The Basics Of Jetting *This Expains It* - Yamaha Raptor Forum
 

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Old 04-28-2008, 07:52 PM   #1 (permalink)
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Default The Basics Of Jetting *This Expains It*

Pilot Screw - Idle
Pilot Jet - Idle to 1/4 throttle
Needle Jet and Jet needle - 1/4 to 3/4 throttle
Main Jet - 3/4 to full throttle

1) Idle Screw
Allow the engine to reach operating temperature. turn to smooth idle. If unable to obtain smooth idle, go to Step 2, Pilot Screw

2) Pilot Screw
If unable to obtain smooth idle: turn the pilot screw in all the way GENTLY until it bottoms, then turn the pilot screw 1/4 turn at a time until achieving a smooth idle. The pilot screw should be correct between 1- 2.5 turns. If idle is between 1-2.5 turns, the jump to step 4, Main Jet. If idle is NOT smooth between 1-2.5 turns, then jump to step 3, Pilot/ Slow jet.

3) Pilot/Slow Jet
If idle is not smooth with pilot screw between 1-2.5 turns, then the pilot/slow jet needs to be changed. There are 2 choices. either A or B
A - If the pilot screw has been turned more then 2.5 turns to smooth the idle, then install a bigger Pilot.
B - If the pilot screw has been turned less the 1 turn to reach smooth idle, then install a smaller Pilot.

Note! A larger pilot jet will eliminate the popping sound that may occur when letting off the throttle.
A larger will improve cold-weather starts.

4) Main Jet
Start with the richest (largest numbered) main jet and test at full throttle. the engine should "stumble" at wide open throttle (If engine does not stumble, then the engine is not rich enough and a larger main jet should be installed). Once you have a rich "stumble", install the next smaller jet one size at a time until full throttle results in normal operation. Once main jet step is complete, go back to step 2, Pilot Screw, and perform pilot screw adjustments one last time.

Note! If quad runs faster at 3/4 throttle then at Full throttle, then the jets are lean and a larger jet must be installed.

Needle
If carburetor has an adjustable needle, this step CAN be performed. If you have a slight hesitation but NOT a hard stumble between 1/4 -3/4 throttle, then the needle is probably lean. Raise the needle by lowering the clip.

Reading Spark Plugs
Once the main jet is set correctly by dropping a main jet size after the "stumble", confirm the jetting of the engine by reading the spark plug. This works best in top gear in a slight uphill area. After running at full throttle, do the following at the same time: pull in the clutch and stop the engine, DO NOT LET THE ENGINE IDLE! Remove the spark plug amd examine the color. It should be a Light Tan. If it is white, then the engine is running lean and a bigger Main Jet must be installed. If the color is Black, then the engine is running rich and a smaller Main Jet must be installed.

This is taken from the jetting sheet received when ordering from JetsRUs!
Decided to type this for anyone who needed it. Hope it helps!
Joe
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Last edited by joequad16; 03-18-2009 at 06:41 PM.
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Old 04-28-2008, 08:01 PM   #2 (permalink)
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Default Re: The Basics Of Jetting *This Expains It*

Karma for that
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Old 04-30-2008, 07:38 PM   #3 (permalink)
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Default Re: The Basics Of Jetting *This Expains It*

Can we get a Sticky on this 411?
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Old 04-30-2008, 07:45 PM   #4 (permalink)
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Default Re: The Basics Of Jetting *This Expains It*

that does explain things pretty good.....
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Old 04-30-2008, 10:56 PM   #5 (permalink)
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Default Re: The Basics Of Jetting *This Expains It*

thank you!!!
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Old 05-12-2008, 12:51 AM   #6 (permalink)
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Default Re: The Basics Of Jetting *This Expains It*

Need a little clearification. What would the definition of smooth be for idle? I think i have an idea but I'm not positive either. Anyone able to possibly get sound clips. That might be a good added feature to this write up. I'm not dense but I'm in the process of trouble shooting/adjusting my carbs for my new exhaust. and I am seeing this write up being extremely helpful
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Old 05-12-2008, 07:07 AM   #7 (permalink)
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Default Re: The Basics Of Jetting *This Expains It*

