Carb guys and/or fuel issue
 

Guys,

Need a little advice - first, the back ground.

Twin carbed HP500s. Port engine running fine. Starboard this year, had seal in belt driven water/ fuel pump go out. While rebuilding, noticed scuffing on fuel pump cam arm due to lack of lube/ water contamination. I'm in there, so might as well replace. New fuel pump installed.

Put the boat in the water and engine runs fine up to 3200rpm. Even barely trying to bump throttle beyond that, the engine acts like its running out of fuel (sneezing, dying, loss of rpm). Pull back on throttle, engine comes right back up to rpm.

Thought I might have a partially plugged fuel filter. Changed those out. No luck. Thought maybe the brand new fuel pump was faulty, swapped out, no luck.

Thinking it through, the fuel starvation issue comes right at the point the secondaries in the carb should be coming into play to increase the rpm. So, pull the carb expecting to find a float bowl needle stuck, partially plugged jets, or debris somewhere in the bowl or carb passages. I didn't find anything like that, everything looked clean and functioned properly. I went ahead and rebuilt the carb, but am at a loss to explain the fuel issue. Boat is now in storage for the winter, so I can't run it to determine if I mysteriously fixed it with the carb rebuild and just missed something.

I thought through possible debris in the tank, but engine runs fine all the way to 3200ish rpm. Repeatable every time no matter what conditions of fuel level or wave action rocking the boat (like LOTO). I was thinking maybe my ignition module could be whacked and not advancing timing, but if I remember correctly, the Thunderbolts are full in by 3000rpm and engine accelerates fine. It will even initially go past 3200rpm, but under steady cruise, will eventually start to starve for fuel. I back off the throttle to where engine is < 3200rpm, engine runs great. Give it a little more, starts dying...

Any thoughts or suggestions from the fuel system experts?

1st thing to do is hook up a fuel pressure gauge at the carb, see if you are maintaining pressure when falls off, you will know quick, if pressure is ok swap carbs and see if follows the carb, then you will know where to look..Rob

I agree with above.

How did the lobe (in the sea water pump)and thing-a-ma-bobby (wear ring?)that rides on it look ? I ask since you said the fuel pump arm was scuffed, plus people have had to replace that in the sea water pump due to wear.

http://www.mercruiserparts.com/Show_...sp?dnbr=823293 92&ivar=images/COMMON/47532.png&inbr=1616&bnbr=270&bdesc=SEA+WATER+PUMP

http://www.mercruiserparts.com/images/COMMON/47532.png

I agree with SB. The eccentric on the shaft (part # 15 above) may be worn not giving you enough action on the fuel pump lever.

Bad coil

Wear ring on the pump cam shaft lobe looked fine. No pitting, scuffing, etc. Looks like normal camshaft operation.

Good thought... the cam lobe, however, is protected by part #16, a wear ring, which all parts look fine.

I can rev engine, on the hose with no load, past 3200rpm (ie no load, lower fuel requirements).

Means nothing other than valvetrain is not creating this 3200rpm deal..

No load = extremely little air/fuel/ and spark needed. Like ...enough to support 25hp. That's it.

Pull the fuel pump back off and verify that the return spring on the pump lever is still there.

Mark