the oil around the gasket, thats it.

El Toro · May 14, 2011

Have you adjusted the main jet adjusting screw near the bottom of the carburetor? This is where you enrich the fuel mixture by opening that several turns. Even garden tractors and my big Toro mower has this feature. Hal

wilamayb · May 14, 2011

I have adjusted the idle screw for best idle qualities and also adjusted the load needle to prevent sputtering but to also eliminate black smoke. It runs fine under load...

I did check for vaccuum leaks with an aerosol can of brake cleaner. It appears to have none. My symptoms do scream vaccuum leak which is why I checked it first thing.

Still no comment on the Threads per inch of the tappet adjustments?

rustred · May 14, 2011

pull those rockers off and have them resurfaced

at the machine shop. or even yourself can do it.
valve setting is .017 hot.
original timing is tdc at idle and 40 degrees advance at full throttle.
just tying to figure out what chugs means as one word can throw a person way off.are you referring to surging? might have to get a video going?

rustyfarmall · May 14, 2011

(quoted from post at 10:09:34 05/14/11) I'm working on an M for a friend that said he was sick of looking at it. Since he was so disgusted with it not running he pushed it outside the barn about 2 years ago where it endured the elements during that time.

His problem was that it would sputter anytime the throttle was opened up or it was under load. I found that problem. It was mud dobber nests in the air cleaner.

The tractor had all the issues of sitting outside for 2 years....

I had the carburetor cleaned and regasketed by Popin John in Kerhonkson, NY who has always done EXCELLENT work on my Deere magnetos and carburetors so I trust that it is done correctly. He also rebushed the distributor and tested it afterwards.

It has electronic ignition and produces a very very hot spark all the way to the new Autolite plugs so I'm feeling like that is not an issue. Run timing is set to 20 degrees BTDC by timing light.

The problem.....It chugs while idleing and also under full speed if not under load. It actually chugs even under partial load. It really doesnt clear up until under full load.

I suspect tappet clearance but the rocker arms are so cupped from wear that feeler gauge adjustment is out of the question.

I've pondered adjusting the tappets by using the adjusting screws as the gauge but I dont know TPI's to be able to convert rounds of turning into a measurement. Does anyone know that info?

Any suggestions? I've tried adjusting the idle screw on the carb in both directions and it is effective in both directions but it does not ever allow it to run smooth.

Any thoughts or comments? I'm a two cylinder guy and am open to any IH suggestions..

Thanks

I really don't understand what you mean by "chugging", but I would make absolute certain the valves are NOT sticking, and then I would play around with the ignition timing. You may NOT be able to set the timing according to the book and have it run correctly.

The valve clearance gap is slighty irrelevant at this point. as long as you still have some clearance when the engine is warmed up, the incorrect gap should not be causing those problems.

I think the info you want on how far the rocker adjustment screw travels per turn will be in a "Machinery's Handbook".

wilamayb · May 14, 2011

I made a video but the fan is so noisy that exhaust sounds cant really be heard so I didnt post it.

No, doesnt surge at all. It's not like its swapping back and forth between load and idle circuit. It's just a rough/uneven idle. Engine speed does not change. It's just not a smooth idle.

It acts like a vaccuum leak but I've already sprayed brake cleaner around the manifold and carb connection to check for leaks. Maybe I should get some ether and try again?

Surfacing the rockers is not really an option. there wont be anything left if they are flattened. This tractor is a play toy only for my friend. The only thing it does is pull a trailer with fencing supplies. They farm with modern CIH equipment but have this thing around to play with. They have alreay spent a pile of money taking it to "professional mechanics" who couldnt find the problem (dirt dobbers in the air cleaner). (I'm the 5th guy to work on it). I have found the problem and repaired the issue of it stalling under load but I'm just not happy with the idle quality. I want it to purr like a kitten when they get it back.

Does no one adjust their tappest by number of turns? Its just the norm for two cylinders. It compensates for worn parts where feeler gauges are not able to be used.

rustyfarmall · May 14, 2011

(quoted from post at 13:28:50 05/14/11) I made a video but the fan is so noisy that exhaust sounds cant really be heard so I didnt post it.

No, doesnt surge at all. It's not like its swapping back and forth between load and idle circuit. It's just a rough/uneven idle. Engine speed does not change. It's just not a smooth idle.

It acts like a vaccuum leak but I've already sprayed brake cleaner around the manifold and carb connection to check for leaks. Maybe I should get some ether and try again?

Surfacing the rockers is not really an option. there wont be anything left if they are flattened. This tractor is a play toy only for my friend. The only thing it does is pull a trailer with fencing supplies. They farm with modern CIH equipment but have this thing around to play with. They have alreay spent a pile of money taking it to "professional mechanics" who couldnt find the problem (dirt dobbers in the air cleaner). (I'm the 5th guy to work on it). I have found the problem and repaired the issue of it stalling under load but I'm just not happy with the idle quality. I want it to purr like a kitten when they get it back.

