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02 Sep 2019
Your admin has not been on the site much as of late as my main computer apparently seized up :( I have had limited access, but will be back soon. Thanks. Mako
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TOPIC: JT2M? compression pressure

JT2M? compression pressure 18 Aug 2019 04:50 #1

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I have a yellow JT2M that has been languishing in the basement for 22+ years. Years ago I got it running but it had very little power, not enough to move itself. I suspected compression was bad. Does anyone know what the measured compression pressure should be for the JTs? I looked at the shop manual and didn't see a compression pressure. I want to measure it before I do anything else.

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JT2M? compression pressure 18 Aug 2019 17:46 #2

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Could have rings stuck in the grooves but if it runs & you started it via the kick start i'd say your lack of power is elsewhare elseware elsewere other place whatever [spelling lol]. My first guess is the exhaust is 99% restricted in the tail pipe & where its welded into the main part of the pipe. Not only but also--drop the pipe off & lay bike over & closely eyeball the exhaust port right up against the piston as it grows there to the point of closing right off [takes years but]. & there's more lol, the rotary valve inlet on similar models like YG1 & stuff YG1T i had a few where the rotary broke the way its formed into the center drive piece. Then the rotary valve just free wheels so only getting inlet where it can rather than timed inlet. Easy way to check that is off with the carb & eyeball the disc. I'd probably very very carefuly slide a screwdriver past it when its open & turn the mag flywheel carefully untill it bumps the screw driver. That'll tell you straight away if its still attached, I'm guessing it probably opens a bit before bottom dead center & closes around tdc--wild guess. I grew up assembling JT1's from new--bring back the "70's--well, that's why we're here haha.
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JT2M? compression pressure 18 Aug 2019 18:17 #3

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In another post, Mark T wrote: "Testing compression for a specific psi reading was never a recommended test for these bikes which is why you won't find written benchmarks. Too many variables and too much difference between gauges."

I guess that is why I never could find a compression pressure in any of the Yamaha JT documentation, and Yamaha does not even show a compression gauge as a tool to be used anywhere.

So my next steps will be to follow RT325s advice and check the exhaust, cylinder and rotary valve. I will take a compression measurement to satisfy my curiosity!

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JT2M? compression pressure 18 Aug 2019 18:23 #4

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My DT50 only has 50 psi on a gauge that reads about 130 on a 175.

DT50 runs great... in fact I rebuilt the top end because of the low compression even though it was supposed to be a low mile engine by the speedo... runs the same and compression didn't get any better.
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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

JT2M? compression pressure 18 Aug 2019 18:27 #5

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JayB wrote: I have a yellow JT2M that has been languishing in the basement for 22+ years. Years ago I got it running but it had very little power, not enough to move itself. I suspected compression was bad. Does anyone know what the measured compression pressure should be for the JTs? I looked at the shop manual and didn't see a compression pressure. I want to measure it before I do anything else.


All I can offer is what my two JTs read for compression. My better running JT reads 120psi and my not so great running JT reads 90psi. Don't know if that helps you at all.
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"If it's possible to do it two ways, do it the other way first to save time"

JT2M? compression pressure 18 Aug 2019 18:37 #6

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Schu wrote: All I can offer is what my two JTs read for compression. My better running JT reads 120psi and my not so great running JT reads 90psi. Don't know if that helps you at all.


^^^ there's some good data, if you have a similar gauge.

P.S. The DT50 has a "slot" that raises the center of the exhaust port quite a bit... I think that's the main reason the cranking pressure is so low? A plus is you can literally start it with your hand, very easy to turn over and start.



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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

JT2M? compression pressure 19 Aug 2019 16:05 #7

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My totaly stuffed MR50 with almost no compression to feel, will start & run & also redline down the road but not enough comp to idle haha. Actualy i sold it about 3 years ago for $350 to a collector for his museum [i'll find a link for that if i can] & he never picked it up. Paid for it though. He's in his 80's. Neat old guy, quite a character but lives in the other island [south island of nz].
virtualexhibit.marlboroughmuseum.org.nz/...hibit1/vexmain1.html

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JT2M? compression pressure 19 Aug 2019 17:52 #8

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I measured the compression at 105 PSI. Based on comparison with Schu's compression numbers, I should be able to at least cruise around on flat ground. So I think the top end is coming off for an inspection. Maybe the rings are stuck? I will also have a bonfire for the exhaust pipe while it is off.

Thanks for the replies Schu, MarkT and RT325 !

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JT2M? compression pressure 19 Aug 2019 18:04 #9

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should go like the clappa's so physically look for the cause before ripping it apart [well i do anyway] but i'm used to things like--Oh it's severely constipated so head straight to exhaust port to give it a poke & try to wriggle some wire etc through the tail pipe until its free into the main chamber. Another thing i forgot is if the points are barely opening--like almost invisible gap--then the timing will be near tdc so very retarded, but would still go better than you're reporting. No need to reply--i'm just talking to myself [old people eh--who'd have them lol]My phone rang yesterday--i cut them off--threw phone to wife--she tried to ring him back- ended up ringing a guy i sold a buggers kodiak quad to months ago lol so ended up hearing how bad it was. No hope.

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JT2M? compression pressure 19 Aug 2019 18:15 #10

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It might just be loaded up from sitting... especially if it sounds like it's "four stroking".

Check the air filter too... foam can go bad (if it has foam) and common place for mice and other critters to make nests.
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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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