Pony motor compression

[RABBIT2]

I just got my RD4 last Thursday. The pony motor does not seam to have any comrpession. Can someone tell me what the compression should be on a 1937 Rd4 s/n 4g4344 should be. Thank You

[Old Magnet]

There doesn't seem to be any specs on pony motor compression. They seem to work as low as 40 psi (tired) with new maybe getting 90 -100 psi. If compression is really low, valves are usually the problem (mostly stuck from rust or carboned up) or stuck rings or just plain wore out. If you are running a compression check be sure to have the throttle wide open and squirt some oil through the plug holes to help with sealing the rings.

[RABBIT2]

Thanks for the info, I've ordered a operators manual and have not got it yet. I have a 4 1/4x4 Cylinder engine service manual. I looked and could not find compression info anywhere. When I turn the pony motor over by hand you can hear is hissss but didn't seam to have much compression. I'll do like you said and squart some oil in the cyclinders and check it again. Thank you again. Lou Ray

[Old Magnet]

You won't get much out of the pony just rolling over the flywheel by hand. Takes at least 2-4 power strokes cranking the pony starter or brisk pulls with the rope while capturing the compression gage pressure to get anything meaningful.

[RABBIT2]

Thanks again for the info. I'll sure take your advice and use as long a rope as I can to get as many "wraps" as I can on the pulley so I can get as many power strokes as I can to try and get a good check. I'll take some wd-40 and pull the plugs and see if I can spray the valves in case they may be stuck. Then after its set for a while I'll put a couple squarts of oil in the cylinders and see what I get for compression. Thanks again.

RABBIT2

[dave morgan]

We have had the bores to become egg shaped and had them reground to circular and used 060 over pistons. It made a different pony motor out of the tired one.

[alan627b]

And in all the excitement, don't overlook the obvious and leave the cylinder drains open!
hope this helps, alan627b

[RABBIT2]

Dave & Alan627b Thanks for the info. I'll sure keep it in mind and put it to good use.

[yieldmap]

Rabbit,

I've been playing with the same problem on my D4, compression on one but not the other. Instead of pulling on the rope, use an old electric motor. Mount it on a 3' 2x6 and use a belt about 70" long to spin the pony over. Takes a lot of pulling out of the equation.

Had an old buddy of mine that permanently mounted an electric motor on his D4 to start it. Pretty good idea.

Sam