75hp is very slow

[bigbz28]

Re: 75hp is very slow

Exactly what operates the fuel pump?

 

[MickLovin]

Re: 75hp is very slow

The compression in the crank forces the diapragm in and out, therefore pulsating the pump into pushing the fuel towards the carbies, if your valves aren't working this could also be the problem. take your line off to your filter and have someone crank the motor while you hold a small container under the fuel line from the pump. I will check my manual for the amount you should have in the container, but a reasonably good flow would indicate your pump is ok, if it is pumping ok, try a right angled float not a straight through, I found even my right angle float did not fill up completely.

I also used clear fuel line on my 100hp to check for air in the system.

and on my 75hp

 

[MickLovin]

Re: 75hp is very slow

The first pic I posted had the right angle fuel filter incorporated, just make sure you have no air leaks up to the carbies.

 

[Jiggz]

Re: 75hp is very slow

The fuel pump is either faulty or if it is already replaced and new, try disassembly and re-assembly again while making sure everything is in proper place and properly torqued. The pump is working except it is not delivering enough fuel to support high rpm with load. If after doing so the problem persists, use an external tank (with vent opened) and if possible use different fuel hoses and primer bulb. If problem goes away, you know exactly where to look, the fuel tank, hoses or primer. If problem persists, the fuel pump is still the problem.

 

[MickLovin]

Re: 75hp is very slow

I meant to mention, I found an air leak in my system when I put the clear fuel line in from the pump to my carbies, it had air bubbles around the outlet of the pump, I was able to prime the air through to the carbies and eliminate the air, I found my pick up in the tank had a leak at the right angle outlet on top of my tank.

 

[jerryjerry05]

Re: 75hp is very slow

Pump it up and take the fuel line off as you start it.
It should have fuel coming out as it runs.
The middle cylinder operates the fuel pump.
It needs good compression to make it work.
The gasket behind the pump in place?
The filter if it's between the pump and carb and it goes empty, then there is still a problem with the pump.

 

[MickLovin]

Re: 75hp is very slow

Thanks jerryjerry, I was unsure which cylinder it was behind.

 

[bigbz28]

Re: 75hp is very slow

Compression is 155 psi on #2

 

[bigbz28]

Re: 75hp is very slow

Thinking about upgrading to a low pressure electric fuel pump. Any tips there?

 

[Jiggz]

Re: 75hp is very slow

Some forum members had done just that. Here's a good thread about 12V electric fuel pump.

12-volt-fuel-pump-my-1989-125-force-outboard-412571.html

 

[bigbz28]

Re: 75hp is very slow

Some forum members had done just that. Here's a good thread about 12V electric fuel pump.

12-volt-fuel-pump-my-1989-125-force-outboard-412571.html

Thanks! That's exactly what I needed to read. I know I could make it work and would be quite easy for me due to my automotive and electrical experience but the safety and risk involved are not worth putting my family at risk in the middle of a lake.

Guess that even though I already replaced the diaphragm and then rechecked it again that I might as well just try another one and go from there

 

[Jiggz]

Re: 75hp is very slow

Maybe we are missing something why your fuel pump cannot deliver properly. From the pick up tube, can you verify you only have the following in your fuel system: suction hose from the pick up tube, primer bulb, fuel hose to fuel pump inlet, fuel pump, fuel hose from the fuel pump outlet, an optional filter (a clear fuel filter), fuel hose to the carbs inlet. Are all connections zip tied or hose clamped especially from the suction side of the fuel pump all the way back to the pick up hose connection? If you can post pics it will be very helpful.

 

[bigbz28]

Re: 75hp is very slow







As you can see from the pictures, everything is clamped. I even tried reversing the hoses on the pump just in case I got it backwards. Next I'll remove the pump again and clean the ports from the #2 cylinder to the pump and while I'm there I'll spin the engine over to try to detect if there is any pressure to operate the pump.

 

[Jiggz]

Re: 75hp is very slow

All seems to be looking good. How about the fuel tank, have you tried running the motor with the fuel tank cap loose? This is just to make sure the tank vent works or is sufficient for high rpm operation. A good indication of lack of vent is the primer bulb softens or even collapses while motor is running. Although the same collapsing of the primer bulb can also be attributed to faulty check valves on the primer bulb itself.

 

[bigbz28]

Re: 75hp is very slow

Yeah I tried all that. As stated earlier, when I had the see through filter between the tank and pump it would stay full, but when I moved it in between the pump and carbs it goes empty.

 

[Jiggz]

Re: 75hp is very slow

Oh that's right, which means it is the fuel pump that is faulty or at least not delivering the right amount of fuel. This is quite an interesting problem and diagnosis. Please do post what conclusion you'll find in the end to help others also.

 

[bigbz28]

Re: 75hp is very slow

How much pressure should be coming out the ports that operate the fuel pump? I can feel a decent amount of air coming out while turning the engine over but if I hold my fingers over the holes I cant tell if they are doing anything. In other words I tried the old compression trick of plugging a spark plug hole and expecting the pressure to force my finger away.

 

[Jiggz]

Re: 75hp is very slow

I'm not really sure how much pressure the fuel pump puts out but it cannot be that much considering it only works on crankcase pressure differential. Which means it also relies on the operation of the reed valves in the intake manifold. If reed valves are not working properly, the operation of the fuel pump is also affected. This is a short description of the fuel pump operation:
1) Piston upstroke creates a low pressure in the crankcase thus diaphragm flexes inward creating suction to the fuel suction tube
2) When piston reaches TDC, the crankcase pressure is at norm or atmospheric pressure and the diaphragm flexes back to neutral position
3) On piston downstroke, the crankcase pressure goes positive and the diaphragm flexes outward hence pushing the previously suctioned fuel to the outlet tube connected to the carbs.

In conjunction with all this crankcase pressure differential are the reed petals which act like check valves for the intake of air-fuel mix from the carbs. In your case, I believe the fuel pump is situated at the middle cylinder or #2. If you have checked everything there is with the fuel pump and just about replaced every parts conceivable, maybe it is time to check the reeds.

 

[bigbz28]

Re: 75hp is very slow

Not asking what the fuel pressure is but actually the pressure that operates the pump. So as you said, it is operated by crank case pressure which isn't a whole lot so I may be OK there. About to go get a new diaphragm for the pump and drop it in the lake real quick. Maybe I'll figure something out

 

[bigbz28]

Great! iBoats.com is back in the water!

Now, I never made it to the lake after my last post because the boat shop didn't have pump kit. Instead the mechanic told my to put an electric fuel pump on it since my qualifications should mean that I know how. (his words not mine lol). Still hovering over that idea but if I did then this whole thread is a waste and we'll never know what is wrong in the first place. So, I went home and started over with my diag. First thing I did was disconnect the fuel hose to the carbs and crank the engine. Gas pumped out like it should and seamed to be plenty of it. Hooked the hose back up and ran the engine and sure enough the filter after the pump would empty. Is there another way for gas to enter the engine besides the carbs? It doesn't look like it's pumping when hooked up. I also noticed that the engine wouldn't return to normal idle speed after I reved it a few times. It wouldn't go below about 2000 to 3000 rpms. I also tried to by pass the pick-up tube in the tank by just putting the bare hose in through the filler cap. Didn't help... To me, it's like the engine is pulling in more gas than the pump can handle, or more than it's suppose to pump...