Does no one adjust their tappest by number of turns? Its just the norm for two cylinders. It compensates for worn parts where feeler gauges are not able to be used.

You don't want to flatten the rockers. They are supposed to be slightly rounded. I set the valve gap on mine at 0.020" cold which results in pretty darned close to 0.017" when hot. I've never tried setting that gap by turning the screw a certain amount, but unless a feeler gauge is used, how are you going to know how much to turn that screw? You've got to have a starting point, or reference point if you will, and I don't know how you would establish that point without using a feeler gauge.

wilamayb · May 14, 2011

Starting point is from touching the top of the valve stem.

I can get a thread gauge and figure it out. I was just thinking someone had been here before.

rustyfarmall · May 14, 2011

(quoted from post at 13:57:42 05/14/11) Starting point is from touching the top of the valve stem.

I can get a thread gauge and figure it out. I was just thinking someone had been here before.

Even if you can do it that way, with the rocker tips being roughed up as they are, you won't be any closer to being accurate than if you used a feeler gauge.

rustred · May 14, 2011

They are NOT suppossed to be flattened! thats why i suggested machine shop, as they know what to do. got to follow the same contour as they are. i have done this before. AND if they are pounded out that much then get another set from a parts machine.
no one adjusts manual lifters by no. of turns. there is no way of determining where your at. if it says .017 hot i will bet my paycheck that you will not be on .017 by guessing. you will be close but not that close. takes a very slight movement and your .001 out, even when tightening the nut if your not careful you can decrease the clearance.
hydraulic lifters are adjusted by turns from zero lash, not manual lifters.
get another carb and try on it, just because its been o/h dont guarrantee its 100 percent.
i have seen some o/h jobs worst than to start with.

D Slater · May 14, 2011

Clear up the defination of run timing? What RPM is 20 degrees set at. If set at 20 BTDC at idle it probably not going to run smooth at no load.

JWCarp · May 14, 2011

It certainly sounds like you have checked all the right things. To tell you the truth I have never really found these engines to be to picky about valve clearance as long as you get it close. I actually sold an M because I could not get the exact same problem fixed. I had a 450 with the same issue and changed the coil (twice) and that fixed it. Why? I have no idea why the first new coil did the same as the old one. That chugging sound when is the most annoying thing there is to me. Hope you can find the problem and sorry I could not help.

rustred · May 14, 2011

you are not telling us what you have the adjustment screws set at.
to start with: the main jet is set 3 turns open.
the idle adjusting screw 1 turn open.
clock wise richens the mixture by reducing the air.
with these adjustments they run pretty darn good and require little fine tuning.

tractor vin · May 14, 2011

I agree with rusty throw the timming light away and the other thing is try it without the muffler that might over do the chugging for you good luck

David G · May 14, 2011

My valve grinding machine has an adapter to mount rocker arms on and grind the correct surface. I have not done that in forty years, but if you are in eastern iowa come over.

pete 23 · May 14, 2011

I would like to know what the timing is at this poor running idleing speed. It should be at tdc at low idle. Far as full throttle timing it depends on what pistons and head are on the tractor. Std stock M with gasoline head was 40 degree btdc. Increased bore, flat head aluminum pistons is 30 degrees btdc. If it has fire crater pistons timing is 22 degrees btdc full speed. Having had the distributor rebuilt, who knows what advance you have in there. You need to know that and what head and pistons to make it run right. The idle mixture screw on an M is bass ackwards from a lot of carburetors and the farther out you turn it the more air (leaner) the mix. That was changed on carbs used on 400 etc to be a mixture screw, out means richer on them.

wilamayb · May 15, 2011

I defined run timing at WOT which is only about 1380 on this tractor. Governor spring must be weak in its old age.

redgems · May 15, 2011

I had what sounds like same problem on my SMTA a few mounths back. Did all the things mentioned here to no avail.plus rebuilt dist,several new caps, several sets of wires, etc,etc. Then out of shear desperation I bought set of ss center, puller wires and a 55000 volt coil from dennys carb and autolite 3116 plugs; problem solved with little tweaking of timing. I am not sure if this right way to set timing but I set tractor at medium trottle and move dist slowly until I find the sweet spot and its timed. Tony

wilamayb · May 15, 2011

So where did those numbers come from?

Any distributor with centrifigal advance will have some advance even at idle which means that 0 advance could not be achieved at anything other than when the engine is stopped.

The advance on my distributor is not as linear as it should be but it is sufficient.

pete 23 · May 15, 2011

These are numbers from IH service manuals used over the years and on their shop tuneup charts. A good distributor will not advance at low idle speed and will have a nice steady advance as engine speed increases up to the internal limit in the distributor which is a slot (hole)machined in the plate on a IH distributor.Been repairing these over 50 years.

M stumbles and chugs until under load

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