Compression numbers?

[Gary H NC]

I did a compression test on my 1981 175.

The 3 cylinders on the left side were all around 95,
All three on the right side were around 105.
Are these number acceptable? why would all three cylinders on that side read lower?
Would it be worth pulling the heads and checking then new head gaskets?

 

[luckyinkentucky]

Re: Compression numbers?

New head gaskets wouldn't hurt especially on an engine of that age. Although, a good decarb could help some, but it won't cure you. The numbers aren't bad, but could be fixed with a little fine tuning. You want to stay within 15 psi with all cylinders, and you are there.

 

[Gary H NC]

Re: Compression numbers?

Thanks, I did a decarb a few weeks ago.
It looks like head gaskets would be pretty easy to replace..It runs ok but just seems like it should have more power.Trying to diagnose why my WOT is only 4900 to 5000. That is with 2 different props and different loads in the boat.
Checked the throttle linkage and it is correct.

 

[Gary H NC]

Re: Compression numbers?

BUMP!! Any advice on the compression numbers?

 

[iwombat]

Re: Compression numbers?

Compression numbers look good. Your power problem, if there is one, may have more to do with how the engine is set-up on the boat. Have you tried adjusting the height at all?

 

[Molaker]

Re: Compression numbers?

Thanks, I did a decarb a few weeks ago.
It looks like head gaskets would be pretty easy to replace..It runs ok but just seems like it should have more power.Trying to diagnose why my WOT is only 4900 to 5000. That is with 2 different props and different loads in the boat.
Checked the throttle linkage and it is correct.

You say you checked the throttle linkage, but are you sure the throttle stop adjust is correct? Are you sure the throttle plates are fully opened at WOT?

 

[Gary H NC]

Re: Compression numbers?

Yes,Have moved it in 3 different holes.Still the same WOT..
I can trim out to get the prop to blow out now.
Top speed 31 mph..
Thought the boat could do better than that.Maybe not
Yesterday about 4800 rpm was all i could get,one passenger,the dog and a half tank of fuel.

 

[Gary H NC]

Re: Compression numbers?

You say you checked the throttle linkage, but are you sure the throttle stop adjust is correct? Are you sure the throttle plates are fully opened at WOT?

Yes,Checked the plates.They go full open...

 

[HighTrim]

Re: Compression numbers?

I agree, wouldnt worry bout those numbers, within the 10 percent. Could always try a seafoam test again. In a portable tank, mix 3/4 gallons of premix gas, an 3/4 can of seafoam, (assuming you don't have the vro connected). Put the remaining seafoam in a spray bottle, run the motor up to temperture, remove the plugs, one at a time, spray some seafoam into each cylinder and replace plug. let sit 15 minutes, remove the air silencer from the carb while waiting, restart motor, and spray seafoam into each carb, until it start to stall. repeat till seafoam is gone. run out the rest of the gas. it is going to smoke like it is on fire, and drip goo on the ground. replace the plugs with new ones, and then do the compression check again. this procedure burns the carbon out of the motor and been known to free up rings, and increase compression.
What size prop are u using?? I wombat may have nailed it on the head with the motor height, is this a new motor to you, did you use to get the 5200-5700 rpm?

 

[Gary H NC]

Re: Compression numbers?

Already did that Seafoam method.The spark plugs look uniform and clean now.
I was getting 5100 to 5200 out of it before.Now its 4800 to 4900..
I can tell by the sound its not getting the rpms it was getting..
No difference between a 17 and 19 pitch props...thought that was strange..

Maybe load the cylinders up with Seafoam and let it set overnight?

 

[iwombat]

Re: Compression numbers?

This sounds like what I'm seeing in my Deteriorating Performance thread next door. Is your idle getting worse like mine is also?

Also, if you haven't checked you may want to inspect for waterlogged foam. That's a common cause for just what you're describing.

 

[Gary H NC]

Re: Compression numbers?

Idles a little rough when cold,kind of spits a little.After it warms up idle is good.
Foam is dry,pulled the floor up and checked that when i bought it.New transom...so no wet rotted wood.
It seems to have started running worse after the decarb..?
It took forever to burn the crud out of the engine. I smoked the neighbors out for 2 evenings in a row

 

[iwombat]

Re: Compression numbers?

Hrm, well hopefully someone will have some insights for the both of us.

 

[Solittle]

Re: Compression numbers?

What troubleshooting have you done so far?

If you haven't already get a spark testor and see if you get a bright blue snap jumping a 3/8" gap to each plug - - thinking you may not be running on all six cyl.

 

[Gary H NC]

Re: Compression numbers?

I was thinking that Solittle. One thing i noticed was when it is running and i pull a plug wire one at a time some cylinders stumble more than others.All of them do stumble to some degree.
The coils look ok,no cracks that i can see.Wires look fine also.
Could some coils just be on the weak side?

 

[emdsapmgr]

Re: Compression numbers?

Take the engine to the water with a friend and a timing light. Get the engine up on plane and check the spark on each plug wire. You are looking for strong, constant flashes from each wire. This will show misfires and weak spark. I'd check it again later, as some electrical problems show up after the system heats up.