1972 Mercury 402 40HP boggs down at full throttle

[dannyaustin]

Hey guys,

I have a 1972 Mercury 402 40 HP that runs great at idle, but when you hammer down on it, it boggs down and goes very slow. Some people are telling me that it sounds like the high speed jet but if om not mistaken, there is only one jet for both low and high speeds. Here is a drawing so you can see the internals of my carburetor. the only one i see is item #30 on the drawing.http://www.crowleymarine.com/mercury_parts/460/6.cfm

I have also noticed that at full throttle when it is bogging down, if you cut the wheel sharply to the right or left, the motor winds up and sounds like its supposed to at full throttle. Then it boggs back down when the boat levels back out. Hopefully this info will help in diagnosing the problem.

Recently replaced items:
float
needle and seat
plugs

 

[crappiejig]

Re: 1972 Mercury 402 40HP boggs down at full throttle

Item 30 is your high speed jet. Item #27 is your idle mixture screw,low speed if you will. Was the carb properly cleaned when those items were replaced? If so I would look at the float level adjustment.

 

[cell]

Re: 1972 Mercury 402 40HP boggs down at full throttle

you might have to adjust the mixture screw while out on the water.I think it should be screwed all the way in lightly and unscrewed out at 1 1/2 turns out,then goose it.If it fails 1/4 out more then goose it....ect.

assuming everything is good with the carb.

 

[dannyaustin]

Re: 1972 Mercury 402 40HP boggs down at full throttle

I did have it out on the water. I had my brother driving while i was messing with the adjustment. I screwed it all the way in, backed it off a turn and a half and had him hammer down. kept backing off 1/4 turn til all the way out, pausing in between each adjustment to let motor catch up to each adjustment. Still no improvement.

Yes everything was clean when I reassembled the float and needle. I also ran into the problem of the float arms not being adjusted right and gas coming out of the overflow hole. I have fixed that problem by bending the arms corectly so that the needle will seat.

Can I just pull the high speed jet and clean it, or should it be replaced?
On the parts list in that link I gave, there are 4 sizes, any recommendations on a size to get if it needs to be replaced.

Also, there is a small gas leak coming out of fuel pump and I've noticed air bubbles in the in-line gas filter. Are these 2 issues linked and could they be part of my problem( i've got a fuel pump rebuild kit on the way to stop the leak)

 

[dannyaustin]

Re: 1972 Mercury 402 40HP boggs down at full throttle

Anyone have an idea of why the motor picks up when making sharp turns? Could this be linked to the fuel pump leaking?

 

[cell]

Re: 1972 Mercury 402 40HP boggs down at full throttle

On the parts list in that link I gave, there are 4 sizes, any recommendations on a size to get if it needs to be replaced.
[/QUOTE

Different sizes for different altitudes.

Get the fuel pumps replaced,then post back with results.you will most likely have to do the carb adjustment again on the water...so have a 6 pack ready for your friend

While turning it could be going up in rpm up due to cav with the prop.

 

[dannyaustin]

Re: 1972 Mercury 402 40HP boggs down at full throttle

I put the new gasket kit in the fuel pump,it seems like it is still acting up with the muff on but i haven't taken it out on the water yet to really tell. When you say "cav with the prop" do you mean air cavitation? I did fail to mention that the prop is pretty worn. its got some cunks missing on the blades. So the motor's winding up on turns due to the lack of back pressure on the prop?

 

[dannyaustin]

Re: 1972 Mercury 402 40HP boggs down at full throttle

Well, good news! I took the boat out after work yesterday to do some bowfishing, and it ran great! I guess the fuel pump was the culprit. When I overhauled the fuel pump, I also put a new prop on it. Between the new prop and the rebuilt fuel pump, you can tell a huge difference! I runs so smooth and so quiet. I had the boat loaded down with a generator, full cooler and 3 healthy sized buddies plus myself and it still had a sick hole shot and was skootin' across the water pretty good at wide open throttle.

Thanks for yalls help!

 

[cell]

Re: 1972 Mercury 402 40HP boggs down at full throttle

Good news


Make sure your running in the right wot rpm for your model.