'91 4.3 mercruiser is weak especially out of the hole

[irishfootballboy]

91 monterey 189 bowrider with 4.3 mercruiser. boat has always been reliable but never a speed or power demon. as far as i know this is the least powerful version 4.3 with 175 rated and about 155 actual. best ever top speed was about 38. now i struggle to get to 30. and it is terrible out of the hole. with 4 adults in the boat and no one behind this past weekend could only hit about 12mph out of hole and took a minute to get up on top and plan out. then about 24 max. recently i had issues with it sputtering after about an hr and a half. would finally get to the point it would shut down or wouldnt go more than 10mph and 1500rpm. finally founhd water in fuel and that issue is gone. has clean fuel tank, all new fittings and hoses, new fuel pump, running 5-6psi fuel pressure, new plug wires, new distributor(not sure about ignition coil). carb was rebuilt. today i checked compression and all are between 146-150. didnt think too bad for a 30yr old motor. have new plugs ordered. engine cranks great and runs smooth but seems sluggish and weak. i have been running regular 89 octane so i knew i was getting non-etanol. could that be the issue? if so could i just continue with that and add octane booster or would be better to go to 93? i had planned to add booster next time out but not sure that will make the difference im hoping for. just looking for a good solid pull out of hole for tubing, wakeboarding, etc with 4 or 5 people in the boat. any ideas? thx!

 

[Bondo]

carb was rebuilt.

Ayuh,..... Welcome Aboard,...... Sounds like the fuel system needs further tweekin',.......

Could be carb tunin', or it got more water in the fuel,.....

 

[achris]

Sounds like a propeller size problem.

What RPM were you getting to when it was all running 'right'? And what are you getting now?

BTW, higher octane fuel is not 'more powerful', it's just better at resisting fuel knock, which means engine designers can run slightly higher compressions. And higher compression (with other things) is what gets more power... In fact, there's a very good case for the fact that higher octane fuel creating less power in an engine designed to run 89. One characteristic of high octane fuel it that it burns slower, so by the time all the fuel is burnt, the piston is that much further down the cylinder and less able to use the later amount of energy released in the burn. Often see reports of car users putting high octane fuel into their tanks, expecting more power and better economy, and actually getting less power, and burning more fuel. If the engine is designed to use 89, use 89, nothing more, nothing less.

Chris......

 

[nola mike]

Also make sure that all cylinders are firing, esp at high speed. I was able to track down a high speed coil miss by putting a timing light on the coil wire. Might want to do that on the plug wires.

 

[Rick Stephens]

Things I would check, if was mine:

1) I'd take the boat out light, just you in it, and see what RPM and what GPS speed you get. (use a GPS phone app if you don't have a GPS)

2) After running for 5 minutes, shutdown and pull a couple spark plugs - the easiest to get to. Take pictures of them. See if they are lean or rich looking.

3) Run a vacuum gauge on that motor. Just on muffs. See what your vacuum says.

4) Use an electronic advance timing light and see what kind of advance you are getting at 2500 RPM.

5) Pull the fuel water separator and dump in a clear container. Make sure it is still 100% clean. Takes very little water in a carb to screw it back up.

 

[Scott Danforth]

make sure you are running on all 6 cylinders.

also check to make sure you didnt drop a flapper which could be blocking the exhaust