1995 merc 90

[brandonlegault]

Merc service manual has contradictory info. 1 1/4 turns from lightly seated on all three carbs or 1/4 turn on no. one and 1 1/4 on two and three<br />engine does not respond to turning no. one all the way in or out. falls on its face when hammered then takes off well

 

[brandonlegault]

Re: 1995 merc 90

no. one carb has a hole in the idle circuit that carb two and three does not have

 

[seldont]

Re: 1995 merc 90

My brother has a 95 model 75 horse Merc. It was running badly when he purchased it. I cleaned carbs and put in new needle valves. Adjusted top carb 1/4 turn out and the others 1 1/4 turns out. Starte fine and all I had to do was do a little fine tuning on them while it was in the water. I think the 75 and 90 horse 3 cyl. motors are basically the same if I am not mistaken with the exception of some porting changes within the cylinders and maybe carb size???.

 

[WillyBWright]

Re: 1995 merc 90

That has to be a typo. I'd expect all three carbs to be about the same. One and one quarter turns out. Slightly more or less, but all three about the same. I can't imagine just one quarter turn on any carb.

 

[brandonlegault]

Re: 1995 merc 90

Is this normal for there to be no response at all when adjusting the no. one idle

 

[seldont]

Re: 1995 merc 90

Was not a typo. I had to ask regarding carb adjustment. Top carb was 1/4 turn and the others were 1 1/4 turns from lightly seated positon. Try and see if it does not work. Worst you have to do is set them back.....

 

[dilligafda]

Re: 1995 merc 90

Setting the top carb 1/4 turn out and the others 1 1/4 turns out is an old trick used by techs to get the idle to smooth out. Something I have been doing for a few years. For whatever reason, when the carbs are all set at 1 1/4 turn out, the fuel recirculation system overloads the top cylinder and causes a rough idle. With the top carb at 1/4 turn out, it allows wenough fule for lubrication and the recirc system provides the rest. Hope this helps!

 

[Clams Canino]

Re: 1995 merc 90

I wonder how this would / could apply to the older inline 6's? My complaint when tuning those is that by the time you have the idle needles opened up enough to get a solid hole shot, it's too rich for extended trolling.<br /><br />They use 3 carbs - one for each pair.<br /><br />Thoughts?<br /><br />-W<br /><br />

Originally posted by dilligaf:<br />Setting the top carb 1/4 turn out and the others 1 1/4 turns out is an old trick used by techs to get the idle to smooth out. Something I have been doing for a few years. For whatever reason, when the carbs are all set at 1 1/4 turn out, the fuel recirculation system overloads the top cylinder and causes a rough idle. With the top carb at 1/4 turn out, it allows wenough fule for lubrication and the recirc system provides the rest. Hope this helps!

 

[dilligafda]

Re: 1995 merc 90

Wow that is a good question, I've never tried that. Usually when I find an idle problem with an older I-6, it's caused by an internal engine problem. I will have to check that out when the water isn't so hard around here!

 

[brandonlegault]

Re: 1995 merc 90

thanks for your input! does anybody know if these carbs are identical

 

[seldont]

Re: 1995 merc 90

The carbs I rebuilt on my brother's 75 horse seemed to be the same. I did however place them back in their original positions when reinstalling them. By the way his motor had no holeshot until I did that. Could it have been the kits and cleaning or could it have been the adjustment?? I do not know. But it sure does run good now.

 

[dilligafda]

Re: 1995 merc 90

No they are not identical. They should be stamped -1 -2 and -3 somewhere on the carb. Make sure you get them in the proper order so the jetting is correct for each cylinder.

 

[Clams Canino]

Re: 1995 merc 90

Ya, sloppy reeds can cause a funky idle. But on top of that, the idle mixtures are very touchy. Lean enough to troll great and the hole shot suffers. Rich enough for a great hole shot and it loads up on extended trolling. And finding the sweet spot in between can be a mission from Cod.<br /><br />-W<br /><br />

Originally posted by dilligaf:<br />Wow that is a good question, I've never tried that. Usually when I find an idle problem with an older I-6, it's caused by an internal engine problem. I will have to check that out when the water isn't so hard around here!

