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

Agreed - it's there.

Per earlier suggestion, will just Tee into fuel line to check fuel pressure supply. That eliminates the pump.

A weak pump at idle will still fill both bowls and allow you too get her on plane.. maybe.. But I must agree that either you fuel gauge them or swap carbs from one to the other ..

Best to do that then to throw parts at her. If no tachometer bounce is noticed during the condition stay with the fuel diagnostics.

Hopefully there are some days for you there to take her out and try all that's been recommended by everyone.

just a dumb and out there thought but when you had stuff apart you didnt by accident hook the vent hose to a vacuum source? just asking as I have seen that before and if you pull a vacuum on the pump it will lay down and not pump much.

I went through a very similar problem this summer. When diagnosing with a fuel pressure gauge, remember that pressure does not equal flow. In my case, I was able to maintain between 5.5 and 6 psi while still starving the engine for fuel. It turned out to be too much restriction caused by the fuel filters at the carb inlet. They were not the correct ones, collapsed and severely limited the amount of fuel that could flow through.

On the dyno I ran to 6000 rpm fully loaded and in full marine trim, but was using the dyno's fuel pump. When installed on the boat with no changes other than fuel pump, I could not get over 3100 without starving. The aitrtex pump I used (I thought it was a merc equivalent) was not able to push enough fuel past the filters - Imagine trying to push a mouthful of water through a stirring straw, you can have quite high pressure with very little flow coming out the end.

Ultimately, I wound up buying a new merc pump, have the Keith Eickert base but not installed, and upgraded all the lines to -8AN (and removed the incorrect filters) It seemed to do fine with the merc pump and new lines so far without adding the upgraded bottom half.

Since you only have the problem on one engine, the line size is probably okay. Have you checked to see they are all clear from the pump to the carb? Are the filters in the carb okay? Partially collapsed? Lines clear? Could the fuel water separator be restricting the fuel available for the pump to push to the carb?


Ryan

i would switch the coil from the other engine,if the problem goes away you are golden,if not you ruled out a coil problem.

I've also seen fuel hose collapse under suction if it is deteriorated from ethanol, etc.

I had a -16 supply one time that a spider built a web in. Motor would start and run pretty good go to get on it and it would nose over.

really should not be running small filters at carb, larger ones before/after pump