1987 Yamaha 115 v4

[Allen1984]

I was doing some adjustments on the carbs due to the previous owners failed attempt to rebuild the carbs. The trouble I’m running into is adjusting the high pressure side. Is the drain screw and high pressure side the same thing like all the diagrams show?

[99yam40]

I never heard of a high pressure side carb adjustment, so not sure what you are talking about.
and not sure of what diagrams you speak of also

[Allen1984]

Sorry I should’ve said the main jet. What the boat is doing is it will idle fine with the muffs in the driveway. It will go in and out of forward/ reverse and throttle up like it should in the driveway. I take it to the lake it idles as it should but as soon as I put it in gear and give it throttle it dies like it’s starving for fuel. Things I have done are checked compression top 2 cylinders are at 130 bottom 2 are at 125. Checked spark changed inline fuel filter and changed the fuel water separator filter. It has a new line and bulb also

[99yam40]

the back pressure from sitting in the water and the load on the motor with the prop in water are affecting the motor.
needs a little richer fuel to air for the transition would be my guess.
I do not think there is a high adjustment on outboard motors like the old 2 stroke weed eaters and chainsaws use to have.

either the carbs are not completely clean or they are not set properly.
did you do the link and sync shown in service manual(not sure if you can find one for that old motor)?
carbs all need to be set to exact same throttle position, set pilot screws to spec, and make sure timing is in spec and advancing as it should also.


motor needs more fuel(richer mix) to help it overcome the load being put on it.

but then main jet may have some plugging

[Allen1984]

The guy I bought the boat from had a manual but some pages were missing. Unfortunately it seems like most of the carb section is gone. So with that being said I haven’t done a link and sync. I do have the screws on the side of the carb set at the exact same position but without having the manual they probably aren’t at spec. I just set them to where the motor idles best

[boscoe99]

Sorry I should’ve said the main jet. What the boat is doing is it will idle fine with the muffs in the driveway. It will go in and out of forward/ reverse and throttle up like it should in the driveway. I take it to the lake it idles as it should but as soon as I put it in gear and give it throttle it dies like it’s starving for fuel. Things I have done are checked compression top 2 cylinders are at 130 bottom 2 are at 125. Checked spark changed inline fuel filter and changed the fuel water separator filter. It has a new line and bulb also

Please don't do that. It serves no purpose whatsoever. It is not a valid test of anything. Unless someone just wants a motor that can be run at high RPM with no load I fail to understand doing it.

The symptoms seem to be that of a clogged carburetor or misadjusted carburetor. Just as a test, have someone mist gasoline into the air intake system as you are giving it some throttle. Does the engine now spool up or does it still bog down and/or die?

[Allen1984]

I will try the mist theory later when I get home. I do know when it tries to bog down I can hit the electric choke and it will catch up for a second then bog down. I know by doing that I’m sending more gas in the carbs. I just don’t know if there is something I can adjust to send more gas

[TownsendsFJR1300]

. I just don’t know if there is something I can adjust to send more gas

To send more gas, clean the carburetors as noted above.

Here's the parts diagram for them, the main jet, part #14 (two on each carb): https://www.boats.net/catalog/yamaha...tlh/carburetor
is fairly easy to get to. I would dis-assemble completely as you'll likely find crud in the orifices..

***Make sure you have a correct (snug) fitting screw driver when removing those jets as time / corrosion likes to seize them together. Some penetrating oil, clamping down in a vise, etc helps...

[Allen1984]

Ok that makes since I was thinking those were my drains for draining the bowls out. That’s probably my problem I have them seated all the way down. Is there a certain amount of turns from lighted seated? I have the manual for this motor but all the pages for the carbs are gone.

[Allen1984]

Also #17 on that diagram. What do those do? I believe those are seated all the way down also.

[TownsendsFJR1300]

Also #17 on that diagram. What do those do? I believe those are seated all the way down also.

There is a drain plug that your looking at on the OUTSIDE of each bowl(with a gasket-each side of the carb). Those should be snug-not leaking.

The jets mentioned are NOT adjusted by turning a screw driver. The main jet IS THREADED. Again just snug.

Part #17, the nozzle, likely just sits in there. There are usually VERY small holes that go sideways. You can see by the pic the direction of the nozzle
Again, you cannot adjust with a screw driver.

Follow the picture closely, their usually VERY accurate what goes where.


ANY and ALL jets should be removed and cleaned. You should be able to see light thru them.

[Jason2tpa]

Not sure if the '87 and '98 V4 115 have same carbs/specs.....'98 spec is 5/8 +/- 1/4 turns out. Use it as a starting poimt then tune.

[Allen1984]

Thanks for all the help guys I really appreciate it. I figured before I go any further I better tear the carbs down and clean them. I only got to one last night. Everything seemed clean. Only thing I could find that didn’t seem right was one of my floats was sitting lower then the other one. I know the float has a tab I bend to adjust it but which tab is it? There’s one that holds the valve needle and one that sits behind the needle valve housing. I tried both of them with no luck lastnight

[TownsendsFJR1300]

Holding the carb UPSIDE DOWN so the float (when seated on float pin) should be about level.

There is an adjustment on the "backside" of the float to adjust "drop" (carb in normal position), float shouldn't drop much farther than with the bowl installed.


There is a set # to measure from the carb body to the float (don't know what it is for your model), but pretty much should be level, NOT pointing / angling towards the carb body...





This carb (upside down), the float is way too high, resting against the needle. It SHOULD BE level.


Now IF this carb upside, that drop would be about right..

[Allen1984]

Thanks for the info! I pulled the bottom carb off today while trying to get to the bottom bolt on the carb I thought it would be easier if I took the fuel hose that comes from the fuel pump. When I took it off I seen some plastic in that line. What happened was the liner for the fuel hose was detaching from the actual hose. I’m thinking this is causing a minor fuel restriction and hopefully the cause of my motor stalling under load. I have new fuel hose and carb mounting gaskets on the way. Hopefully I have some good news for you guys this weekend.