What would be considered "Good" compression, or "Bad"

[Jim Werner]

Hi to all! I'm in the process of working-beating on a 79 700 skier. I rebuilt the carbs,replaced the impeller and gasket, and the lower end seal and "O" ring. I had to replace the two plastic pieces on the distributer linkage because the were broken and wired together. I checked for spark then I mixed up some 50:1 and gave the motor a good priming. hit it with a little S.F. and ........and.........and........just alot of smoke but no ignition to speak of. I then replaced all the fuel lines, because I felt that this is a fuel issue at this time and checked the fuel pump diaphram for holes etc...checked the fuel filter, and rechecked the carb bowls for debris ..nothing found, and decided to give it a go....still nothing..Now comes the first hand full of my hair on to the shop floor. So now, did a compression check and this is what I foundn #1 160, on #2 160, and 115 on #3, got gas, got spark, got compression it should start, or run for a few seconds right, or am I sadly mistaken.

 

[fucawi]

Re: What would be considered "Good" compression, or "Bad"

Apart from its a 3 cylinder we dont know what it is ...force Chrysler ? year Serial no your no 3 compression is low but will not stop it running .....Is the timing right ?

 

[jerryjerry05]

Re: What would be considered "Good" compression, or "Bad"

That amount of difference will make it harder to start.
Try Seafoam or Power Tune to remove some carbon and see if that helps.
Try squirting some oil in the bottom to bring up the #s.If that works the rings are stuck/broken.
The Seafoam don't help then remove the head and inspect.
He put in the year 79 700/70hp Chrysler. No Forces before the early 80/s
On the 3 cylinders it will make it a lot harder to start if the comp is low on 1 cylinder.

 

[Frank Acampora]

Re: What would be considered "Good" compression, or "Bad"

NAH! Three cylinder engines will start and idle almost normally on two cylinders..

First remove all three plugs and squirt a good amount of premix into each cylinder. Replace the plugs and crank without choking. You will have flooded the engine so it needs to crank a bit but should clear out, start, and run until the prime runs out. If it does this, then there is a fuel problem. If it does not fire, then it is electrical.

If it fires and runs, use it for a while to see if compression comes up. If it doesn't, then try a decarb to see if you can get the compression up on the low cylinder. If none of that works, pull the head and check the head gasket.

If it doesn't fire, then pull out the top plug and ground it. with the ignition on, turn the flywheel by hand, looking for spark. In neutral, this should happen somewhere around TDC. If no spark, then troubleshoot ignition. If upon turning slowly you get multiple sparks ( like a buzz) on the top plug, then the CD unit is bad.

 

[Jim Werner]

Re: What would be considered "Good" compression, or "Bad"

Wow thanks guy's..the serial is;70HOAN it comes up as a 79 skier 700. So the low compression will cause the difficult start..so what would be a good number for compression?

 

[snwwlkr]

Re: What would be considered "Good" compression, or "Bad"

I've got a 70hp Chrysler that is low on psi about like yours is on one cylinder. I pulled the head and found that cylinder had a good amount of very light corrosion, i'm guessing from a leaking head gasket. Nothing a light honing wouldnt remove but I would think that running it like that would have trashed the rings and more. Eventually I will eventually get to honing it and really think its going to run great after that. Pulling the head takes all of about 5 minutes max and if you're lucky you might not even need a new head gasket.

 

[Jim Werner]

Re: What would be considered "Good" compression, or "Bad"

Oh, BTW I pulled the head and there is a few scratches and a small detent in the number 3 on the upper part of the cylinder wall right at the top. The whole part of the run -no run chapped me. I think that a good hone and over size ring job is in order here. I don't want to be caught out on the lake- (Michigan) that is with my pants down so to speak. The top of the piston is ok..i.e. no dents or depressions as to where there was a part of the ring that came off and was banged into the top of the piston. When I rebuilt the carbs and took a reading, rather counting the number of turns that the jet screw was out, I took note that the # 3 was right about 3/4 turn out from bottom, so I'm under the impression that this cylinder was run lean and cooked the rings a bit. I still agree with Frank that it still should run on two cylinders not well but enough to warrant more time and $ investe din this thing.

 

[jerryjerry05]

Re: What would be considered "Good" compression, or "Bad"

I didn't say it wouldn't run,It will be harder to start and not run as good as it should.
On my motors.The first time it doesn't start or starts harder than usual then I know something's going on.
Port enging started missing.Comp 145-150 all cyl,good spark.new pump etc.Bad spot on the head,from salt buildup.
Filled the head(weld) new gasket no problem!! Same with carbon.

 

[Jim Werner]

Re: What would be considered "Good" compression, or "Bad"

So what would be considered "Good" compression 150#,160# ?

 

[snwwlkr]

Re: What would be considered "Good" compression, or "Bad"

I'm no expert like some on here but I have been around outboards a decent amount. That mixed with doing a lot of reading on here and other sites I would say.

150psi on all 3 would be killer.
115psi on all 3 would even be acceptable.
145, 140 ,135 would be ok..
150, 150, 115. or something similar= not so good.
The general consensus and also the Seloc manual says.. its better to not deviate more than around 15psi from highest to lowest. Not saying its not possible that it will not run decently if thats the case its just the motor will be more or less "unbalanced" and thus hard to tune, etc. One low cylinder also points to some sort of problem. Could be a hard parts problem or it could just need running and a decarb..

 

[JB]

Re: What would be considered "Good" compression, or "Bad"

"Good" compression is when all cylinders compress within 10% of each other and over about 100psi.

The psi is almost irrelevant because of the many variables that can make as much as 50psi difference in measured compression. It is the sameness that matters most.

The triples I have had will start and idle smoothly on one cylinder with no load.