Mechanical fuel pump gph question

[dmiles452]

I am still struggling with the rpms on my 1989 Mercrusier Stern Drive 350. The engine runs fine but tops out at 3500 rpm. The ignition timing is good and the engine runs really smooth. I have made sure that there are no restrictions in the fuel line My question is: are there more than one mechanical fuel pump gph rating available for this motor? My thought process is that the engine is just not getting enough fuel to go any higher in rpm. I have not checked the pump pressure at the carb, only because I dont think its a pressure problem but maybe a volume problem.

 

[alldodge]

Did the motor ever get higher then 3500?
Need more info?

Most likely it's not the fuel pump, it's probably a tired motor or to much pitch

Edit: being same boat that need a bolt replaced in the transom could also mean the boat is water logged

 

[Scott06]

I am still struggling with the rpms on my 1989 Mercrusier Stern Drive 350. The engine runs fine but tops out at 3500 rpm. The ignition timing is good and the engine runs really smooth. I have made sure that there are no restrictions in the fuel line My question is: are there more than one mechanical fuel pump gph rating available for this motor? My thought process is that the engine is just not getting enough fuel to go any higher in rpm. I have not checked the pump pressure at the carb, only because I dont think its a pressure problem but maybe a volume problem.

a couple of things to verify-

Did it ever get above 3500 rpm?

What do spark plugs look like?

check that throttle blades and choke are fully opening
double check firing order
compression test
verify ignition timing both intial and total advance
check fuel pressure

 

[Bondo]

My question is: are there more than one mechanical fuel pump gph rating available for this motor?

Nope,.... Only one choice,.....

 

[QBhoy]

If she wasn’t getting enough fuel…you’d know all about it, I’d say. She would likely pop lean and/or not tend to hit such an exact ceiling of rpm in such a way.
The truth is, that more details may be required around the issue. Things like how she performed previously, or even if that’s an unknown. What about the propeller on her ? Could she be over propped ? What speed are you seeing when she reaches the reported 3500 rpm ? Is the tachometer known to be accurate? Does she accelerate well or otherwise ? Is the choke sticking on ? Have you verified that the back two secondaries are operating ?

 

[Lou C]

I don’t know what ignition system you have but I’d be curious about your total timing advance….

 

[Scott Danforth]

The same mechanical fuel pump will support well over 550hp.

Assuming the pump isn't broken, it's something else

 

[dmiles452]

Did the motor ever get higher then 3500?
Need more info?

Most likely it's not the fuel pump, it's probably a tired motor or to much pitch

Edit: being same boat that need a bolt replaced in the transom could also mean the boat is water logged

Yes the motor has run higher than 3500 in past, the motor was rebuilt 3 years ago with maybe 50 hrs on it. I doubt its tired yet. I drilled a few small holes in the transom to check for rot or water. I got wood shavings out of the holes with no sign of rotten wood.

 

[alldodge]

Motor was running fine above 3500 and now it's not.
There is no backfiring it just won't go any faster

Don't see it being the fuel pump, if it was the motor would start to back fire from running lean.

I would do this as Scott mentioned

What do spark plugs look like?

compression test
verify ignition timing both initial and total advance