Starting Engine Compression

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dogger

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I have a D 2 5U whose starting engine starts either the first or second time every time. It has a cylinder compression of 80 psi. I also have a D2 3J whose starting engine almost never starts that carries 55/50 psi even after I replaced the rings with standard size rings after determining there was almost no wear of the bore, ground the valves, and replaced the old head gaskets with new.
Is there anything that affects the compression in a starting engine other than bore size, rings, valves and head gaskets?
Is there a difference between the design compression ratio of the J models verses the U models?
Any help will be appreciated! Thanks in advance.
Dogger

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Fri, Jul 25, 2014 7:03 AM

Old Magnet

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80 psi is exceptionally good, 60-65 psi is typical, 45-50psi is low.
I'm thinking there is some change in compression with cylinder head changes but I'd have to do more research.

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Fri, Jul 25, 2014 7:40 AM

neil

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Reply to Old Magnet:

80 psi is exceptionally good, 60-65 psi is typical, 45-50psi is low.
I'm thinking there is some change in compression with cylinder head changes but I'd have to do more research.

Longer spark plug that consumes more combustion chamber volume - that might gain you a pound or two, shave a few thou off the head and/or shave the deck

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Fri, Jul 25, 2014 9:44 AM

Dozerman51

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Reply to neil:

Longer spark plug that consumes more combustion chamber volume - that might gain you a pound or two, shave a few thou off the head and/or shave the deck

Have you checked the timing? How about the mag? Does it give a a good blue spark or a weak yellow spark. I always make sure I have a good ignition system before anything else. Just my 2 cents worth. Come to think of it, that 2 cents in old world money is probably only worth 1/2 a cent . LOL

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Fri, Jul 25, 2014 11:59 AM

catsilver

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Reply to Dozerman51:

Have you checked the timing? How about the mag? Does it give a a good blue spark or a weak yellow spark. I always make sure I have a good ignition system before anything else. Just my 2 cents worth. Come to think of it, that 2 cents in old world money is probably only worth 1/2 a cent . LOL

Yup, If the bores, rings and valves are good there is no point in trying to raise the compression, the problem is elsewhere, there were thousands of these things made and their most important requirement is gas and a good spark.

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Fri, Jul 25, 2014 6:12 PM

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