Cub won’t start unless roll starting

[Duwop]

Cub doesn’t want to start but it does crank and turn over. I’ve been having problems keeping it running after it warms up let’s say 30 to 60 min. After it cooled down it would start and run again until it warmed again.
So I started with checking carb and decided to replace with a new one.(done) I checked spark at coil and I have spark thought it might be weak, I don’t know how to tell for sure maybe someone could help?
Checked points, condenser, plugs, dist. Cap plugs and I replace rotor as it was scorched hood.
Currently I’m charging the battery as I wore it down trying to get the cub started.
I’m confused as to why it won’t start unless I roll start it.
I’d appreciate any thoughts it’s a 55 Cub

[Don McCombs]

Sounds like a classic case of a failing coil or condensor.

[k hutchins]

Replace the coil and you should be fine if the other distributor components are good

[tst]

check engine compression ?, a worn engine will warm up being towed to make it start

[Glen]

Hi,
If you do a compression test, remove all 4 spark plugs, have them out of the engine at the same time. Open the throttle fully, so the engine can get air for the test.
Use the starter for the test.
I would charge the battery fully before doing the test.
The Cub service manual says a Cub engine has 120 lbs of compression, that is probably for a new engine.
I write down the numbers, so there is a record.

You could do a spark test.
Remove the center spark plug wire from the distributor cap, and hold it by the insulation, with the metal end about 1/4" away from a bare place on the engine. Put the transmission in neutral, turn the ignition switch to on, and run the starter. The spark should be strong and blue, or white. Yellow is weak, and the engine may not run, or not run well.
I would charge the battery fully before doing the spark test.

The ignition coils on Cubs quit sometimes, they get weak, or quit after running for 15 or 20 minutes, and warming up.
Then some of them work again when cold. That could be what yours is doing.
TM Tractor has new 6 or 12 volt coils. The modern coils have all 3 connections at the same end.
They are a bigger diameter than the old original coils, with connections at both ends.
You need a new coil bracket for the bigger diameter coil.
TM Tractor has a new bracket also.

The condenser can go bad too.

Part of a tune up is checking and setting the ignition timing. The Cub service manual says set the timing using a timing light.
The timing should be set right so the engine runs good, and has good power.
Below is a page from the Cub service manual showing the timing marks newer Cubs should have.
A 1955 Cub might only have mark number 1 in the pic, the TDC mark.
Using the timing light, align mark number 1 in the pic with the pointer when the engine is running at slow idle speed, which should be about 500 RPM.
As you speed up the engine the timing should advance, so be sure it is running at about 500 RPM.
Use only the pic and the description under it, they are talking about other things on the page.

http://www.farmallcub.info/manuals/gss- ... 001-43.jpg

You can also check it at full speed, which should be 1800 RPM for a 1955 Cub, up to serial number 192113, mark number 2 should be aligned with the pointer at full speed.

You probably know how to change the timing, loosen the 2 clamp bolts at the round base of the distributor, and turn the distributor.

The engine may need a tune up.
The ignition points need to be good, not burned, and their gap needs to be set right, which is .020" for a Battery Ignition unit.
TM Tractor has new points and condenser.

[Duwop]

Thanks to all, I’ve been charging the battery and added water.
Changed the condenser and coil as suggested by post#2 Don McConbs
I’ve only roll started twice since problems began.
I’ll post back if it doesn’t start and will check other items suggested

[SONNY]

6-v or 12-v??? My 55 is a 6-v and was starting kinda weird so it got everything new in the dist along with wires, cap, plugs and battery. --- well after that we had a no start! ---Battery cables were bad , ya turn over--well sort of but no go. Cut old ends off, put repair ends on for the test, cleaned the other ends at starter and ground, pulled starter rod and we got running motor. I figured my old starter was getting bad, but with new battery and ignition it now starts first time over. Check your cables too. also have you checked the valves?? bet they need adjusted too!! mine did! makes a new tractor out of them!

[Duwop]

6volt- Sonny Ok got battery charged changed coil and condenser was slow to start it did start. I ran a bit til warm shut it off bingo started right up. Go out this morning no start, pulled plugs all sooted up so I changed plugs bit slow to start but it did so I shut it off waited few minutes started up. Take a nap go back out this afternoon very difficult to get it started 25 min. I think there’s a problem with the repop carb.
I’ll check the valves and timing this week

[Duwop]

All new battery cables and wiring harness

[Don McCombs]

Did you check the float setting before you installed the repro carb? What brand and number spark plug are you using?

[Duwop]

I had auto lite 386 plugs but changed to champion 516,
No I didn’t check float setting, so please explain

[Don McCombs]

The Chinese carburetors frequently come with the floats set out of spec. You'll need to remove the carb from the tractor and take it apart, top and bottom. Remove the four screws holding it together. Gently pull it apart, being careful not to twist it, side to side. There is a thin brass tube inside that is easily damaged. Once you have the top free, use the diagram below to check the float settings in the up and down positions. Notice that the settings are measured without the gasket in place. If you need to adjust the settings, remove the pin holding the float and slightly bend the tabs on the float to make the settings right. Be gentle. It is usually a trial and error process to get them correct. Reassemble the carb and reinstall on the tractor. You will then need to adjust the carb to governor linkage setting, also. Follow the instructions in the second image below.





The Champion 516 (D-16) spark plug is a fairly "cold" plug. If you're having fouling problems, I recommend going to a "hotter" plug, like the Champion 502 (D-21).

Let us know if any of this helps.

[Duwop]

Ok compression test number but this is a cold engine as I can’t start it to warm it as book calls for #1=80 #3=85 #4=80
#2=85
I ordered another carb as the one I just got kept flooding engine
Now I’m checking spark to plugs it’s got good spark at coil

[Duwop]

Timing mark on crank pulley with rotor point to #1 wire or 2 o’clock, spark at all plugs.

[Eugene]

Ok compression test number but this is a cold engine as I can’t start it to warm it as book calls

Which book calls for a warm engine?

#1=80 #3=85 #4=80 #2=85

Was that the dry test? Compression is low. You need to do a dry test and then a wet test.

Static time the ignition system. I don't think your spark/ignition is correctly timed to the engine.