Mikuni Jet Issue ?

Thanks for the diagram Patton, that is what I was looking for. Should there be gap between the part #45 that the idle screw touches when screwed in and the back plate, part # 59 when the idle screw is not in ?

I took the carbs off today and cleaned all 4 carbs. All of them checked good when I cleaned them. I was able to bench sync the carbs using a paper clip on the engine side. I have the idle adjustment screw screwed down about half way, which would give me enough room to adjust the idle either direction. I started the bike with the choke on and the bike started but doesn't sound like it is running on all r cyclinders, even though all 4 pipes are very hot. Is it true what Zoro said that the Mikuni 29 mm carbs to not need to do a running sync on them ? That is the first time I have heard that so I am not sure.

Would do water-spray test on the exhaust headers with engine running, to determine which cylinder lacks combustion.

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I started the bike today and after letting it run for a few minutes, I sprayed water on the pipes and all 4 of them sizzled and then evaporated, so all 4 cyclinders appear to be working correctly. The bike also sounded good. There is asome grey smoke coming out of the header, which I think I should be able to get rid of by adjusting the air screw. Does the Mikuni 29 smoothbores need to have a manual sync done on them ? Someone said earlier in this thread that it does not, but I had never seen this anywhere until then. I didn't ride the bike to see how it runs through the gears.

Despite Sudco's advice, many of us perform running syncs on Mikuni smoothbores.



The important running sync vacuum manometer readings are NOT while the carb top covers are off.

The important running sync vacuum manometer readings are with the carb tops fitted in the normal running position. Which, ideally, will be equal among all the carbs at idle rpm.

What are your current manometer readings at idle rpm with the carb tops fitted in the normal running position?

When cleaning the smoothbore carbs, an easy to overlook item is removing and cleaning the air jet BS30-97 shown as #12.



Cleaning procedures for the pilot circuit are basically similar among the Mikuni manual throttle slide carbs.




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Courtesy of Z1 Enterprises:



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Patton said "The important running sync vacuum manometer readings are with the carb tops fitted in the normal running position. Which, ideally, will be equal among all the carbs at idle rpm." How do you adjust for proper sync with the Motion Pro tool with the caps on ? Or am I reading what you said incorrectly.

There's a difference between "adjusting" and "reading."

Just read it with the caps on.

If the carb vacuum signals are reasonably equivalent at idle rpm, the sync is acceptable.
So don't worry about attempting further sync adjusting with caps off.

The important sync reading is when all the carb caps are on.

What are your current manometer readings at idle rpm with the carb tops fitted in the normal running position?

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OK, now I got what you are saying. I will have to do the running sync on the carbs tomorrow, with the caps on. Thanks for clearing it up for me.

With the sync tool connected, will adjusting the air screws affect the reading on the sync tool ?

I warmed the bike up today and after it warmed up, I turned the choke off and the bike was idling just over 1000. Turned the bike off and connected the Motion Pro sync tool with all 4 screws turned all the way out. Saw very little fluid from the #1 carb, but nothing from the other 3. I disconnected the tool and calibrated it using the #1 carb. I believe that I was told before that I shouldn't have to do that we the Mikuni 29 smoothbore carbs, but I did it anyway since I was not getting a reading with the screws turned all the way out. After calibration, I put the tool back on the bike and had it idling just over 1000 again. This time I was seeing fluid from the #1 and #4 carbs up to the second line, but nothing from the other 2 carbs. I pulled the plugs a couple of days ago and the #3 plug was greyish color and the other 3 were black and sotty. Is it possible that the neddle needs to be lowered ? The bike has 120 main jets and 25 pilot jets. Is that correct for a bike with a 4 into 1 Vance and Hines pipe with no baffle and K&N pods ? Sorry for the long entry.

BIGKEVIN wrote: ...Is it possible that the neddle needs to be lowered ? The bike has 120 main jets and 25 pilot jets. Is that correct for a bike with a 4 into 1 Vance and Hines pipe with no baffle and K&N pods ?....

For purposes of carb sync, it's all pilot circuit.
Main jet size has no influence.
Jet needle clip position has no influence.

Would suspect the 25 pilot jet is okay for sync purposes.

Here's a better copy of paragraph 8 from Sudco's smoothbore sync instructions.



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