Low WOT MERCRUISER 5.7l Thunderbolt ignition

[Profittakr]

I just bought a 1992 Sea Ray 240BR . Took it out last week and it ran poorly, coughing and sputtering and not much power. It only ran at 3200rpm at WOT. I took off the Rochester Quadrajet 4bbl carb and rebuilt it. It was very dirty and gummed up so I figured that was the problem. I reinstalled the carb tonight, replaced the fuel water separator
​​​​and took it for a run. It runs smooth and predictably but it still only runs at 3200rpm WOT. I know it should run at 4200-4600rpm. The Alpha one drive is spinning a 19p prop which I believe is the recommended size.

What could be the reason for the low WOT? Carb problem? Prop? Ignition? Other?
Could the 4bbl not be opening properly and not giving enough fuel? I will need to check that with someone else in the boat. Suggestions?

What top speed should I expect out of this boat? I think I am only getting about 30mph now. I would have expected closer to 40mph.

 

[thumpar]

Did you take the carb completely apart to wash it all and blow out the passages?

 

[Profittakr]

Yes, all torn down, cleaned and reinstalled using new carb kit.

 

[Scott06]

Probably in this list in the stickies https://forums.iboats.com/forum/eng...6-list-of-possible-causes-of-low-wot?t=295070

 

[Profittakr]

Thanks how would I test #6 and #8? Proper gear ratio and restricted exhaust?

 

[dubs283]

Thanks how would I test #6 and #8? Proper gear ratio and restricted exhaust?

The gear ratio should be stamped on the port side of the drive upper housing

You can remove the exhaust boots and drop tubes from the risers/y-pipe and look into the y-pipe for blockage and the water shutters. After the y-pipe its into the exahust bellows and the drive - pull the drive and inspect the bellows and drive exhaust cavity

 

[Scott06]

Drive ratio probably isn't an issue in this case your boat should have a 1.47/1.5 or maybe a 1.62 if it's a heavier boat. This is more for the higher ratio smaller motors like 4.3/3.0 where some one could have swapped a lower drive and the motor won't have enough power to get it up in rpm.

Hes got you pointed in the right direction on the shutters. Over heating can damage them where piece break off

 

[Profittakr]

I will check out the shutters and the gear ratio this weekend although I am not aware of any overheating issues in the past and it runs between 160 and 170 now and I also believe the drive is the original as well.

I also called the local marine store/mechanic shop and he immediately said that the prop was WAY too big for that boat. He suggested dropping down to a 15p or a 17p prop assuming that the motor is running well and has good compression (something else I will check this weekend). Looking at a 'prop selector' website, 15,16,17 does seem to be a reasonable size.

I believe that I had read that this boat should have a 19 pitch on it and that is what the previous owners had used as well but they also complained of low top speed. They said they were only ever able to get about 30mph out of her. I hate to go buy props on a shim and keep trying until you get it right but this mechanic said that would be the 1st thing he would be looking at. Thoughts?

 

[Scott Danforth]

the 5.7 should have no problem spinning a 19P prop on that boat.

however I am not trying to sell you a prop.

your down on power

check the compression, check the shutters, and the fuel system

if the drive was swapped, only the very short lived 1.32:1 alpha from behind the BBC's would dog your 5.7 all other gear ratios are 1.47 or higher. the 1.47:1 is correct, and any of the ratios from a 5.0, 4.3, or 3.0 would make you bounce off the rev-limiter not lug your motor.