Hello! I’ve had a cold starting issue forever now and I thought I’d throw it out to you guys for some advice.

I have a 2014 KLX 351 w/ a TM 34 carb, Two Brothers slip-on, snorkel delete. Jetting is 142.5 Main, 40 Pilot, slide needle is in middle position. Mixture screw is 1 3/4 turns out, NGK CR8EIX (for the higher compression piston). My elevation is 700 ft and I run ethanol-free fuel. Cold starting has always been an issue, sometimes even when it’s when warmed up a bit it still wants half-choke for a sec. To me this behaves like a lean pilot circuit but my spark plug is showing pretty rich. I’ve played with the mixture screw with very limited success. The carb could not be cleaner. It’s possible the plug looks dark due to the Main jet since I think I might be a little rich on that side. I’m open to any suggestions you might have but I have specific questions regarding the Air Jets. I haven’t found any reference on how to select the proper size Pilot or Main Air Jets. Is there an ideal Air Jet to match up to the Fuel Jet that’s installed? I’m interested in using all the adjustments available on this thing to get it dialed in as perfectly as possible.
I appreciate any help you can offer! In the meantime I’m going to put it a fresh plug and play with some settings while keeping it off the Main Jet.

 

You might check with B and B Cycle Restoration (Bill Blue) if it is their carb setup, a bored out TM33-8012.

I'd try idle mix adjustment first, richening the fuel mix, because the need for choke indicates it may need a richer mix when cold.

If you have the TM34 pumper the idle mix screw adjusts fuel intake. Opening up the screw richens the mix (more fuel), closing it leans fuel mix (less fuel. The OEM carb on my bike was at 2.5 turns with a 140.

If you have the TM34-2 flat slide without accelerator pump, the idle mix screw adjusts air intake, so turning the screw out will lean out the mix (more air), turning it in will richen the mix (less air). I am really interested in your results if you have the TM34-2, because most riders have installed accelerator pump carbs, but back in the 80s on the SR/XT/TT 500 Yamahas they used the plain flat slide, no pump. I have always wondered why the non-pump carb hasn't been used, because it is a whole lot less expensive. They worked on the 70s-80s four strokes like the 500 Yamaha and they should work on the KLX. I wanted to try one, but I came across a good buy on a TM36-68 that a lot of riders have used for about the same price as the TM34-2.

When you adjust the idle mix you will be working the idle mix until the mix is too rich where the idle drops, to too lean where the idle drops as well. The key point is with a proper pilot jet there will be a range between around 1 turn out to 3 turns out where the mix will be right. You are targeting the middle of that range. It may be small, possibly a range of 0.5-1 turn.

• First carefully turn the idle mix screw in until it gentlystops, too tight can damage the carb.

1. Turn the idle mix screw out 2 turns,
2. start the engine and warm it up to full operating temperature.
3. Slowly turn the idle mix screw in until there is an rpm drop, (If the rpm drop immediately, turn the screw the opposite direction until the rpm stops rising.). On the modified 33 pumper this will be leaning the mix, on the TM34-2 this should be richening the mix.
4. If you had an immediate rpm drop and turned the screw the opposite direction, opening, until the idle drops in that direction.
   1. If turning in until it is around 1 turn from seated, does not have the rpm drop then the pilot jet needs changed. On the pumper the pilot is too rich, requiring the next size or two smaller jet. On the non-pump carb the pilot is too lean, requiring a size or two bigger jet. Change and start over.
5. Next, from the second idle drop, counting the turns, turn the idle mix screw in/out until there is the other drop in rpm.
   1. When turning the idle mix screw out the screw should not be turned out more than 3 to 3.5 turns from full in (because there is a spring that holds the screw in place that may be too loose and allows the screw to fall out). If it goes more than 3 turns out without idle drop the pumper has too lean, the non-pumper is too rich. If that happens, change jet and start over.
6. Then divide the turn count in half and turn it out that amount which gets it right in the middle of best range. You should find the right range between 1.5-2.5 turns out with the right pilot jet.

I hope this isn't too confusing. It's hard to explain when the adjustment is functionally reversed on the two possible carbs. This sounds complicated, but it isn't. You're just looking for the right position to have the best idle mix and going either too near seated or too far out is the indicator of the need for a different pilot jet.


If that doesn't work, I believe there is a starter jet. The OEM carb has one and starting improves when drilled out.

 

Thanks for that! I should’ve specified, the B&B TM33/34 pumper carb is actually the one I have. It’s fantastic (once the bike is running). None of my local dealers have Mikuni Air Jets in stock so I’m going to place an order for some today. I did find an interesting link on another forum that supports my theory about what’s going on here. Seeing how I’ve already played around with a variety of fuel jets and needle positions I think this is the avenue to pursue.
This guy explains his situation really well:

https://www.thumpertalk.com/forums/t...ir-jet-tuning/

 

I'd still contact B and B about it to see what Bill says. Can't hurt and might be a simple fix.

 

I definitely will and I’ll update the thread when I get somewhere with this

 

So I’ve made some excellent progress on this the last couple of days and the bike is starting instantly now and running beautifully. I’m going to take it out today and run it for awhile then check the plug again to see how the mixture is looking.
Here’s a summary of adjustments I’ve done:

• Pilot Air Jet, from 1.1 to 1.3
• Pilot Fuel Jet from 40 to 45
• Main Air Jet from .7 to 1.0
• Main Fuel Jet from 140 to 142.5
• Mixture Screw 2 turns out