Acceleration under load

[Troublehead73]

I am new to this board and recently back into boating after a 12 year hiatus... I do have the manual and have read it but have some issues I am looking for pointers on. As an owner of three antique Harleys I am not afraid of turning wrenches... Bleed, Cuss, Drink, Repeat!
Boat- 1987 Marlin 18.5' open bow w/ 4.3L/175 Mercruiser V6 Alpha One Stern Drive 14.5" 19p
1) Boat seems to idle a little high and you have to be quick to get it in gear or it will die- I think this can be easily corrected with an idle screw adjustment but ask the advice of the pros
2) Boat will not get up to speed while under way- I have read many topics and discussion on this and have thoroughly confused myself to the point of replace the boat...
3) Boat will get out of the water on a plane if I constantly rock the lever from half throttle to full- You can actually hear the motor roar to life for a second but if you stop rocking the throttle it will slow back down to about 10MPH- Doing this seems to burn gas faster than dumping it into an open pit and setting fire to it

The prop seems to be good, there are a couple of knicks from the previous owner but it does not vibrate. Boat runs great in the driveway and will rev smoothly in neutral.
Any advice will be well heeded.
Thank you

 

[Scott Danforth]

Welcome aboard

issue 1 sounds like your shift cable needs adjustment/replacement as it is hitting the shift interupt switch incorrectly

issue 2 could be a variety of items, start with the fuel system (see the check list in the stickies about low WOT)

issue #3 is tied to issue #2. your accelerator pump has an issue

 

[Rick Stephens]

Shift cable adjustments are easy.However, note that when the lower shift cable gets worn and starts dragging the engine will cutout and die when you shift. The shift linkage has an ignition kill switch that momentarily unloads the gears to get it out of gear in the water. That interupt system will kill your engine if the lower shift cable is sticky.

Sounds like you need to do a carb rebuild. And/or check filters and drain the fuel if it is old.

Rick

 

[Troublehead73]

Thank you, I will update as I go through everything. Where are these stickies???

 

[Rick Stephens]

Where are these stickies???

Top of the Mercruiser forum:
http://forums.iboats.com/forum/engi...ance/mercruiser-i-o-inboard-engines-outdrives

 

[Troublehead73]

Thank you, I will check the stickies! I have just spoken with the guy I bought the boat from and he said the shift cable is brand new but he had problems with it before he sold it (forgot to mention). He thought there was an adjustment issue that was not allowing the butterflies in the carb to open up so it was not getting air, causing it to essentially run in choke.
Please correct me if I am wrong but, IMO this would explain the excessive gas use and low speed under load. But does this explain why I can rev it cleanly in neutral? He said he had a buddy of his drive while he sat back by the motor and manually opened the butterfly and the thing jumped out of the water... Thoughts???

 

[Rick Stephens]

Well, the choke has to work right. Should be obvious running on muffs.

If you already have a new cable, follow the directions in this video to readjust the linkage.
Mercruiser Alpha Shift cable adjustment

 

[JerryIrons]

I have just spoken with the guy I bought the boat from and he said the shift cable is brand new but he had problems with it before he sold it (forgot to mention). He thought there was an adjustment issue that was not allowing the butterflies in the carb to open up so it was not getting air, causing it to essentially run in choke.
Please correct me if I am wrong but, IMO this would explain the excessive gas use and low speed under load. But does this explain why I can rev it cleanly in neutral? He said he had a buddy of his drive while he sat back by the motor and manually opened the butterfly and the thing jumped out of the water... Thoughts???

I would do my own trial and error troubleshooting and stop paying attention to what he said at this point. From my experience, it's easy for an engine to run ok in neutral sitting in the driveway. But under load in the water is when you really find it how it's running. You've got an older boat, and you really don't know the maintenance on it. If it were mine, I would probably do a carb rebuild. At least then you have a clean carb for a baseline. (and use new fuel as well, 100% gas no ethanol) Something is amiss in the fuel/air delivery. Might as well change fuel filters, and check your timing while your at it.

 

[Troublehead73]

I agree with your statement about the neutral and under load difference, JerryIrons. Upon further review I will be adjusting the shift cable tomorrow when I get home. I believe this will help with the shifting issue where it is almost dying (creates an issue when trying to get it loaded on a trailer)... I will also check the choke for operation to ensure that it is opening and closing to the specification in the manual and visually verify the accelerator pump is functioning. I am not seeing anything anywhere to indicate that there is a independent adjustment for the throttle (aside from idle) so I assume this is conditional upon the proper adjustment of the shift cable, am I safe in this assumption? I have all of the previous maintenance records since the boat was new but a carb rebuild could be in order...

Where do you find gas with no Ethanol?

Thank you for all of the help so far!

 

[Troublehead73]

Well if anyone is still tuned in I found some time today to work on the boat. Fuel filter stone on top of fuel pump was pretty packed up with debris, the inline filter in the carb was full of varnish flakes, the lower shift cable measured 7 1/4" center to center, and the bolt for the ignition cut out was torqued down so it took a lot of effort to even move it! It seems as though the accelerator pump is working well so that is nice! I checked the end play in the lower cable and had 3/8" adjusted stroke to 6" set the upper cable exact fit minus 4 turns of the barrel, freed up the switch and lubed it so it functions. While in there I noticed the throttle cable at the carb was bound either a bad adjustment on the cable or the idle adjustment was way too far... I opted to back off the screw so the linkage was free and then called it a night!
Any guesses or theories ? to what will happen tomorrow when I hook up the hose?

