Slight Miss

[Underwood]

Cub S/N 6697 I have a slight miss at idle, just enough to be noticable and a little annoying. The odd thing is that under even the slightest load the miss goes away. Even the increased load of pushing in the clutch causes the miss to go away and the engine runs smooth. Engine starts easily, over the past couple of years I have replaced all the usual stuff just as general maintenance, am I being anal here or missing something?

[Urbish]

Clogged idle jet or overly lean needle valve setting? A load opens up the throttle engaging more of the high speed jet. That's where I'd look first.

[Glen]

Hi,
You could adjust the idle mixture screw on top of the IH carburetor, and see if it helps.
The Cub owner's manual says warm up the engine to adjust it.
Also the engine should speed up smoothly from slow idle to speeds above slow idle.

Below is a page from the 1947 Cub owner's manual telling about adjusting the idle mixture screw.

http://farmallcub.com/rudi_cub/www.clea ... age-24.jpg

My Cub's adjustment screw needs to be about 1 1/2 turn open.

[Underwood]

Thank you gentelmen for the ideas. We are getting some much needed rain today so I will have to wait a day or 2 to try your suggestions. I will report back.

[Eugene]

Try the carburetor issues first.

My thought, the miss is an ignition problem. Mark the front pully at the TDC mark and again half way around. Second mark only has to be close. Timing light at low idle, check the timing marks from each plug wire.

[Underwood]

Opened the idle mixture screw to almost 1 3/4 turns which helped. Next step is probably to check for partially plugged idle jet to see if that is why the sccrew has to be opened that many turns. Thanks again for everyone's help.

[Rick Spivey]

If you are saying it is open to 1 3/4 turn in total that is fairly normal for a cub. We usually start at 1 1/2 out and fine tune from there. Remember that this is an air adjustment, so out is leaner, in is richer.

[SONNY]

Might not hurt to check valves. got one here starting to miss and valves were getting too tight. ---- just another area to check.

[Underwood]

Valve lash is a good thought. I know the lash is off by a couple of thousants (both too tight and too loose on the various valves) but i didn't adjust them at the time. Is a few thousants enough to be a problem?

[Eugene]

Engine at low idle, missing. Pull one spark plug wire at a time. See if the miss is related to one or more cylinders. One or more clinders, Check the plug wires, rotor to cap gap, etc..

Your posted problem indicates an increase in engine RPMs and the load on the engine. The increased load requires a much higher voltage to fire the spark plugs.

[Jackman]

I like Sonny’s suggestion , valve adjustment might be it and if not we’ll you will know the valves are set to spec.

I have same problem with my Cub slight miss, I call it a flutter, have tried everything to eliminate nothing seems to work.

Valve adjustment is my plan for my slight miss Cub

[NJ Farmer]

It could possibly be a bad (weak) spark plug however I would lean toward a fuel/carburetor issue. Like the post listed above I would check the idle discharge tube or idle air bleed holes….

NJ Farmer