I have a 1979 Yamaha IT175, rebuilt the motor after buying it. Top end got new piston and rings and a honing because it wasn't too worn (yeah I know, should have bored it). Ran ok for a summer. This week I went to drop the carb needle a notch and during that found an alarming amount of sand in the intake. Obviously I need to figure out how it's getting in there, but I feared for the health of this motor and did a compression check. 90 psi cold. I have no idea what it was right after the rebuild. Is this number too low to hope for another season? What numbers would you guys expect for a bike like this at its best? I realize it probably should be tested hot as well. Thanks!
sand in intake sounds like a leak past your air filter. Or not seating/sealing the filter base properly. I have lots of that era MX, YZ, etc. and the boot connecting carb and airbox tends to harden and shrink making it very hard to get a complete seal on carb or airbox.
Also, if you are running a K&N filter in airbox those can pass fine silt like particles if not using an outer foam skin. Especially vulnerable if you are riding in sand washes, riverbeds or very dusty conditions a lot.
90 psi is barely starting.
Bad gage I suspect.
What is the compression ratio?
90 psi may be fine.
The spec for my TZR is 100 psi (7.0 kg/cm2), with 71 psi (5.0 kg/cm2) being the minimum. The compression ratio is only 7.5:1.
I got 85 psi when kicking it over with both plugs out.
Starts first kick and idles good.
im not sure on an IT BUT id imagine you'd want at least 110. here's the thing : low compression = worn rings. worn ring rotates or breaks id be concerned about damage if that ring lets go.
as a side point, my tzr has a skimmed head but is 145 psi. the dead stock 70 r5 is 130- 135 psi. my 67 c2tr, sock aircooled 120 single is at 110-115 psi
Thanks for the replies so far. I work in a test lab so I'll check the gage and make sure it's on. Probably also do a hot test. Thanks Mark and JKV45 for the numbers to compare with, some depress me and some don't. The bike runs ok, but my gut tells me I'm looking at a full top end treatment.
I think the foam filter element wasn't fully seated or was previously crushed and thus all the sand. Northern Michigan trails are full of (unwanted) stretches of pure sand.
I have one of those IT175's. Just out of curiosity, stock cold engine, no oil, hasn't ran in months, 150psi. They are kind of a high-performance motor.
Quote from: 1976RD400C on March 06, 2026, 02:03:38 PMI have one of those IT175's. Just out of curiosity, stock cold engine, no oil, hasn't ran in months, 150psi. They are kind of a high-performance motor.
Well thank you for the comparison! That's certainly telling. Sounds like new top end here we go. And maybe new bottom end. Crap. It was a new crankshaft.
I was curious why my readings were so low at 85 psi, and I found this calculator that shows compression ratio and gauge psi.
https://www.readycalculator.com/compression-ratio-to-psi-calculator
My 7.5:1 ratio says 90 psi gauge reading - so my 85 psi reading is pretty close.
Does seem pretty low though, but it's within the spec range for my engine.
yeah. id be rebuilding. compression ratio numbers in a conventional sense really don't translate to 2 strokes in the normal way :twocents: 1st pic one mine was :rebuild time.