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iff7378 
Bronze Level
Joined: 12 Oct 2015
Location: Rochester NY
Points: 16
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 Topic: Rebuilt B Low Compression
Posted: 12 Oct 2015 at 6:39pm |
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I just finished a rebuild on a 1939 B with 3-7/16 overbore pistions. New sleeves, rings and all that too. The tractor runs quite well except for the occasional miss and is rather tricky to crank start (no electric start). I checked the compression and I've only got about 40 lbs in each cylinder. Any ideas why my compression is so low or things to try to see where im losing compression? The valves and guides are also brand new and I gave them all a good lap so they should be sealing well.
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Dick L
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Joined: 12 Sep 2009
Location: Edon Ohio
Points: 5087
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Posted: 12 Oct 2015 at 7:11pm |
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Lapping is not a valve grind if that is what you are indicating that you lapped rather than grinding the seats. Lapping is a final mating surface polish after the seats are cut or ground. I spark all new valves on my valve grinding machine before installing. I never put a new valve in out of a package.
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WF owner
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Joined: 12 May 2013
Location: Bombay NY
Points: 5026
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Posted: 12 Oct 2015 at 7:23pm |
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It's pretty hard to get a good compression test on a crank-only engine. To get a good compression test, you should turn the engine over repeatedly with a good starter (which you don't have).
If everything is new, the valves are adjusted properly, and it seems to run ok, I'm betting compression is ok. My 39 (crank-only) B seems like it has very little compression when I'm cranking it (but it still wears me out when it doesn't start quickly!!!). Mine is a hard starting engine, too, which is why it usually gets towed.
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Bill_MN
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Joined: 12 Sep 2009
Location: Sioux Falls, SD
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Posted: 13 Oct 2015 at 2:06pm |
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Yes, sounds like the compression test was done incorrectly. A proper test is done by putting the tractor in a low gear and towing it with another tractor or truck with the choke and throttle wide open, and having someone walk beside or get a screw in gauge.
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1951 WD #78283, 1918 Case 28x50 Thresher #76738, Case Centennial B 2x16 Plow
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DougS
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Joined: 03 Nov 2011
Location: Iowa
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Posted: 13 Oct 2015 at 2:18pm |
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Check the timing and the magneto impulse timing. That can make a tractor very hard to start. A few years ago I was hand cranking a C and it would start when I would crank it very very slowly, but not if I cranked it at a normal speed.
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Dick L
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Joined: 12 Sep 2009
Location: Edon Ohio
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Posted: 13 Oct 2015 at 2:55pm |
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Quick check would be to put a little oil in a plug hole and rig a compressed hose to each cylinder. Set the pressure at around 100 pounds and listen for leaks with both valves closed. If you have pressure regulator start lower and increase pressure until it will not hold.
If the leak sound is in the exhaust below 80 pounds you have a bad valve seal.
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Orange Blood
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Joined: 29 Nov 2010
Location: ColoradoSprings
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Posted: 13 Oct 2015 at 8:20pm |
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If it's a brand new overhaul, as you suggest, I imagine the rings have not seated in yet either, which will also drop compression some.
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Still in use:
HD7 WC C CA WD 2-WD45 WD45LP WD45D D14 3-D17 D17LP 2-D19D D19LP 190XTD 190XTLP 720 D21 220 7020 7030 7040 7045 3-7060
Projects: 3-U UC 2-G 2-B 2-C CA 7-WC RC WDLP WF D14 D21 210 7045 N7
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