Does this compression results make sence

[rdhj]

trying to figure out why my 87 dt9.9 stalls at low idle in gear, so i did a compression test on it...both cylinders are showing only 5 psi...it starts up pretty easily and runs in neutral. I was thinking that maybe i need a new head gasket or the rings are stuck....question is, wouldn't it not even start if the compression was only 5 psi? or can it run that low?

 

[99yam40]

No it would not run with only 5 psi.
need to find a different compression gauge

 

[rdhj]

just did a leak down test on the top cylinder and found fluid pushing out the bolt hole for the crankcase seal...also getting about 5 psi loss as long as I'm actually at TDC ..can also hear the air going through the lower end and out the prop....cant get the bottom cylinder to stay put as i increase the pi

 

[jimmbo]

I would also say try a different compression gauge. As for your leakdown test, are the valves closed during the test? You also have to prohibit rotation of the flywheel crankshaft during the test. Also make sure the crankcase is vented
You are always going to have some blow-by during a leakdown test. The rings, in almost all cases do have a gap so the seal isn't complete. During operation, the compression stroke is so brief that the pressure doesn't have the time to bleed off

 

[rdhj]

i didn't think two strokes had vales, just the inlet and outlet....i believe i had them closed all the way....i was messing with it at low pressure by starting at the bottom of the stroke and going all the way till it closed and started to open again. at no time was it quite indicating no passage of air...going to have to try another gauge....used this on my car when i got it and it was fine...go figure it runs strong so it can't possibly only have 5 psi...i did open the throttle all the way.

it looks like i have around 10% leakage...is that good enough for a 1987 set of rings? or is that enough to cause it to have problems saying running at the low idle of 650 in gear?

 

[rdhj]

By the way, why is there no gasket between the two halves of the crankcase...thought maybe it cracked where its leaking out this bolt, but i guess not as there is no gasket according to the part diagram

 

[racerone]

??-----There is no gasket !!----It is a precision metal to metal " half joint "---Just uses a bead of sealer.----Get another compression tester !

 

[rdhj]

when out today and used some glue gun glue to plug up the end of the bolt hole where I saw gas coming out the threads, then turned the mix screw out a little and increase the RMPs a little. Wouldn't you know, it ran totally fine...wonder if my little tach is lying to me and that little air leak was the problem all along....

 

[QBhoy]

Can’t be 5psi.
No valves on a 2 stroke.
Make tester is decent. Seal is decent on fitting. If need be get a socket and drill onto the cowling nut to turn over faster than possible with pull chord.

 

[QBhoy]

5 bar maybe ? 14.7 psi in a bar.

 

[jimmbo]

I assumed it was a 4 stroke. Me bad