'98 115hp 2 stroke Idle Adjustment

[Jason2tpa]

Hey guys. Advice for tweeking the idle lower about 100-150? The manual says to loosen the idle screw first, then adjust both pilot screws, etc. Taking a shot one of you master mechanics can suggest some different for a small tweek. Thanks.

[ausnoelm]

Why would you want something different to what the manual suggests? Doesn't make sense to me, you have a manual, it tells you a method, but you want a different method? Remove a couple of plug leads, that might slow it down a bit......

[rodbolt17]

find a real manual.

the A/F screws have a setting.
the idle speed has a range idle speed in gear in the water is 650 + or - 50 RPM.
yep its that simple.
set the pilot air screws at 5/8 of a turn from lightly seated and it works or something is wrong.

[boscoe99]

Hey guys. Advice for tweeking the idle lower about 100-150? The manual says to loosen the idle screw first, then adjust both pilot screws, etc. Taking a shot one of you master mechanics can suggest some different for a small tweek. Thanks.

Are you just wanting to get the motor to idle as low as possible? Sounds like you might be a slow *****er.

What manual are you looking at? When it comes to adjusting the engine idle speed the Yam manual says to first adjust the engine timing. Then adjust the carburetor synchronization. Then set the engine idle speed. No mention is made of messing with the fuel/air ratio.

[99yam40]

give motor info and the problem you are having if you want people to help.

If motor is idling too high, has someone been jacking with it and caused it to do this?

[Jason2tpa]

Rodbolt and Boscoe...I was hoping you guys would reply. The manual I'm referring to is a Yamaha service manual specifically for my motor year and hp. Once the engine is warm it typically runs on the higher side of normal. Here is verbatim from manual:

Idle speed specification: 750 +/- 50
Loosen idle adjusting screw and fully close throttle valve. Turn in pilot screw until it is lightly seated. Then turn out pilot screw to specification.

If my assumption is correct, the pilot screws affect the mix. If the idle was way off then adjusting the pilot screws would be necessary. What I was trying to convey in my first post was can I make a small idle adjustment using just the idle adjustment screw WITHOUT messing with the mix?

And Boscoe, I do slow ***** and fish bridges with engine running, bumping in and out of gear a lot. When it idles high transitioning into gear can be a thump, not a gentle tap, if you know what I'm saying.

[boscoe99]

OK. I found your manual. It does say to adjust the idle mixture screws. I think this is just a preventative step that really does not have much to do with the idle RPM. Later manuals for the 115 leaves this step out. Reason being the idle mixture screws have tamper resistant plugs in them.

Here is the later version of the idle adjustment procedure. Just follow this, make the setting to the low end (750 minus 50 = 700) and you should be good to go. But if this is not what you need then try 650. Then try 600. The lower RPM's may or may not be satisfactory.

You got 1/4 turn on the idle mixture screws as an allowance. You might need to open them up or turn them down a bit to get the idle quality that you are seeking. Like they say in NASCAR, speriment man.

But once again, are you reading the part about the need to check the ignition timing and to synchronize the carburetors before you move on to the idle RPM adjustment?

[rodbolt17]

hahahaahaah
your gonna have a fubar using just that part.

FUBAR


that so called throttle stopper actually controls idle ign timing.

there is NO a/f mix adjust.
A/F is con*****ed by the fixed pilot fuel and pilot air jets.
all that screw does is allow more or less of the fixed ratio.

ya gotta remember,that Yamaha manual is written for a trained technician not the average joe with a couple of wrenchs and a screwum driver.

[boscoe99]

hahahaahaah
your gonna have a fubar using just that part.

FUBAR

that so called throttle stopper actually controls idle ign timing.

there is NO a/f mix adjust.
A/F is con*****ed by the fixed pilot fuel and pilot air jets.
all that screw does is allow more or less of the fixed ratio.

ya gotta remember,that Yamaha manual is written for a trained technician not the average joe with a couple of wrenchs and a screwum driver.

Therein lies the problem with Yamaha data. What are the hundreds of wannabee mechanics to do that want to work on Yams and who want to do a good job? Just like a number of courses that mechanics are taking (anyone actually) the material is usually designed to meet the needs of the weakest member. To make sure that all get it. Yes, the more knowledgeable have to be a bit bored some times and wait for the lesser trained to catch up. It is the way of life. The manuals should be written so that the most dense mechanic can do the job.

What do you do when you come across a motor that needs to be worked on and which you have never worked on before and have no specific training on whatever it is that needs to be worked on? What did you do years ago when most/all of it was new to you?

Now on to the air/fuel mixture. Are you saying that turning the screw inward or outward will have no bearing whatsoever on the air/fuel ratio that is seen within the combustion chamber just prior to light off? If nothing is happening, why is an adjustment screw there? Why does Yamaha offer plus or minus allowances? Why does turning the screw outward a bit on some motors help the idle quality? Why is there a plug installed to prevent folks from gain access to the adjustment screw?

