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J13 compression numbers? "help a brother out"


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Hey PaL, thats a good question, but there's a couple things you jumped over in asking it as a new thread. First doing some research on your own, either here on Ratsun, or google would have given you your answer.






If you do that on your own and still don't find what you're looking for, ask it in a j13 specific *Engine* thread rather than posting a new thread. 


Don't take this personal, it's just friendly advice on how to keep the moderators from biting your head off.  ;)

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Compression ratio is around 8.2 but compression readings in PSI are different and lower as the engine wears out. A reading of 120 is getting low, but as long as it is not smoking and using oil and all cylinders are within 10% if each other, less is better.


While the compression ratio can easily be calculated the actual cylinder pressure cannot easily be done. This is because the intake valve does not shut at bottom dead center so not all of the air in the cylinder is compressed. A rule of thumb is 15 to 20 X compression ratio. So 123 to 164PSI, likely 140-ish, which is half way.





At very high engine speeds the air is moving so fast that it can't stop easily when the piston passes bottom dead center and starts to rise up the cylinder. Air rushes in, even sometimes exceeding atmospheric pressures. (I don't think you need to worry about that)

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120 is on the lower end.. but if it's uniform.. okay it'll probably run. It's not a new motor.. so that's probably about what you'd expect. Would like to see it more around the 140-150 mark.. but largely depends on the compression ratio as Mike mentioned.


Uniformity is key.. and no smoking. lol

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As a couple of replies have stated, 120-ish is a bit low but usable if you're not pulling stumps up. Our E1, which is basically the same engine had 120 +/- 5 psi on the four cylinders when I acquired it. I drove it for a year while gathering all the required stuff to rebuild.


Of more importance would be the oil pressure/crank & bearing condition on an engine with approximately 120 psi compression. Mine took about five seconds to put the oil light out when it sat overnight. Also had the beginning of a death-rattle just before the light went out. When I tore the E1 down, the crank definitely need attention. I found a nice crank in lieu of a regrind. 


Correct me if I'm wrong, but if you put a pressure gauge on it and have at least 30-40 psi oil pressure and don't have any hammering noisesdown in the basement, you should be good for around-town driving. Just so you don't work the little thumper like a Viking ship rowing slave. 

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Thanks guys really appreciate all the help I'll try and get some photos up but I jus wish I could get it to run it jus cranks up turn over idle roughly then dies out as soon I give it throttle it died instantly I'll still do more research and searching for more info on what why when and HOWS Thanks again RATSUN OHANA ......ALOHA.......

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A good idle and running, needs a good spark and a correct ignition timing.


For the carburetor to work at it's best the intake vacuum must be as high as it can be. For now we have to assume the valves are in good shape and sealing properly on their seats but the valve clearances must be checked and set so that they can properly close. No gasket leaks at carb or intake manifold and any vacuum lines not cracked and leaking.


The carb needs to be correctly set up when running. Idle mixture and speed set. Furel level correct and jets clean and not plugged.

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Thank u so much Mike and the rest of RATSUN family I'll keep u all posted Mike appreciate it very much wish I had your knowledge in these awesome machines by the way I'm trying to learn how to use photo bucket I'll try and get a picture of my 520 aloha ......

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