Compression questions

RMedus

Cadet
Joined
Apr 27, 2003
Messages
18
ARGHHH....I got the lower unit repaired but the motor doesn't want to run very well. I did a dry compression check on all 3 cylinders and all 3 were low, My battery is a little on the low side, could that cause low compression readings?<br />I took out all three plugs and squirted some oil in the cylinders and all 3 came WAY UP. See readings below.<br /><br />Cyl <br />Dry W/Plugs in <br />Top 85 <br />Middle 83 <br />Bottom 90 <br /><br />Wet w/ plugs out<br /><br />Top 125<br />Middle 150<br />Botttom 145<br /><br />Keep in mind the battery was low as well so it spun faster on the wet checks. <br />What should I do now? Have it rebuilt? Shoot the guy I bought the boat from? (I recently bought the boat and the seller said the motor ran great.) I have already had to clean out 2 of the carbs and replace fuel line to the carbs.<br /><br />HELP!<br /><br />Ron.<br />The Fishin Musician
 

Hooty

Rear Admiral
Joined
Oct 2, 2001
Messages
4,496
Re: Compression questions

The top cylinder looks low but it may just need a de-carb job. Go to "Search" and look for "de-carb" or "decarb". Easy to do and can make a big difference in the way an engine runs.<br /><br />g'luck & c/6<br /><br />Hooty
 

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: Compression questions

It would be a good start to tell us what engine you are talking about, Ron.<br /><br />Dry test on a cold engine doesn't mean much. Numbers were close enough on all three.<br /><br />Warm the engine up and retest with all plugs out and throttle fully open.<br /><br />I wouldn't do anything else until I had done a thorough decarbonization (yeah, I know that is a long word, but a lot of folks don't know what decarb means. :) )<br /><br />Let us know the results you get.
 

petryshyn

Commander
Joined
Oct 3, 2001
Messages
2,851
Re: Compression questions

don't even consider your results without a good battery.... :(
 

RMedus

Cadet
Joined
Apr 27, 2003
Messages
18
Re: Compression questions

The engine is a 1978 evinrude 75 HP I have the battery on the charger and will check the compression again when I have a full charge on the battery. <br />Now....can you reccomend a product for decarbonizing? I have heard seafoam is good but I haven't been able to find any in this jerkwater town I live in. <br /><br />Thanks.
 

Hooty

Rear Admiral
Joined
Oct 2, 2001
Messages
4,496
Re: Compression questions

Seafoam is good and NAPA handles it.<br /><br />c/6<br /><br />Hooty
 
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