Fine tuning a Cub

[Larry in WNY]

I rebuilt my 1958 Cub several years ago but it never ran smooth. Plugs kept getting black and fouling. Carb was adjusted right. It was hard to time with a light. The mark seemed to move around. So this is what I did and now it's running like a swiss watch. Note: Valves were adjusted and rechecked previously.

1. Quit running after warm up. New coil solved that problem.
2. Removed distributor, took it apart, cleaned, oiled and greased moving parts, new points and condensor. It's stable timing again. Good YouTube videos out there for distributors.
3. Put original carb jet back in it. I got a bigger one because I read they run better. Nope just fouled the plugs. Added new Autolite 3116 plugs.
4. Installed a screen on the top of the sediment bowl inlet. Good insurance against any rust flakes plugging it.

It hasn't done much work other than running a sickle mower around the yard. Now that it's running well I'll be moving wagon loads of firewood and then check compression to see if new rings have increased compression. Before the rebuild it was in the 90 to 100 psi range. Checked after rebuild and all around 100 psi. Hoping for more. One of my other Cubs has 130 psi.

I hope this helps anyone who may dealing with the same problems.

Larry

[NJ Farmer]

Black sooty plugs means too much fuel or too little air.....one of the two are out of balance. Check all of your intake connections/Gaskets. then I would go through the carburetor again perhasps someone bored out the main jet and your injecting to much fuel into the maniforld.

Either way you are on the rich side of the fuel air mixture ratio....

NJ farmer

[Eugene]

Compression to see if new rings have increased compression. Before the rebuild it was in the 90 to 100 psi range. Checked after rebuild and all around 100 psi. Hoping for more. One of my other Cubs has 130 psi.

Conduct dry then wet compression check.

When you installed new rings, did you do the valves?

[Larry in WNY]

Yes new valves installed. I honed each cylinder to get a cross hatch pattern. Staggered the rings and set end gaps. I did several bore measurements and wear was low. This was used as a mower most of its life before I bought it.
Larry

[Larry in WNY]

Did a wet and dry compression test. No change. Warmed the engine up and got 100, 105, 100, 100. It runs really smooth so it's not terrible.

Larry

[inairam]

Did you do cut new seats, lap the new valves in, and adjust the valve gap? I would have expected more if you put fresh rings in it. the wet-dry shows you did a good job with the rings so I would look at the valves.

I would do or redo the valve adjustment first. I think the book says 0.013" gap. I personally use 0.0150" which I think is spec for the later years or the number low boys.

[NJ Farmer]

You might want to recheck your carburetor internals…..there are two different 3/4 IH carb models and yes it does matter!
The main jet, discharge tube are different in the models. If you have the wrong parts installed the performance will definitely change the performance of the carburetor. I suggest you get a copy of the carburetor manuals (that are free) in the manual section of this website and look up the different specs on the carburetor models. The model number is stamped on the flat portion of the top portion of the carb.

NJ Farmer

[Waif]

Reads good to me.
Time under load might get rings and bore married better , but I could live with 100.

Congrats on the gains. Lil buggers can run smooth! Till ya get to showin them off or something and a stray "putt!" comes out. l.o.l..

They'll burn lousy pump gas and everything in it even if they cough. But these Cubs are the first larger than small yard equipment engines I've run on recreational fuel without the ethanol consistently after tune ups , and feel it's part of how they perform.
Seems to make a difference in how they idle. Where the fuel I was running prior seemed to have times of too much non gas crap being digested ,with indigestion following.

[Larry in WNY]

Did you do cut new seats, lap the new valves in, and adjust the valve gap? I would have expected more if you put fresh rings in it. the wet-dry shows you did a good job with the rings so I would look at the valves.

I would do or redo the valve adjustment first. I think the book says 0.013" gap. I personally use 0.0150" which I think is spec for the later years or the number low boys.

I didn't recut the valve seats. I bought new valves and lapped them in until there was a circle like the manual shows. I did find an old valve grind tool after it was assembled.

Larry

[inairam]

I would redo the value adjustment and then redo the wet-dry compression test. I think ~100+ is a good number but you would have hoped for more after all of the time and money on what you did. It may just be the adjustment is off a little.