Jetting question

MikeT

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Jan 17, 2001

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• Sep 12, 2002

• #1

When I raced last weekend (on my 1996 YZ125 with a 350 main) the power at FULL throttle would fall off a little (this was on a long uphill 4th gear straight). It didn't pop or just outright shut down, the RPM's would just fall off a little. I don't think it was the rev limiter I think it was the main jet not being perfect. The plug was medium dark chocolate. It ran crisp and clean everywhere else. Do you think my main jet is rich or is the power falling off a sign that it is too lean? :think:

 

H

helio

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Dec 19, 2001

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• Sep 12, 2002

• #2

I am not answering to your question MikeT...But I felt the same 3 weeks ago in my last rider(like the engine cutting the power in 5 gear at full throttle!!!!)...I changed the reeds and rejet the carb before this race in order to have more top and the plug shows a gray color almost white after wide open for 6,7 seconds(I think I am not running lean because I can see a black ring close the bottom of the insulator...), so I decided to order another main 1 size up to test this situation. I am waiting for it.

BTW I remember now to read sometime ago that some CDI´s could be programmed from factory to turn back after reaching the max of rpm...In order to keep the engine under safe conditions...

I am very interested to hear some explanations for your question MikeT

 

W

whyz

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Nov 18, 2001

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• Sep 12, 2002

• #3

So it went flat?
When the YZ we have was doing it,it was alittle lean on the MJ.
It was alittle colder back then too.

I'm still learning.
See Ya guys.....

 

Last edited: Sep 12, 2002

MikeT

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• Sep 13, 2002

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• #4

Anyone?

 

J

JTT

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Aug 25, 2000

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• Sep 13, 2002

• #5

Mike, I would try going one richer on the main and see how it works in the same situation. Going richer is not going to hurt anything, and it almost sounds like you are lean.

Where on the plug were you seeing the colour? Remember Rich's threads on plug reading, and how many variables effect it, and where you should be looking ;)

 

MikeT

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Jan 17, 2001

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• #6

I remember Rich's thread. The color I am seeing is all over the porcelin. The first two threads on the plug are also wet. No spooge out the back though.

 

Jaybird

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Mar 16, 2001

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• Sep 13, 2002

• #7

Mike...Mike...Mike.....I thought you had hashed out jetting on many occasions before?
The color on the insulator will tell you just about squat. You want to go full throttle for a bit under a good load and then chop the throttle while holding the kill switch in. Then you need to look ALL THE WAY down inside to the bottom of the insulator for a black ring. At 1mm or less of ring you will be close to correct main jet wise. Get a flashlight and a magnifying glass to see this....almost impossible otherwise.

 

MikeT

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Originally posted by Jaybird
Mike...Mike...Mike.....I thought you had hashed out jetting on many occasions before?

Click to expand...

So did I. The jetting on the bottom and middle is perfect, but until last weekend I hadn't had a long enough straight to do a full throttle run to check the main. I will try the magnifying glass and flashlight idea.

 

truespode

Moderator / Wheelie King

Jun 30, 1999

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• Sep 13, 2002

• #9

I would definately try the ride and feel. Take a larger main with you next time you ride a track like PVMX and try it out.

Good luck man... I hope you don't get too much more power. You left me pretty good as it was :)

Ivan

 

J

James

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• Sep 13, 2002

• #10

Well I can vouch for the fact that TOO RICH will make it loose power on top too. My guess would be that it was a touch rich considering the temp and humidity. Even after I got my main jet back in, mine felt a little weak. I think it was the pure race gas that I was running (forgot to mix it with pump) becuase I was exactly on that previous Monday.

 

Rich Rohrich

Moderator / BioHazard

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• Sep 13, 2002

• #11

Fuel requirements are most significantly affected by the throttle position and load you place on an engine. If you jetted the bike spot on for WOT on a relatively flat surface the fuel required at WOT in fourth gear going up a hill (greater load on the engine) will be MUCH GREATER. One more time. :)
If you aren't jetting your bike under the same types of loads you'll see when riding then the jetting won't be right if the load changes significantly.

Run the hill with the choke pulled up a bit and see if it improves the performance. Either that or bump the main 2 steps richer so there is no mistaking if you are going in the correct direction.

 

Last edited: Sep 13, 2002

MikeT

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Jan 17, 2001

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• Sep 13, 2002

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• #12

Check Commander! I will switch the mainjet next time out.