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90hp 2 stroke fuel pump problem

M

Matt04

Member
Hi everyone, I posted a thread a while back about power cutting in and out on my '95 Mercury 90hp motor, at the time I thought it was a electric problem, but I have since discovered it is a fuel problem so I wanted to start a new thread to try to isolate the problem. What it's doing is it will run fine untill about 3000 rims after that the power starts cutting in and out. If I disconnect the fuel line going into the engine and put in in a fuel can 1' from the engine it runs mostly fine, still a slight miss at wot. And from there I have disconnected everywhere up to the tank and the problem gets worse the further away I disconnect it, (like it's having trouble sucking the fuel the further away it is) I have also installed a clear hose between the fuel pump and first carb to look for air bubbles (per the service manual), and didn't have any bubbles even at wot. I also rebuilt the fuel pump and sealed every connection along the fuel line and nothing fixed it. I am stumped about where to go from here.
 
Pump the ball while it is running, if it improves, the odds are you need a fuel pump. If it does not improve, clean the carbs.
 
I used 3/8" fuel line, when I pump the bulb while it's running it improves, but stiil doesn't run perfectly. The rebuild kit for the fuel pump included a new diaphragm, new check valves and gaskets, and the rest of the pump looks OK, it does seem like a bad fuel pump, but I don't know what else could be bad in it, or how to diagnose that further.
 
You need to temporarily install a fuel pressure gage at the carbs. You should see a reliable 5 psi at idle, and more at high rpm.


If not, it could be an air leak before the pump, bad check valves/ diaphragm, or a restrictive filter before the pump. (Inlet filters should be a minimum coarseness of 25 microns.)

Jeff
 
There's no mention of checking fuel/water separator and filter, Everything seems to point to fuel delivery, an obstruction,possible air leak, faulty pump. as fast jeff says check fuel pressure
 
I will try to get a fuel gauge, I have checked the fuel water separator and bypassed it as well, that didn't make any difference. I will also check and see if it has a filter the engine fuel line before the carbs and see if that's clogged. I also did try to take the cap off with no improvement.
 
Could a crankcase vent obstructed by gasket cause this? When I rebuilt my old carbs, I just slapped the gasket on between the carbs and the block, not paying attention to the alignment or the hole for such venting. Not good. Motor ran like ass and would eventually conk out if I didn't continually supply the fuel manually. Guys on here would know way more than me. Just throwing one of my mistakes out there.
 
Mill686, something like that seems possible to me, because even with a good tank on it it doesn't run perfectly, better but not perfect, and the further I try to hook up the tank to it the worse it runs. I bought a gauge, but since it takes all day to take it to the lake for a test run, I don't know how much more I want to mess with it. Maybe I'll just find a electric pump to install
 
Last time I checked it was 115-120 across all three cylinders, wouldn't bad compression be hard starting and low power? It starts easy and has plenty of power if I'm pumping the bulb
 
What is compression today ?---------Reason for the question is simple.---------The fuel pump runs off crankcase compression pulses.-----Problems with cylinder compression can give a clue to crankcase compression problems.
 
Just did another compression check, all 3 were still in the 115-120 range. Also removed the fuel pump and both crankcase holes are clear, I'm going to install the fuel pressure gauge and take it to a lake soon to see pressures under load, any other ideas, would like to have a plan to figure it out when I go to the lake next time.
 
Due to the fact compression is good,Threads#11 & #13 refer to pumping primer bulb to keep her running,everything points to a fuel restriction or a weak fuel pump,a gauge will tell the whole story we hope. Thread#6 stated 5psi at idle and higher psi as throttle advances. Have you tried holding tank above engine to gravity feed motor to eliminate fuel delivery issue,just an idea
 
I'm having a similar problem my compression is 118 on all 3 cylinders its boggin under load if I go into the throttle slowly it will eventually plane?
 
Jared lance,you need to start your own thread, Home page top left under forum home click on new posts, You will get better responses. Check your fuel pressure. Have you tried pumping primer bulb while engine is running to bypass fuel pump
 
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