Broke Down

Tue Jul 23, 2013 9:42 am

Okay, so, I'm driving my '48 Cub down my street on a check ride, and each time I put it into 3rd gear, it starts to die, like it's not getting enough fuel for the speed. When I downshift to 2nd gear it runs okay for a few minutes, then I shift back into 3rd gear and it dies. Kaput. Won't start. It backfires, chugs, and backfires each time I try to start it.

Sooo, I get my daughter to tow me back to the shop. :oops: When I try to start it there, same thing--loud backfires, maybe a couple of chugs, but won't stay cranked.

Is this a fuel/carb issue? Does backfiring mean too much fuel, that the carb bowl is stuck? I mean, I get a loud POW! each time I crank it. I'm a novice with this Cub that I inherited from my father-in-law.

Thanks for any advice.

Re: Broke Down

Tue Jul 23, 2013 10:59 am

It sounds like weak spark to me. Does your Cub still use the original style magneto coil or has it been changed over to an external coil? Either way it may be time to check over the ignition system, points and coil in particular. I would check the points for pitting, file and reset as necessary. If it uses an external coil it could be low voltage getting to the coil. If it uses the mag coil it may be that it has gotten weak and needs replaced. My 2 cents.

Re: Broke Down

Tue Jul 23, 2013 11:54 am

Agree with SmokeyCub, back fire, not start = ignition fault some place.

Weak performance in 3rd gear, compression test results were?

Re: Broke Down

Tue Jul 23, 2013 12:39 pm

Good, you guys are giving me a starting point. I'll check those out when I get off work. Thanks

Re: Broke Down

Wed Jul 24, 2013 7:19 am

Since you have a sudden no start condition with a lot of backfiring, check the mag rotor gear teeth and the small pinion gear that drives it. It might be that your rotor has jumped time. If that's the case, you'll need to replace one or both.

Re: Broke Down

Thu Jul 25, 2013 2:13 pm

Gary Dotson wrote:Since you have a sudden no start condition with a lot of backfiring, check the mag rotor gear teeth and the small pinion gear that drives it. It might be that your rotor has jumped time. If that's the case, you'll need to replace one or both.


Good advice. Will check on that as well. Thanks!

Re: Broke Down

Fri Jul 26, 2013 2:49 pm

Okay, got the points out, look bad. Will replace, also the spark plug wires are burnt.

What are the points gap and dwell for the '48 Cub??

Re: Broke Down

Fri Jul 26, 2013 4:36 pm

Gap is .013 for a magneto, .020 for a distributor. I believe the dwell should be 60 degrees.

Re: Broke Down

Fri Jul 26, 2013 10:01 pm

Smarg wrote:Okay, got the points out, look bad. Will replace, also the spark plug wires are burnt.

What are the points gap and dwell for the '48 Cub??


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Re: Broke Down

Sat Jul 27, 2013 6:13 am

Take the time to take out your spark plugs and read them to see if that helps. Sounds like fire related, get that all right them see if you have the problem solved. Oh, nothing wrong with being towed back to the shop. It happens with new vehicles, so more expected on a 65 year old.

Re: Broke Down

Mon Jul 29, 2013 3:14 pm

Okay, so...back to square one.

Replaced the points, coil, distributor cap, and plug wires. Checked the basic timing over TDC, the teeth inside the distributor are not broken or sheered, so I know it didn't skip time.

Tried starting it...got the same POW! backfire, no start. Rats!

Required deeper examination with my very limited skills....sigh. Took out the plugs and looked at them...they are only about 45 minutes old (running time), but they already looked like they were 10 years old with the carbon deposits and darkness of worn plugs. Turned it over while the plugs were out, using a flashlight, to see if the valves were working...it appears that the exhaust valves on #2 and #3 cylinders are not lifting. Camshaft??

Do I have any other options than a engine rebuild at this point?

This made me finally break down and order the Cub owners manual, shop manual, and parts list manual from Steiner--YES, RUDI, YOU WILL NOT BE ABLE TO LECTURE ME ANYMORE. :mrgreen:

Re: Broke Down

Mon Jul 29, 2013 3:42 pm

You can remove the side cover from behind the carburetor and watch the valve train more easily from there.

Don't panic and go into engine rebuild mode yet. She may still have some life left in 'er and you've got nothing to lose (and a lot to gain/learn, plus some tools to buy) by trying to resuscitate.

Re: Broke Down

Mon Jul 29, 2013 6:20 pm

Are the valves stuck open? There's a thread in here somewhere about how to free them up. I agree with Matt - pull the side cover off and see what's going on. My money is on stuck valves, which can be freed without major work.

Re: Broke Down

Tue Jul 30, 2013 5:38 am

Did you by any chance check the gap on the spark plug? Sound like it could be from the Plugs with too much gap.

Re: Broke Down

Tue Jul 30, 2013 6:27 am

I agree with Matt as well. Pull the side cover off so you can watch the valve action. An intake valve hanging open can cause your symptoms.