1950 cub

[Mht]

My 1950 cub still won’t run right. First a little background on me and my cub might be in order. I’m a former heavy equipment mechanic so I know my way around diagnostics and engines. I bought my cub with a stuck engine and when dissembled I determined the block was not salvageable so I bought a parts tractor with good block and built myself an engine with new bearings rings and valve train including hardened exhaust valve seats. Installed engine in tractor with rebuilt carb (Macdonald carb kit and I’ve triple checked float height). Installed magneto with new points condenser rotor cap wires and plugs ( parts from brillman). I static timed the magneto and tweaked with timing light after tractor was running. Carb to governor linkage has been checked and set according to service manual and governor spring is new. The tractor idles great and runs great at full throttle. That is where the he good ends. Rapid acceleration and the tractor stumbles and dies. It then restarts fine. Low speed (less than half throttle and the tractor drives and operates well in all gears. Full throttle and it dies about as fast as you let out the clutch except in first gear or reverse. I’ve double and triple checked everything I can think of where do I go from here

[tst]

sounds fuel related, check for plugged up jets are maybe the wrong size jets

[Jim Becker]

Tst is probably sending you in the right direction. However, I have to come back to this part:

. . . I static timed the magneto and tweaked with timing light after tractor was running. . . .

How did you do that? A properly working magneto has an impulse coupling with a built-in 13 degree advance. The advance is in play any time the engine is turning fast enough for the impulse to not lock (when the clicking sound goes away). Until Cubs came with battery ignition, the front pulley had one notch, at TDC. Later Cubs had 2, one at TDC and a second at 16 degrees advance.

When you used the timing light, what notch on the pulley did you use? If you used the single existing notch, you have it retarded 13 degrees from where it belongs. If you happen to have 2 notches and used the advance notch, you have too much advance (which I wouldn't expect from your symptoms). If you added your own mark at 13 degrees or eyeballed the approximate position on the pulley, you might have it about right (which would be back where it static timed to).

[Mht]

I only have one notch in my pulley. I used a timing light with adjustable advance and made sure it was 13 degrees bdc while running

[Waif]

Interesting....

Choke lever on backside of carb between it and engine could be checked for position. Plate position could be off if lever is off from where it should be set to convince operator that choke is open or closed or partway. Or choke plate could be at a goofy angle from stock configuration with the new carb..

Air intake could be reviewed. Symptom acts like a delay in fuel supply catching up , but air could be similar.

The gravity fuel system could be reviewed for obstruction/reduction.

Tractor performance lags at midrange is why yours is interesting.

Have you tried higher gear take-offs (letting clutch out ) at lower throttle setting? I get an engine bogging ,then the governor gets involved in higher gear take offs. But full throttle would be quite a jump...

When I had a single bad (mashed about closed) plug gap on one Cub the engine did not care for rapid throttle advance. Along with bogging out , it would sputter a little before loading up too rich.
It ran alright at high end of throttle range (minus a well spark fired cylinder) and at low throttle too. Just didn't like change quickly. More so when going from mid to high range in a hurry .
Stalled it out a couple times when working the tractor fairly good and was about to suspect a coil failing. But another tractors bad coil symptom was to cut out (die) with no sputter/struggle when hot.
Maybe that one cylinder was dieseling at high and low range but flooding when the throttle was changed fast?

[Waif]

Bear with my revisiting the choke lever...For my clarity if for nothing else.

When reinstalling a carb the set screw on the choke lever clamp (round head slotted) was cranked down until stout resistance was encountered , but unbeknownst to me ; barely gripped the throttle/choke plate through rod on carb..
As it slipped during use it created a random choke plate position. Using choke rod from operators station made that situation worse.
Lever position could say choke was open (or closed ,or partially on) but it was not. Looking at the rectangular piece on the other side of the through rod ,(the same one used when handcranking ,and the same one used when there is no operator station choke rod) confirmed plate position did not jive with choke lever position.

The resistance of the set screw was not at the clamp where it binds against through rod of carburetor.
Rather , the edge of the screw was bound against a seam on the cast choke lever's clamp.
(Interesting and varied symptoms resulted to be sure before the dim light came on and the other side of the carb was considered).

