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92 190 PS
11-15-2009, 09:37 PM
Just bought a fishing/bass boat this weekend. The motor is a 20HP Mercury and it runs relatively well. Starts on 2nd or 3rd pull every time, idles very smooth, runs well under power and compression of 102 and 98. Only issue is the motor lacks power. Seems like it's running at about 25% of it's capacity. It's a 15 foot flat bottom bass boat and the motor only pushes the boat about 7 or 8 MPH on GPS.

I tried to spray a stream of gas into the carb while going max to see if the motor was starving for fuel (thought this would tell me carb/fuel issue) and the boat would cut out as if I'd pulled the choke. Spark plugs look good. Also, tried to push the throttle on the side of the motor manually to make sure the throttle was fully engaging the fuel and it appears it is.....

So questions is what are the next couple items to check? Could it be a bad coil? Linkage from throttle to motor? Suggestions?

Thanks in advanced for helping with my non-mastercraft second boat....

JimN
11-15-2009, 09:47 PM
Just bought a fishing/bass boat this weekend. The motor is a 20HP Mercury and it runs relatively well. Starts on 2nd or 3rd pull every time, idles very smooth, runs well under power and compression of 102 and 98. Only issue is the motor lacks power. Seems like it's running at about 25% of it's capacity. It's a 15 foot flat bottom bass boat and the motor only pushes the boat about 7 or 8 MPH on GPS.

I tried to spray a stream of gas into the carb while going max to see if the motor was starving for fuel (thought this would tell me carb/fuel issue) and the boat would cut out as if I'd pulled the choke. Spark plugs look good. Also, tried to push the throttle on the side of the motor manually to make sure the throttle was fully engaging the fuel and it appears it is.....

So questions is what are the next couple items to check? Could it be a bad coil? Linkage from throttle to motor? Suggestions?

Thanks in advanced for helping with my non-mastercraft second boat....

What is the highest RPM you've gotten under load? Does it sound like it's hitting a rev limiter at any time?

Find out what the normal compression would be- 98-102 seems low and that could indicate bad reeds.

92 190 PS
11-15-2009, 09:52 PM
Not sure what the max RPM is at this point. Today was first trip out and tools were limited. Does not sound like it is anywhere close to rev limit. I'll check on compression specs...

Just checked a couple message boards and it looks like normal compression is 120 to 140. What would be some common issues/indications associated with low compression? My very novice understanding is that as long as the compression of both cylinders is close it's less of an issue than the opposite over time.

Long story short this boat was meant to be a toy to fish with and I don't want to put a lot of money into it. Just wondering if there are some things I can do relatively cheap to get the boat performing better. Not expecting miracles, as I understand the engine is 33 years old...

JimN
11-15-2009, 09:59 PM
Not sure what the max RPM is at this point. Today was first trip out and tools were limited. Does not sound like it is anywhere close to rev limit. I'll check on compression specs...

If it seems like it's hitting the rev limiter, the temperature sensor could be bad or the high speed section of the stator may be toast.

chico
11-15-2009, 10:07 PM
Is it too low on the transom?

coz
11-16-2009, 08:37 AM
http://forums.iboats.com/Mercury-Outboards/

vogelm1
11-16-2009, 02:27 PM
A friend of mine had the same problem with his 35 hp Merc...he was told to check his exhaust (not sure if it was ports, or the tube that takes exhaust down to lower unit) for blockage. Sure enough it was plugged/restricted by carbon build-up. He did do a lot of trolling/low rpm running with it, which may explain why it happened. Just a thought.

flipper
11-16-2009, 02:33 PM
A friend of mine had the same problem with his 35 hp Merc...he was told to check his exhaust (not sure if it was ports, or the tube that takes exhaust down to lower unit) for blockage. Sure enough it was plugged/restricted by carbon build-up. He did do a lot of trolling/low rpm running with it, which may explain why it happened. Just a thought.
Same thing I was thinking. Check at the head too. I've seen them build up right there too.