Unstyled B running rough

[Matt Ammentorp]

I have a 1937 unstyled B that has had a complete engine rebuild, and has a new magneto and a professional carb rebuild. While it runs, it is difficult to start. It runs pretty well when under load, but misses and backfires when idling or running downhill. I have confirmed that the mag timing is correct with respect to the flywheel (LH Impulse mark), but feel there must be a timing problem of some kind. I'm now thinking it is possible that my crankshaft-camshaft gearing might be one tooth off. It makes sense to me that I could be off with that timing and still have the mag timing (driven off the governor) correct to the flywheel. I would love to get any advice or input before I start tearing things apart! Are there other possible causes I've overlooked?

 

[John T]

For best starting, the mags snap/impulse must occur right at or very near TDC (to fast she kicks back too slow harder to start) and that can still happen if a gear is off a tooth provided theres enough adjustment in the mag to governor rotational timing and/or the rear lag angle adjustment.

The rear lag angle adjustment ONLY affects start/impulse timing and lag angle NOT run timing.

The run timing is a function of which drive cup it has (like a 25 or 30 etc) plus the mags rotational adjustment in the governor flange, (forward CW retards, backwards CCW advances) plus the points gap plus wear n tear

Turning the mag in the governor flange affects BOTH start n run timing

1) If the flywheel is on the correct crankshaft spline????????? when the flywheels Left Hand Impulse timing mark is at the flat tractor side mark at 3 o clock the left piston ought to be at TDC IS IT??????????? One may be able to tell with the left spark plug removed and a wire in the plug hole to find if TDC is when LHI is at 3 o clock

2) If the crank is then timed on the right cam tooth when left hand exhaust mark is at 3 o clock the left exhaust valve should just be tightening up ready/starting to open.

3) Finally if the cam to governor timing is right, at that same left piston TDC time, LHI at 3 o clock, the open female slot in governor shaft end (where mag drive coupler mates) ought to be flat horizontal for n aft

NOWWWWWWW IF THERES A tooth off somewhere and you can still by adjusting the lag angle or turning the mag get her to snap when the piston is at TDC you ought to get her to start okay and then put a timing light on and see just where shes firing at once running?????? and if its a gas tractor and shes at 20 to 30 BTDC when running, she ought to do fairly well but if its like 10 to 20 or maybe 40 BTDC she needs re adjusted.

Ive seen more gears off time at the cam to governor then other places so Id check that one first !!!!!!!!! It requires removing the oil line and raising the governor in cases to get the governior shaft open female slot fore n aft flat horizontal.......

For good starts the mag needs to be good n hot, good plugs n wire core wires, snap/impulse at TDC, carb correct.......

Thats enough for starters

John T

 

[David G]

Sounds like it is running lean to me.

 

[PopinJohn]

Your problem could be too much timing advance if you have a 35 deg drive cup on the magneto and are using gasoline fuel. Once the engine starts and the impulse is out of the way, the run timing is what the drive cup is. The drive cup timing for use with JD all-fuel tractors was 35 degrees. When gasoline became more popular, the drive cup for 25 degrees was more appropriate.

 

[Ken11487]

I thought there was enough adjustment to usenan35 deg drive cup with the mag turned to give 25 deg?

 

[Matt Ammentorp]

Thanks everyone for the suggestions. I will do some further investigation when I can get back to the farm. I feel confident the flywheel is mounted correctly on the crank. Some work with the timing light is in order. How do I tell whether I have a 25 or 30 degree cup in my Wico mag? All I'm familiar with is long-lug/short-lug.

 

[PopinJohn]

Most all the long lug Wico's were 35degree. Sometimes they are marked with a part number that can be looked up on a Wico chart. The part number and sometimes the degrees are stamped into the drive cup. Should be visible when the mag is removed from the governor housing. A lot of the drives weren't marked at all. Also if changing you have to make sure it is the correct rotation drive cup, since not all Wico's rotate the same way.