Author Topic: Mikuni HSR 42 tuning question  (Read 938 times)

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Offline timvan

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Mikuni HSR 42 tuning question
« on: Wednesday, March 25, 2009. 05:58:02 AM. »
Trying to get my mik sorted out...all seems well but starting and idling.. The rps will stay up after revving the motor a bit no matter where I turn the air bleed. Im assuming the pilot is too large? No air leaks either.

Thanks In advance

Tim

Offline ederdelyi

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Re: Mikuni HSR 42 tuning question
« Reply #1 on: Wednesday, March 25, 2009. 06:56:16 AM. »
From the Mik HSR Tuning manual (available for download from the Mik website):

NOTE:
Make sure that the stock Harley-
Davidson choke cable is fully seated in
the metal elbow at the carburetor end
of the cable assembly (see Installation
Instructions). Harley ‘s cable is stiff and
can fail to fully seat in the elbow. This
condition results in poor mileage and a
poor idle.
The Mikuni choke cable is more flexible
and less likely to jam. Still, it is best to
check to be sure the cable is installed
correctly.

The HSR idle system has two
tunable components: the Pilot Air
Screw and the Pilot Jet. The air
screw’s purpose is to fine-tune the
mixture at idle. The pilot jet controls
the total amount of fuel passing
through the idle system. The pilot
jet can be exchanged for a richer or
leaner one if needed (see the note
on the following page).
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.
Use the Idle Adjuster to re-set the
idle speed.
If the engine becomes too hot
during the adjustment procedure, the
resulting idle mixture will prob
ably be on the lean side of correct.
If you have a large fan, use it while
adjusting the mixture. If you do not
have one, you may need to take time
out for a short ride to cool the engine
back to normal temperature.
NOTE:
1. If the best idle is achieved with the air
screw less than one turn out, the pilot jet
is too small and should be exchanged for
a larger one.
2. If the engine speed does not slow after
two-and-a-half turns out, the pilot jet is
too large and should be exchanged for a
smaller one.

Offline timvan

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Re: Mikuni HSR 42 tuning question
« Reply #2 on: Wednesday, March 25, 2009. 01:58:55 PM. »
Guess im correct in my pilot jet assumption

Offline ederdelyi

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Re: Mikuni HSR 42 tuning question
« Reply #3 on: Wednesday, March 25, 2009. 02:40:48 PM. »
If the pilot air screw has no effect at either extreme then the problem may lie elsewhere. That's why I included the note on the enrichner cable. The air screw should affect idle at one extreme or the other. How big is the motor and what size pilot jet do you have in it now?

Offline timvan

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Re: Mikuni HSR 42 tuning question
« Reply #4 on: Wednesday, March 25, 2009. 05:09:04 PM. »
The air screw does make a difference..its actually the rivera dual carb setup, ones a dummy with a pilot and needle...other is the full carb. Dont really like the setup but its my Friday night bike. Motor is a 117" S&S, not sure on current pilot, have to pull the bowl

Offline ederdelyi

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Re: Mikuni HSR 42 tuning question
« Reply #5 on: Wednesday, March 25, 2009. 05:23:01 PM. »
Aha! That do make a difference. I assume that they are linked together (both open/close together) and not progressive? I've seen both setups. In either case, you need to make sure that both carbs are pulling the same amount of air and the throttle plates are truly in sync. Ever heard of a tool called a Uni-Syn? Don't know if they still make 'em but they sure did work good for syncing multiple carb setups. Perhaps the best way without a vacuum guage or Uni-Syn type tool would be to make sure that both carbs hit the throttle stops at WOT and the the slides are equally open, then set the idle speed as close as you can and again try to ensure that the slides are open the same amount ... then attempt to adjust the idle mixture. That setup can be a bear to get right. Good luck with it.

Offline timvan

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Re: Mikuni HSR 42 tuning question
« Reply #6 on: Wednesday, March 25, 2009. 05:38:27 PM. »
yes they are synched, it is a complete pita system which the previous owner put on. Guess he liked the looks. Id pull it but cant see spending the $$ on my toy bike right now