50 hp 4 stroke starving for fuel

[HonkyDoryFishingTeam]

I have a 2000 Merc 50/4 stroke. I just rebuilt the carbs. It has fresh fuel,new fuel line to motor, racor fuel/water separator, sufficient fuel in tank, fresh plugs, etc.. It runs fine to 3/4 throttle, but when you pin it, it starves and quits. It restarts easily. The primer ball is not collapsing.(indicating restricted fuel flow) I tried pumping the primer ball while advancing to WOT and same thing. I water tested it on the trailer on a ramp. WOT in reverse yielded 5500 smooth rpms(not as much load in reverse). WOT in forward caused it to starve. I suspected the fuel line from the fuel pump to the #4 carb(which then supplies #3,2 & 1) is collapsing because ethanol has softened the inside to a point in which it collapses in on itself, however, I cut this line in half, pushed a screwdriver in it, twisted it a couple times and removed it and it came out clean. I expected to see gummy black stuff on the screwdriver blade and did not. Whatta you think????????

 

[fastback4]

Re: 50 hp 4 stroke starving for fuel

I would first keep it simple and disconnect the racor fuel/water separator even if it ran fine before, install a new primer bulb and drain the carbs at the drain plug on the bottom of the bowls if possible. Check the gas supply line in the gas tank if possible. It should be made of hard plastic or copper and not made of rubber fuel line. If it's rubber fuel line inside the tank, it could be collapsing closed when you nail it. .......Tom

 

[HonkyDoryFishingTeam]

Re: 50 hp 4 stroke starving for fuel

I'm using a portable 6 gallon tank for testing. If there was a restriction on the negative pressure(vacuum) side of the fuel pump it would collapse the primer ball. Now what?
Thanks!

 

[fastback4]

Re: 50 hp 4 stroke starving for fuel

True it would be collapsing. Say you rebuilt the carbs? Is the high speed jet installed in the right place. When it sputters and quits on a WOT attempt can you feather the throttle back to 3/4 throttle and maintain the motor to run? Can the motor achieve WOT if you gradually work the throttle to get it there? I'm wondering now if you have the float levels in the carbs adjusted properly. With the carb held upside down the floats should be level with the carb housing seating surface.
Check this out also. Some Mercury Marine bulletins i came across.
http://forums.iboats.com/attachment.php?attachmentid=26608&d=1240244937

 

[HonkyDoryFishingTeam]

Re: 50 hp 4 stroke starving for fuel

Thank you FB4 for your response.
The motor will clear up if you back out a little. I'm the 3rd mechanic to look at this thing, so I'm trying to pick up the pieces. Everyone has claimed to rebuild the carbs. One guy says he replaced 2 of them(for reasons I can't imagine or verify). The float levels do not appear adjustable because the floats are plastic, even the piece between the floats where the metal tang you would normally bend to adjust float height is,is plastic. I'm scheduled to work on this again tomorrow. I'm replacing the fuel line from the pump to the carbs, just to eliminate it, but I don't think this will do it. I considered vacuum leaks, but at WOT there's no vacuum anyway. All fuel lines are clamped securely, so I don't think it's sucking air at the fuel pump(on the vacuum side). I'm considering running WOT till it quits, without restarting, then pulling the bowl drains to see if it is sucking the bowls dry.
Your turn..........
Wayne

 

[fastback4]

Re: 50 hp 4 stroke starving for fuel

Checking the bowls i would consider but i asked if you could gradually increase the throttle to achieve WOT, and if not what rpm range can the motor achieve before it quits. When it quits I would then pull the spark plugs and check for condition, then drain the bowls.......Tom

Forgot to ask....Are the fuel floats level with the carb housing and if not do they have fuel in them?

 

[HonkyDoryFishingTeam]

Re: 50 hp 4 stroke starving for fuel

As I recall, even with a gradual ramp up to WOT in water,on trailer would cause it to starve and die. I'm unsure of rpm's, I was advancing the throttle by hand at the engine.( couldn't see the tach.) As I put the carbs back together I took note of the float position when the carbs were upside down and made certain the valves opened and closed freely. The floats were parallel to the gasket sealing surface for the bowl. The valves were ok.

In my first post I noted the motor ran at WOT in reverse, but not in forward.
It was cavitating at about 5500 rpm's, but I did this for a reason. I know it's not as great of a load due to slip and I wanted to see if it was throttle position or load or rpm's that caused the stall. Moments later I tried it in forward and it starved out. I was nowhere near 5500 rpm's yet, due to a good bite. It only acts up when you go for that last 1/4 throttle. I'll get better numbers tomorrow.
Thanks a bunch...........

 

[fastback4]

Re: 50 hp 4 stroke starving for fuel

What are you pushing with this motor. What is the shaft lenghth of the motor. What is your prop size. I'm checking to see if there is a rev limiter relay on this motor.........Tom

 

[HonkyDoryFishingTeam]

Re: 50 hp 4 stroke starving for fuel

I did not get to work on it last night, but am doing it now.
It's a short shaft on a pontoon. Not sure of prop size at this time. I'm gonna put it in and try it again. I be back w/ details.

Thanks again,
Wayne

BTW. This motor has a restrictor plate between the carbs and manifold just like a NASCAR would. It reduces the hole by about 1/3. The owner said the motor was a "program motor". It was used by an outfit in Canada for 1 year, then Merc bought it back, serviced it and sold it with warranty at a reduced price. Could the restrictor plate be to detune it a little so the first user wouldn't hurt it or is it to detune a 75 to a 50(for example)?

 

[fastback4]

Re: 50 hp 4 stroke starving for fuel

This story gets better all the time...lol.....I want some pictures. Short shafts usually are not used on pontoons. This configuration is a bit baffeling. Who's certain what Mercury engine this is. Yes, a restrictor plate like Nascar uses decreases the full output of a motor. Question is what were they researching. Need that prop size. Check the plugs appearance after it stalls out. I still believe the motor is over reving.....Tom

 

[HonkyDoryFishingTeam]

Re: 50 hp 4 stroke starving for fuel

My mistake on the shaft length, I guess. I measured about 22 inches to the cavitation plate. If a long shaft is 25, I assumed this was closer to 20 so I thought it was short.

The good news is it runs like a sewing machine now! I replaced that fuel line from the pump to the carbs. I also ported the restrictor plate to the intake runners. The owner said it used to at 5300 rpms at WOT. Today, I got 5900-6000 rpms.(no cavitation) It was also a couple of mph faster, as you would assume with the increased rpm's. It performed flawlessly.

My guess is the fuel line was defective due to ethanol and couldn't keep up to WOT demands, and porting the restrictor plate gave a few extra ponies.

I cannot guess what kind of research they did with the motor, if any. It kinda sounded to me like a demo deal. The outfitter in Canada gets new motors every 12 months. They must be dealer serviced and maintained during this time. At the end of 12 mos. he gets new motors and Merc gives him trade-in value for his 12 mo old motors. Then Merc services them and resells them with a warranty. At least, that's how the story goes. I can't verify that.

Lastly, is 6000 rpm's too much for this motor? It never sputtered like it was against the rev limiter, but I have no literature on this motor.

Thank you for your time and effort helping me with this job. I may have gotten frustrated too soon. I shoulda' trusted my instincts first, then cried for help.

All the best,
Wayne...Vero Beach Florida..6 miles from Sebastian Inlet