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Mikuni HSR 42 - how to set pilot air screw (idle mixture)

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Old Sep 17th, 2011, 02:52 PM     #1
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Mikuni HSR 42 - how to set pilot air screw (idle mixture)

Hello
Today I tried to set the pilot air screw in Mikuni HSR42. In the Mikuni manual we can read:
"The pilot air screw is set at two turns open from the factory. This is the position we have found to be correct most of the time. If the screw position has been altered, gently bottom it and re-open it two full turns. Next, run the engine until it reaches normal running temperature. With the engine idling smoothly, adjust the pilot air screw in slowly until the idle either slows or becomes irregular, then turn the screw out until the engine again slows or begins to idle irregularly. Count the number of turns between the two positions. Set the air screw mid-way between these too-rich and too-lean positions."

I tried do like this so I set two turns out and set about 1000 rpm. Than I adjusted screw in and after 1 turn in the idle risen to 1300 rpm, after 1,5 turn in was about 1450 rpm, two turns in (bottomed screw) it was again about 1000 rpm. All time idle was regular and it didn't slow. Than again I re-open air screw two full turns and I started turn the screw out. After 1-1,5 turn out there was about 850 rpm and when I screwed it out it was still about 850 rpm. But all the time the idle was regular. So how to set it????

I know that these factory Mikuni settings are for "normal" Harley. I have "normal" air filter but my pipes are just stright tubes. The pipes are oryginal but without connection between them. Than there are stright pipes about 22 inch lenght and 1.75 inch diameter.

Is here anybody who really know how to set it, anybody who did it before??
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Old Sep 17th, 2011, 03:08 PM     #2
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Re: Mikuni HSR 42 - how to set pilot air screw (idle mixture)

From the description of your problem it would indicate to me that the pilot jet was too large and perhaps a smaller size would be more suitable for your bike

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Old Sep 17th, 2011, 05:23 PM     #3
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Re: Mikuni HSR 42 - how to set pilot air screw (idle mixture)

My Mikuni instructions state "the engine should have a smooth steady idle between 1/4 and +3 turns out. If more than 3 turns is required, the pilot jet is too rich. If it requires less than 1/4 turn, the pilot jet is too small. I think you are ok at your 1450 rpm setting.
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Old Sep 18th, 2011, 07:55 AM     #4
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Re: Mikuni HSR 42 - how to set pilot air screw (idle mixture)

I set it today. I warmed the engine and took a ride about 8 kilometers, than I connected induction tach to plug wire and I started adjust the pilot air screw. I choose the highest rpm and than I adjusted rpm to 1000 rpm. Than I took again a ride about 8-10 km, and I try adjust it more correctly. I repeated it one more time after short ride. The engine has the fastest idle between 1/2 and 1 full turn out. I set it to 3/4 turn out. But when I measure rpms sometimes a little bit faster were at 1/2 and sometimes at 1 turn out. I think it depended on the temperature of engine so I decided to set 3/4 turn out. One thing is for sure: when I set less than 1/2 turn and more than 1 - the rpms decrease. And I have to say that it seems that now the bike acelerates faster.

What do you think, will it be OK?? Or maybe should I go to shop and check if at this setting the lambda is about 0.9 and CO reading about 3.5%???
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Old Sep 18th, 2011, 03:35 PM     #5
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Re: Mikuni HSR 42 - how to set pilot air screw (idle mixture)

Quote: Originally Posted by Piotra View Post
I set it today. I warmed the engine and took a ride about 8 kilometers, than I connected induction tach to plug wire and I started adjust the pilot air screw. I choose the highest rpm and than I adjusted rpm to 1000 rpm. Than I took again a ride about 8-10 km, and I try adjust it more correctly. I repeated it one more time after short ride. The engine has the fastest idle between 1/2 and 1 full turn out. I set it to 3/4 turn out. But when I measure rpms sometimes a little bit faster were at 1/2 and sometimes at 1 turn out. I think it depended on the temperature of engine so I decided to set 3/4 turn out. One thing is for sure: when I set less than 1/2 turn and more than 1 - the rpms decrease. And I have to say that it seems that now the bike acelerates faster.

What do you think, will it be OK?? Or maybe should I go to shop and check if at this setting the lambda is about 0.9 and CO reading about 3.5%???
Seat of the pants testing IMO will be fine, save your money for more useful things, you are very close to perfection on the setting now report back any changes Best of luck
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