1974 - Mercury 1150 RPM Drop @ WOT

[Big_Totoro]

Hello!

I'm new to boating, and have bought my very first boat. It's a 1971 Sidewinder. It's powered by a 1974 Mercury 1150.

Engine starts fine, even when cold. From just in gear (barely moving) I can punch to WOT (instant throttle response) and the engine goes up to 5000 rpm. It's all I can do to hold on sometimes. Get up onto plane easy and very quick.

Once I'm just up onto plane the RPMs drop to 3600 rpm. I on;y stay up at 5000 rpm for a second or 3 maybe. I'm running a 17 pitch prop. I've had someone prime the fuel bulb while at WOT and there is no change (good or bad).

It almost seems like it's flooding itself? I ease off the throttle all the way back down to barely moving, but then quickly goto WOT again and it then will bog and slowly climb onto plane and up to a max of 3600 rpm.

I'd appreciate any help. I don't know alot about boats or engines, but I'm not afraid to get in and start pulling things apart if I have too.

thanks!

 

[Chris1956]

Your issues is a bit unique, as those motors usually starve for fuel, rather than flood at high speed.

My first assumption is a restriction...Are there any fuel filters that could cause restriction? That motor should have a banjo filter under the carb covers. Maybe that is restricting flow? The fuel line should be 5/16" ID. 3/8 is better, but was not original.

If that all checks out, check float heights and condition. When you first accelerate, the carbs will empty after a few seconds. The fuel pump needs to fill them fast enough to maintain your RPM. I think some sort of restriction or float adjustment is the issue.

 

[Big_Totoro]

My first assumption is a restriction...Are there any fuel filters that could cause restriction? That motor should have a banjo filter under the carb covers. Maybe that is restricting flow? The fuel line should be 5/16" ID. 3/8 is better, but was not original.

Thanks,

I have a shop manual that shows all the fuel filters etc. I will check those. The fuel lines are you talking about the whole system or the hoses that run from the fuel pump and connect each carbeurator?

thanks!

 

[Chris1956]

The fuel lines that connect the carbs to the fuel pump should be rubber with crimped brass ends. If that is what you see, they are original, and should be OK, provided they are not falling apart. If they are replacements, I would expect to see 1/4" ID brass nipples, fuel lines and clamps. The fuel supply line is 5/16" ID.