PaL Posted February 26, 2017 Report Share Posted February 26, 2017 I don't know if it's a stupid question but I going ask anywayWhat's the compression reading for a j13 engine is 120 good or not for all cylinders is what I got???Any help would do????Mahalo Thank you RATSUN ohana...... 1 Quote Link to comment
paradime Posted February 26, 2017 Report Share Posted February 26, 2017 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. J13 CYLINDER COMPRESSION http://community.ratsun.net/topic/21089-j13-cylinder-compression/ Second, 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. ;) 1 Quote Link to comment
PaL Posted February 26, 2017 Author Report Share Posted February 26, 2017 Ok will do. I appreciate your help...... 2 Quote Link to comment
datzenmike Posted February 26, 2017 Report Share Posted February 26, 2017 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) 1 Quote Link to comment
paradime Posted February 26, 2017 Report Share Posted February 26, 2017 Or a site Administrator might offer a bunch of extraneous information about calculating compression ratios... You just never know what you're going to get around here. Welcome to the funny farm. 2 Quote Link to comment
datzenmike Posted February 27, 2017 Report Share Posted February 27, 2017 Just averaging things out. Maybe you're having a bad day and I'm having a good one. 1 Quote Link to comment
PaL Posted February 27, 2017 Author Report Share Posted February 27, 2017 Thanks guys I'll be looking into it tomorrow after work for sure.... Quote Link to comment
DanielC Posted February 27, 2017 Report Share Posted February 27, 2017 One factor that has not been mentioned yet is cranking speed. The faster the engine cranks, the less time for pressure to leak away. Quote Link to comment
Draker Posted February 27, 2017 Report Share Posted February 27, 2017 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 Quote Link to comment
difrangia Posted February 27, 2017 Report Share Posted February 27, 2017 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. Quote Link to comment
difrangia Posted February 27, 2017 Report Share Posted February 27, 2017 You're new here, Pal. How bout some photos of that 66 truk? If you need any tutoring on hooking the photos up here, read through this: http://community.ratsun.net/topic/54932-photobucket-ratsun/ Quote Link to comment
PaL Posted February 28, 2017 Author Report Share Posted February 28, 2017 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....... Quote Link to comment
datzenmike Posted February 28, 2017 Report Share Posted February 28, 2017 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. Quote Link to comment
PaL Posted March 1, 2017 Author Report Share Posted March 1, 2017 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 ...... 1 Quote Link to comment
PaL Posted March 2, 2017 Author Report Share Posted March 2, 2017 I been trying guys if I got this one correct on how to send a picture I'll send more tomorrow....Aloha...... Quote Link to comment
difrangia Posted March 2, 2017 Report Share Posted March 2, 2017 Bingo !!! More !!! Quote Link to comment
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