Stock Carburetor

Jetting the stock Roadstar Mikuni 40 carb

Created by: BubbaKahuna

Published: 03-05-2018, 04:45 PM

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Jetting the stock Roadstar Mikuni 40 carb

PRINCIPLES OF MODERN MOTORCYCLE CARBURETOR FUNCTION
Specifically in application to the Yamaha Road Star
By Ken “the Mucker” Sexton
August, 2007

Whatever the motorcycle or automobile, virtually all carburetors (or “carbs” for short and not to
be confused with the “carbs” which can affect your waist-line) work on the same principles and use
similar internal systems to deliver fuel in the proper air/fuel ratio to the engine. Depending on the...

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Dialing in your PMS

Created by: Tykes_Place

Published: 03-03-2018, 04:45 PM

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Dialing in your PMS

There are several methods of approaching the process of dialing in the PMS on the stock carb. This article outlines one method that can used, and which is designed to balance out the other circuits in the carb for best driveability.

The Pilot circuit (which the PMS screw controls) is responsible for primary fuel delivery at idle and throttle positions below about 1/4 turn (see Bulldog's article for more complete explanation).

All circuits in the stock carb are functional...

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The WOT test (Wide Open Throttle)

Created by: Tykes_Place

Published: 03-03-2018, 04:45 PM

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The WOT test (Wide Open Throttle)

This is a living document and will be ammended from time to time, as needed to make corrections, and as appropriate to accomodate new things as we learn them. We are still learning the benefits and potential misreads that can come from this test, but at this point the feedback has been very good, and it has helped to identify lean conditions in many main jets.

This is a test that is fairly well known in Harley performance circles, and is now being adopted and adapted to the Road Star....

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The "At witt's End Backfire Cure"

Created by: Tykes_Place

Published: 03-03-2018, 04:44 PM

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The "At witt's End Backfire Cure"

Reprinted by permission from Mr Tidy's tech tips

I report this as a last ditch effort for those who may experience extreme situations where their bikes are persistently backfiring through the intake OR exhaust, AND conventional efforts to fix the problem prove fruitless.

It is not a panacea for problems that might be fixed through normal jetting and adjustments. I do want to stress that this is not the best way to fix MOST backfiring problems. Usually the correct fix is...

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Setting up and Adjusting your TPS

Created by: Tykes_Place

Published: 03-03-2018, 04:43 PM

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Setting up and Adjusting your TPS

The TPS adaptor on the carburetor is responsible for reporting the position of your throttle to the ignition (ignitor). The ignition uses this information to adjust the timing curve on the bike to match driving conditions.

We have seen some broken TPS's that failed to do so, and the bikes they came in on were somewhat sluggish to drive. To assist those who may need to check, adjust, or replace their TPS adaptors we put together this brief paraphrase of the instructions...

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Rejet Your Carburetor

Created by: Tykes_Place

Published: 03-03-2018, 04:41 PM

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Rejet Your Carburetor
Reprinted by permission from Mr. Tidy's Tech Tips

Re-Jet Procedure
Many thanks to "Thor"

This is how to change your jet and needle, for an explanation of how your Carburetor works. Stargazer has a great article by the Mucker at his site. The jetting is not terribly complicated if you remember this:

Do not disconnect the throttle Position sensor from the Carburetor.
Remove the nut that holds the choke cable on the left side, and remove the

...
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Principles of Modern Motorcycle Carburetor Function

Created by: Tykes_Place

Published: 03-03-2018, 04:39 PM

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Principles of Modern Motorcycle Carburetor Function

Reprinted by Permission from Mr Tidy's Tech Tips

Specifically in application to the Yamaha XV1600 Road Star

Whatever the motorcycle or automobile, most carbs work on the same principles and internal systems to deliver fuel in the proper mixture ratio to the engine. The actual components within the carb(s) that use those principles vary quite a bit, but their ultimate execution remains the same. They can be broken down into separate “circuits”, called that because, like...

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Last edited by Tykes_Place; 03-03-2018, 06:16 PM.
Pilot Jet and PMS explained

Created by: Tykes_Place

Published: 03-03-2018, 04:35 PM

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Pilot Jet and PMS explained

Some of you have recently sent me notes asking for clarification between Bulldog's Jetting Primer and Mucker's Principles of Carburetion, with respect to what sounds like different explanations of how the Pilot circuit works in the Road Star's stock Mikuni carburetor.

A careful reading, and a little more understanding about the differences between how gasses flow (air) and liquids flow (gasoline) will help to clear this up.

After a thorough examination of one of my extra...

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Mikuni/Dyno Jet Conversion Table

Created by: Tykes_Place

Published: 03-03-2018, 03:51 PM

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Mikuni/Dyno Jet Conversion Table

Reprinted by permission from Mr. Tidy's tech tips
Mikuni / Dynojet Jets

...

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How To Adjust the Accelerator Pump

Created by: Tykes_Place

Published: 03-03-2018, 03:44 PM

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How To Adjust the Accelerator Pump

Reprinted by permission from Mr Tidy's tech tips

On the 40mm Mikuni Carburetor

One of the problems I've had with the Road Star is a coughing or backfiring out the carburetor through the breather. One of the cures is to decrease amount of fuel injected by the accelerator pump when first opening the throttle.

Look in the lower right-hand portion of the image, this is the area of the accelerator pump.

Duration adjustm...

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