Talk to us.

[99yam40]

hahahaahaah
your gonna have a fubar using just that part.

that so called throttle stopper actually controls idle ign timing.

I agree that the part of the procedure posted looks like it is for adjusting timing, but it does not say to hook up timing light and monitor timing marks just RPM
That Yamaha manual may have a problem.

Reminds me of the manual I have for my C40, the sequence for syncing was not written correctly

Oh yea, IMHO by screwing pilot screw out more allows more of the mix Rod talks of into the intake along with the air moving past the cracked open throttle plates eventually hitting the combustion chambers, causing a richer mixture in the motor

[Jason2tpa]

Awesome boscoe ill check this out thanks. To answer your last question, yes i did read about adj timing and sync before adj idle. And reading that is what caused me to post this original thread. If making a small idle adj requires doing the above, or is it ok to just adj idle screw. I figured you masters would know if such a small adj can be made skipping timimg/sync. I know its tough for you to say not having the motor in front you. But could you say that IF motor is running on high end of idle rpm spec, would there be a timing/sync problem. And if so, what are the other tale tells to knowing if its timing/sync or if its just idle?

OK. I found your manual. It does say to adjust the idle mixture screws. I think this is just a preventative step that really does not have much to do with the idle RPM. Later manuals for the 115 leaves this step out. Reason being the idle mixture screws have tamper resistant plugs in them.

Here is the later version of the idle adjustment procedure. Just follow this, make the setting to the low end (750 minus 50 = 700) and you should be good to go. But if this is not what you need then try 650. Then try 600. The lower RPM's may or may not be satisfactory.

You got 1/4 turn on the idle mixture screws as an allowance. You might need to open them up or turn them down a bit to get the idle quality that you are seeking. Like they say in NASCAR, speriment man.

But once again, are you reading the part about the need to check the ignition timing and to synchronize the carburetors before you move on to the idle RPM adjustment?

[rodbolt17]

still
nope
that's not how you do it.
remember that manual was translated from Japanese.
sometimes poorly translated.
idle ign timing should be about 5*ATDC.
carb pick up timing, the point that the cam picks up the roller, should be about 4*ATDC.
nothing to do with idle speed.

most Yamaha carbs use fixed jetting for air and fuel.
once its fixed its simply fixed.
the A/F screw simply allows more or less of the FIXED A/F mix into the intake.

simply study the carb design.
it aint hard.

[boscoe99]

still
nope
that's not how you do it.
remember that manual was translated from Japanese.
sometimes poorly translated.
idle ign timing should be about 5*ATDC.
carb pick up timing, the point that the cam picks up the roller, should be about 4*ATDC.
nothing to do with idle speed.

most Yamaha carbs use fixed jetting for air and fuel.
once its fixed its simply fixed.
the A/F screw simply allows more or less of the FIXED A/F mix into the intake.

simply study the carb design.
it aint hard.

Does allowing more or less of the FIXED A/F mix into the intake affect the air/fuel ratio that is ignited in the combustion chamber?

I am thinking that it will. And that the mixture will be a tad richer. But...

It surely worked this way on my Mercury 25 HP two stroke with a Tillotson carburetor. Turn in, lean sneeze. Turn out, ran fine. It worked this way on my chain saw once I got the special tool to turn out the idle mixture screw so the damn thing would run. But these are not Yamaha's.

Wish I still had my play Yam F40 carb'd model so I could put an air/fuel meter on it to see what happens as the idle mixture screw is turned in and out.

Oh Mr. Almetelo (man with an F40 carb'd model) we have a project for you.

[rodbolt17]

on the above 115.
A/F is FIXED.
it has a pilot fuel jet in the fuel bowl.
it has a pilot air jet in the carb throat.
actually 4 of each.

you CANNOT change this ratio without physically changing a jet.
you can allow MORE or LESS of this mix.

not all carbs work this way.
idle ign timing,carb pickup timing and WOT timing are con*****ed by the magneto control lever.
engine idle speed is con*****ed by the carb idle screw.

yep
its actually that simple.

[99yam40]

Awesome boscoe ill check this out thanks. To answer your last question, yes i did read about adj timing and sync before adj idle. And reading that is what caused me to post this original thread. If making a small idle adj requires doing the above, or is it ok to just adj idle screw. I figured you masters would know if such a small adj can be made skipping timimg/sync. I know its tough for you to say not having the motor in front you. But could you say that IF motor is running on high end of idle rpm spec, would there be a timing/sync problem. And if so, what are the other tale tells to knowing if its timing/sync or if its just idle?

timing will affect idle RPMs, so checking timing is the smart thing to do and is easy to do.

The sync is making sure all of the carbs are opening the same. If the sync is off then changing how far the throttle valves are open/closed can make a difference in how well it is idleing

But yes you can use the idle speed adjustment screw to change idle RPM a small amount without hurting the running if everything else is to spec.
only way to tell for sure though is to check them