The seam was filed smooth/level with casting side. And the screw still bound. A couple millimeters was taken off the screw head diameter and the set screw could then be seated properly.
A simple device vs a simple mind I guess...

[Jim Becker]

I only have one notch in my pulley. I used a timing light with adjustable advance and made sure it was 13 degrees bdc while running

That should be good to go. Never mind that concern.

. . . When reinstalling a carb the set screw on the choke lever clamp (round head slotted) was cranked down until stout resistance was encountered , but unbeknownst to me ; barely gripped the throttle/choke plate through rod on carb..
As it slipped during use it created a random choke plate position. . . .

Related, I had one once where the hole on that lever was stripped/worn to the point it would not clamp tightly. The area where the screw went through closed up before it was snug on the shaft. Another I just replaced, the portion on one side of the hole had actually broken off. It too would tighten at the screw while remaining loose on the shaft. The reason it wouldn't clamp tightly wasn't evident until the carb was removed from the tractor.

[Glen]

Hi,
I sent you a PM.

Below is a pic from TM Tractor, showing the engine side of the IH carburetor. The choke lever, for the choke rod, usually faces down, when the choke is fully open.

[Mht]

Thanks for the responses. I checked the choke lever and it was loose. I addressed that issue and there is some improvement. I also readjusted the governor rod and again some improvement. I believe this carb and tractor should have an .030 main jet and I have not checked that but from all my previous work on the carb I’ll hazzard a guess that I have a larger main jet in place that is going to be the next thing I check once I find my small numbered drill bits. Thanks for the tips on things to look for when learning the ins and outs of a cub

[Waif]

The idle mix screw adjustment by the book has the throttle lever advanced and retracted repeatedly with engine running to test how carb digests after an adjustment.
Kind of a looking for a compromise setting throughout all speeds deal.
That's a different approach than just setting idle by ear.....

You probably already confirmed the low speed/idle air tube inside the carb is all happy. And carb top is flat , and sealed good by it's gasket.

You get your Cub dialed in , and it'll run great.

[Rick Spivey]

The low speed air adjustment screw (on top of the carburetor) adjust the air, not the gas, so "in" is richer, "out" is leaner. Instead of just adjusting until it idles well, check that there is also no stumble or hesitation when "blipping" the carb to full speed (high idle). Also watch the exhaust to be sure there is no black smoke.

[Mht]

I’m still chasing a problem with my cub. I got an new .030 main jet and new .060 discharge tube and installed them. The discharge tube was the same as the one originally in the carb. The tractor will barely run with the new jets. I changed back to my larger main jet and tractornow runs but still dies out at high end of throttle range. My carb has no numbers on it. How can I determine exactly what carb I have so I can make sure I have the correct jets

[Bob McCarty]

How can I determine exactly what carb I have so I can make sure I have the correct jets

If you've found your numbered drills, the 251234R94 carb has a #57 bleed and .034 main jet. The next version (364579R91) has a #53 bleed and .032 main jet. Unless the carb has been replaced, those two should cover what you have. That info is from the fuel section of the GSS-1411 Blue Ribbon service manual available under PDF Manuals.

[Mht]

The saga continues. I’ve now replaced governor spring main jet and discharge tube. Disassembled governor and checked for free movement and to make sure it was assembled correctly which it was. The tractor will now run with the choke closed almost all the way and seems to have governor response but appears to be running rich based on soot on the exhaust rain cap and the exhaust smell. I have checked for air leaks around the carb and manifold and find none. Anyone have any other suggestions or anybody in the Raleigh nc area that might be willing to take a look at this tractor and help me figure it out

[Waif]

Spark plug gap .023".
Magneto point gap .013".

Solid copper core spark plug wires are what you want on your magneto ignition.
Brillman offers a variety of wires including suppression type. Not saying that is what you have , but if you are quadruple checking anything...


I blew a lot of soot and oil out the 48's stack for a while due to hydraulic fluid leaking into the crankcase.(Gaining oil level had a source...)
Still stinks me up running it ,(Ahhh back in the saddle.) But no more volcanic type activity splattering on the hood.