engine bog at wot

[Rellik546]

so my brother and i have been chasing this issue and it only happens when there are a few 'people or more in the boat, but not with 1 or 2. its a 17' wellcraft classic rated for 7 people. 1985 reman mercruiser 3.0l 140, alpha 1 gen 1

at WOT with anywhere from 3-7 people the engine will bog or loose power and drop several hundred rpms.

we have:
Rebuilt the carb (rochester)
replaced all fuel filters (carb inlet, fuel pump, and fuel water separator)
gas tank is clean, as it was emptied during a stringer and deck rebuild
Replaced :
plugs, and gapped
coil
plug wires
condenser, cap, rotor, points, set dwell using feeler gauge, confirmed with dwell meter
alternator due to bad bearing
checked fuel pump
checked compression
set timing altho i didnt do it where you bypass the interupter switch (base timing mode i think) is this crucial?

It runs fine throughout the range with any amount of load except at wot with multiple people in the boat.

after doing all these repairs the motor as a whole runs better. use to get 35, now it goes about 45 wot with 1 person. what could cause the bog under heavy loads? timing advance? maybe a distributor problem? a spring or bushing? advancing mechanism? when it happens there are no funny or odd engine noises. also im not sure if this may affect the issue but my tachometer will fluctuate and at wot it will pin to the max rpm, even tho i know for fact that the boat is at its recommended wot rmp range. could this be part of the problem, and why would a tachometer do that? is it shorted?

I have all the mercruiser manuals, and have read through them which has gotten me to where i am now. hopefully someone can help or has had a simliar issue.

 

[Bondo]

Re: engine bog at wot

after doing all these repairs the motor as a whole runs better. use to get 35, now it goes about 45 wot with 1 person. what could cause the bog under heavy loads? timing advance? maybe a distributor problem? a spring or bushing? advancing mechanism? when it happens there are no funny or odd engine noises. also im not sure if this may affect the issue but my tachometer will fluctuate and at wot it will pin to the max rpm, even tho i know for fact that the boat is at its recommended wot rmp range. could this be part of the problem, and why would a tachometer do that? is it shorted?

Ayuh,... Disconnect the tach, 'n Try it.....

I'm thinkin' it's just runnin' outa Horsepower....
Just because there's 7 seats in the boat, Don't mean it's capable of runnin' with such a load...

 

[Rellik546]

Re: engine bog at wot

is it possible the jetting in the carb is wrong?... this weekend i tried an experiment. with just myself in the boat after about 3 minutes the bog started, but it was very little maybe a couple hundred rpm. but it would continue to get worse. if i came back on the throttle say where te boat would normally be at 3500 rpm it would operate normally. i added one person at a time and retried with an additional person. as i added more people the bog got worse (as in per person it dropped even more) and it started to happen more quickly.

I have no starting issues, warm or cool out, hot or cold start. after running for awhile and sitting, or right after running for awhile it always starts fine. i feel like this is some sort of fuel issue. is it possibly the float or main metering jets? we burn through a full tank every week and add new gas everytime out. this as been a consistant problem all season. could i have set the timing wrong? does this seem like it could be a timing advance issue?

are there any tests i can do to eliminate possible causes/ identify them? ive been through the merc manual #10 for my motor and #3 for my rochester carb.

any input will help im running out of ideas

 

[Bondo]

Re: engine bog at wot

i feel like this is some sort of fuel issue.

Ayuh,.... Check to see that the tank is givin' up it's fuel, 'n run a pressure test of the fuel pump...

 

[Rellik546]

Re: engine bog at wot

fuel pump (carter low volume pump the one with the fuel filter on the top) was putting out 1-3 psi sporadically at 1000 rpms, at wot it was between 0-1psi. i spliced in a piece of fuel resistant vinyl tubing between the fuel water separator and the fuel pump inlet. at 1000 rpm i had a couple small air bubbles, at wot i had bubbles that spand the entire length of the sight tube 4".

so before replacing the fuel pump, we went through all the fittings from the tank to the pump.

this includes

removing the fuel pick up and inspecting where it meets the fitting for craps, i also plugged one end and blew through it, listening for leaks. (to remove the pick up from the tank keep the big nut that sits flush on tank stationary, then unscrewing the square fitting that is threaded through that big nut) mine does not have a filter/sock on the end of the pick up.

anti siphon valve (replaced it with a new one)

pipe doped all threaded fittings- pick up tube fitting, anti siphon valve, brass fittings in the fuel water separator, fuel pump inlet and outlet fittings, hard fuel line fittings between fuel pump and carb.

