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Conflicting Results on Compression test


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WrenchPincher
New User

May 19, 2019, 9:27 AM

Post #1 of 7 (97 views)
Conflicting Results on Compression test Sign In

Hello,Today I'm trying to diagnose low compression on a 212cc One Cylinder motor that goes in a two wheeled vehicle.
All tests were done with a cold motor (room temp of 80 degrees F)
Using a Snap On MT324 cylinder leak down tester at 75psi (max test pressure is 100psi), I get a reading of 9% which would indicate a healthy motor.There is no sound of leaking at the intake valve at all and only VERY little at the exhaust valve, and very little through the breather (rings)

However, using several different Compression gauges (to ensure accuracy) I get only 35psi (cold)No matter what, I cannot get more than 35psi on the compression gauges.
The motor is quite difficult to start but there is no backfiring at all and it runs ok once started but does seem to struggle as if the power is low.When starting it, it seems there is little resistance. As if the compression is in fact too low.

I'm wondering how the SnapOn Leak Down tester can find that the motor is healthy, yet gauges give me such an extremely low reading of 35psi?
any ideas?
Thanks


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

May 19, 2019, 11:29 AM

Post #2 of 7 (90 views)
Re: Conflicting Results on Compression test Sign In

? Basics: Would be better warm of course. The 35 PSI reading just can't be right or doing it right or fast enough. Your pressure test say healthy it is that way but if you want to see better IDK, try a rubber tipped one OR it's probably manual crank - pull or pedal and just isn't fast enough.
The volume of just the hose on such small engine is probably just too much. IT wouldn't fire at 35 we know that.
Whatever throttle plate must be held open don't forget that and also do some (little as possible) oil inside should bring it up but if that alone totally changed it back to a normal not sure yet?


Also back to school on this way. You really discount the first 3 compression stroke and count the fourth one - right? Can you push or ride this thing and let surface turn engine faster?


On the outside of all reasons could be it's so washed with fuel results are not correct.


Back to school too: Rings actually tilt do have a spec for how much up and down they can move but must move or lose the "scraping" effect saving oil. OMG, that off the top of my head from freaking eons ago including ring end gaps measured and staggered. Also if off with the up/down gap there were wavy ring things or machine work to solve that. Lots more too just save that space here.
The thing is probably fine and hard start is another reason totally. Timing if fixed is that loose? Check it all. If some ancient creation you are making it might not like current fuels with ethanol, lots don't! Some "real old" serious just eats up carb gaskets, needle valves or whatever it has if so just hope that new and tolerant stuff is available,


T



Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

May 19, 2019, 12:05 PM

Post #3 of 7 (86 views)
Re: Conflicting Results on Compression test Sign In

OK, you are trying to do a compression test using a leakdown tester. That IS NOT a compression tester.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

May 19, 2019, 1:46 PM

Post #4 of 7 (74 views)
Re: Conflicting Results on Compression test Sign In

? HT - I can do both with one but do take it apart attach pressure and still have gauge. I can't imgagine what assortment of stuff is out there didn't OP say used several? Tom


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

May 19, 2019, 1:49 PM

Post #5 of 7 (70 views)
Re: Conflicting Results on Compression test Sign In

No, you can't do a compression test using a leakdown tester. The gauge doesn't even read high enough.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

May 19, 2019, 2:08 PM

Post #6 of 7 (64 views)
Re: Conflicting Results on Compression test Sign In

? Not beating on it too much. Not sure how many I have but just put an A/C gauge on it and off you go. If one wasn't able make the manifold so it could, T


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

May 19, 2019, 2:16 PM

Post #7 of 7 (61 views)
Re: Conflicting Results on Compression test Sign In

Well, I'm sure in his case, he didn't change the gauge first. He didn't even do an actual compression test. He did a leakdown test which won't tell you anything about the compression.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.







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