 

[JerryIrons]

Well, hopefully it runs better. With the gunk you found in the filters though it probably dirtied up your carb as well. My guess is that it runs better, but you still may have some bogging on acceleration. Going out on a limb there. You can find 100% gasoline near a lake, boat launch area. Around western ny we have a noco that sells it, only high octane but at least it's 100%. Those varnish flakes you found is what ethanol does, it's like paint thinner cleaning crap off your tank. And if you found any "fibers" that's ethanol eating away at fiberglass.

 

[Troublehead73]

Well, hopefully it runs better. With the gunk you found in the filters though it probably dirtied up your carb as well. My guess is that it runs better, but you still may have some bogging on acceleration. Going out on a limb there. You can find 100% gasoline near a lake, boat launch area. Around western ny we have a noco that sells it, only high octane but at least it's 100%. Those varnish flakes you found is what ethanol does, it's like paint thinner cleaning crap off your tank. And if you found any "fibers" that's ethanol eating away at fiberglass.

Sorry for the delayed response, out of town for the weekend. Did not find any fibers thankfully, my marina near me sells gas with a blend so, no dice unless I go with race gas.
The cleaning of the filters made very little if any difference.

The adjustment of the cables however certainly made shifting easier goes crisply into gear at about 11:00 and 1:00 respectively.

The problem I am having now is getting the idle right so it does not die constantly... I have to give it a quick rev in neutral and make a very fast move to engage the gear. So low idle is bad and WOT is not there, I can only get at the highest between 3000~3200

I was getting ready to order a new carb but started thinking, and I am really kicking myself for not thinking of this before, but the timing is all mechanical advance. If the springs are broken or the weights are gunked up this would cause a low RPM scenario and loss of power that may even to a trained ear sound and feel like a fuel related problem?
I may be reaching here but as I said earlier on if I "pump" the throttle I can get higher RPM and get the boat on a plane but that could be a direct result of the accelerator pump pushing excessive amounts of raw fuel into the carb with every push (also explains the fuel consumption). I'm really reaching back into my big motor, big vacuum robbing cam days but am I onto something????

I lay myself before the pros for advice before I dive too deep, it has a Thunderbolt IV ignition so this may make a difference that I am not taking into account.

 

[Bondo]

I may be reaching here but as I said earlier on if I "pump" the throttle I can get higher RPM

Ayuh,..... It's starvin' for Gas,.....

Rebuild or replace the carb,..?? Fuel pump,..?? crud in the tank,..??

Btw, T-Bolt IV ain't got flyweights, it's electronic,....

 

[Troublehead73]

Starving for fuel sounds correct. It has an aluminum 25g tank, pumped it out its clean. Should I be able to blow through the anti siphon valve. When I try to blow through it it is very difficult. If it should be difficult to blow through I guess I'm just at replacing the carb. Thanks for all the help so far. Look forward to the response!

 

[JerryIrons]

You could use another fuel tank, like an outboard tank and hook it up to rule out some things. You could take the anti siphon valve out temporarily, take off gas filler cap to make sure vent isn't plugged, etc. I bought an ultra sound cleaner from harbor freight and used some purple simple green to clean out my carb and it worked really good.

What I have found is that there are usually several potential causes to any boat problem, and it's just a process of narrowing them down.

 

[Troublehead73]

Well after several weeks of trials and tribulations I finally feel like a complete idiot!!!:grumpy:

I have checked timing, checked the ASV, replaced the fuel line, checked the tank vent, cleaned the carb with spray carb and choke cleaner, adjusted linkages etc, etc, etc!

Yesterday I pulled the carb off to have it rebuilt. I never once messed with the idle mixture screw as it is nearly impossible to get to so I figured the previous owner would not have touched it... Man was I wrong! The idle mixture screw was all the way in and tight and the spring was a jumbled up mess. I turned it out a turn and a half and all of a sudden I had gas running down my arm. EUREEKA!!! I thought about throwing it back on and taking it out to prove to myself that this is indeed the gremlin I have been chasing but, the base gasket was torn upon removal so I am rebuilding it and will test in a couple days.

Moral of the story "Never underestimate the stupidity of others"

I will update when back on the water!

 

[Troublehead73]

Well, I was able to actually go boating!!! I am going to replace the carb however, as the unit is beyond rebuilding in my opinion. The previous owner had tightened the idle mixture screw to the point where the casting on the seat in the bowl is chipped out. I was able to adjust it to a point where I could idle and shift well in the no wake zones and was capable of getting on a plane and running right around 32 MPH. I don't think that is too bad with three adults, two teenage girls a cooler and all the gear... I have a little lag between idle to on demand power but I will go though the stickies if this exists after the carb is replaced.
Thanks for all the help. Happy boating!!!:lol:

 

[JerryIrons]

That's great, if it were mine running fine I might even wait until fall.

 

[Rick Stephens]

If you re-carb, go with the Edelbrock 1409. Really easy to work on and adjust.