Optimal jetting for maximized performance

The first time I read JD's tech article on jetting with needles was in 1992 when I was writing for Dirt Rider magazine and publishing a newsletter called MotoTECH.
He asked if I could influence the editors of DR to publish it, but unfortunatly that magazine is written for a 5th grade reading level and they rejected JD's article. JD has written what I'd equate to a Master's Theisis in jetting. Its probably the equivalent of a graduate level project in mechanical engineering (360 hours) but I'm afraid that its largely wasted on the general dirt biking public and there is no layman's interpretation of his work.

Before you can get the real message behind his excellent tech article, you have to be able to differentiate between rich and lean. That seems to be the biggest stumbling point to riders, even great riders. Thats the biggest mystery to me because my contention is that if you can ride a bike you can jet it. When I was 13 years old I bought a copy of Carl Shipman's book on jetting a motorcycle and Olav Aaen's book on tuning snowmobile carbs.

Those books were heavy on relating throttle position to jet circuits. I still remember taping and marking the half-turn throttle of my Hodaka in joyous experimentation of jetting. Later on I met Bernie Kozad of G.E.M. Products in Carol Stream Illinios. He published the first Mikuni guide to needle diameters at 5mm height increments, and they stocked every needle for Mikuni round slide carbs and I spent the majority of my lawn-mowing income on needles. I had a blast jetting bikes and developed a local reputation as a guy who could work out just about any jetting ill and that later became the basis for my engine building business.

Although I've written articles on the ride and feel method of jetting and given thousands of specific jetting specs in net forums like this one, and provided detailed explanations of why people can't take specs from a magazine or telephone conversation and apply them to their bike and make it run perfect all year round, I still get the daily emails that go like this;
"I'm a 40 year old beginner with a 1990 something 250cc Jap dirt bike and I'm planning a trip with my buddies in a few months, I use the cheapest brand of gas I can get, my bike has a v-force rad valve, and a blown silencer, the trails will probably be muddy and I'll be wearing Fox boots with green socks. What will my jetting need to be in order for me to beat my buddies. PS if you don't email me back in 15 minutes I'll tell everybody on the Internet that you suck!" OK maybe I'm being a bit sarcastic but the point is jetting is subjective. Its specific to every different motorcycle and rider for any given moment of the day as clouds pass overhead and the sun beats down raising and lowering the air density. The atmosphere is like an invisible ocean.

I've tried to encourage people to try different jets in their carb and give them some point of reference to start from. Everybody has a different level of sensitivity for jetting. Take for example the differences between Rich Rohrich and I. Rich shows up for riding equipped with a well defined set of carb parts, a cooler with a pint of MTBE, and a $500 altitude density computer. Meanwhile I've got a sliding choke lever mounted to the handlebar of my KTM. Rich will spend half the day fiddling with his jetting to get the stoicimetric ratio pinpointed while I lumber around adjusting my choke lever becauise I'm too lazy to work on my own bike. Rich lives for that corner to corner acceleration that makes him squeeze the bars like he's on a carnival ride whereas I'm happy just to get a free hour from work to ride a bike.

You wanna get the most from JD's tech article? Buy a bunch of carb parts, tape and mark your throttle, and go find some hills to play on. Have fun changing jets and evaluating the performance of your bike. Once you get the hang of it then re-read JD's article and start playing with different needles and settings. Then maybe if you're lucky you'll experience the same g-forces that Rich does when his bike is digging a trench and spitting up everything off the track while fuel and air particles collide in perfect stoicimetric fashion inside the engine's combustion chamber. G-forces like that tend to distort your mouth into a smile shape :cool:

I lumber around adjusting my choke lever becauise I'm too lazy to work on my own bike. Rich lives for that corner to corner acceleration that makes him squeeze the bars like he's on a carnival ride whereas I'm happy just to get a free hour from work to ride a bike.

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oh my goodness that just tickled me lol I Love It!!!! the vast knowledge , the countless hours of skinned knuckles and metal splinters in the search of perfection and then the real world of " hey its time to ride lets gas up and go"

but sheesh you didn't have to quote me

"I'm a 40 year old beginner with a 1990 something 250cc Jap dirt bike and I'm planning a trip with my buddies in a few months, I use the cheapest brand of gas I can get, my bike has a v-force rad valve, and a blown silencer, the trails will probably be muddy and I'll be wearing Fox boots with green socks. What will my jetting need to be in order for me to beat my buddies.

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by the way its old alpine stars and grey socks which by the way means I had to richen all jets place a 1/2mm shim on TOP of the needle clip and then lower float hieght to compensate.

I think people also have to aware that sometimes perfect jetting does not result in huge amounts of power. they sometimes expect too much from their bikes. Perfect jetting to me means that the bike runs clean, produces the maximum power that is possible for the engine combination and naturally never fouls plugs.

I find my jetting to be much different to others as I run Amsoil synthetic oil at 75:1 (never had a problem in 15+ years). My only issue is with jetting my TMX carburator at low rpm's. I have a large selection of pilot jets (every one available) and have had no success with being able to get any adjustment out of the pilot screw (should be sensitive within a 1/8-1/4 turn of the air mixture screw. It loads up a bit at slow rpms, but never fouls a plug. I live with it because I believe it has a lot to do with the porting/exhaust etc that has been done to the bike. In the end, I just want to ride.

As a side note, I have found that black socks at knee level attract more heat which effectively lowers the air density on either side of the airbox resulting in a richer mixture. I prefer to select my sock colors depending on the temperature (white for hot days, black for cold days). That way I can fine tune the air density at the airbox and resulting air/fuel mixture. LOL :confused:

I was pleased to see a more philisophical discussion regarding jetting! hopefully, if we still have eric's attention and if we get Rich Rochrich's attention i have a suggestion. Please dont think i am being too forward, but why dont you guys put a book together regarding the jetting of modern motorcycles? I am sure you guys are busy people, but i bet it would be a good seller, and there are lots of potential chapters there!

My motivation for saying this was outlined earlier, the problem is, everyone has THEIR setting and it might not be of use for YOU. My YZ was suffering from bad spooge, but rather than post on here for some settings or go to some MXA or FMF setting i contacted one of the more experienced guys on the board (with little experience with YZ's however) personally for help....this gave me a good basic grounding on where to start. A few small adjustments later and the spooge is nearly gone and the bike runs very good (still more slight adjustments to come also), and i now understand more on how MY bike and MY carb works. if i had posted, i might have got lucky, but the chances are i may have got a wrong setting.. ..ive read nearly all of the jetting posts and they can get very messy! then throw fuel, mixture ratio and oil type in there and it gets really messy.

wouldnt it be nice if every time you got the question

"I'm a 40 year old beginner with a 1990 something 250cc Jap dirt bike and I'm planning a trip with my buddies in a few months, I use the cheapest brand of gas I can get, my bike has a v-force rad valve, and a blown silencer, the trails will probably be muddy and I'll be wearing Fox boots with green socks. What will my jetting need to be in order for me to beat my buddies."

you could give the answer:

"send me a check, and i will give you a signed book on how to figure it out yourself!"

Thanks in advance for listening!

BC