If you look back at my previous post Mercruiser did not say the pump was bad but they wanted to sell me a thicker gasket in turn to shorten the the pump arm movement decreasing the pressure.

What is the max pressure these pumps can handle before ??. They are supposed to be explosion proof. Where does the pressure go when holding it back with a regulator. Does the vent tube blow??

Update, I called mercury marine. They called the dealer where I bought the pumps for a return on the bad pumps..They refunded my money. Apparently they knew of this problem. I bought 2 Holley pumps.

the pressure will hold diaphragm and arm up off the pump drive cam, it wont keep building pressure.

the stroke length of any of these mechanical pumps will determine the pressure... so if the eccentric that drives it is slightly different from motor to motor or the boss on the block is machined slightly different then you get different numbers. the thick gasket deal would have been perfectly acceptable presuming the the pump itself truely wasn't bad... which since they noth did it was probably not the case... what this all sounds like is that batch of pumps got designed for and application with a certain x distance from the 0 point on the eccentric to the boss and is now trying to be used in apps where that distance is shorter...

I don't think the stroke length would change the static pressure much, only to the extent of the change in the force created by the spring due to the different compressions.

It would change the volume for sure. So it is possible that would result in lower pressure if the engines requirements exceed the fuel delivery at a given RPM, just like running too small a pump. But that is not exactly predicable. And most likely would do zip at idle.

We did some bench testing on these when we first discovered the problem. If you hold one in your hand you can barely push the pump arm.

I cannot remember the part number with the issue, but it was not a random batch. It was all of them from a new vendor. The PN has since been superceded, the new part seems OK.

geeze mike... are you sure about the spring thing ? every mechanical pump i have ever had apart which DOES NOT include one of these by the way, the arm drives the diaphram directly... the spring only forces the arm to keep contact with the eccentric...

i had a whole bunch of motors in various catagories that required the use of the std mech pump and adjusting the stroke length by virtue of shims/gaskets allowed me to taylor it quite specifically... if you say these pumps are different inside then thats that... but ive never seen an example of it...

Steve it took me a while to figure it out way long ago, but take a take a closer look.

In a mechanical fuel pump the displacement of the arm from the at-rest position is the intake/suction stroke.

The spring, which is above the diaphragm in the housing returns the arm/diaphragm to "at rest" position. That is the pressure "stroke". The output pressure of the pump is directly related to how hard the spring pushes on the diaphragm. You can actually calculate the pressure it if you measure the spring pressure at the relevant height and divide that by the active area of the diaphragm.

So if the diaphragm is 2.5 OD there is about 5 square inches of area, 10 PSI would mean 50 lbs of spring pressure.


If it was the other way around the pressure would be only be limited by how strong the components are until something broke.

Make sense?

sure... but think about it... the total diaphram movement in both verical directions is defined by the arm... if the arm moves a lot, then the diamphram sucks in a lot of fuel but then the pressure spring is also collapsed a like amount so its generated force is higher... f=-kx ... where f is the force k is the spring constant and x is the displacement... and it will be linear.... stroke the dia. 1" get x vol of fuel delivered at y pressure. stroke it 1/2 inch and get 1/2 x volume at 1/2 y pressure... and because of the leverage effects , small stroke changes make big changes to both

the total stroke defines both parameters... thats why when you change the cam or the eccentric you have to make sure that the gaskets and spacers are also adjusted to get youy back the volumes and pressures you need. in the case of the merc pump we are talking about ( and again i say i have never cut one up to look inside) i can't imagine that it isn't exactly the same as every other pump on the planet...

either way... in this guys case, reinventing the wheel would have been the wrong thing to do anyway... just get the right pump and go home early.

Merc # 861678A1 was the old part # for the fuel pump that would cause about 12 psi at 700 rms flooding the carb. It seems the year the problem pumps were made in 2010. I thought these mech. fuel pumps were all made by Carter. I knew years ago that was true.


The new fuel pump # is 8M0058164 and the problem has been resolved.

At first we checked all of this to try to pinpoint the problem as well.

Carb float adjustment - float itself, carb problem, venturi problem.

Spring too tight around the diaphram

vent hose clogged or fitting clogged

too long or changed out fuel pump rod or
changed camshaft - and or cam lobe problem these 2 can cause high fuel pressure by overstroked the mech. fuel pump level.

possible very high crankcase pressure

fuel pump lever arm longer or angle of degree different

mounting gasket and or spacer depth different.

anyways what I was told - use thicker gasket and or a spacer close to 1/8 inch thick and that did take care of the problem - the pump arm had something different about it and overstroked the pump - hense the higher fuel pressure.

I will email back to Merc engineering to see if that answer has change. I dropped the issue because used other pumps and Merc. problem has been resolved.

carter has been in the fuel pump business for 100 years... litterally i think... they make a great product... beyond that, if its not too much hassle, an electric with a regulator... all good quality of course is a perfectly good solution... and a better deal all around i would think. but there might be other considerations in your case i am unaware of..