Low to mid engine stumble when given throttle. Mercarb 4.3l

[Mariah180]

I just rebuilt the mercarb on my 4.3l. Also a new fuel filter. The engine hesitates on application of throttle then takes off. This occurs from idle to 3000rpms. Accelerator pump arm, I've tried all three arm positions.

I looked at a online mercarb rebuild video and it showed a screen and check ball underneath the spring that pushes on the accelerometer pump. My mercarb model did not have the screen or checkball underneath the spring. Did not look like it has a place for a check ball either.

Prior to carb rebuild the engine was running terrible under load or accleration. If I worked the throttle correctly I could pump it up and get the boat on wake and it would run full throttle and full speed. But once it was shut off and I tried to get back up on plane it would stumble and hesitate and might accelerate or not. Nothing in the carb looked dirty during the rebuild. Everything looked good (in my novice opinion) :cold:

About to place it in the lake and run some new fuel through it. :watermelon:

 

[Bifflefan]

Re: Low to mid engine stumble when given throttle. Mercarb 4.3l

Mercarb (if its like mine) only has one idle mix screw, I'm assuming its a 2 bbl carb.
You need to turn the mix screw out about 3.5 to 4 turns as a starting point. This is because you are feeding 2 holes with one passage, thus you need twice the flow through the one passage.
Right now you are running to lean and that is causing the stumble.
Just crank out the screw a few turns and retest it. Once you lose the stumble, go a half turn more and call it good.
I think 4 to 4.5 turns out from lightly seated should be about right.

Also, recheck that you did the float level correctly. Read the instructions and don't try to eyeball it or go from automotive knowledge. That just doesn't work.

 

[Mariah180]

Re: Low to mid engine stumble when given throttle. Mercarb 4.3l

Just bad fuel. It wouldn't come above 2500rpm in the lake. But after several minutes it finally came up on plane but not full rpms. After 20-30 minutes of hard use the engine is now running normal. Guess the bad fuel took some time to burn out of the system and clean fuel mix in.

 

[Mariah180]

Re: Low to mid engine stumble when given throttle. Mercarb 4.3l

I did turn the screw out 1/2 a turn. I will tr another turn on the settings. Thnaks