Out board manual usless

[puddle jumper]

I am trying to help my friend out with his boat motor. We do a compression test and its low. We look in his manual to see what the lowest compression allowable is. Now remember this manual is one just about all of us use. The manual says and I quot ( you did do a compression check when the motor was new) What the heck kind of answer is that. This manual in it has no where were the compression speck is available. Also does not give you a spark plug gap speck.

 

[Bob_VT]

Re: Out board manual usless

LOL I feel your pain......

10% is the rule of thumb between all cylinders and I have seen outboards run with 75 lbs in each cylinder which I consider low.

 

[kenmyfam]

Re: Out board manual usless

What motor are we talking about here ??

 

[Moody Blue]

Re: Out board manual usless

Also does not give you a spark plug gap speck.

Could be that surface gap plugs are used ?

 

[j_martin]

Re: Out board manual usless

The reason that they don't give you a specific compression reading is that it can vary widely in similar motors depending on the port configuration and head volume. Slotted (relieved) ports add even more confusion, effectively varying compression with speed. If you would ask the thousands of owners around the world on this site what it should be for your specific motor, you would get an answer or 2 in a short time.

A service manual usually assumes that you are a serviceman, and have some basic knowledge of the topic. If that leaves you feeling jilted, it isn't the manual's fault, it's your lack of education.

 

[puddle jumper]

Re: Out board manual usless

What motor are we talking about here ??

Its a 115 Suzuki compression is 120 120 120 90 cold and the same hot.

J_Martin its not the lack of education

 

[BLU LUNCH]

Re: Out board manual usless

It's got to be with in 10% of the total readings, at 120 psi that one cylinder is to low maybe a decarb will raise it...........

 

[trendsetter240]

Re: Out board manual usless

Its a 115 Suzuki compression is 120 120 120 90 cold and the same hot.

J_Martin its not the lack of education

Hey puddlejumper,

At this point it doesn't matter. One cylinder is 30psi low so you know you have a problem. That's all you really nead to know.

I've brought my compression tester along to every outboard purchase I've made. Occasionally the seller has one too and we've compared readings. I've seen them vary as much as 20psi between mine and others.

Cheers

 

[puddle jumper]

Re: Out board manual usless

Sorry that I started this thread as a rant. The original point I was trying to make was the reason you buy a manual is to get information not a pass the buck comment.

In my opinion 120 psi on just about any engine is minimum compression. So 90 psi is way to low.

Can any one tell me what would be considered a good compression for this engine. In my opinion it should be about 150 psi.

 

[trendsetter240]

Re: Out board manual usless

In my opinion 120 psi on just about any engine is minimum compression. So 90 psi is way to low.

Can any one tell me what would be considered a good compression for this engine. In my opinion it should be about 150 psi.

Is it a DF or DT? The four stroke should be a bit higher than the 2.

My rebuilt, rebored, brand new pistons and rings, 3 cylinder 70hp evinrude looper has 135psi across the board. Haven't seen too many outboards with 150psi.

Then again, maybe my compression tester might read a bit low...

Cheers

 

[Silvertip]

Re: Out board manual usless

Most manufacturers don't publish compression numbers and haven't for years. Probably because any engine will run just fine with as low as 75 PSI provided the numbers are even across cylinders. yes -- the engine may be down a bit on power. Compression numbers at or above 100 PSI are perfectly normal and the engine will run fine. If the numbers are not even across the cylinders there is an issue. You don't need any more info than that.

 

[JB]

Re: Out board manual usless

Nobody, as in nobody, publishes a spec on psi compression for a 2 stroke outboard. What gives you the idea that such a spec even exists except in a lot of wrenchs' heads?? I have read in an OMC Service Manual that a particular engine should compress over 100psi if. . . .followed by a detailed list of conditions. . .but the range is what is important.

Differences in guages, temperature, barometric pressure, residual lube on cylinder walls, battery condition and technique all contribute to differences in apparent readings. Different people testing the same engine at different times with different guages may get readings differing by as much as 50psi, and almost any engine that meets the +/- 10% spec is in satisfactory running condition.

Many engines in top condition will test as little as 50-60psi.

The true reason to compression test a 2 stroke outboard is to find out if one or two cylinders are failing. The liklihood of all cylinders failing the same amount at the same time is almost nil.

The only 2 stroke I ever actually measured at over 150psi was a kart racing engine set up to run on nitro and methanol.

 

[rodbolt]

Re: Out board manual usless

most of the swirl chamber loopcharged hydro electric 3cyl OMC motors ran about 180 as did most of the 2 cyl hydro electric,
most the inline 6 mercs ran 150.
the early to mid 90 200 and 225 OMC loopers ran 80 PSI when new as to a lot of the replacement powerheads for the carbed 3.1L yamaha.
printed numbers are rather useless but the variation is what counts.
and hear again, we dont know if its a 2 or 4 stroke 115, if its a two stroke its time to remove the head for observation.
if its four stroke its time to run a leakdown test to determine if its valves or rings.

 

[puddle jumper]

Re: Out board manual usless

Its a 1990 155hp 2 stroke :redface:

 

[clanton]

Re: Out board manual usless

Probley had 120 when new, maby 125. I`m have a 2000 OEM Johnson 2 stroke manual and a 2004 4 stroke OEM Suzuki manual in front of me now. No specs for the 2 stroke. The Suzuki four stroke manual gives compression specs.