John Deere model a

[Robsrepair]

I am working on a model a that now kind of runs. I was told that it was running fine until someone opened the throttle from idle to full boar rapidly. He said there was a huge backfire and that was it. I rebuilt the carb (full kit). Rebuilt Magnito (coil cracked) replaced points etc. I got the engine to run but not good. Compression is about 30 psi. I put a differential compression tester on an have about a 90 percent loss. Allot of blow by from the rings. What I can"t figure out if this is appropriate is from the intake. When the engine runs there is a major amount of gas/air blowing out of the front of the carb. Intake valves did not leak back on the differential compression test. Any ideas?..

 

[RLA]

did you check timing?? Do the marks register?
If so, tear it down.

 

[JDNPC]

Check valve lash and visibly look at rocker arm action with valve cover off while rolling engine over. Ensure governor gear is meshed accurately and the weights are not pinched or damaged.

 

[Lee B]

I've only seen this behavior when the camshaft is mistimed by a large amount (1/4 turn), which should blow a hole in the 'running good' words you've heard about this machine before you got it. They generally don't go around mistiming themselves after all. It had help and that person didn't know how to undo it. Line up your LH Exh indicator line on the flywheel at 3 o'clock position with the line on the transmission cover and check your number one exhaust valve (flywheel side, LH I assume is short for Left Hand) to see if it's just starting to lift off it's seat. I suspect it's way off, either well open already or with plenty of the standard .020 valve clearance that shouldn't be there at that position. You can be 180 degrees off when lining LH Exh indicator up, so do check it twice to be sure you are approaching the end of the power stroke. Or just rotate less than 180 degrees in a CCW direction from a confirmed compression stroke TDC position by using your thumb in the spark plug hole to test for compression pressure. This is where the LH Impulse mark should be lined up too just as a side note. And of course that's where the mag should trip or impulse with a very audible clank. Your low compression pressure indicates to me that you are horribly mistimed as well, but the air in front of the carb is the dead give away. I've seen a cloud of gas droplets dancing in the air cloud on the wrong side of the carburator too and when that caught on fire with a back fire resulting in a 3 foot ball of fire, there were plenty of people vacating the general area in an unorderly fashion. With the proper timing restored and a good work out, your blowby may be reduced drastically, but sometimes these old girls just get tired after 30 to 50 years so it may still need a ring job to correct that issue.

 

[Robsrepair]

I am going to check cam timing next. With rocker cover off the valves appear to open close as normal. No. 1 intake adjustment is way different from other three. I'm going to see if valve may be sticking? Thanks for the imput!

 

[Robsrepair]

What is the best way to get to the timing marks on ole bossy?

 

[430WLPG]

Check and see if it blew out the I intake manifold gasket

 

[Mike M]

Given that it is a Model A and the many of them I have seen with bad cam bearings letting the cam hop around I'd sure check the cam bearings !
I bought one once that died in the field and the bolts came out of the cam gear holding it on so check them too.

 

[Robsrepair]

(quoted from post at 21:34:20 04/03/14) I am working on a model a that now kind of runs. I was told that it was running fine until someone opened the throttle from idle to full boar rapidly. He said there was a huge backfire and that was it. I rebuilt the carb (full kit). Rebuilt Magnito (coil cracked) replaced points etc. I got the engine to run but not good. Compression is about 30 psi. I put a differential compression tester on an have about a 90 percent loss. Allot of blow by from the rings. What I can"t figure out if this is appropriate is from the intake. When the engine runs there is a major amount of gas/air blowing out of the front of the carb. Intake valves did not leak back on the differential compression test. Any ideas?..

 

[Kent Petersen]

Crank and cam timming marks are on the right side of the gears view thru top of crankcase look in from the pulley side no 1 rod will need to be just short of bdc

 

[Robsrepair]

I checked cam bearing it was ok. Cam timing was definitely off. Did some more investigating to find cam gear loose on cam. Suspect cam and cam gear are no longer friends. I Will need to do some more investigating and probably need to replace the cam and gear. Project is now on hold for now. Thanks for the help. Rob.

 

[Mike M]

What year is your A ? Mine was a 1946 and I think I heard of some others around that year had loose gears too.
Pretty easy fix. Mine had some fell out and I think what was left of one snapped off. I was able to work thru the top and the side hole where cam bearing goes and I think I had to drill my bolt out but it was loose and spun right out.
Seemed to bolt back up fine.