Hey whats up ceanern?Try hooking a tach up to it. go by that for a smooth idle.Its going to fluctuate a small amount im sure.Just a suggestion. Mine will be running within the next week to 2 weeks, you guys better be ready
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Old 05-12-2008, 09:03 AM   #8 (permalink)
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Default Re: The Basics Of Jetting *This Expains It*

I'm doing some adjustments. I just ordered new pilots and jets from 167 up to 175 I think is what I ordered in increments of 2 just to ensure I can fine tune it. And I picked up 2 more goodies to see what it does for me.
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Old 05-14-2008, 04:45 PM   #9 (permalink)
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Default Re: The Basics Of Jetting *This Expains It*

Guys, not trying to spoil a good post here, but those recommendations apply to 2-stroke carbs.

For one, Slide Cutaway is used on 2-stroke carbs. Carbs used on 4 strokes, either CV or slide type do not use slides with special angles cut into them to improve airflow. The normal 4 stroke carb you will come across on quads and bikes will use slides with sharp edges.

Furthermore, slide carbs on 4 strokes use accelerator pumps, while 2 stroke carbs don't.

From my own personal findings, I have found that on 4 strokes, the pilot jet basically only add a measurable amount of fuel when the throttle is closed. As soon as you open the throttle, the needle comes into play and does the biggest amount of work. Yes, the pilot jet will always add fuel, but it is so little that I cannot even pick it up with a air fuel ratio meter on my dyno. I have had bikes that gave the same Air fuel ratio with a totally blocked pilot jet or with it open at very small throttle openings.

On 2 strokes it is a different matter. Here the pilot jet can make a very noticeable affect in throttle response right of idle and even low down power.

I feel personally that on 4 stroke carbs that the needle have an affect on fuel supply all the way from as little as 2% throttle opening to 100% throttle.

Why I say that is that I have seen carbs with totally blocked pilot jets, supply the right amount of fuel at way less than throttle , plus you can set up the carb perfectly at full throttle and have the correct main jet size in there to give you the perfect air fuel ratio. Then, you change to a needle and install another with a slightly different tip and it will mess up the air fuel ratio at full throttle, when it was just perfect with the other needle.

The raptor 660 is actually a perfect example of this. With a slightly modified motor I tried using the stock needles and for the life of me, I could not get enough fuel into the motor at high revs, even when using size 200 DJ main jets. Switch to the Dynojet needles and you start over-fuelling that same motor at full throttle with jets bigger than 170. So, needles do have an affect on full throttle.

I would agree that the main jet on a 4 stroke carb will only makes a noticeable difference in fuel supply at 3/4 throttle and bigger, same as a 2 stroke carb.

I have to warn you thought, that using the biggest main jet till you get a stumble and then back off one jet size is not the way to go. Some motors will stumble and splutter at just under 12.7:1 A/F ratio and if you then back off a jet size or two, it will be perfect, but on other motors, I find that they splutter at say 12:1 and richer and you need to back off 3 to 5 main jet sizes before that motor will make its best peak power. Again the 660 raptor can handle loads of fuel before it complains, but it does not perform well if you add to much fuel. A YFZ450 for example complain and start to misfire here and there if you are just slightly rich at around 12.7:1.

Accelerator pump is another matter, which will take to long to explain right now, plus you don't have them on the stock raptor carbs
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Old 05-15-2008, 03:56 PM   #10 (permalink)
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Default Re: The Basics Of Jetting *This Expains It*

I'm running at 145 150 needles with stock pilots and its running lean, i'm not getting full power and the headers turn red after long period of time, i was told to try 150 155 jets and 25 pilots. . . What do you think? would this help my problem? i have a slip on DG pipe and an after market air filter its no K&N its a spongy one with the aluminum clamps (was on bike when i got it), not sure what one though. I'm at 300ft above sea level.

-Kyle
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