Correct Idle Jets for 90 HP

[Prairie Dog]

I'm new to the forum today, hello to everyone. My motor is a 1982 Johnson 90 Hp V4 (MLCNB). It has two double barrel carbs.I have the carbs apart for cleaning and rebuilding.I noticed the upper carb has two #35 idle jets, while the lower carb has two #36 idle jets.All the schematics show this carb having #34 idle jets, the #36 being for 140 HP motors. Is this a concern, should I replace these jets to #34?

 

[Prairie Dog]

Is there anyone out there that has carb tuning experience? Is there some reason these idle air bleed jets would be leaned out so much? And is there any reason the two carbs would be jetted differently?

 

[Tim Frank]

I'm new to the forum today, hello to everyone. My motor is a 1982 Johnson 90 Hp V4 (MLCNB). It has two double barrel carbs.I have the carbs apart for cleaning and rebuilding.I noticed the upper carb has two #35 idle jets, while the lower carb has two #36 idle jets.All the schematics show this carb having #34 idle jets, the #36 being for 140 HP motors. Is this a concern, should I replace these jets to #34?

You don't offer any background....how long you have had the motor (i.e. is it new to you?) ....why you are messing with the carbs....what problem(s) caused you to be messing with the carbs....in other words, pieces of the puzzle that you are looking for help to solve.

As a guess, I'd say that someone has already "messed" with the carbs and re-jetted them for some reason, or found some plug -and-play carbs that are not playing all that well. People are forever trying to "up-carb" as a way to get extra horsepower. It does not always work and the next owner gets left to try and pick up the pieces.
Are you having a specific problem? Has the motor EVER run well for you?

You say that each carb has a pair of identical jets, presumably plus a high speed jet. That sounds odd. I helped a friend a few years ago with his 90 HP, and those carbs had three different size jets.

Maybe recheck your manual or the schematic for those carbs and verify that you do not have a real mishmash from the previous owner.

 

[Prairie Dog]

The motor is new to me.It idles very rough and low speed is very rough. There are two carb bodies,with two carbs in each body.Each carb body has two high speed jets, two mid speed jets, and two idle air bleed jets (three jets/carb). I disassembled the carbs to see if a good cleaning and rebuilt would help the rough idling. This is when I found the two #35 idle air bleeds and the two #36 idle air bleeds.According to the schematics, both these jet sizes are too large for the 90 HP. Could running too lean an idle air bleed cause this rough idling?

 

[racerone]

Everybody jumps all over the carburetors when a motor is running rough.-----On that motor the first thing to do is a compression test.---Report the numbers.--Then check for spark on each lead.

 

[Tim Frank]

^^^^ +1.
Low compression or a spark issue and the idle will be all over the map.
Two easy tests.

Did you check what numbers were on the other "orifice plugs"? Are all the other ones per spec?

 

[Prairie Dog]

Compression is 120-130 in all cylinders, there is spark on all four. The other orifice plugs are #26 and #56 which are all correct to specs. The only jets not matching spec are the #35 and #36.

 

[racerone]

The reed valves are rarely a problem on the crossflows.---But have you checked them on yours?

 

[Prairie Dog]

No, I haven't checked the reed valves, I will do that. I suspect like Tim Frank said, that someone has "messed" with the carbs, and I'm inclined to put the idle bleeds back to spec size #34, unless someone can give me a reason why they might have been changed and mismatched as well.