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When I quickly open the throttle at high rpm the engine

Customer Question
When I quickly open...

When I quickly open the throttle at high rpm the engine hesitates and looses power before accelerating.

Mechanic's Assistant: How old is the transmission? Have you checked the fuel filters?

Transmission has 20k miles and is 18 yrs old

Mechanic's Assistant: What is the model of your '18 motorcycle?

Cleaned and synched carbs and replaced fuel filter ~ 4k miles ago 2001 yamaha vstar

Mechanic's Assistant: Are you fixing your bike yourself? What have you tried so far?

I am fixing the bike myself and I have done nothing beside pm.

Submitted: 2 months ago.Category: Motorcycle
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Answered in 9 hours by:
6/1/2018
Motorcycle Mechanic: Louie, Technician replied 2 months ago
Louie
Louie, Technician
Category: Motorcycle
Satisfied Customers: 13,498
Experience: 25+ yrs. experience
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Hi and welcome to JustAnswer; an independent site with members providing product information, repair tips and technical assistance as a form of paid Q & A service.

My name is ***** ***** I'll be assisting you with your question.

I apologize for the extended wait time you experienced, I've only just come online now and would like to assist you with your question.

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Motorcycle Mechanic: Louie, Technician replied 2 months ago

Initially, let's try check and review the basics. Proposed preliminary tries (if have not been done) and even if just for test purposes:

  • • remove the spark plugs to be able to check the burn color of the tips;
  • • to determine whether mix is lean or rich;
  • • ensure original air box and filter correctly/tightly fitted;
  • • with no possibility of an air leak.
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Customer reply replied 2 months ago
Rear plug is on the left front on the right in both pictures
Motorcycle Mechanic: Louie, Technician replied 2 months ago

Appreciate the pictures and yes, front appears to be lean.

If no air leak, then possibly adjustment needed on the needle valve height. Consider/try raising the front carb's needle by lowering the circlip lock to compensate/correct lean condition.

The alternative is to redo the (front) carbs.

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Customer reply replied 2 months ago
I wasn't able to work on the bike this weekend I will get to it next weekend. If the needle valve adjustment does not fix the hesitation should I look at float valve height and fuel pump next? Could valve lash being out of spec cause the hesitation?
Customer reply replied 2 months ago
Should I make the needle adjustment on front only or both carbs?
Motorcycle Mechanic: Louie, Technician replied 2 months ago

JUst on the front please.

And will start with this adjustment as both an attempt of a fix and as a test by process of elimination.

The objective is adjust to make both burn colors nearly the same first and then make both slightly rich eventually (if hesitation persists).

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Customer reply replied 2 months ago
Can we keep this question open until next weekend when I can get to adjusting the needle and testing the bike
Motorcycle Mechanic: Louie, Technician replied 2 months ago

No prob on my end; post an update when conveniently able and I would get a prompt to reply soon as able too.

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Motorcycle Mechanic: Louie, Technician replied 2 months ago

Appreciate the pictures and I understand.

Yes on use of a shim / small flat washer to raise the needle.

Also consider (even if just for test purposes) adding a layer or 2 of (generic) air filter over the "...k&n washable filter...".

The general idea to correct the lean condition is to allow more fuel and/or restrict air entry.

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