Acceptable Compression

[stikman63]

Have a 1975 70 HP Evinrude, just did a compression check on it and the results were a little disturbing. Top Cylinder- 90, Middle Cyl- 105, Bottom Cyl- 105. Is this acceptable and if not are there any tricks out there that might raise the compression in No. 1 <br /> Also, put a Stingray on the boat this weekend, was having problem coming to plane, but now planes out good and quick but I lost about 6 MPH on top-end. Is this just a matter of trim adjustment or am I going to lose top-end for for quicker planing? Didn't mess with trim while I was out..

 

[12Footer]

Re: Acceptable Compression

The compression seems a tad low on all, but within 15PSI is acceptable. Try running some decarb thru it, and re-test.<br />It wouldn't hurt to play with the trim a bit,by lifting it one notch. The stabilizer should not scrape-off that much speed.In fact, in most cases, it adds a bit of MPH, by keeping the hull drag constant,and trim and wieght redistribution can adjust where the hull planes. But if the compression is low, the HP will not be up to advertised spec iether.

 

[stikman63]

Re: Acceptable Compression

I lied... Compression on No # 1 is a little less than 80.. I sprayed some Deep creek in the cylinder and it rose to 90.. I went out a little later and rechecked and it read 60 on 2 seperate checks..

 

[stikman63]

Re: Acceptable Compression

Should the compression be checked on a warm engine or does it matter?

 

[G DANE]

Re: Acceptable Compression

Hi Stikman63<br /><br />Usually compression should be taken at warm, as this is the working condition. However a motor in good shape will show correct compression figures cold too, but a little bit lower. As your motor got some years on it if it wasnt rebuild, it could be a very good idea with decarb, carbon can make rings stuck and build up around rings in pistons can make rings leak comresssion by. The age doesnt always affect compression, i have a 1973 20 Hp, never rebuild, 1000´s of hours, it still shows correct compression, even cold. Try to do a decabs ( do a search on subject ) and follow instructions carefully. If compression still low - how is your headgasket - are one of the pluge very clean ? How was you aware of this issue, does it not run properly ?

 

[stikman63]

Re: Acceptable Compression

Believe it or not all plugs look the same. The boat runs a little rough at idle and at times tends to die. Was doing a general checkup on it when I found the compression this way. Also I went out later and after letting it set awhile and got a 105 reading out of the weaker cylinder. I then rechecked and got a 90.

 

[what200]

Re: Acceptable Compression

spraying oil in the cylinder to raise the number does not give you a true result. If using oil does raise the number then that usually shows worn rings or some cylinder scoring. You might want to pull the heads to check the condition of the cylinder wall and piston, but 60 sounds like time to freshen the motor. One thing is that you should find the reason that cylinder is lower than the others.