merc 120 WOT problem back again HELP

[rpatton]

Hi guys i have had a wot problem earlier this year which i thought i had fixed but i was wrong. Heres what it is doing. The motor starts great all the time cold or hot. It runs great up to about 3700 rpms . If i go over it acts like it is runing out of gas in the carb bowl. I mean it will run flat out for about 30 seconds then start to sputterand drop rpms drasticly until i pull the throttle back to idle and let it catch up. Then it will run great to 3700 but when i go over that it dies out again. I thought i had fixed it by replacing the fuel line to the tank. it ran great at all rpms for two weekends but now its doing it again. Does anyone have any ideas. I thought fuelpump but would it not do it all the time rather then intermitinly. Water in fuel? Can the float in the carb change adjustment on its own ? please help this is driving me nuts. 1984 mercruiser 120

 

[achris]

Re: merc 120 WOT problem back again HELP

It does sound like a 'running out of fuel' type problem. What I would do, get a small section of clear hose and put that in the line between the fuel pump and the carb. Then run the engine 'normally, and have a look at the clear hose. See bubbles, you have an air leak. It may not be that, but it's a quick and easy check that cost almost nothing to do.. and can rule out a whole swag of issues. That other thing worth doing, even at the same time, is to put a vacuum gauge on the suction side of the fuel pump. You need to see less than 2" of vacuum on that gauge....

Just some thoughts for you...

Chris......

 

[rpatton]

Re: merc 120 WOT problem back again HELP

Thanks chris i was already going to trythe clear hose trick like you have suggested many times that i have read. Sounds quick and easy. Could this be as simple as water in the fuel or a float coming out of adjustment, if thats even possible. Do you think its worth pulling the top off the carb to take a look?

 

[achris]

Re: merc 120 WOT problem back again HELP

I'd do the clear hose and vacuum gauge test first, less invasive... And it will point you to where you need to look.... It's unlikely to be water in the carb, as water sinks and would go straight into the jets and through to the engine... The float going 'out of adjustment', also unlikely as you'd get a change in performance at all revs, and it would take 30 seconds for the symptoms to display. No, this sounds like the pump can't keep up at high demand.... And that smacks of delivery problems...

Chris.....
Do you have a water sep fuel filter mounted in the line between the tank and the pump?

 

[rpatton]

Re: merc 120 WOT problem back again HELP

I was also thinking pump but why would it straighten its self out for a couple of weeks. Can they be rebuilt. And no i do not have a seperator . I am planing on it though

 

[achris]

Re: merc 120 WOT problem back again HELP

Some can, some can't. Depends on the one/brand you have.... Easier to do the tests first, we'll know where to look then...

 

[rpatton]

Re: merc 120 WOT problem back again HELP

Ok i will test first but i do not have a vacume tester though

 

[achris]

Re: merc 120 WOT problem back again HELP

ok.. Just do the 'air' test for the moment. Do you have a fuel pump pressure gauge?

 

[rpatton]

Re: merc 120 WOT problem back again HELP

By the way i have the old style pump with the filter on top

 

[rpatton]

Re: merc 120 WOT problem back again HELP

No presure guage . Sorry

 

[achris]

Re: merc 120 WOT problem back again HELP

Have a look at this... it's only $20, much cheaper than just throwing parts at the engine, ah? -> Lisle 20300 Vacuum Gauge AND Fuel Pump Pressure Tester | eBay

By the way i have the old style pump with the filter on top

Diaphragm kit is available for the units with the filter on the TOP.... Merc part # -49089A1

 

[rpatton]

Re: merc 120 WOT problem back again HELP

Thanks i will check it out

 

[rpatton]

Re: merc 120 WOT problem back again HELP

UPDATE Chris I ran the motor at wit with a 8inch piece of clear fuel line. This is what I noticed ,at idle there was solid fuel with no bibles that I could see but as I gave it more throttle bibles began to appear and at the 3700 rpm mark it got a huge air bible or lack of fuel and died unless I pulled the throttle back and left it catch up. Does this mean a bad pump or am I sucking air from somewhere. I checked all of the connections and they are tight.I should mention the clear line was place at the carb right above the carb. Thanks

 

[harleyman1975]

Re: merc 120 WOT problem back again HELP

Boy sure sounds like a fuel pump to me and the rebuild is not too diffecult...