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Cub timing

grandpa Love

grandpa Love

Well-known Member
Got a magneto coming in the mail today for the slightly burnt cub. Compression stroke is when both valves are up? Or down? Looking in #1 plug hole. Thanks.
 
Compression stroke would have both valves close and piston at TDC. As it is coming put on the compression stroke the intake valve should be open but starting to close and be closed at TDC
 
Can't go by the stopped position of the valves. They are always closed when the piston is at TDC.

The surest way to tell, hold your finger over the #1 plug hole, turn the engine until you feel compression, keep turning until the #1 piston is at TDC.
 
Here's a pretty good video of the procedure.
Make sure you check that 2 and 8 position of the tangs.
It will run if they are at 3 and 9 but it will start hard and lack power.
Don't ask. LOL

YouTube
 
valves will be closed as said, BUT if you rock the crank back and forth and both the valves open and close then you are not on compression stroke. these valves are not fully closed, neither will you have any rocker clearance. compression stroke the rockers are loose, and do not move when rocking the crank.
 
When one valve is opening and one is closing the motor is exactly 180 out of time and the timing mark should be visible turn the crank till you see the timing mark again and you will be at tdc.
 
(quoted from post at 16:04:54 01/23/21) valves will be closed as said, BUT if you rock the crank back and forth and both the valves open and close then you are not on compression stroke. these valves are not fully closed, neither will you have any rocker clearance. compression stroke the rockers are loose, and do not move when rocking the crank.
Click to expand...

"compression stroke the rockers are loose, and do not move when rocking the crank"

SOUNDS good in theory, but a "Cub", being a "flathead" doesn't have rocker arms!
 

Copy and paste the following link into your browser and you can view the factory mag timing/installation procedure.

http://farmallcub.com/rudi_cub/www.cleancomputes.com/Cub/Blue%20Ribbon%20Service%20Manuals/GSS-1012%20Electrical%20Equipment/index.html

Look at Pages 4, 5, and 6.
 
But that mark will be there twice in the 4 cycles. One time on the exhaust stroke and the other time on the fir stroke so you have a 50/50 chance of having it right. That is why I say watch the intake valve. It opens at the bottom or close to the bottom of the compression stroke to suck in air and fuel then closes close to being at TDC for the fire stroke
 
I said that a bit wrong. Intake stroke the piston is going down. Compression stroke the piston is going up so the intake vale closes close to when the piston start back up on the compression stroke
 
Hold a finger over the spark plug hole. Turn the crank. When the piston comes up on compression it will blow your finger off the hole. Then the timing mark (TDC) is when the notch in the front pulley lines up with the round pointer in the block.
 
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