no compression

[dave2092]

merc xr6 150. compression good in all cyc. except no compression in cyc #3 started motor to warm up before test. pulled all plugs plug #3 was pretty dry with a little oil on it. no compression in #3 any help

 

[dave2092]

Re: no compression

is powerhead shot??

 

[blue90HP50]

Re: no compression

Dave: The experts on this forum will need more info on the history of the motor. Serial number, year if known, what were the symptoms when started running bad, has motor been sitting a long period, has motor run hot. No need to address all the above, but you get the idea. One other thing, a listing of compression #'s for each cylinder is preferred.
Just trying to help out.
Blue

 

[dave2092]

Re: no compression

lower end rebuilt last year. compression in all other cyc. was around 120. except #3 (0) PSI. boat sat for about a year. I will get motor #'s for you. it is a 1993 if that will do for now. Did not get a temp reading on motor. but it seemed to run hot. would have at 3/4 throttle runs great. but just boggs and dies. I sure its because of overheat. I am just hoping the cyc is not swelling? if need more info let me know.

 

[bryanwess2000]

Re: no compression

If your getting 0 compression on a cyl. you probably have a broken rod. Pull
all the spark plugs and rotate the flywheel by hand while looking in the
cylinder with no compression to see if the piston is moving if your unable to
tell use a wood dowel or screw driver to push against piston while turning
the flywheel.
The other things it could be are, a big hole burned through the top of your
piston, completely broken/missing rings, or a badly blown head gasket. all
of these are less likely than the broken rod. The other posibillities would be
more likely to show SOME compression. Do not run the motor like this unless
you want to risk ruining your block and crank. If the broken rod catches on
a port you could be looking for a new powerhead.
Pull the head to assess the damage. Head gaskets are cheap powerheads
aren't.

 

[dave2092]

Re: no compression

will test the piston. but how could the boat get up 55 60mph with a broken rod. is that even possible??

 

[j_martin]

Re: no compression

will test the piston. but how could the boat get up 55 60mph with a broken rod. is that even possible??

It's a V5. Wasn't meant to be, but that's how it's running.

 

[dbrew]

Re: no compression

when you take the plugs out shine a flashlight in the hole and look for holes in the piston, no matter what even if last time it ran ok dont turn it on the powerheads can run around 2500 and if you arent getting any compression then something will give sooner or later. pulling the head is easy and doesnt take that mutch time and the head gasket is cheaper than a powerhead if you get the head off you can inspect walls, play in piston and obvious holes. good luck

 

[dave2092]

Re: no compression

well rod can not be broke Just checked movement on the piston from the flywheel. flashed flashlight in there did not see a hole burnt threw. Could I have got a false reading from gauge. checked it twice. when did the test all cyc. sounded strong. The #3 seemed to have a higher pitch when under pressure. They all were around 120psi #1 seemed to be 90 on other side. could that be bad head gasket. also just talked to mech. said be around 2800$ for replacing a piston he said may as well do all 6 while he is in there for a extra $200. Headgaskket around 500$

 

[dave2092]

Re: no compression

I heard that a leak down test on a 2 stroke is useless. Anyway to test the headgasket I heard to use soapy water on the head to see if any bubbles show. any ideas??

 

[dave2092]

Re: no compression

any one know where go for online manual. I ordered the online selco manual but it does not give instructions on headgasket removel pos

 

[scottkell]

Re: no compression

I have the same situation with my V6 200hp. It appears that I have burned the rings up on #6. The cylinder has no compression at all. I took the head off and inspected the cylinder. It doesnt look that bad. There is some ring debri stuck to the cylinder wall that I think will hone off. I wonder how often people just slap a new piston and ring kit in a cylinder that has lost compression. Especially a cylinder that has little or no damage. I used to race dirt bikes and any time a bike lost compression we would just do a new top end (piston and ring kit). Unless of course the cylinder was destroyed.

 

[bryanwess2000]

Re: no compression

I didn't realize you actually ran the motor on the water. According to your
post I thought you may have just been running on muffs and it would die.
I still don't see how your getting 0 psi on a cylinder without a large hole in
the piston or broken rod. My 90hp read 50-60psi with large chunks missing
from the piston rings.
You should be able to find a factory service manual on ebay for $30-40 or
order one from merc for $99. I can't help you on an online manual but even
the clymer should have the head torque sequence and powerhead
disassembly procedure outlined.
$500 for a headgasket replacement sounds crazy high. The $3000 for a
rebuild is probably inline if it includes boring and honing all cyl. and all new
pistons, rings, bearings, and gaskets. If your going to spend that much
just look for a drop on powerhead. You could probably find a higher hp
one around the same price or less even, and part out or sell your powerhead
to offset some of the cost. Look on scream and fly in the for sale section,
also contact some of their vendors for rebuild costs. Good luck on which
ever direction you choose. I know how bad it sucks to loose a motor this
time of year.

 

[sschefer]

Re: no compression

I heard that a leak down test on a 2 stroke is useless. Anyway to test the headgasket I heard to use soapy water on the head to see if any bubbles show. any ideas??

Leak down tests are a good indicator of engine wear just like a primary and secondary compression tests but very few folks understand how to do either properly or how to properly interpret those readings, so you're right in that sense.

The old soap trick is an old flat head check and all though these are flat heads, the sealing surface is surrounded by water passages so air doesn't leak out the edges of the head unless it's lifted.