1996 ProMax 200 compression ?

[Hank W]

Can anyone tell me what compression a stock 1996 200 Promax in good condition should have? Mine has 4 years of average use on it and has only 105 psi per cylinder per my gauge. this seems low to me. Is this normal? The good news is that all 6 are within 2 psi.

[T-REX]

Tha only 200 promax I ever checked wuz 125#'s across tha board, with thick head gaskets...it wuz a stockie that had never been in...But I don't remember what year it wuz...REX

[Rickracer]

Just to be sure, you might want to try checking compression with another guage to see if it yeilds any better numbers. If so, you can stop worrying, and just look for consistancy whenever you check compression again.

[Hank W]

Thanks, I picked up a new gauge yesterday and I'll be trying it tonight. If it is still that low I'm planning to go to 35.5 cc heads. Would this be a good idea? Should I use the thick gaskets with those?
This is the motor that I have posted about lately that has been evidently sending one hell of a flame out to the tuner. EGT 1275 degrees at times. I pulled the plugs to check piston top color. It is black and nasty in there. I use the best Merc oil available to me here. I am still using the oil injection. My mechanic thinks that maybe the compression is so low that it just will not burn the fuel well in the cylinders. I am hoping this was a low compression motor from the factory and not a badly worn one.

[Rickracer]

I am no Merc expert, but if you're getting that much oil, maybe there's a problem with rings carboned up and sticking and/or possibly an overdose of oil from the injection system? Have you decarbonized anytime recently? Have you noticed any excess smoking? Have you checked timing? All of these things could have some effect on the combustion process. I'm sure some sharp Merc guys will jump in here with more specific advice, but these are all things I would check. On the heads issue, if you did decide to go to 35 or 36 cc heads, I believe that wouldn't be a problem with the thin gaskets as long as you stick to premium fuel, but again, hopefully some sharp Merc guys will have more specific info.

[us1]

Those heads will work fine but you can’t fix low compression from bad rings with new tighter heads. Most compression gauges read are not accurate so you might want to do a leak down test or try several compression gauges. I believe that motor should have about 120 LBS. If your combustion chamber temperature is truly 1275 you might want to get that under control before raising the compression.

[Hank W]

I tried a new gauge last night and got 111-114 psi on all 6 cylinders. I guess I'm pretty close to where I should be. The marina that I launch from has a near 1/2 mile no wake zone so maybe that is why my motor is so dirty right now as that is the last running it did Sunday. I remembered that I had removed the exhaust liner late last season, so I reinstalled it last night. I looked up the tuner and it is mostly dry in there, a little wet in the port side up high.
I thank all who answered this post. I am going to try the 35 1/2 cc heads. I'll let you know the results in a week or two. Racemore.. It does smoke a good bit at Idle and off plane speeds but oil consumption seems about right with average fuel consumption...that is I'm using a gallon of oil about every 50 gallons of gas. If these heads wake this thing up I'll probably get the ECU "Rapaired" and remove the oil pump and let it spin some RPMs.
John, I'm going to see what temp it runs w/ the liner in and if it is still hot I'll check the timing. If still hot it will be my mechanic's turn to figure it out for me. He seems to think this motor came from Merc w/ low compression heads and that it is just not burning the mixture in the cylinders but in the exhaust. Do you think removing the exhaust liner in a stock motor might be letting or causing the flame to run down the pipe? It is back in now, so we will know soon. Thanks again

[Edited by Hank W on 08-30-2001 at 10:32 AM]

[us1]

It’s not likely to me that the liner will effect EGT much. But on the other hand no one including myself has been able to come up with an explanation why some motors 1300 plus on the EGTs and don’t melt. Obviously the combustion chamber temps are not 1300 plus yet the gauge read high. Disclaimer, this is not true of all motors, some that read high are really high and should be tuned to run cooler.