Carb Jetting Tutorial

[Deleted member 12676]

A few main things reduce engine power such as intake air leaks, head gasket leak, incorrect ignition timing, and incorrect carb jetting. If you have been smart enough to put a fully adjustable carburetor on your bike then you might want to check out this great tutorial on carb jetting: http://dirtrider.net/forums3/forums/eric-gorrs-moto-tech-articles.198/ written by Eric Gorr, a two stroke tuning expert.

 

[HeadSmess]

fast forward to page 4 or dont bother at all, because there is NO tutorial....

also there is not much info on small pathetic engines that need to be held WOT permanently to do anything, regardless of what modifications you have made

far better to look at http://www.iwt.com.au/mikunicarb.htm which explains everything in one page clearly without sifting through 7 odd pages of useless information.


sure, read it, then make your own decision on which one actually explains WHAT TO DO.

 

[Deleted member 12676]

Notice he doesn't say anything about looking at spark plug color:

"The most basic method of determining correct carburetor jetting is "ride and feel." This method requires you to determine if the carburetor tuning is too rich or too lean by the sound and feel of the engine. The first step is to mark the throttle body in 1/4-throttle increments, from closed to full open. Then, this method requires that you ride the motorcycle on a flat, circular course. To check the carb jetting for throttle positions up to 1/2 throttle, ride the motorcycle in second or third gear. Roll on the throttle slowly from 1/4 to 1/2 open. If the engine is slow to respond and bogs (engine makes a booooowah sound) then the carb jetting is too lean. You can verify lean jetting by engaging the carb's choke to the halfway position. This will make the air-fuel mixture richer and the engine should respond better. If the carb jetting is too rich, then the engine will make a crackling sound; the exhaust smoke will be excessive and the engine will run as if the choke is engaged. Careful engagement of the choke can help you determine if the jetting is rich or lean. Another important tip is to just change the jets one increment at a time, either rich or lean, until the engine runs better. Most people are afraid to change a jet because they think that the engine will be in danger of seizing. Believe me, one jet size won't make your engine seize but it could be the difference between running bad and running acceptable.

To check the jetting for throttle positions from 1/2 to full open, ride the motorcycle in third and fourth gear. (You may need to increase the diameter of the circular riding course for riding in the higher gears.) Check the jetting in the same manner as listed above. The carb jets that affect the jetting from 1/2 to full throttle are the jet-needle, main jet, power jet (electronic carbs) and the air jet (on four-strokes).

 

[HeadSmess]

the seat of the pants method is a good method, as plug colour can be dependent on the correct heat range as well as mixture, so isnt always the best indicator.


he also never mentioned slide cut away angle, depth of fuel in the bowl, back drilling the jet and associated stem to increase fuel "storage capacity" for when you give it full throttle... (think, the stem floods with fuel at low speeds...that lil bit more fuel down there than the jet would normally allow through gives a brief "extra rich" period just when its needed, without resorting to pumps)


unfortunately, there arent many places (in this country at least) where you can hold something like my mates honda cbx in top gear at full throttle... in fact, even my road registered pit bike from a few years ago would break every speed limit in the country! and that was before they lowered the northern territory limit from 140km/h...

taking it to a drag-strip is pointless. pay out, then spend the rest of the day pulling covers off to get to carbs rather than any actual testing... why i like the bench mounted dyno idea


popping choke on only works with butterfly type chokes, useless on a mikuni "bypass" type choke. but is a very good indicator of which way to go though, partially closing off the intake. which ever way one actually does it. i know this lil zeedee (tgb) quad bike i was given needs a larger jet. currently its running right due to the piece of tape partially covering the intake


i did like one guys take on it... video camera. compare videos. best out of three sort of affair, then make a mod, film and compare against previous best...etc etc.

not sure if you saw the last post or two on my dyno page, but i did find software that simply uses a microphone or other signal source, plus some figures about weight, gear ratios and so forth, to figure out the rate of accel and therefore power... basically making all my machine work pointless but hey...


i might actually mount it to a bench one day now that its basically finished and working.... so far, ive had to...replace a roof, rebuild a welder, rebuild an air-compressor, all in between working and yeah... i hate thinking its been more than 6 months now...sort of embarrassing :wacky:

 

[Jerryjcz]

fast forward to page 4 or dont bother at all, because there is NO tutorial....

also there is not much info on small pathetic engines that need to be held WOT permanently to do anything, regardless of what modifications you have made

far better to look at http://www.iwt.com.au/mikunicarb.htm which explains everything in one page clearly without sifting through 7 odd pages of useless information.


sure, read it, then make your own decision on which one actually explains WHAT TO DO.

That tutorial has got to be the best description I’ve come across to date. Finally someone found where it describes everything pretty clearly and to the point. Definitely a HUGE HELP for beginners and someone wanting to understand how it works better