General Jetting Tips

[husker525]

Found this on the web, thought it was basic but to the point:

Note: Jet sizes increase in different increments. A 125
main jet may have a 127, 127.5, or 130 as the NEXT
larger jet size.
Keihin, Mikuni and Aftermarket jet kits use different
numbers to indicate jet orifice size. A 125 Keihin jet is
not the same orifice size as a 125 Mikuni jet. Jets also
have different thread sizes. Always use the same jet
design and brand as original.
Starting point for stock engines with bolt-on
modifications: Increase main jet size by at least 3
sizes. Move needle clip down one position (richer).
Starting point for modified engines with larger
displacement: Increase main jet size by at least 4.
Move needle clip down one position (richer). Pilot
jet usually stays stock.
JETTING BASICS
It is the responsibility of the owner to determine
proper jetting for their engine.
These jetting specifications are designed as a rule of thumb. They are
in no way absolute. Variations in air density, specific gravity of fuel, altitude
and other engine modifications play a large part in jetting. Newly
built engines need rich jetting during break-in. Do not jet for power
until an engine is broken-in.
Plug reading may not work. Revving the engine while it is sitting in the
garage doesn't work. Other than Dyno testing, the steps below is the
simplest way you can jet your engine.
1. Find a gentle slope that you can ride in 2nd or 3rd gear. Look for
something that will put a reasonable load on the engine. This will be
your “dyno”.
2. A basic outline of which jet is active at a particular throttle setting:
Pilot Jet = 0 to 1/4 throttle. Needle = 1/4 to 3/4 throttle. Main Jet = 3/4
to Full Throttle.
● Changing the Main Jet size won't affect how your engine idles or
runs at 1/4 throttle.
● Engine RPM isn't what determines which jet is active - throttle position
does.
3. Make the recommended jetting changes. Always start rich and
work leaner.
4. Start and warm up the engine, then ride your "dyno hill". Any point
where you feel the engine stumble or hesitate indicates a tuning problem.
Note the throttle position and modify the corresponding jet (1/4 to
1/2 throttle = needle. 3/4 throttle or more = main jet).
5. Only change jetting by 1 step at a time, and 1 circuit at a time (don't
change needle and main at the same time). Re-check after every
change.
6. Once the engine runs smoothly throughout, you're jetted!
If you ever notice an abrupt loss of power, or
engine sounds change, shut it down.

[RevUsry]

Pin it.

[Terse]

Nice find Bobby, and I want this pinned too. Soooooo...........

[fever2ride]

NICE!! This helps a LOT!!!! Jetting has always confused me.. I'm a late model car guy, so carbs are new to me..

Based on this, the advice I got from here is pretty dead on.. No problems other than off idle.. but that can be fixed now.. :D

[grimraptor250r]

Originally Posted by fever2ride

NICE!! This helps a LOT!!!! Jetting has always confused me.. I'm a late model car guy, so carbs are new to me..

Based on this, the advice I got from here is pretty dead on.. No problems other than off idle.. but that can be fixed now.. :D

Topic bookmarked
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[jman05]

if i have popping at open throttle does that mean my main is to lean or rich? i cant seem to get it right....

[jman05]

if it helps i have a little more than average red headpipe

[burkemotopros]

Originally Posted by jman05

if it helps i have a little more than average red headpipe

You are running too lean when it is popping and the red head pipe is another give away.

[jman05]

it seems like its popping more 1/2-3/4 throttle. i am running 45 pilot. 175 main. 4th clip(im assuming from top...) and 2.5 turns out on a/f screw. any idea or any help..

[jman05]

also, screwing the a/f screw out would give me more fuel, correct?

[foxyone1277]

hey all, im having some problems myself. i put on a k and n filter and pre filter. after they were put on i cant go from zero to 100 any more i have to ease into the throttle. i tried the fuel screw, same result. stepped up the main, same result...... any help would be appreciated...... please pm me if you would.

[smock19nathan]

maybe your bike is bogging ( when you nail it off an idle it wants to die, but if you ease in and then nail it... it will go ahead and stay running?) thats got a little to do with other things than just jetting...

[Joester]

can you guys help me too? :P My 2007 has a DMC full exhaust, pro design intake (possibly being replaced with an FCI) and i had it jetted 45/168/2.5 out IIRC. that was the wring jetting too, i think that might have been a little rich, im not sure. Now, i have a 12.75:! CP piston, and im getting a mild port job done on it. how do i jet for HC pistons and porting?

thanks!

[Bonesx0x2x4x]

ok i got a question...how much of a size difference have some of you seen from an 80 degree summersay to a 40 degree winter day?...im talking ll jets not just main

[husker525]

Originally Posted by Bonesx0x2x4x

ok i got a question...how much of a size difference have some of you seen from an 80 degree summersay to a 40 degree winter day?...im talking ll jets not just main

That totally has to do with air density, and humidity. If you want max performance, then technically you could change the pilot jet, the needle setting and the main jet to achieve the ultimate jetting for each temperature change. The 4 stroke motor is much more resiliant to temperature and barometric changes than its 2 stroke cousin. A 4 stroke can run a little rich or a little lean, and not damage the motor, unlike it's cousin.

[heatscore87]

can you guys give me an idea of what i should do with my jetting to fix my bog. elevation is roughly 2500 feet,k&n intake with airbox lid off, slip on exhaust, and cam mod. and is there ay real diffrent from slip on to full exhaust?

[DwnDak]

I have an issue with my 04 I just bought.... The sleeve is a bit different color then the head... and we're assuming its rebuilt as internally it looks great... from what we can see.

Anyways, I'm at 550ft above sea level, its average temps outside, low humidity etc... But the pipe glows at idle, and it works good for the most part but after about 15 mins of average riding (not hard on it or much reving) it stumbles on its face and dies out.. Almost like its flooded itself, and it won't start for a good 5 mins afterwards of just sitting.

So I checked the carb its mint clean... the guy had I believe (tryin to remember) a 145 main, and a 65 pilot in it. The clip was on number 4 down from the top, mixutre screw was out 3.5 turns.

From what I found online that setup is... lets say... a bit off.. ?? So what I did was put the stock 155M and 45P back in it... on third clip from the top, and the air screw out 2.5 from closed *I know stock is 1.5 but a bit richer can't hurt performance too much..

I'm waiting to get my thumb throttle in from the dealer to test this new setup as the bike had a twist, single cable, and it was dangerous... I'm also wondering since it was a single cable twist if it wasn't closing quick enough/correctly and as I'm gearing down into a corner it was staying open too much and falling on its face???

I've got a FMF pipe and powerbomb header, and a complete K&N Cone and lid setup. And the plug looked good when I pulled it out... real good.