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Author Topic: Jetting help  (Read 906 times)

Offline Flyinrazrback

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Jetting help
« on: June 05, 2018, 08:46:29 am »
All, looking for some starting jetting recommendations. 87 top end with moderate porting, o-ring head and dome set to run on 110, stock carb bored to 41.5mm, Vforce 3 reeds. SB Designs Airbox Eliminator, Dr. Q 2.0 pipe. I was thinking maybe a 40 pilot and 460 main.

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Offline Q2W

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Re: Jetting help
« Reply #1 on: June 06, 2018, 07:53:34 am »
Man, do the 2.0 pipes need that much main?

How worn is the slide on your carb?

If it was me, I'd start around 35/440-450

What position is the needle?

Offline Flyinrazrback

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Re: Jetting help
« Reply #2 on: June 06, 2018, 11:11:19 am »
The slide is tight and current the needle is on the 2nd clip.

Offline MotorGeek - Jerry Hall

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Re: Jetting help
« Reply #3 on: June 07, 2018, 07:37:16 am »
Man, do the 2.0 pipes need that much main?

How worn is the slide on your carb?

If it was me, I'd start around 35/440-450

What position is the needle?

A worn slide or slide channel will not affect the main jet size, it affects the idle mixture and the repeatability of the idle speed each time the throttle is closed. 

The main jet size is affected by the restriction of the muffler/stinger and the restriction, resonance of the air filter and intake pipe design.

An exhaust system that has more restriction in the muffler and stinger will require smaller main jets than an exhaust system that has a larger and or higher flow rate outlet.  An intake system that has more restriction will require smaller main jets.

Porting does not affect the main jet size very much unless the porting is really screwed up, it is the exhaust and intake restriction that has the largest effect.

Stingers and mufflers that have a lot of time on them will have more restrictions because the layer of carbon inside of the these passageways reduces the flow. 

Highly developed pipes will often need the inside of the pipes cleaned/ decarbonized every 50 hours or so.  Pipes with too much carbon on the inside of the pipe will also raise the RPM where the engine makes its peak power and torque.

Offline MotorGeek - Jerry Hall

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Re: Jetting help
« Reply #4 on: June 07, 2018, 07:57:35 am »
All, looking for some starting jetting recommendations. 87 top end with moderate porting, o-ring head and dome set to run on 110, stock carb bored to 41.5mm, Vforce 3 reeds. SB Designs Airbox Eliminator, Dr. Q 2.0 pipe. I was thinking maybe a 40 pilot and 460 main.

It should have a 6DK3 needle in the 2nd or 3rd clip position from the top clip position with a R-2 to R-6 389 series needle jet.  If the carb does not have these two items in it, none of the below information is valid.

Start large enough on the main jet to make it miss-fire at high RPM in the 3rd or 4th gear and then lean it down so that it does not miss-fire.  The Q pipes seldom use main jets smaller than a 450 or larger than 500.

Put a pilot jet in it than produces the correct running characteristics at an idle when the air screw is about 1.5 turns out.  The porting, pipe, and intake system do not usually affect the pilot jet size.  I seldom see the TM38/41.5 Mikuni use a pilot jet outside the range of 22.5 to 30.


Offline Flyinrazrback

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Re: Jetting help
« Reply #5 on: June 07, 2018, 07:59:32 am »
Jerry, it has an 88+ carb, does that affect the pilot jet size?

Offline MotorGeek - Jerry Hall

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Re: Jetting help
« Reply #6 on: June 07, 2018, 08:04:11 am »
No.  I would expect it to run correctly with a pilot jet somewhere in the range of 22.5 to 30.

Offline Q2W

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Re: Jetting help
« Reply #7 on: June 07, 2018, 08:42:04 am »
Man, do the 2.0 pipes need that much main?

How worn is the slide on your carb?

If it was me, I'd start around 35/440-450

What position is the needle?

A worn slide or slide channel will not affect the main jet size, it affects the idle mixture and the repeatability of the idle speed each time the throttle is closed. 


Right, that's why i asked the question.  More often than not on these old bikes the slides are so worn it makes jetting the pilot a headache.  I replaced my slide about 10 years ago and was able to drop from a 42.5 to a 32.5.

I guess it makes no difference now because the slides have been discontinued.

Offline Flyinrazrback

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Re: Jetting help
« Reply #8 on: June 07, 2018, 12:42:30 pm »
.

Offline mike42

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Re: Jetting help
« Reply #9 on: June 12, 2018, 06:54:42 pm »
Man, do the 2.0 pipes need that much main?

How worn is the slide on your carb?

If it was me, I'd start around 35/440-450

What position is the needle?

A worn slide or slide channel will not affect the main jet size, it affects the idle mixture and the repeatability of the idle speed each time the throttle is closed. 

The main jet size is affected by the restriction of the muffler/stinger and the restriction, resonance of the air filter and intake pipe design.

An exhaust system that has more restriction in the muffler and stinger will require smaller main jets than an exhaust system that has a larger and or higher flow rate outlet.  An intake system that has more restriction will require smaller main jets.

Porting does not affect the main jet size very much unless the porting is really screwed up, it is the exhaust and intake restriction that has the largest effect.

Stingers and mufflers that have a lot of time on them will have more restrictions because the layer of carbon inside of the these passageways reduces the flow. 

Highly developed pipes will often need the inside of the pipes cleaned/ decarbonized every 50 hours or so.  Pipes with too much carbon on the inside of the pipe will also raise the RPM where the engine makes its peak power and torque.

How do you decarbonize a pipe?  Im a noob.

Offline MotorGeek - Jerry Hall

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Re: Jetting help
« Reply #10 on: June 13, 2018, 10:09:26 am »


How do you decarbonize a pipe?  Im a noob.

Decarbonizing a pipe is a time consuming dirty job.  The best method I have found is to take a torch and heat every square inch of the whole pipe surface to a dull red color.  This will crystallize the goo and hard carbon and loosen it's adhesion to the inner pipe wall.  Now put about a quart of 1/4 inch size sharp edged gravel inside the pipe and shake until all of the scale is gone.  The three major problems with this method is it sometimes distorts the pipe to the point that it no longer fits the bike, it destroys the nickle or chrome finish and may weaken the metal if you get the metal too hot. 

Another way is to find an engine shop that has the old fashion hot tank with the caustic cleaning solution.  The problem with this method is finding a way to pump the cleaning solution through the inside of the pipe so that it contacts every square inch of the inside of the pipe for a long enough period of time to remove all of the carbon and goo. 

The silencer's perforated core and the tube from the pipe to the silencer must be decarbonized.  This is very important because a pipe that is a little restricted in these areas can cause severe piston overheating and piston failure!!!
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Offline All American

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Re: Jetting help
« Reply #11 on: July 17, 2018, 01:10:51 am »
I didnít know any of that motogeek thanks
1-1985 SUZUKI LT250R
1-2013 HONDA TRX450ER

 

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