Carb Jetting 101: Terms, Tips and Jetting Theory

[jk5blazer]

Alright, I know this post is kinda old but Im really needing some help. Is there a link someone has that shows stock jets? My 86' 250 sx is sputtering/bogging at WOT, the plug is black, and the air/fuel screw has no affect. I just bought a Shindy Carb re-build kit for the 86-87 250sx and put it toghter. The old jet that was in there was a 125 and the new one is a 130. I dont know of the pilot right now, have it writen down in garage but it idles pretty well. I put the 125 main back in because the bogg/ sputter got worse i think. It was noticable before now I cant ride it. Any advice? Should I go even further down in main jet size?

[scottp597]

If it is sputtering at WOT and your plug is black then your main jet is too big. I would buy a clymer manual, search for an owners manual on this site, or see if someone can chime in on the stock recommended size for that machine.

After you change the main jet and your WOT feel is correct, you may need to then adjust your air screw and needle position to get the off idle throttle response dialed in.

[scottp597]

http://www.oscarmayer.net/atc/manual...vicemanual.pdf

[jk5blazer]

I just found out that it runs great 1/3 thur WOT if I have the air box lid off, just not from idle to 1/3. So I guess that sums up the fact that its running rich.

[jk5blazer]

So the main jet stock is 130 and there is a 125 in it and its still running rich. Does that happen? How many different sizes should I get? It runs great if the air box lid is off, just idles real high. I was thinking 120 and 122. any other thoughts? I dont want to just throw money at this thing, well anymore then I need to that is! Thanks

[Dammit!]

Just wanted to mention that this thread doesn't get checked often. If you have questions that you can't figure out from the info posted here, it would be best to start a thread in Trikesylvania to ask for help.

[jk5blazer]

to be honest I have! I know i cant expect an instant responce but..... I will try again, thanks for the 411 though.

[RIDE-RED 250r]

i will admit that i didnt take the time to read the entire 9 pages, but i noticed on the first post with definitions that piston wash was not included as a means of determining burn condtions. plug chops are good, but piston wash is better if you dont mind wasting a head gasket or 2. i would say, plug checks to get close, and piston wash check to verify and determine thumbs up, or thumbs down when you think you are dialed in. on a 2-stroke a good wash pattern will show about 1/8 of an inch of clean piston around the outer circumference of the top of the piston. and where each transfer port is, you should see clean or "washed" piston about the size of a thumbnail. if the entire top of the piston is black with no wash anywhere to be found you are lean. this occurs because the cumbustion chamber temps are so hot from being lean the the fuel coming in starts to burn immediately. when you have proper wash, (proper mixture/combustion chamber temps) the fuel comes in and keeps the top of the piston in the areas i mentioned clean. and abviously, too much wash indicates a rich condition. basically, if no wash occurs, it indicates things inside are getting hot enough to start to damage the piston, particularly on the exhaust side of the crown. hope this helps some, and that i didnt just repeat a previous post.

check out this link, i know its about sleds, but the same principle applies and i think this is a really good write up with pics on piston wash....
http://www.dootalk.com/forums/index....post&id=115122

[Dirtcrasher]

^ Great post!!

Unfortunately most guys are lucky to get the jug on once, let alone twice!!

Hopefully starting rich and reading the plug will do the trick.........

Great advice to share though

[RIDE-RED 250r]

^ Great post!!

Unfortunately most guys are lucky to get the jug on once, let alone twice!!

Hopefully starting rich and reading the plug will do the trick.........

Great advice to share though

happy to help in any way i can....as to removing the jug, i think in most cases with the tank out of the way, you could get a good enough look at the piston crown by just removing the head and bringing her up to TDC. of course this may not be true of all machines with different engine/chassis layouts.....

[hossola]

Hey bro thanks for all the great info! I know that these post are some years old but I hope that you are still around. I recently bought a bike that had been sitting. I cleaned out the carb real well and it started right up. The bike runs well but once I have top gear/ top speed it backfires and bogs out. It wont start for about 5 minutes. It doesnt seem to ever die like that when I am not gunning it. It also NEVER backfires at low speeds/ gears. Any ideas? I bought a carb gasket kit that is coming in a couple days. When i replace them I am going to make sure that the jets arent clogged and that they float is at the right level. Anything else I should check or alter when I have the carb apart?
Thanks much.
Rob from PHX

[skypilot]

Hey guys, I'm running a stock (except for Cobra header and muffler) "85 Honda big red. I built a snorkel out of 1 1/2 pvc pipe and installed it like the one mondex is running here on the site. When i installed the snorkel, my trike ran terrible in anything above 1st gear, fine at low idle. When i get up to speed in say third or fourth gear, engine bogs down, and starts hiccuping back through the snorkel. Going down the road, their is air blowing out of the snorkel? But at idle it is sucking air, I can kill it by placing my hand over the snorkel inlet. Is this a jetting issue? Trike ran great in stock condition, except for backfiring when you let off the gas going down hill!?! Crazy I know! The stock air filter is a steel mesh type? I'm thinkin, no foam--it was this way when i bought it. So what's up? Is this related to the air filter or jet's in the carb. I live in east texas, and this is primarily a mud machine and trail cruiser. Thanks for any help from the trike docs!

[RIDE-RED 250r]

sounds like the foam element is missing. the steel mesh is doing nothing to filter out small dust particles, its just there to support the outer foam element that catches all the harmful debris. get the foam outer filter on there...and its possible your snorkel is restricting airflow and you may be running too rich. do a plug chop at half and full throttle (if possible) and take plug reads...this will tell you alot. if the chops arent possible the way its running now, get it to the point its spitting and carrying on and bop the kill switch. then pull you plug and see what it looks like...my bet is that it will be black and wet.....meaning rich

[85 250sx]

ok so i thought i had everyting dialed in and running good. it doesnt bogg out or spudder but i just noticed that if i run out of gas it starts to rev like crazy! first time it happend i thought it was gona blow up but luckly i hit the kill switch pretty fast. this is on a 2 stroke by the way. does this mean its running to lean?

[Dammit!]

It only does it when you're running out of gas? When it runs out of gas, it is by definition running lean. REAL lean. lol