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18 Feb 2019
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TOPIC: Desired Compression?

Desired Compression? 11 Feb 2019 10:05 #1

  • EnduroTEX
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Gents I have searched my manual(s) as well as done some searches and can't find definitive desired compression for several bikes. I just did my first compression tests (ever ha) on 2 bikes and have a third I will be doing later today. Any direction where I may find this info would be greatly appreciated.

None of these bikes can start so all of this is being done on cold motors using a OTC (5606) brand compression tester.
Method- Connect tester to spark hole and open throttle all the way- kick until pressure stops increasing.
Bike 1 - 74 DT 250 =
1st test= 150
2nd test = 151
3rd test = 151
4th test adding just a wee bit of oil through spark hole for lube on future start attempt this week = 152

Bike 2 -75 DT 175 =
1st test= 120
2nd test = 121
3rd test = 121
4th test (same as above with wee bit of oil added) = 122

The 74 360 will get tested later after I determine how to disable the Decompression cable but would be good to know the compression ranges desired for this model as well..

Lastly I am building my leak test tool now (Thank you G French for your design I will be using) to check each bike as carbs are getting cleaned and cleaned and cleaned. Gas tanks were done this weekend and boy did they need it! Hopefully will attempt some first starts soon... Thanks in advance for all answers.
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Last Edit: by EnduroTEX.

Desired Compression? 11 Feb 2019 10:16 #2

  • darinm
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I think what you'll find is that with such small displacement engines, the gauges are not really that accurate, I've heard 10% variance based on the gauge used.

It's better to keep a record and see how the engine trends over it's lifespan using the same gauge, so you'll know when it needs to be rebuilt.

That said, if your gauge reads like mine does, both sound like they are in a fine range to be running well.
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Desired Compression? 11 Feb 2019 18:31 #3

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Thanks darinm - This is good to know and can see how the gauge could effect readings. I purchased the gauge new and will continue to use the same one for future comparisons and yes document that as well is wise. Good to know that I am "In the good range" on both bikes.

I am still not clear of what that good range would be for each bike : example 120-160 good/below 110 bad- same for both bikes or does this vary. Seems to be no written benchmarks I have been able to find.

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Desired Compression? 11 Feb 2019 19:55 #4

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A good read on compression here...

2 stroke compression testing
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Desired Compression? 11 Feb 2019 20:25 #5

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I'm not too experienced, but it seems that most feel around 100 is on the lower end and signs forthcoming work, with 130-150 pretty typical for a fresh top end, depending on the gauge. On my 1972 CT2 170 I saw just under 100 before a rebuild and 130-120, depending on the day, over the next 1500 miles after the work was done.
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Desired Compression? 11 Feb 2019 20:55 #6

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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 agree with what darinm said. The most valuable thing you can do is make your own benchmarks using your gauge on your bikes using your method (hot engine or cold? Throttle open or closed? Kick over or push? Hot day or cool day?)

Real life example: I have a few DT50's, one that's extremely low miles in excellent condition. I can "kick start" it with my hand easily. The highest compression reading I can get on a gauge is 50psi. I took it apart and piston, rings, bore are perfect and in spec. The other two DT50 engines I have also have the same "low compression". Yet they start easy and run strong (for a 50). The one I have licensed gets me up to 50mph pretty easy and I'm no lightweight. Even did pretty well on the trails at Hungry Valley at a Full Moon Fever event a few years ago... just had to keep it buzzing 9,000 to 10,000 rpm on the hills.

Most people will tell you it shouldn't even run with only 50 psi... but it does run, and runs well. And so do the other two. If I use the same gauge on a 175 I get between 120 to 135 psi average... even on the same bike on different days it varies a little up or down. So I usually don't even test compression anymore and if I do, it's only to see if it's "good enough" from my experience with my gauge on a similar bike.

P.S. The 250 readings almost sound a little high. I'd pop off the head and check for excessive carbon buildup... or possibly the crankcase has filled with oil and you're trying to compress some liquid with each kick? (which will raise the readings)
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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
Last Edit: by MarkT.

Desired Compression? 12 Feb 2019 06:45 #7

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Thank all of you for the replies. I am much more educated than I was 24 hours ago on compression testing/gauges etc... I have spent the last 14 years riding/racing and wrecking (alot it seemed at times) in Enduros and cross country events all on modern 4 strokes. Never had bikes as a kid so I am backward from the majority I speak to who "grew up" on 2 strokes . A real newbie in this regard but not a stranger around bikes

The article sent by RT360 and all your comments make it clear gauges, methodologies etc... vary so widely that as MarkT said- it was never meant as a recommened reading. However, the flip side is a low compression reading could be a sign of attention needed. I will lose the OCD on the need for "a number" and keep plugging away at cleaning up, leak testing and preparing these bikes for first starts to see where I go from here. Keeping good records on readings, using same gauge and methodoligies to the test for directional changes in compression readings is best bet vs chasing a magical benchmark - got it.

MarkT- Not even having numbers to shoot for I thought the same that 151 seemed high on the 250 for a bike that had clearly not started or run in many years with no mention of any new piston work by the PO. I hope I am ok to keep moving forward to simply getting it running for the first time. Once I have a running motor (if possible through my prep) then I will plan on pulling heads, checking pistons/rings etc... which was part of the plan.

Thanks again to all- I am still in the honeymoon phase with this whole effort, i hope that lasts. Crankscase full of oil possibiity , yikes... Honeymoon would be over

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Desired Compression? 12 Feb 2019 11:48 #8

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I take compression readings but always with a grain of salt and always with the same gauge setup.

My recently acquired IT175 showed a reading of about 145psi cold, throttle wide open. And It has a worn out cylinder/top end. So it is what it is, at least I have a baseline though.
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Desired Compression? 12 Feb 2019 11:54 #9

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$.02 The more consistent readings come from a gauge that does not have a disconnect. The disconnects seem to leak (slightly) or have bad flow. IMHO American, German or Japanese made will be more reliable and closer to your actual pressure. You only need three things to make your motor explosion. Fuel, Spark & Compression. Increase one of these a little & power goes up slightly, decrease one of these a little and power drops a lot. You sir, are on the right track!!
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1970 DT1-C Grand Prix race bike
1970 DT1-C Street tracker
1968 Honda CL90
1971 Honda Sl 125
1973 Honda CR250
1974 Honda MR-50
1966 Triumph Tiger Cub
1967 Moto Guzzi V700
1970 Suzuki TC90
1947 Mustang

Desired Compression? 12 Feb 2019 16:33 #10

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Crankcase full of oil is not uncommon and often a good thing... might prevent bearings from rusting. Over a long time the autolube pump can slowly let the oil leak into the crankcase.

It can make the bike very hard to start and on rare occasions the hydraulic pressure can pop out a crank seal... for the most part it's not a big deal. :likey
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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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