engine dies under load

79floater03

Recruit
Joined
Apr 26, 2003
Messages
3
I have a '79 115hp Mercury. When I put the engine in gear (forward or reverse)and increase the throttle it really boggs down. Sometimes it will even die. It does fine at an idle (in gear). It also runs great out of the water, just does this under a load. The lower carb. seems to be leaking some fuel when the engine is trimmed up to the point that the carbs. are at a sloped angle. Any ideas what might be my problem?
 

12Footer

Fleet Admiral
Joined
Mar 25, 2001
Messages
8,217
Re: engine dies under load

Welcome to iboats.<br />It sounds like you have lost one or more cylinders. After running it for a little while to warm it up, touch the head by each plug, until you find one or more cooler than the others. That cylinder is not firing.<br />These inline motors run pretty steady with one cylinder down. Now, you have to find ouit what is missing (fuel or fire).<br />Pull the plug, and it should reveal which is missing.
 

79floater03

Recruit
Joined
Apr 26, 2003
Messages
3
Re: engine dies under load

Checked the cylinders as suggested. It appears that cylinders 5 and 6 are not getting fuel. however fuel will leak from the third carb. when tilted (trimmed up). when i was at the lake and this problem was occuring, i trimmed motor all the way down and the engine started running a little better (still bogged down and sluggish). from what i can figure so far is that when the motor is trimmed all the way down, the carbs are tilted so that fuel would towards the cylinder rather than run out of the carb itself. any further suggestions?
 

Hooty

Rear Admiral
Joined
Oct 2, 2001
Messages
4,496
Re: engine dies under load

I think, at this point, I'd do a compression check. <br /><br />c/6<br /><br />Hooty
 

Hooty

Rear Admiral
Joined
Oct 2, 2001
Messages
4,496
Re: engine dies under load

You'll need a compression gauge. They're available from almost any auto supply. Cost about $40.00. Use the search function of the forum here and look for "compression test".<br /><br />c/6<br /><br />Hooty
 

fluidjuice

Seaman
Joined
Jun 12, 2008
Messages
54
Re: engine dies under load

Hi
i have a 40 hp classic mercury 1991 4 cylinder with the same problem,
the carbs have been cleaned and i have good spark on all 4 and good compression but the bottom two cylinders dont run, engine runs on top two fine when idleing the put it in gear and give it some revs and it stalls every time.Unless you give it fast idle and bang it in gear which isnt the done thing i know.
have you found out what the problem is yet as the marine engineer at my local boat shop said it would cost me hundreds of pounds for him to strip engine to find out what the trouble was and wasnt worth the money to do so.
I really want to get this motor running properly as i think i have been sold a bad engine.
 
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