question about compression v6

[fishingdave]

Hey everyone I just boat a new boat with a 1999 bluewater series 150hp. I had a compression test done on the motor and found out that all pistions range from 116 to 120 psi. Is this normal for a motor that is said to only have 100hrs on it ? ALso what is the correct way to do a compression test on this motor.<br /><br />any information would be great<br /><br /><br />thanks<br />dave

 

[crem1]

Re: question about compression v6

Looks a little low to me. Warm up the engine first then insert into each cylinder, one at a time, a good compression gauge. You need a 360 deg turnover of the flywheel. A six would be hard to do by hand, try it with a short burst with the ignition key. <br /><br />Alex

 

[andy6374]

Re: question about compression v6

Your numbers are great. It's not the value of the compresssion so much, but the range of compression values which is important. They should all be w/in 10%, which you clearly are.<br /><br />Like crem1 said, the test should be done with a warmed up engine. Take out all the spark plugs,ground the ignition system (pull lanyard switch), and test each cylinder one at a time. Try to test each cylinder with the same amount cranks of the flywheel, this will give more meaningful numbers. In other words, if it took 10 revolutions to get up to 120 psi on one cylinder but only 3 to get up to 120 on another then there might be a leak. But I'm sure your fine.<br /><br />-andy

 

[crem1]

Re: question about compression v6

Well then the argument for having 110 PSI and no worries should hold true?<br /><br />Alex

 

[fishingdave]

Re: question about compression v6

Thanks guys, I feel better now. The motor is on a 19ft robalo, and is fast as hell. The compression reading where given to me by the dealer and said that they where good. I just want to make sure i didnt get coned. The boat has an hour metter on it that says 98hrs, the power head look brand new and the only thing that was ever changes seems to be the starter(arco) and some cooling hose lines. Would a starter go with in 100hrss<br />dave

 

[rodbolt]

Re: question about compression v6

hours are teats on a boar hog on most outboards and marine equipment.<br /> show me a 5 year old motor with 100 hours and ill show you a pile of neglect, almost 100% of the time.<br /> your numbers look about normal for what I have seen and the spread is good. how is the oil system maint? are the lines brick hard? when was the last fuel pump rebuild? they dont last forever and if you run it till it quits it may take a crank or piston with it. how is the water pump and t-stats and pressure control diaphram? these rubber parts deteriorate with or without use, hours become meaning less which is why the service schedule is set up as hourly or CALANDER WHICHEVER occurs first.<br /> on a two stroke its not critical to open the throttle nor warm the motor as your looking for a spread not a number. I always watch the gauge and see if the pressure builds equally for each stroke and 5 strokes should be all it takes.<br /> be aware that motor uses a nylon oil pump drive gear. it does not tolerate sitting much as the driven shaft will rust and take out the drive gear. if you ever get an oil alarm and everything looks ok immediatly switch to 50-1 premixed fuel before continueing. ya never know if its a faulty module or did the shft actually quit rotating. but try to find out the maint history, ya may have a bit of catching up to do.

 

[fishingdave]

Re: question about compression v6

how would i find out the maintance history on the motor??? the serial number is OG868018<br />let me know if anyone can do a look up on that.<br /><br />The water pump seems to be working strong, the thermostats open and close correctly. The lines seem to be okay.. What is the maintance schedule for this motor ??<br />thanks<br />dave

 

[rodbolt]

Re: question about compression v6

the only way is to ask for the maint history and reciepts from the previous owner(s). the schedule can be found in the service manual. if your not sure then go ahead and do it. sometimes ya only get one shot at the overheat or the no oil rotation or a bad fuel pump diaphram. will be a good time to test all the alarm systems as well.

 

[andy6374]

Re: question about compression v6

rodbolt-<br />As far as the crapped out fuel pump diaphragm, won't the engine stall out of fuel starvation before it throws a piston at the back of your head.<br /><br />-andy

 

[rodbolt]

Re: question about compression v6

not always, if its a small leak it can allow excessive raw ful to by pass the oiling system, if its a weak or strectched diaphram it can allow a high speed lean out, both can be fatal to the engine.<br /> if it were just to quit it would be better but its rare its a total instant failure.<br /> and thats why ya just rebuild them every 2 or 3 years and be done with it. that tiny little squre pump MUST be capeable of moving an excess of 15 gallons per hour.

 

[andy6374]

Re: question about compression v6

Gotcha!!<br />I guess I will be rebuilding mine this winter. I wish everything was as cheap to rebuild.<br /><br />-thanks<br />andy

 

[crem1]

Re: question about compression v6

I'm confused. The compression is lower than when you started out with but even across the board then there is no worry?<br /><br />Alex

 

[andy6374]

Re: question about compression v6

crem1-<br />Don't get me wrong, if you are reading 20 psi across the board then you're in trouble. But generally speaking, a good even compression of say 90+ psi (pulled out of thin air) is fine. You will find that most gauges don't even read the same pressures.<br /><br />In other words:<br />90 psi across the board and the engine runs great, then you are fine.<br />90 psi across the board and the engine runs like crap, then it's probably time for a rebuild.<br /><br />Do to the nature of the beast, you will probably rarely find a engine that has evenly worn cylinders/pistons.... More likely if an engine is bad 1 or more cylinders is ok where 1 or more is out of wack.

 

[rodbolt]

Re: question about compression v6

if its 90 across the board and runs poorly odds are graet rebuilding just the powerhead will leave you with a rebuilt crappy running motor, compression is not magic. the piston/rings either seal or they dont. on a two stroke you will never see it just wear and go lower and lower across the board, not long after the rings quit sealing the hot combustion gasses will take the piston out.<br /> forget any and everything ya ever read about car motors, other than they burn gas and cleanliness is crucial on reassmbly they really share no technology.

 

[crem1]

Re: question about compression v6

When you say rebuilding the powerhead, do you mean new pistons and rings, reboring of cylinders, if need be, and refurbishing the crankshaft?<br /><br />Alex