Re: Tuning Carbs 1995 Merc 115 HP 4 cylinder
I just finished "diddling" with the carbs on my 3 cyl 90...just decided to open the jets 1/4 turn for really no reason......WRONG! After the fact, the manual clearly says that once set, leave the low speed adjustments alone. Changing timing advance, via the idle screw associated with the throttle linkage is how you adjust the idle if you don't like it.
Manual says idle rpm for both engines is 675 +/- 25 in F in the water pushing the boat.
I have a manual and read it's contents. One thing I learned is that you want to make very small adjustments, like the blade width of a screwdriver when you get close to optimum performance, not 1/4 turn at a time.
For my engine it says 1-1 1/2". After the dust settled, I started with 3/4" open by finding the sweet spot the engine liked on muffs. Serv Manual numbers were too much fuel. I then took it to the lake and with the boat on the trailer, backed into the water in normal launching position, but with the boat secured, I slowly opened each carb the same amount and that being the width of a large screwdriver blade that I used to turn the screws until I could put the engine in gear and attempting to push the boat and truck up the ramp such that it would take the throttle in gear and not bog down....got that from Charlie B on here. Worked just fine and got my performance back.
For yours "9/16 turn +/- .015in" is what the service manual says....note the ref. to turn when referring to the setting and in. of tolerance. Really screwy numbers. Let's see.....a circle is 360 degrees so 9/16 is 202 degrees, so half a turn is 180 and 3/4 turn is 270 and 5/8 is half way between them and 9/16 is half way again between half turn open and 5/8.....I guess I could swag that.
On the .015in. tolerance one has to know the circumference of the circle where one would vary the setting and make that adjustment.....to me it sounds just like Charlie told me, 1 screwdriver blade width at a time.
Your engine only has screws for the upper 2 cylinders (no low speed adjustments for the bottom two carbs...course you already know that) and feeding the lower two at elevated rpms, making the transition seamless seems to be a real trick; buttttt when you get the adjustment right the big guns on here say that you can't tell when the other two kick in at somewhere around 2500 rpm as I recall.
If you didn't mess with anything but pull your carbs, clean and replace, they you probably don't have to tweak your timing other than to vary your idle knob to suit you.
Best I can do. If I screwed it up you can bet that Charlie B, Faztbullet and the other "guns" on here will jump in and correct the error of my ways. Grin.
Mark
