3.0 liter 140 hp Mercarb 1389-9562 rebuilt but no power, lacks speed, plugs with carbon

Hi

I rebuilt my Mercarb 2 barrel and the engine has no power, it takes forever to plane the boat and I noticed fuel spots on the transom from the carb. The engine misses also.
The engine ran fine before I rebuilt the carb. I noticed the Power Piston Assembly (has a long rod with a spring over it) was loose when I tried to get the piston to move more freely.
I did not think much of it until I tested the boat.

How is the Power Piston Assembly and the Power Valve Assembly suppose to work, I believe the Power Valve Assembly is suppose to give the engine more gas when the engine starts to accelerate, but how does it do that. My RPM max is around 3900, about 400-500 lower than it should be the boat is a Sea Ray Seville and it only goes about 39 MPH normally it should be 45 MPH..

I really think the Power Piston Assembly is the problem, any comments?

Thanks for your help

The over amount of fuel leads me to think the float is set wrong, or the needle is not seating.

There are different needle designs, did you use the same as what was in there? The float setting is different for each type, one needle assembly has a spring the other does not.

Also did the needle have a small spring/wire attached to it?

The power valve functions from vacuum, when you are running at high rpms the valve will open based on the spring (different colors would represent different compression forces) which translates to inches of mercury (Hg) to over come the spring to allow more fuel, when you "nail" it, it will close as there is no vacuum.


The fact that you see excess fuel means something is letting more fuel thru the carb than it needs.

Start with the choke........is it fully open when warm?
then recheck float level.

Also what year is the boat/ motor serial number?

we need that info to see what carb you are speaking about.

Thanks!!

I have the original float needle and seat or valve body, so I can replace them.

The engine is B487501

I forgot to say that after I rebuilt the MerCarb and I still had the same problem, I replaced the original mechanical fuel pump with a Holly 712-8011 Red Electric Marine it puts out 5 psi
I hope that's not to much.

I still have the old Power Valve, should I put it back in, also the Power Piston is loose should I stake it, it is sticky too. The choke is open when warm

Had trouble posting,

I have a spring with the needle

Do you think the Holly pump may have too high of a pressure and going around the needle valve,

I think the mechanical pump was good, I found my original problem, it was bad check valve on the gas tank !!!

sorry for the second post