1973 mercury 402 runs very rough when throttleing up

TJ Moore

Recruit
Joined
Aug 6, 2016
Messages
1
Hello Everyone,
I am completely new to this forum and new to boating. I picked up an old boat with a 1973 mercury 402 (serial- 3594641). It had good compression, but I had to put a new water pump, trigger, and carb rebuild. I got it running and it idles very smoothly. However, when I take it out to the lake and give it throttle it runs extremely rough then dies. It will then start up again and idle just fine. Any help on this issue would be greatly appreciated. My thoughts are the main jets? If the thought is the jets, is there a way to determine the size the main jet should be?
Thanks again in advance for any feedback.
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Acceleration is provided by the low speed jets coming off idle and getting up to moderate rpms. Low speed jets too lean (screwed in too far) will limit the available fuel as the rpms increase, especially under load, like getting the boat to plane out and the engine will stall. Back it/them out a little at a time 1/16 turn, till you have been out ? turn over preset which is 1 ?-1 ? area. If you get out that far and still no help then you have fuel limiting problems elsewhere. In running your tests, tweak and goose. When the engine starts to die, cut the throttle back to idle, let it recover, open more and repeat. No sense in waiting till it dies to cut the throttle. Your low speed needle valve should be a perfect cone, no deformations of any kind.

The non adjustable high speed jet will/should be a brass pellet somewhere in the carb or the bowl with an obvious means to remove it from outside the carb/bowl, like a brass hex nut looking thing. It has to be clean and varnish is a good reason for it to be clogged. Other thing is a small piece of debris. I'd remove it and put it up to the light ensuring that you can see through the tiny hole then reinsert and blow out with comp. air.

The venturi tube and other small tubes should be run through with fine wire after soaking in a good carb cleaner (Berryman's Chem Tool....most auto parts stores is my favorite....dries out cleaner for me than Gunk brand). After that, one more shot with cleaner then a good dose of comp. air.

Last, ensure that you set the float to the correct value per instructions in the kit. If no kit instructions and no manual, turn the carb upside down with the bowl and needle valve intact with the float arm resting on the closed needle valve. Bend float arm to position float body parallel to bowl mating flange on carb.

Give that a go.
 
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