Compression Testing

[PDCII]

Just finished an install on a SBT engine on one of my 2 1998 GTX's. Did a compression test and had some very low readings in the 120 psi range and was very concerned. Cross checked it with my other GTX and was getting about the same readings. I know that ski runs great. Have been reading a lot of threads on here during my install and have gotten some good advice. Also, have read several times and recomendations by DRHONDA that when compression gets below about 135 an upper end rebuild is recomended to prevent lower end damage. I thought I was looking at not only a problem with my new install but also a upper end rebuild on my other. The way the ski ran had me wondering though.
I ordered a new compression gauge from my SnapOn dealer and re-checked. Bingo, both skis reading in the 145 range.

Point is, make sure your tools are accurate. If something doesn't seem right, double and tripple check before you make a major decision...

 

[Dr Honda]

the 951 engine is only 135 on a fresh engine. I wouldn't worry yet. Go run it, and get the rings to break in a little, and then check it again. (cold engine, open the throttle)

 

[spx800]

Just finished an install on a SBT engine on one of my 2 1998 GTX's. Did a compression test and had some very low readings in the 120 psi range and was very concerned. Cross checked it with my other GTX and was getting about the same readings. I know that ski runs great. Have been reading a lot of threads on here during my install and have gotten some good advice. Also, have read several times and recomendations by DRHONDA that when compression gets below about 135 an upper end rebuild is recomended to prevent lower end damage. I thought I was looking at not only a problem with my new install but also a upper end rebuild on my other. The way the ski ran had me wondering though.
I ordered a new compression gauge from my SnapOn dealer and re-checked. Bingo, both skis reading in the 145 range.

Point is, make sure your tools are accurate. If something doesn't seem right, double and tripple check before you make a major decision...

Here, i'm assuming he's talking about the 787.

 

[Dr Honda]

Here, i'm assuming he's talking about the 787.

Sorry... you are right. I thought I read 98 XP.

Yes... 135 on the 800 engine is low.

 

[PDCII]

I'm an ASE Master Tech and been in the automotive industry for nearly three decades. First time to really play with a two stroke to this extent though. I appreciate the tips. Yes, cold engine, throttle open, full battery charge and both pugs removed. Just wanted people to realize that equipment and testing procedures can give erronious reading and be certain before they jump into a rebuild without verifying the readings...

Test was done on the engine after about 10 gallons of fuel for break in. Reason I did is because I was a little concerned with the top end speed. I'm going to give it a little time, fine tune and see how it goes.

 

[HighTrim]

Yeah I have 3 gauges and they all read completely different, withing 10 to 15 psi of each other.

 

[w.o.t.]

Yeah I have 3 gauges and they all read completely different, withing 10 to 15 psi of each other.

Typical of any three compression gauges. I switched to leakdown testing 20 years ago. A leakdown tester is calibrated to available shop air pressure with its own the gauge right before testing so lots of variables drop out of the diagnostic process that can obscure what is happening when one is relying on a compression tester.

 

[brock1]

im so lost i dont even know what you guys are talking about anymore ...LOL 98 xp 951....787....i think he is saying that he has a 787 and the first time he checked it it was low ...then he bought a new guage and now its fine..so moral is dont believe the first reading

 

[Dr Honda]

Typical of any three compression gauges. I switched to leakdown testing 20 years ago. A leakdown tester is calibrated to available shop air pressure with its own the gauge right before testing so lots of variables drop out of the diagnostic process that can obscure what is happening when one is relying on a compression tester.

On a 2-stroke... a leak down is a totally different test. It's not like doing a leak-down on the 4-stroke, where you are only looking at the top-end. On a 2-stoke... the leak down test is for checking for crank leaks. Since we only run 1 or 2 rings... and since we don't have the oil rings... the top end doesn't seal the same, and you will loose top-end pressure in about 5 seconds.


PCDII: Check the squash band. the test is easy... get some solder, and put in thought the spark-plug hole, and direct it at the edge of the piston crown, over the wrist pin. Then, roll the engine over and smash it. Pull it out and measure it. do both sides, and then do the other piston.

I'll have to look up the specs for you... but if it's on the big side... then the compression will be low.