High Speed Miss (long post)

I've got a Farmall 340 that is missing pretty bad at high speed (over 1500 RPM) while under load. Doesn't miss except at high speed while under load. It runs nice at idle and all the way up to full throttle when not under load. I've been looking for this miss for awhile and have replaced the plugs, wires, points, condenser, coil, cap and rotor. The point gap is at.020, plugs at .023 and the timing is on the mark. Gone thru the fuel system also. Soaked and blew out the carb, set the float level and checked for good fuel flow. No plugged elbow screen either. All seems good on the fuel/carb system.
I have set the valves at .014 while warm. Engine reads 125 lb of compression in each cylinder and a vacuum gauge reads steady at 20 inches while at idle. While at high speed and under load the vacuum gauge bounces downward about 4-6 inches (to 14-16 inches). So, I'm thinking I might have a valve problem? Or another possibility might be a worn bushing on the distributor shaft. I can feel some radial play in the shaft, but I'm not sure how to better tell if this is the cause of the miss. Both a valve problem and a wobbly rotor seem likely at high speed to cause a miss, but why only under load. Or maybe I'm on the wrong track? Any ideas?
Pete

Might try the exhaust valves at .017 hot. They might be causing the miss.

Sounds like a really GREAT opportunity to have a look at what's going on with an ignition 'scope.

I'd try checking your intake manifold for leaks. It will skip bad if you have holds or carb or manifold is loose.

The ideas below are good. I will add that the distributor shaft is bad if you can feel any play at all. Measure with a dial indicator in Up down/in out directions. .001 is max deflection. Any more and the dwell will be way off. If that still does not fix it, try the idea below.

If the altitude you are located at is above 2500ft, it might need a size larger main jet. (try setting the float 1/32 to 1/16" higher than stock, this will also deliver more fuel. Jim

(quoted from post at 04:12:34 10/14/10) The ideas below are good. I will add that the distributor shaft is bad if you can feel any play at all. Measure with a dial indicator in Up down/in out directions. .001 is max deflection. Any more and the dwell will be way off. If that still does not fix it, try the idea below.

If the altitude you are located at is above 2500ft, it might need a size larger main jet. (try setting the float 1/32 to 1/16" higher than stock, this will also deliver more fuel. Jim

Click to expand...

At higher elevations since there is less oxygen, you need less fuel. Consequently you need a smaller not larger main jet and lower not higher float evels.

Hi Jim: I debated a comment to your post since you always write very good info, but I'll ask why you suggested adding more fuel above 2500 foot altitude? About every airplane I've flown that had a carb improved when leaned as I went higher. Your suggesting richer mixture.. I suppose I forgot to consider something simple... By the way, most aircraft I am talking about have updraft carbs as do most IHC older tractors such as the letter series A to M. THanks in advance. ag

I had a high speed miss with my 706 d282 a while back. Turns out it was just a clogged air filter sucking the rubber intake tube shut at high RPM. I had blown out the air filter but it didn't change until I put the new one in.

I was incorrect. It is reality to thin the mix when at altitude. My thinking went from what if it was a high altitude tractor at low altitude, and I got crossed up mid thought. Thanks. Jim

My mediocre thought processes gone wild. See below. Jim

Hi Jim: I am glad your normal.. I get crossed up quite often and my "friends" say too often.
Thanks. ag
Ps: By the way, in most carb equiped small aircraft engines, the mixture adjustment is pulled out to "cut off" to stop engine and only after crankshaft stops turning, is the ignition shut off. If not done this way, "after run on" is fairly common and it can really kick backwards at times. Cutting off all fuel gives a quick clean(fuel free) stop. ag