roy prins

Well-known Member
I have this M that came in the shop and the Guy says tune er up good it doesnt have any power -- OK says I - I put a complete pertronics kit in it and it started and run real nice - I put it on the dyno - got her warmed up good and started putting the coals to her and run good at 45 HP for about 2 minutes and started to backfire out thru the carb -- backed up the dyno and it cleared its self -- no carb adjustment cured it -- checked valve clearance timing and all I know is it must be a stickey intake valve -- now what -- I put sea foam in the gas and the oil -- any other thoughts out there? Roy
 
What is the total advance of the spark. I will assume it has battery ignition and not a magneto. Just for kicks I would retard the spark while it is under load until it starts to lose horsepower. It may be over advanced.

I saw a fellow mechanic work all day on a tractor that would act just like that, run good, then act like it was going to die, spitting and sputtering, backfiring. In the end the timing was over advanced and it ran fine. Might be worth a try. It happens.

Of course if low rpm timing gets too late that way some repairs to distributor advance would be in order. The original cast iron piston M was 40 degrees advance, increased bore should be 30 and fire crater pistons way down to 22degree advance.
 
sounds like the fuel flow is not keeping up with the consumption. Check the strainer assembly under the fuel tank, and make sure there isn't something stuck in the fitting that screws into the carburetor.
 
didn't think a lack of fuel causes a carb backfire. it would just cut out. pretty sure its a tight valve also. when heats up it gets sticky. and don't close.
 
Fast, what is the train of thought asking about non-detergent oil? I doubt that would have any effect causing this problem. Maybe if the oil was poor or not up to spec wearing a cam lobe down might be a possibility to his symptoms.
 
I had am M that did the same thing. Ran good with no load to a medium load. When it got to a heavier load it would start to backfire. If you eased up on the load quick it would go back to running good after a short time. If you didn't ease up on the load, it would die. I spent a quite a bit of time trying to figure it out. Finally pulled the head and the valves. Put a drill in a vice then, put the valves in the drill. One intake valve had a tiny bend in the stem. When the engine was under a heavy load, the valve must have either expanded or warped more just enough to make it stick. Replaced the valve and guide. It ran perfect after that.
 
Exhaust valves under load run ORANGE hot at the valve head, the stem is a little cooler, but cheap oil partially cooks on the valve stem inside the valve guide forming a layer of carbon, when the valve gets hot under load the stem expands and sticks holding the valve open until the load is reduced and the exh valve cools off.

IH #1 oil reduces and will eventually clean the carbon off valve stems. There may be other oils that clean the carbon too.

Had a 10 hp Kohler in a Cub Cadet that ran exactly that way, ran great at half and 3/4 load, would run at full load for a couple minutes then stick the exh valve, idle it down for 2-3 minutes and it would unstick the valve and run fine. When I took engine apart I scrapped the carbon layer off with a pocket knife.
 

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