Compression too high??

[kicker]

Picked up my first RFI yesterday 2000 GSX 787, Upon further inspection last night I am pretty happy with it. Did a compression test and it came up at 165 and 170 psi. The motor was rebuilt last summer.

Proceeded to check my 96 GSX which was rebuilt 2 years ago and same thing, 179 psi in both. I figured the gauge (which is brand new) must be in accurate. Borrowed one from a shop next door and same (within a pound) readings.

Is it normal to rebuild a motor with higher then factory compression? With a higher then factory compression, is there other tuning factors that I should be aware of on an RFI or a carbed ski.

 

[Dr Honda]

an 800 engine should only be showing 150 psi. (give or take)

If they have been sitting for a while... you may just have extra oil sitting on the rings. Start them for about 10~15 seconds... let it cool for a few minutes, and recheck.

If it's still that high... I would verify the squash gap (correct it if needed)... AND... make sure to run premium fuel.

If the heads have been cut properly, to raise the compression... there is nothing wrong with high static compression. BUT... tuning gets more critical, and higher octane fuel is needed for safety. (to prevent detonation, and hot spots)

 

[Spimothy Leary]

an 800 engine should only be showing 150 psi. (give or take)

If they have been sitting for a while... you may just have extra oil sitting on the rings. Start them for about 10~15 seconds... let it cool for a few minutes, and recheck.

If it's still that high... I would verify the squash gap (correct it if needed)... AND... make sure to run premium fuel.

If the heads have been cut properly, to raise the compression... there is nothing wrong with high static compression. BUT... tuning gets more critical, and higher octane fuel is needed for safety. (to prevent detonation, and hot spots)

spot on. do exactly that.

actually if the heads have been cut, its not a bad thing if it was set up right, i think you can still safely run 93 gas .. (but don't quote me on this)

 

[griz400]

im guessin, goin by your adress, probably foggin oil and winterized ??? as doc said, run it some and retest it -- i find it strange that you have 2 motors with that high of compression, when i test compression, i watch needle fluctuate 5 times, and thats it -- by the way, dont read in too much with test, cause a compression test really is just a tool for diagnosing a problem, and your other ski has high compression for 2 yrs now ???? also, i couldnt tell you what compression is in mine, in my truck, or wifes car -- lol

 

[kicker]

Thanks for the help guys.
I did put fogging oil in a while ago. I ran them both and came up with the same results give or take 1 psi. Both these machines were bought in my area and there is only 1 guy (other then the stealership) that does the rebuilds around here, so I am guessing that he more then likely did both of these skis but not sure. He is not that helpful and I cant get any info from him........ hence the reason I joined this forum.

The RFI I just picked up yesterday but the carbed GSX I have had for a while and runs like a champ and yes the compression is still high. I just like to know what is exactly going on and what has been done so I can treat and maintain both properly to avoid problems.

I will check the gap and post the results.

 

[donkey]

my 96 has hi compression,180/182,,on a snap on gauge and a mac.
it worried me too,the dr had me check the squish,which is in spec.
i run super in it,seems to be living happily,AND it will outrun my 98xpl,a bunch!

 

[kicker]

Good to hear donkey. I never cheap out on fuel or oil in my toys, which i think goes a long way for the longevity of any 2 stroke motor. If it weren't for cheap (or the wrong) oil and bad gas mechanics would have far less work in their shops i'm sure.

I did check the squash gap on both skis
2000 787 RFI was;
1.49mm and 1.47mm

1996 787 carb
1.38 mm and 1.4 mm

Spec for the 96 is 1.2mm - 1.6mm, I cant find the spec in the manual for the 2000, i am assuming it will be the same.

So if they are both within spec, what is giving me the high compression? Possibly not doing the test correctly?
Done as follows
- remove plugs
-thread in tester
-strong battery
-WOT
-crank engine till needle reaches max PSI

Maybe i am over analysing, from the info above as long as I am running high octane fuel full synthetic oil, I should not have to make any other adjustments, just giver? Or is there other special tuning practices that I should take into account to make sure the engine lasts?

 

[Dr Honda]

Yep... the 800 engines havn't changed. So, the gap will be the same.

I wouldn't worry too much for now. But... next season... just listen to the engines. They will tell you if they are happy. (and use premium fuel)

I've safely run up to 180 PSI on pump fuel... but the ignition timing has to be PERFECT.

 

[donkey]

hey kicker,,how hard does your 96 pull out of the hole?
mine kinda feels lazy on the bottom end,but the big end is WAY too fast.
thinkin a smaller pitch prop might be in order for mine

 

[Minnetonka4me]

im pretty sure 160 was stock on the 787s...140-150 on SBT motors.

Squish is the main concern, as long as that is right and you are using the correct gas...enjoy the extra power!

 

[kicker]

Thanks guys, The squish is within spec and no other adjustments to make so HAMMER DOWN, I will be sure to have a keen ear next season ( I actually have a 14 year old that hears things happening before I do). Ill use Snipes Corner to help read the plugs.

Donky, the 96 is a rocket out of the whole no issues at all, the power is smooth all the way through the range. Its possible that your raves are set different, I keep mine flush. Ill hit 57-58 mph on the top end w/gps ( i know the forum rule is you need a picture but I m not bragging so believe me if you want lol). I wouldn't change your prop unless you are NOT hitting max rpm or riding the rev limiter it wont make a difference, I think my 787 GSX is 7250 max rpm.

Edit; it will make a difference but if you are hitting max rpm you will just hit it sooner and loose max speed by hitting the rev limiter, but you will gain bottom end.