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John Deere Tractors Discussion Forum
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What is the compression for a 51 B

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Andy Kavanaugh

03-21-2008 16:41:50




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I've got a 51 B and I was wondering what the compression should be, or the ballpark anyway. I tested the tractor today and it read 86 P.S.I. It read this almost exactly in both cylinders. Is this right, high, or low?? Thanks.




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Mike M

03-22-2008 05:50:50




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 Re: What is the compression for a 51 B in reply to Andy Kavanaugh, 03-21-2008 16:41:50  
Sounds kinda low if it is a gas model. Did you have the choke open,throttle open ? to let the max. air in. Did what you get build fast ? or take awhile to get to that reading ? Should be enough to get it running to test for blow-by and low power.



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Andy Kavanaugh

03-22-2008 08:06:00




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 Re: What is the compression for a 51 B in reply to Mike M, 03-22-2008 05:50:50  
What do you think it should be?? ballpark???



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Andy Motteberg

03-22-2008 17:51:26




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 Re: What is the compression for a 51 B in reply to Andy Kavanaugh, 03-22-2008 08:06:00  
110 for Gas engines



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Mike M

03-22-2008 14:19:36




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 Re: What is the compression for a 51 B in reply to Andy Kavanaugh, 03-22-2008 08:06:00  
I think someone posted a few threads back about 110 lbs. in a bulletin. I was thinking I had some do better maybe upwards of 120 ?



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Duane Larson

03-22-2008 16:38:50




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 Previous post info in reply to Mike M, 03-22-2008 14:19:36  
Deere Field Service Bulletin 212 (2/54) gives 70/110 for All-Fuel/Gasoline engines. That is at the "rated rpm", which is 1250 for the B, and is obtained by belting the tractor up and spinning the crankshaft at 1250 rpm.

The I&T manual JD-4 took that number and listed it as the compression at "motoring (cranking) rpm", which is the rpm obtained with the starter with no plugs in and the throttle wide open, thus misinterpreting what Deere published.

Deere does not give a "motoring rpm" number for the B, but for the 50 they do, and the 70/110 would be reduced by about 0.8 on the average, so would be ~56/88. Probably more than you wanted to know....

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wilamayb

03-21-2008 20:53:00




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 Re: What is the compression for a 51 B in reply to Andy Kavanaugh, 03-21-2008 16:41:50  
Also be aware that compression testing is to be performed at WOT (wide open throttle)



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d. coleman

03-21-2008 19:19:43




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 Re: What is the compression for a 51 B in reply to Andy Kavanaugh, 03-21-2008 16:41:50  
alot depends on conditions during comression test, starter condition, battery voltage, condition of wiring, even air dencenty ect. what I would look for is difference in pressure between cylinders. just my 2 cents. good luck. d. coleman



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Andy Motteberg

03-21-2008 17:59:57




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 Re: What is the compression for a 51 B in reply to Andy Kavanaugh, 03-21-2008 16:41:50  
If you have a Gasoline model B, the compression at cranking speed should be 110psi, or 70psi on All Fuel tractors.

Andy.



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