1988 4.3 omc wont get on plane

[lasko1]

Boat idles and runs fine at trolling speed but when I throttle up to get on plane it starts speeding up and then bogs out. Here's some background of what I noticed the day it started. While running out into the lake in 2' waves I noticed the engine would bog down a bit intermittently at 3/4 throttle. We would fish for an hour or so with engine off then it became difficult to start back up. After a couple time of this, I called it quits and decided to head in. We were able to get up on plane but then the bogging became more frequent and eventually slowed the boat to 10 mph with less frequent bogging at that speed. I can put boat in gear and begin to get on plane then it completely bogs out until I bring throttle to "slow" position. It seems like carb fills with fuel while I'm going slow, then when I throttle up it sucks the fuel out and starves itself. Pulled fuel water separator and didn't notice any water but only let it set for a minute. May need to let it settle out more?
Looking for other causes of the bogging issue and ways to check them.

 

[lasko1]

Forgot to mention that this was the second trip on that tank of fuel and there were no overheating issues. Boat was not overloaded weight wise.

 

[southkogs]

Fuel pump? Are you getting good flow through the fuel line?

 

[lasko1]

I'm not sure what good flow would look like. To check this, do I undo the fuel line from the carburetor and slip a hose over that and and pump it into a 2 L bottle? How much fuel should I see?

 

[lasko1]

Reason I ask is because it's putting out enough fuel to run the boat at idle and up to ~7 mph but can't go much faster than that without it bogging down

 

[southkogs]

I guess the question I'm asking would be better asked, does it sound like it's starving for fuel? I've always found that engine that isn't getting fuel sounds a little different than one that is missing or has a blown cylinder.

If it were me, I'd probably check spark on each cylinder - check compression on each and then make sure I didn't have a any clogs in the fuel line.

 

[lasko1]

Yes, the boat sounds like its starving for fuel to me. It's not a quick "miss" or studder , it's that slower bogging down sound. I read it could be a bad anti siphon valve so I just removed the floor and found where the rubber fuel line attaches to a barbed fitting. I thought that fitting was the anti siphon valve but i can see clean through the fitting. That barbed fitting then threads into a 90 degree fitting which threads into the fuel tank and I think that is the pickup tube. If I am correct about identifying the anti siphon valve, it appears someone may have already changed it out to just a barbed fitting. I'm starting to lean towards having a bad fuel pump.

 

[lasko1]

Seeing that I have no anti-siphon valve in line, how would I go about testing on mechanical fuel pump for failure issues?

 

[southkogs]

I guess you would test if for pressure. But I'm not sure what that would be on an 88. Does it look like fuel is leaking anywhere around the pump?

 

[Bondo]

Ayuh,.... Carbed motors need 4 to 6 psi of clean fresh fuel,....

It could also be the carb needs a cleanin', 'n rebuild,.....

 

[Silvertip]

The anti siphon valve is not "in-line". It is at the outlet of the fuel tank where the fuel line connects at the tank. There are two things to check on a fuel pump. 1) pressure and 2) volume. Although the pump may show 4 - 6 PSI at the outlet, it may not have enough volume to keep up with engine needs.

 

[lasko1]

No leaks around the pump. Carb was rebuilt 2 yrs ago. Anti siphon valve...is it about 3" long with threads on one end that go into the larger fitting that threads into the tank and hose barb on the other end which that ends hose goes to the fuel/water separator?

 

[southkogs]

Do you have good compression?

I'm not a guy to just toss parts at something, but I did have a situation several years ago where I gambled on a part and it solved my issue. Replacing the fuel pump wouldn't be a bank breaker, but it's kinda' 50/50 it would solve your problem.

 

[lasko1]

~170psi on each cylinder for compression

 

[lasko1]

To replace fuel pump do I just remove the rubber vent hose, fuel inlet and outlet, and 2 or 3 bolts holding it onto the engine and then reverse the process for installation? From pictures of the fuel pump, there is a shaft that sticks into the engine and I am wondering if there's anything special I need to do for that or will it line up correctly with whatever actuated it?

 

[lasko1]

Silvertip, I suspect I will have to disconnect the metal fuel line from my carb and clamp on a hose with a pressure gauge to measure the discharge pressure of the fuel pump. Since this is a mechanical pump I will have to roll the engine over to get fuel to flow. Is there anyway to tell what is sufficient fuel flow? One thing I did not mention is that approximately two hours after shutting the boat off at the lake, I pulled the fuel water separator and it was only half full of fuel. Not sure if that is vital information for you guys or not. For the last 5 -10 minutes of the boat trip, I basically idled up the channel to the dock.

 

[southkogs]

My pump was super easy to replace, undo the lines - two or three bolts - reverse process with the new pump. A mechanical pump has an arm that actuates the diaphragm.

 

[lasko1]

Thanks for the info Southkogs. I was doing some more surfing on the net and found a post where a fella had the "exact" issue as mine. His ended up being points and condenser. I never would've expected that, but with mine not starting very well I'm gonna try that first since its been ~3 yrs since they've been replaced.​ Anybody know how often points and condenser should be changed out for periodic maintenance?

 

[Bondo]

​ Anybody know how often points and condenser should be changed out for periodic maintenance?

Ayuh,.... Every Spring,..... Once a season,....

 

[southkogs]

... His ended up being points and condenser ...

Ah. That's why I asked about the starvation vs. missing question. I was trying to work the classic "spark or fuel" routes ... zigged when I shoulda' zagged

Bondo's right ... though I usually cheated mine an extra year :nono: