Carb Jetting 351
#1
10-02-2011, 10:06 PM
Join Date: Jul 2011
Location: Cottonwood, AZ USA
Posts: 1,728
Carb Jetting 351
2007 250 KLX..... OK, took my newly installed 351 & TM33 flat slide carb up into the hills in CO, it died @ 3/4 throttle at 10000ft. So I change the main jet from 140 to 120 and it ran OK for the trip. The bike is stock, except for bore/carb with recycled air system removed.
Now back home @ 3300ft & ready to fine tune the carb. On the way back from the hardware store today, @ wide open full throttle accelloration, at high rpm, the bike is stumbling about every 1 second.
I am thinking the main jet is too lean now & considering 130 main jet for home riding. The bike was running pretty good otherwise and pulled good at low rpm, I got the air pilot about 1.5 out, so that seems almost right.
Any suggestions or comments on jetting?
Other mods are not anticipated.
Now back home @ 3300ft & ready to fine tune the carb. On the way back from the hardware store today, @ wide open full throttle accelloration, at high rpm, the bike is stumbling about every 1 second.
I am thinking the main jet is too lean now & considering 130 main jet for home riding. The bike was running pretty good otherwise and pulled good at low rpm, I got the air pilot about 1.5 out, so that seems almost right.
Any suggestions or comments on jetting?
Other mods are not anticipated.
Last edited by durielk; 10-03-2011 at 11:52 PM.
#3
10-03-2011, 05:40 PM
You can cut off part of the air entering the engine or reach down and pull the choke out to see if it makes a difference. If it does, you need a bigger jet....
You an also shut off the fuel and watch how it acts. If it gets worse, its too lean, if it gets better for just a second, you are too rich. Do this at throttle position you are working on.
You an also shut off the fuel and watch how it acts. If it gets worse, its too lean, if it gets better for just a second, you are too rich. Do this at throttle position you are working on.
#4
10-03-2011, 11:50 PM
Join Date: Jul 2011
Location: Cottonwood, AZ USA
Posts: 1,728
OK, thanks, good suggestion.
Also, I was thinking I'll take the air cleaner off, which should lean it out and test it.
That carb wedged in there is a pain to adjust the main jet, you have to pull the carb as it does not have a bottom access (which means, snorkle, seat, tank, throttle cable). Even if it did, I don't know if you would have enough room to do anything.
All the info I read sayes if it stumbles it is too lean, but I think I am way too rich, as when I was @ 10000' the engine just died over 7000rpm. Maybe I over shot with the 120 main jet, but the service man at the KTM dealer said they usually drop 2 sizes for getting up high in the hills.
I'll try it tomorrow & post.
Also, I was thinking I'll take the air cleaner off, which should lean it out and test it.
That carb wedged in there is a pain to adjust the main jet, you have to pull the carb as it does not have a bottom access (which means, snorkle, seat, tank, throttle cable). Even if it did, I don't know if you would have enough room to do anything.
All the info I read sayes if it stumbles it is too lean, but I think I am way too rich, as when I was @ 10000' the engine just died over 7000rpm. Maybe I over shot with the 120 main jet, but the service man at the KTM dealer said they usually drop 2 sizes for getting up high in the hills.
I'll try it tomorrow & post.
Last edited by durielk; 10-03-2011 at 11:56 PM.
#5
10-04-2011, 11:03 PM
Join Date: Jul 2011
Location: Cottonwood, AZ USA
Posts: 1,728
OK, warmed the bike up and did the 7/8 Roll off test first. The bike was stumbling at full throttle and when rolled off, did not fell much different, but after the 1 sec it kinda cleared. I think I was into the needle settings then.
Removed the air cleaner & left the air box lid off. Big change, anything over 7/8 throttle killed the engine, just would not run. Back off into the needle & it picked it up.
I did a lot of carb research on the www yesterday. The most logical & seamingly knowledgeable info:
Plug test does not work, high volt ignitions & modern fuel always gives a white plug unless wayover rich.
7/8 throttle test, when you rolloff, if it clears for 1 second you are too lean.
Did not do the fuel shutoff test.
I don't know how the choke works on this carb, so did not do that test either.
If it is running lean, it will not miss, but lack pull.
If it is rich, it will sputter & blow back through carb.
Regardless of that, I think I am off the chart lean, when I cut the throttle to 7/8, I think it is creating more vacum & starts sucking more fuel.
I think I'll try a 130.
Any comments?
Removed the air cleaner & left the air box lid off. Big change, anything over 7/8 throttle killed the engine, just would not run. Back off into the needle & it picked it up.
I did a lot of carb research on the www yesterday. The most logical & seamingly knowledgeable info:
Plug test does not work, high volt ignitions & modern fuel always gives a white plug unless wayover rich.
7/8 throttle test, when you rolloff, if it clears for 1 second you are too lean.
Did not do the fuel shutoff test.
I don't know how the choke works on this carb, so did not do that test either.
If it is running lean, it will not miss, but lack pull.
If it is rich, it will sputter & blow back through carb.
Regardless of that, I think I am off the chart lean, when I cut the throttle to 7/8, I think it is creating more vacum & starts sucking more fuel.
I think I'll try a 130.
Any comments?
Last edited by durielk; 10-04-2011 at 11:07 PM.
#7
10-06-2011, 06:47 PM
Join Date: Jul 2011
Location: Cottonwood, AZ USA
Posts: 1,728
OK, Thanks for the info.....
Installed a 135 today & everything is running good @ 3300', 70D F. 351, 300 snorkel, TM33 carb, no air recirc, everything else stock. Air screw @ about 7/8.
I was thinking the only time I have problems is at near full throttle, so I focused on main jet. I figured if it ran @ 400' with a 137.5 (unknown mods (assuming a muffler upgrade)), I should be able to run a 135 almost stock @ 3300'. Maybe a little lean, but at higher elevations it will be richer (where I'll be using more throttle) and be ridable at 6000' on up. I don't run big pulls with the throttle wide open anyway, so if it is a little rich at the low end with Bills low speed carb components, I'm thinking I'll be ok.
Time to take it into the big hills, which may be a while. I'll update when I learn anything else.
Installed a 135 today & everything is running good @ 3300', 70D F. 351, 300 snorkel, TM33 carb, no air recirc, everything else stock. Air screw @ about 7/8.
I was thinking the only time I have problems is at near full throttle, so I focused on main jet. I figured if it ran @ 400' with a 137.5 (unknown mods (assuming a muffler upgrade)), I should be able to run a 135 almost stock @ 3300'. Maybe a little lean, but at higher elevations it will be richer (where I'll be using more throttle) and be ridable at 6000' on up. I don't run big pulls with the throttle wide open anyway, so if it is a little rich at the low end with Bills low speed carb components, I'm thinking I'll be ok.
Time to take it into the big hills, which may be a while. I'll update when I learn anything else.
#9
10-19-2011, 08:22 PM
Join Date: Jul 2011
Location: Cottonwood, AZ USA
Posts: 1,728
I am now interested in to adj the needle clip. Can you do this without pulling the carb? Do you go in through the top or through the slide access on the rear?
