1989 DT200 - Typical Compression Readings

chackett

Seaman
Joined
Mar 1, 2012
Messages
52
Hi All,

I have read many of the questions "Is xyz a good compression reading for my motor" along with all the responses suggesting that the actual PSI in the cylinder isn't as important as having a consistent reading across all cylinders. I understand that having 100 psi on one cylinder and 80 on another and 90 on a third, etc is poor.

I have never taken a compression reading before, but yesterday I borrowed a compression tester from my local O'Riellys auto parts store and checked the compression on my 1989 DT200. All 6 cylinders read 90 PSI and seem to hold it.

I tested both cold and at operating temperature. The tester said to check compression with all plugs removed and the tester inserted into one cylinder at a time. Hopefully that's the correct procedure.

I hate to be another "is 90psi an ok reading" poster, but gosh .. that just seems low. It is very consistent though .. all 6 cyliders read 90 pretty exactly.

Thanks!
Chris
 

robert graham

Admiral
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Apr 16, 2009
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6,908
If I had an older motor(and mine is a 1999) and it was running reasonably well(mine is) I wouldn't even give a thought to compression readings....Now since you've run the tests and they're all about 90 PSI (seems O.K. To me) and you can't stop worrying about it then maybe buy some Ringfree and run a tank of gas with the "carbon removal" dosage....then run the tests again....but in any event my guess is you're in good shape!....good luck!
 

chackett

Seaman
Joined
Mar 1, 2012
Messages
52
Thanks Robert, I appreciate the response. My guess is that it's ok too .. but I figure since I"m trying run down this other issue, I would check everything while I'm at it. Thanks for the reply!

Chris
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
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8,851
The problem with using equipment that tons of others have used and abused is you have no idea if it is accurate.
Might find a different gauge and see if it gives same reading. This is why they do not give specs, not all gauges read the same

The motor does not hold the compression the gauge does with a check valve so throw that out of the equation.

I would expect around 100 to 120, but have no idea what a DT 2000 should read
 

chackett

Seaman
Joined
Mar 1, 2012
Messages
52
The problem with using equipment that tons of others have used and abused is you have no idea if it is accurate.
Might find a different gauge and see if it gives same reading. This is why they do not give specs, not all gauges read the same

The motor does not hold the compression the gauge does with a check valve so throw that out of the equation.

I would expect around 100 to 120, but have no idea what a DT 2000 should read

I can't believe I didn't think about the check valve .. even though I had to press it to release the pressure betwen each cylinder . sheesh :)

robert graham said:
Just curious, but what's "the other issue" that you mentioned?...

Here's a thread with a bunch of detail, but my main issue at this point is that my tach seems to be readin 2x the actual motor rotation (measured with inexpensive handheld non-contact laser tach).

Thanks!
Chris
 

robert graham

Admiral
Joined
Apr 16, 2009
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6,908
Regarding the tachometer/RPM readings, there's a small slot head screw on the back of tachometer with several settings....with a small screwdriver and the motor running switch that screw back and forth until your tachometer reads correctly......you'll be able to tell when it's right....
 

chackett

Seaman
Joined
Mar 1, 2012
Messages
52
Regarding the tachometer/RPM readings, there's a small slot head screw on the back of tachometer with several settings....with a small screwdriver and the motor running switch that screw back and forth until your tachometer reads correctly......you'll be able to tell when it's right....

Thanks Robert, I'll absolutely look into that when I get up there this weekend to continue the effort!

Thank you,
Chris
 

jerryjerry05

Supreme Mariner
Joined
May 7, 2008
Messages
17,923
Doing a compression test.
The gauge could be a piece of crap so the results could be off.
But even if it's a junk tester it should give a reading.
The one you have might give different readings than mine.
Yours read 90 on all cyl.
Mine might have read 120 but it probably would have read that on all cyl.
Did you let the motor turn over 5-8 times, or let the needle stop on its own?
Some just let the motor turn over 1-2 times and not allow the gauge to read right.
At 90# on all cyl. that tells me the motors in good shape.
Get a different gauge to compare results.
Mercury makes a product called Power Tune.
It's a de-carb. Get a can of it and follow the directions.
 

chackett

Seaman
Joined
Mar 1, 2012
Messages
52
Thanks for the response JerryJerry90, I appreciate it.

Yeah .. because I just borrowed a compression test kit from the local auto parts store, it definitely is possible that the gauge is not accurate or worn out, or just not good quality.

As for how I performed the test .. I screwed the adapter into the spark plug hole and turned the gauge so I could see it from the ignition. I turned the motor over until the gauge didn't move any more. The gauge has a check valve, so it hold highest PSI until released. I feel pretty good that I turned it over enough to get a good reading .. not just a couple times.

Definitely will look into the Power Tune ..

Thanks!
Chris
 
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