replaced all hose clamps with new stainless steel ones, each fitting was given 2 clamps (be sure not to pinch the fuel line by clamping beyond the barb on fittings. (did this on accident on one fitting and it caused problems, found out quickly tho and fixed it)

ran the boat with the gas cap off while under load to test for a bad vent line.

looked through the rubber fuel lines into light to see if they were collapsed (they were fine)



After doing all of that we still had air bubbles in the clear sight line we spliced in. It turned out to the the fuel water separator filter gasket... i used regular motor oil when we installed it to its housing. and it must have been leaking air. we removed the filter and put grease both below the rubber gasket and on top. Bingo no more air bubbles in the sight tube.


turned the motor over and had a solid reading of 2.5 psi at 1000 rpm and between 2.5 and 3psi at WOT. the fuel pump at 1000 rpm is suppose to be between 5 - 6.5 psi. At this point we hadnt tested under load on the water. we removed the fuel pump and took it apart, we cleaned everything and sprayed out passages with compressed air, inspected the diaphragm, rocker arm and spring. Everything seemed fine. put it back together and had the same fuel pressure as before. took it off again and by using my hand to pump the rocker arm and my fuel pump gauge attached the the fuel pump outlet i was able to get 5-6 psi.

after doing a little bit of forum searching i stumbled upon a thread about a guy getting too much fuel pressure and how mercruiser had told him that they make different size gaskets between the fuel pump and the block. if you have too much fuel pressure get the thicker gasket, if you dont have enough use the thinner gasket. well i looked at our gasket and it was about 1/8 thick.

i installed the fuel pump without the gasket to the block temporarily to just see if i could get more pressure. i turned the motor over and ran it for 5 seconds. i had 4.5 psi instead of 2.5-3psi with the gasket installed. there were a couple drops of oil that were sprayed out due to no gasket. so i bought liquid gasket and just applied to the mating surface on the block and re installed the fuel pump.

I took her out on the water and it no longer lost power at WOT (which was caused by giant air bubbles being sucked up into the system, and the bowl in the carb would empty then get just enough gas to keep running but we were loosing 500 rpms and it would start to surge as it received fuel, and die again as the bowl in the carb emptied again because of another air bubble.

The boat now runs better than it ever has. i put 7 people in it yesterday and ran WOT for 10 minutes without any hesitation or surging, it also idles better, before i would have to adjust it daily with the idle speed screw. it also fixed a slight hesitation on hard acceleration that was as bad as a bad accelerator pump(already replaced), but it was noticeable

I must say that these forums are a vast source of knowledge, the adults only section has really helped me out.


I would like to ask a moderator however to add using heavy grease on the fuel water separator instead of motor oil or similar, i have noticed a few other people that have had this problem and its probably worth listing under the HOW TO test a fuel system section.

i should note that because i no longer use that gasket between my fuel pump and block, that i did listen for noises with a stethoscope and i also removed the pump after using it part of the day to inspect the rocker arm that contacts the cam for any wear. everything is in good order. Maybe this should also be added to a MAYBE of that same how to test a fuel system section. It saved me from having to buy a new pump so maybe its worth mentioning.

i think it worked because over the 20 years that arm has either worn, or bent slightly and by being able to slide another 1/16 - 1/8 inch it contacts more of the arm now so the throw on the diagram is longer creating more pressure as confirmed by my hand pumping. i watched the throw with the gasket on and compared it to the length that i was moving the pump up and down with my hand. there was a noticeable difference. this worked for me, it may not work the same for everyone and i would use caution if you attempt to do this.

 

[Bondo]

Re: engine bog at wot

i should note that because i no longer use that gasket between my fuel pump and block, that i did listen for noises with a stethoscope and i also removed the pump after using it part of the day to inspect the rocker arm that contacts the cam for any wear. everything is in good order. Maybe this should also be added to a MAYBE of that same how to test a fuel system section. It saved me from having to buy a new pump so maybe its worth mentioning.

i think it worked because over the 20 years that arm has either worn, or bent slightly and by being able to slide another 1/16 - 1/8 inch it contacts more of the arm now so the throw on the diagram is longer creating more pressure as confirmed by my hand pumping. i watched the throw with the gasket on and compared it to the length that i was moving the pump up and down with my hand. there was a noticeable difference. this worked for me, it may not work the same for everyone and i would use caution if you attempt to do this.

Ayuh,... I think anybody else, includin' Myself woulda just bought a new fuel pump....

I rarely feel Bad, 'bout replacin' 20 Year old parts....