Okay, I write this post in hopes that it helps someone out if they are having the same symptoms I was.

So about 2 months ago I installed a BB 351 on my bike. Before that I had a DJ kit and was running a 124 main with a stock pilot. Bike ran absolutely fantastic, I don't think there was a better running bike out there. After the big bore, Bill Blue (plus a lot of other posters on here) say to go up on pilot jet size and two main jet sizes. Well, I live at 3300' elevation, so taking that into consideration I went with a 128 main jet and 38 pilot. Bike ran great but I couldn't get the idle to settle down. Everytime the bike got nice and warm, when I pulled up to a light or something, the idle would surge. I tried and tried to get it to quit, couldn't tune it out.

So last night, I pulled the carb and went to a 40 pilot and a 132 main. Yuck, the bike ran like crap then. No matter what needle clip position, it was just rich rich rich.

So tonight I pulled the carb once again, went to the stock 35 pilot and back to a 128 main jet. Bike runs great now!

How the heck is everybody running a bigger pilot jet? Especially on a 250 ?

Maybe it is my elevation? But I wouldn't think it has that much effect on it. I could have probably even left my jetting alone for the big bore.

My 2 cents, don't touch the pilot unless you have to. Unless you really want to become good at pulling the carb. By the way, had the carb out, jetted, and back in 36 minutes. I am getting pretty good at it.

 

What i heard from Bill Blue was to go up one size on the main, and leave the rest alone. that's what i did and I had good luck. I think TNC recommends what you just found out, he has the 300 cyl and i think he pretty much left the stock jetting alone. When you think about it, the larger cylinder is just pulling more of the air fuel mixture in. The air pulls the fuel in, so the more air you pull the more fuel you pull with it. I can see how the stock jetting satisfies the bigger engines.
Dan

 

What a coincidence that you posted on this subject today!

Just this evening I stepped back down from a #38 to a #35 pilot and also back down from a #128 to a #125 main jet.

Also installed a N1TC needle (clip in 3rd pos.) versus a stock needle that was shimmed at .060".

Can't tell any difference between the #35 or the #38 so will likely keep it at #35. On test ride towards dark, may have had some lean surge with the newly installed #125 main but will have to check again tomorrow after I mark the throttle grip and run it again. Will also have to see how the new needle is doing after marking the throttle and doing a proper test ride.

No issues with idle, at all. BTW my elevation is around 1600 ft.

All this is for a 250. Will probably do a BB351 this winter after riding time shuts down for the winter. After that, more fun and games with jetting but like you, I'm getting proficient enough at carb R/R that it's not really a factor anymore.

 

I learned this when tuning my FCR35 carb on my bike... When the engine is up to running temp and you pull up at lights (would be safer to just pull up on the side of the road while testing) you say it surges. Does the surge settle down? Or does it get worse? Or does the bike then die? If it dies is there a bit of a fuel smell in the air?

If it surges and does not settle down, it is probably running VERY lean. Check your air/fuel screw is appropriately set somewhere between 1.5 to 2.5 turns out from fully seated.

If it surges and then settles down without stalling, you are close to having the air/fuel mixture right. It is probably a little lean so try turning the a/f screw out a quarter turn at a time to see if you can get it right.

If it surges and then stalls (possibly with a smell of fuel in the air) your pilot circuit is too rich. If your a/f screw is already only 1.5 turns out, you need to go down a jet size. If the a/f screw is anything more than 1.5 turns out then try running it down to 1.5 out. If you have to have the a/f screw anything less than 1.5 turns out then drop a jet size.

Please note, I am just talking about the pilot jet and a/f screw here. While getting your idel right, DO NOT MAKE ANY OTHER CHANGES TO JETTING, FUEL NEEDLE, ETC. You need to make only 1 change at a time and see its effect to correctly diagnse and correct problems.

Please see my comment above about only making 1 change at a time. Secondly, the bike idling should be solely tuned by making changes to the pilot jet and air fuel screw (otherwise known as the idle circuit). Changing the needle height and main jet size should have no effect on the bike idling unless you have a seriously oversized main jet or you needle is so high it is allowing fuel to be drawn through the main jet at idle.

 

The main jet LIMITS the amount of fuel that can get into the air stream. The density of the air flowing through the carb can vary, allowing more air to pass through the carb with a larger displacement cylinder. Fuel doesn't change density and therefor limits the amount of fuel available.

Ride on
Brewster

 

So I think I see what you are saying.....With a larger bore sucking more air through the same carb, the air will be more compressed, the liquid fuel can't be compressed. Now you have more air with the same amount of fuel, and may be running lean. ??
Dan

 

That's correct on the fuel side. The air flow varies due to pressure differentials from ambient to near total vacuum.

Ride on
Brewster

 

I don't know, B...if it were that simple, you'd always have to jet up with just an increase in displacment. Now there are a lot of generalities that apply here that you've mentioned, but I think there are other variables that apply. While I know that volumetric efficiency is mainly discussed in terms of intake and exhaust sizes and how they flow more volume through a given displacement, I think there's a sum-of-all-parts kind of thing going on too. Many of us here have experienced a displacement increase that didn't require any rejetting. Now some will muddy the water with how they did have to up the jetting noticeably when installing a bigger bore, but I will bet if truth be known, many of those did not have an identical intake and exhaust setup before the bigger displacement was added. Granted, some will, but as you well know all engines are not in exactly the same condition, in the same locations, or tuned to the same preference. I notice some folks with rejets, pipes, and with or without bigger bores had noticeable drops in fuel mileage. I'd bet most all of them are jetted too rich in part or all of the throttle range.

One other issue with the KLX that I've been curious about that you may have an opinion on. The KLX250S...at least for sure the '06/'07 models, has the valve reliefs running down the crown of the piston to almost touch the first ring land. Now, it's not the first time I've ever seen that, but it's usually not the ideal design for combustion chamber/fuel burn efficiency from what I've been told and read. When you install an OEM 300 piston or bigger, the reliefs are in what is normally the more common and probably more efficient combustion chamber to valves to piston configuration. And yes, I realize there are variables to this concept too. Still, I'm wondering how much this might affect the jetting discussion we're having here.

 

351, sea level, 40 idle jet. no problems, bike runs great.

 

Yeah, it would idle at the standard 1400 rpm, then just start to idle at 1900 rpm for maybe 10-20 seconds, then maybe go back to 1400 for a few seconds, then back up to 1900.

Yeah, I tried every amount of turns on the kouba screw, and the idle would not flatten out. It would just be lean or snap crackle and pop a lot when it got rich. I thought the same thing, it has to be lean. Thats why I went to the bigger main and pilot. It had the same symptoms, but was even worse. That told me I went the wrong direction. Like you said, I was probably pretty close, so I went back to the 128 main because it ran great besides the idle. Went with the smaller stock pilot and it made all the difference.

Yeah, I have been playing with it for two months now, I was making one change at a time. Systematically going through all the adjustments. When I made jet changes, I was just making educated guesses.