100hp falters

[beckoning]

This will take some explaining. '88, Mercury 100hp, 4 cyl, oil injection. Motor starts, warms up in the hoist at 1000rpm, move out to big water through the channel (4 minutes) at 1200rpm, open to 3000rpm in open water. Within 30-40 seconds rpms drop, motor falters, acts like it's running on 3, then 2 cylinder. Mate steers, and I press the fuel bulb 3-4 times and rpm's return for 10-15 seconds, then falters, loses rpm's. I've rebuilt the fuel pump, replace filter, used clean gas from a clean (no water) auxiliary tank, replaced bulb. I can replicate this in the hoist, start, run at 3-4Krpm, motor falters after 1 minute. Background: Early 2024, carbs professionally rebuilt, water found in carbs and tank. Poured red bottle of HEET in tank (18 gallons) to dissipate water. Motor is being starved of fuel. I will appreciate feedback please. I'm running out of things to check. Good advice, please. No crackpots.

 

[Chris1956]

Gee, you had me 'till no crackpots

Air leak in fuel line?
Remove anti-siphon valve for test.
Fuel line diameter is min of 5/16".

What kind and quantity of fuel filters do you have?

A lot of those Mercs went on SeaRays of that year. My SeaRay of that year had a fuel filter inside the fuel pickup pipe. The pipe unscrewed from the pickup fitting, exposing the filter, which was partially clogged.

Don't use Heet in outboard fuel. Use E10 instead, as it will absorb water as well.

 

[beckoning]

Gee, you had me 'till no crackpots

Air leak in fuel line?
Remove anti-siphon valve for test.
Fuel line diameter is min of 5/16".

What kind and quantity of fuel filters do you have?

A lot of those Mercs went on SeaRays of that year. My SeaRay of that year had a fuel filter inside the fuel pickup pipe. The pipe unscrewed from the pickup fitting, exposing the filter, which was partially clogged.

Don't use Heet in outboard fuel. Use E10 instead, as it will absorb water as well.

 

[beckoning]

This is helpful. Sorry for the long reply, but I'm trying to narrow thing down. Yes, it would seem there is either an air leak somewhere, or something is blocked. Where is the anti-siphon valve? My 18 gallon tank is under the floorboards that are screwed down to hull braces, and the floorboards are covered in poly-plastic flooring (fishing boat). Fuel line is 5/16". The fuel filter is new marine grade and quality (can't recall brand) that is typical white plastic canister that runs vertically up to carbs (4). My boat is Sea Nymph, Great Lakes Special (18.5', heavy aluminum, high freeboard). When I disconnect the fuel line from the engine, the squeeze bulb will squirt out a projectile of fuel, so the pickup in the tank seems clear. Last thing I want to do is tear out the floor to get at the tank. When I put in the floors, I recall a vent, but I don't know where the end it to check if it's clogged (partially?). Back to anti-siphon: The fuel line from the tank connect to the motor at bottom right. The fixture it connects to seems to leak once in a while after the motor is turned off. If it leaks fuel out, it can leak air in. What is between where the fuel line connects to the motor and fuel pump? When running, I can clearly see fuel easily passing through the filter. Please don't give up on suggestions.

 

[racerone]

Low crankcase compression??-----It is that PRESSURE pulse that drives the fuel pump.

 

[Faztbullet]

Replace fuel line to motor , most likely the liner has deteriated and restricting flow.

 

[beckoning]

Low crankcase compression??-----It is that PRESSURE pulse that drives the fuel pump.

I understand that is how the fuel pump works, sort of a push-pull on the 2 diaphragms in the fuel pump. However if the compression test is good ( about 120 all around as I recall from last Spring) how else could crankcase pressure fall?

 

[beckoning]

Replace fuel line to motor , most likely the liner has deteriated and restricting flow.

I have some spare new, black, marine grade fuel line, and that's any easy swap. Any ideas on how to check the fuel tank pickup other than removing the floor? I used compressed air to blow back into the fuel tank, and there must be a check valve because I couldn't blow back, even with compressed air. llAny ideas on where to look for the fuel tank vent? Sea Nymph is no more.

Hopefully, someone will weigh in on the 2 fixtures at the bottom of the engine compartment. They are 1) the fixture to which the fuel line connector connects, and 2) the fixture that is towards the rear of the engine to which the fuel line goes, and from which another fuel line goes to the fuel pump.

 

[racerone]

???----You can have zero compression in one crankcase and still show 120 PSI in the that cylinder.----Checked the reed valves ?

 

[airshot]

Your 100 hp only has a 5/16" fuel line ? My 40 hp has a 3/8" fuel line....
Should have an inspection hatch for the built-in fuel tank where the connections are. Open that up and pull the cover where the fuel line connects and check the pickup tube. Also look for white snot around the pickup from water being in the tank. That could restrict fuel flow as it gets sloshed around. Worse case is to pump the tank out completely and flush it out. Then replace all fuel lines, I would go with 3/8" - A1/15 fuel hose.
Gotta start at one end then work your way back. My .02 anyway.....

 

[beckoning]

This is not a test. My need is real. My 100hp runs great at 3500-4000 rpm for 20-30 seconds, then begins to falter. I don't adjust the throttle and squeeze the bulb 3-4 times and the motor revives for 15 seconds, then the cycle repeats...falter, squeeze, run, falter, squeeze, run.... I
- rebuilt the fuel pump,
- switched to a small clean tank,
- replaced the fuel lines.
What next? l'm open to knowledgeable suggestions.

 

[racerone]

Fuel pump is not doing it's job.

 

[beckoning]

Gee, you had me 'till no crackpots

Air leak in fuel line?
Remove anti-siphon valve for test.
Fuel line diameter is min of 5/16".

What kind and quantity of fuel filters do you have?

A lot of those Mercs went on SeaRays of that year. My SeaRay of that year had a fuel filter inside the fuel pickup pipe. The pipe unscrewed from the pickup fitting, exposing the filter, which was partially clogged.

Don't use Heet in outboard fuel. Use E10 instead, as it will absorb water as well.

- Where is the unit-siphon valve?
-

???----You can have zero compression in one crankcase and still show 120 PSI in the that cylinder.----Checked the reed valves ?

Fuel line has been 5/16 for years. Fuel filters are SeaWay(?) marine grade -- visually gas flows freely while running. Which is what you call the "fuel pickup pipe?" Is that the tube in the tank? Mine is accessible only if I remove the flooring cover and the floor. Agree that Heet is not preferred. I'm running the tank down so there's little fuel-Heet mix left. Bottom line, the problem seems to be that the fuel supply cannot keep up with 100hp motor demands. The lower rpm, the longer the time it takes for faltering. The higher the rpm, the shorter the time to faltering. Somehow, the fuel supply cannot keep up with motor demand.

 

[beckoning]

Fuel pump is not doing it's job.

Seems you may be correct. Where is the anti-siphon valve?

 

[racerone]

Outlet of the tank.