 

[seldont]

Re: 1995 merc 90

If the jetting is indeed different that must be the reason the adjustment is so different on the number on (top) carb. Does that make sense??? I will try to look at them next time. Come to think of it my jetski has a totally different jetting on the number one carb and when I did a rebuild I questioned it and even considered replacing the jets in it. Consulted a factory manual and found it was equipped that way from the factory. But the question to me is why is it so different on one cylinder??

 

[dilligafda]

Re: 1995 merc 90

Originally posted by Clams Canino:<br /> "And finding the sweet spot in between can be a mission from Cod."<br /><br />LOL mission from Cod! Funny!!! <br /><br />I'm sure the jetting differences have to do with all the engineering stuff like expansion rates and temps of cylinders, that kind of stuff (hey, I'm just a wrench you know!) It is not uncommon to see different jets in multi carb, multi cylinder 2-strokes. I see it all the time!

 

[brandonlegault]

Re: 1995 merc 90

Do you think the hole in the no. one idle circuit has anything to do with the merc service manual saying 1/4 turn out? For an expirement i plugged the hole and engine responds well to any change on idle screw; before there was no response.<br />major plug fouling going on now

 

[UpstNYer]

Re: 1995 merc 90

Uhhh, I really don't know this engine but am assuming it's the 2-stroke 3 cylinder Merc 90. If so, I did alot of reading on it before ordering a new one up for March delivery. My understanding is that the engine idles on only 2 cylinders...for reasons that were never explained. The third cylinder comes online somewhere around 1500rpm. Makes for a characteristically rough idle.<br /><br />If this is true, It could be that the No. 1 cylinder is the one that's offline at idle. Therefore the idle mixture screw is almost closed compared to No. 2 & 3. Probably open just enough to ensure the cylinder gets some lubrication but not enough for combustion. <br /><br />If I'm wrong on this, let me know. Would like to understand what I'll be getting this spring.<br />Bob

 

[Bob Kimber]

Re: 1995 merc 90

# upstNYer,<br /><br />To the best of my knowledge none of the 3cyl mercs (75 or 90 hp ) have ever idled on 2 cyl, they have been around more or less unchanged except for better electrics & ignition systems since 1988. I think you are confusing this motor with the 4cyl 100hp (superseded) or 4cyl 115 - 125 hp which all use the 2 + 2 system, idles on 2 and the other 2 come in at aprox 1600 - 1800 rpm.<br /><br /># elcapitano,<br /><br />I have heard this debate on the mixture screw on no 1 carb for years, as Clams said it can be hard to strike a balance for trolling against stumbling on acceleration, these carbs only have a primary pickup some other motors have additional secondary stages, Also at some stage the carbs were modified, I don't know what year and I don't have the manuals immediately accessible, the early models had an idle backdraft vent screw (or similar title) just slightly to the left and above the mixture screw, on models since at least 1999 on, these screws have been removed on all carbs so now just a threaded hole is visible.<br /><br />On the models since 99 i have found the best all round performance - idle & holeshot has been with the top carb about 1 to 1 1/8 turns out and the other 2 carbs approx 1 3/8 turns out. As stated by other posters, mixture settings seem to be very sensitive, also ensuring correct clearance between cam roller and throttle linkage is extremely important, this clearance varies a few thou between model years so must be confirmed for your particular year. The mixture settings I referred to above have seemed to be a reasonable starting point before final tuning on the water, taking into account my climatic location. The motor will appear to run rich if run with muffs or in a test tank but idle smoothly in gear with no stumble on holeshot when tested in the water.<br /><br />Only my local observations <br /><br />Bob