Fuel Pump Diaphragm

[TriadSteeler]

'74 Merc 500 4cyl two carb setup.

I am having the same issues as others with running the fuel lines dry.

Pumping the bulb springs the motor back to life. I have rebuilt the carbs and the fuel pump.

I'm thinking this is either a stuck float/needle or the fuel pump.

When I first connected everything up, I messed up and connected the fuel pump backwards. The motor obviously wouldn't start, but I cranked it over 5 times, about 5-6 seconds per try. Would that destroy the FP diapragm/check valves and/or anything in the carbs. I just want to eliminate the easy stuff (fuel pump) before I tear the front end back apart to get at the carbs.

 

[hkeiner]

Re: Fuel Pump Diaphragm

Mercury issued a Service Advisory (#2001-17) that provides some troubleshooting guidance on fuel starvation. All you need is a vacuum gauge, a piece of clear hose, and a t-connector to do the troubleshooting. The tests help confirm (or eliminate) various common causes of fuel starvation, such as a failing fuel pump, a restriction in the fuel line upstream of the fuel pump, or an air leak in the fuel line. Once you confirm (or eliminate) the various likely causes, you can then limit your repair effort to what is most likely wrong and avoid spending time and money on those components that are OK and not causing the problem. Systematic troubleshooting is how I would approach the problem anyway.

Good luck



http://www.marinepartsman.com/Mercury-Marine-service-bulletins/Mercury-Mariner/2001/EN_17.PDF

 

[HTG]

Re: Fuel Pump Diaphragm

I have that problem when running on low fuel.

 

[TriadSteeler]

Re: Fuel Pump Diaphragm

I think I know the problem. However today was one of those days when I should have put down the wrenches and walked away.'

When I reassembled the fuel pump, It was before I ordered my torque wrench. I didn't tighten the bolts all the way and said I'd do it once the torque wrench came in.

Needless to say, I forgot. The pump must have been sucking air from the gaps in the gaskets.

So I pulled out the torque wrench, set it to 85in/lbs, and started torquing. All of a sudden it was really easy to turn. On the bright side, (It broke in the middle. I was able to take the other two bolts out and pull the pump off and get the rest of the bolt out. It was here that my assumption was correct. The gaskets and diaphragm were all wet with gas and wrinkly on the outer surfaces. At least that is a cheap kit.

Now for the questions for the experts. The bolts that were in there are obviously different lengths, but they are 8mm and there were little plastic spacers about 1/4" long that the bolts go thru that sit in the pump housing. They kinda looked homemade. Are they supposed to be there? and are the bolts supposed to be 8mm?

Looks like another trip to fastenall.

 

[Faztbullet]

Re: Fuel Pump Diaphragm

Should be 10-32 screws torqued to 10-15 inch lb, ...1/4-20 would go at 85 in lb

 

[TriadSteeler]

Re: Fuel Pump Diaphragm

I have searched high and low for those bolts. I can't find them anywhere. I even special ordered some from fastenall (The guy measured the originals and they were definately metric 8-32 X 55mm, 50mm and 30mm), but they won't thread in.

As far as I can tell, those holes do not go thru to the block. What I am wondering is it possible to tap out those holes? I don't want to crack the block. I was planning on using the same diam tap with 32 tpi. Would this be advisable, or will I create a monster.

EDIT: Never mind. I ran the 8 X 32 tap in there and it threaded easily and cleanly. The guy at fastenall must have mis-measured the originals. They do look awful close, but they just don't fit.