86 merc 115 carb work

[IronOutlaw]

I finally got the boat out on the water yesterday for a test run. Last trip out was a failure because of a bad float. I replaced two of the floats and took it back out.

Carbs were rebuilt in 08 by previous owner and only ran 1 time.

It's hard to start cold but warm it cranks right up if I just touch the key. When cold it also struggles and boggs in low rpms but once over 3000 it's like something hits me in the back and it takes off like a bat outta hell. Once warm it doesn't bog as bad but it's still there.

Boat runs a little over 40 on the speedo but I would think there is more in it. It get there really fast and just stops accelerating at around 40-45. But is a 17 foot ranger.
When I went to lad back on the trailer it kept dying. Sounded like it was choking itself out and I would have to rev it to 2500 in neautrL to clean it out.

Does this sound like a poorly adjusted float? I just dropped the new one in and didn't adjust it.
I don't want to completely build the carbs because everthing n them is like new. I just don't know what to adjust and don't want to start turning screws and cook this motor.

 

[CharlieB]

Re: 86 merc 115 carb work

Float height is critical on all carbs to have the correct pressure difference between the fuel level and the throttle opening/intake vacuum.

Idle mixture gets real iffy if the floats are not set correctly.

Bog, hesitation, and dying on rapid throttle opening is often mostly caused by too lean an idle mixture. A low float setting will also contribute to a lean idle condition.

Get a copy of the factory service manual and read it carefully, clean the carbs and pay close attention to the float setting.

Initial idle mixture is one an a half turns out from very lightly seated. This should give enough fuel to get the motor started and running. Final adjustment must be done IN THE WATER, not on the hose. Adjust IN Forward gear at idle, adjust each mixture screw to obtain the best possible motor speed while in gear, then back each one OUT an additional 1/8 turn to provide the additional fuel necessary for acceleration.

Test by slamming the throttle wide open, ANY hesitation, bog, or lean cough, back out each screw just the width of the screwdriver blade slot and retest.

Once ALL traces of hesitation are gone put the cover back on and enjoy a job well done!

 

[IronOutlaw]

Re: 86 merc 115 carb work

Thanks for the reply. I have a clymers manual I'm using. I pulled the carbs and the middle float setting was off about a 16th of an inch. I cleaned with carb cleaner and blew out with air compressor but the carbs look super clean.

Tomorrow I'm gonna set the idle screws in the driveway and do a final test n tune Saturday or maybe one evening this week if I get off early. I'm guessing I don't want a very smooth idle because that will mean its too lean?

And also to delete the backdraft all I have to do is insert a bb from a bb gun into the vacuum lines? I don't like anything with the word lean in it on a twostroke.

 

[CharlieB]

Re: 86 merc 115 carb work

And also to delete the backdraft all I have to do is insert a bb from a bb gun into the vacuum lines? I don't like anything with the word lean in it on a twostroke.

??????????????

Do you mean the small hoses from cyl to cyl on the crankcase?

NEVER plug these lines. make sure that they are clear and in good shape, the fittings have one-way valves in them to recirc the excess oil/fuel, assist the main bearings to stay properly lubed. Plugging any of these WILL cause $$$$$ damage.

Plugging any carb vent is also a NEVER do, if atmospheric pressure cannot reach the float bowl the pressure differential that causes fuel movement is lost, causing a MAJOR leanness, smoking the motor at WOT.

 

[IronOutlaw]

Re: 86 merc 115 carb work

I guess I called them the wrong name. I talking about the little hoses on the carbs that lean out the engine for better fuel economy. Someone told me I need to delete them or stop them up.

 

[CharlieB]

Re: 86 merc 115 carb work

You can remove the hoses but do NOT plug the carb vent, doing so WILL cost you a motor.

 

[IronOutlaw]

Re: 86 merc 115 carb work

Just wanted to follow up and say I tested the boat yesterday and it's running great now. No hesitations and it's got alot better topend. Before I was running 5000 rpms at 40 and now I'm running 5900 at around 50.

However I can't get it to idle dependably under 1000 rpms. It will idle but will die if idling long distances in gear. I don't want to mess up the lower unit but 1000 in neutral is where it runs best.

Also I think i may can go with a steeper prop. I can turn 6000 wot and that with 4 people in the boat. It has good holeshot now but I'd like to knock the rpms down a few hundred rpms when I have to buy a new prop.

Prop doesn't feel like it's slipping neither. It's a quicksilver 48 72500 a5 21p.

I think I can go 23" with no problem.