Frustrating little Cub — Now Running!

[lagrue]

I have a little Cub which I’ve finished restoration on recently.

Unfortunately, I cannot get it to run more than a few seconds. I’ve had a couple nice long runs which have given me hope. I’m a former small engine mechanic, and currently work for one of the largest engine mfgs in the world, so I really don’t want to make this just a man cave ornament! Somewhat a matter of pride!

What have I tried?

Engine has a converted Merc chainsaw mag, per Bob D’s great instructions. Interestingly, it tests out well, and shows spark on my main tester, but not my backup tester.

Tank and fuel system are clean. Fuel lines are new. Made a leather washer as a packing washer for the needle. Seems to run most often and longest with needle about half a turn open.

I re-doped the float and it floats well. However, it does seem that the seat does not seal the best.

Poppet valve is set to .050" lift. Made a new gasket for the base of the needle poppet assembly.

Points are clean (whole assembly) and set to .018". I’ve tried .020 as well and found no real difference.

Runs on a J6 plug, but not a J4 or J12. I don’t have an H10 to test. All are gapped to .025.

Fuel is all non-eth 16/1.

I’m interested to see if there is anything that I’ve missed.

[frankr]

I gotta suspect the ignition is not up to snuff. I say that because it ought to run on any of the plugs you mention. Maybe not long term,
but it ought to at least start and run.

Next, I suspect the condition of the piston, rings, and bearings (crankcase sealing)

[garry-in-michigan]

I would suspect crankcase sealing. Try using SAE 40 wt. low ash oil. . . 🙂
\

[The Boat House]

I would also suspect poor sealing of the bearings .
Add somemore oil to the fuel.
May help, may not depending on the condition
of the bushings.

[bob-d]

Possible carb flooding? You did mention the seat did not seal the best.
Did you lap the seat needle with either Bon Ami, or a 320 lapping compound? You can also lap the poppet valve as well. Make sure to clean off the compound completely when done.
Worth a try.

[pm-t2]

I’ll suggest something I’ve seen somebody else do in irder to get one of the Cubs to start.

Try injecting a shot of 90 weight gear oil into the combustion chamber through the spark plug hole, then try starting it, and run with 8:1 oil mix.

The compression on these motors is usually marginal, and the heavy oil helps seal the rings better for starting. Once its running, you will probably be OK…… until it quits or you shut it off, then you might have to oil it again.

Good luck with it eh.

best,
PM T2

[lagrue]

Thanks everyone for the suggestions.

Bob, I did lap the needle in with some heavy metal polish, but it did not seem to have much of an effect. Probably need to go back and do that again more effectively.

I spent some time this morn going through the ignition system. The spark is inconsistent. One pull, I have a good fat spark, next pull nothing? Indicative of a condenser? Coil is new and tests out as good, so I hate to think that it’s the coil…

I went ahead and threw it back in the tank got it to fire every pull for about 20 pulls, but would not stay running. Tried to mix in a little 30wt (don’t have any 40 on hand) and it did not start after that.

I believe both issues are at play here, but without consistent spark, the other is a moot point.

[bob-d]

download/file.php?mode=view&id=11896
Lagrue, here is the capacitor that I used on mine, along with the chainsaw coil.
They were recommended by Mumbles as replacements for old motors,and they work well.

---

Attachments:

[mike-stroz]

I think I’ve got the same problem with my Pal. Could it also be the gasket between the cylinder and block? I know I had a pretty good seal with the bearings.

[fmanracer]

Hi, I would like to ad that the piston cylinder fit needs to be tight on the smaller motors. I’d say .003 skirt clearance on a cub and the rings need to tight on their butt gap also. I’d say.005 would be nice. The piston that is loose can be built up with Teflon coating today where knurling was the only way years ago. There is nothing you can do with poor fitting rings except replace.

[Author]

Posts