Hi all. I'm looking for some (recent/new) feedback regarding pumper carbs to improve throttle response and impact on mileage.

Some background:
Bike description is in my sig. I live at 2900' and typically ride anywhere from 2500' up to 8000'. Sometimes I get up to 9000-10000', but it's somewhat rare I get up that high. I also rarely ride on the highway in excess of 50-60. If/When I do, it's really only for about 5 miles at a time. More often, my max speed is around 40 give or take, on forest dirt roads and the like.

I have a 9 year old that's riding now, and we are going to do a lot more single track in the coming years. Nothing too technical, but certainly more technical than the Jeep roads and ATV trails we've been riding up to this point. A dedicated 2-stroke would be fantastic for this, but I intend to keep my plated KLX as a do-it-all machine. For single track, through, I desire better throttle response -- mainly for lifting the front tire over roots, rocks, logs, etc.

I fully acknowledge and understand that CV carbs are not known for their crisp response. I've been considering a pumper carb, but I don't want to dramatically sacrifice my fuel mileage. I've dismissed the notion of a big bore kit, as my research and reading strongly suggests a 351 alone would likely drop me from 65mpg current average to 55 or less. Also, it's been my experience that the 250cc's will haul me where I want to go with relative ease -- especially with the 13/50 gearing.

As stated, I'm currently getting about 65mpg riding dirt roads. Surely a pumper carb will impact mileage, but to what degree?? Riding single track (LOTS of throttle on and off) will have a more signficant penalty with a pumper. I get that. But what about every day riding where one isn't constantly hammering the gas so much?

For those who have done a pumper carb swap on a 250cc jug, does a 5mpg average penalty seem reasonable?

 

I installed a pumper carb on my 351, I redid the connections to the pump to virtually take it out of operation. It does have better throttle response than the cvc. IMO, lofting the front end with a klx is pretty difficult. But, I don't try to do that much and can't really comment on that. IM experience, you need a 2 smoker to do much of that.

 

I have an '06 with a factory 300 cylinder and piston installed. Has a full Muzzy exhaust and basically lidless on airbox...the perimeter of the top is still in place. I run 14/50 gearing. I have a Trailtech Vapor instrument panel that is set for speed and mileage via GPS...it's accurate.

I've been running a Mikuni TM36-68 pumper carb for years now. I did not find any mileage drop going to the pumper carb over the CVK34, and frankly I was surprised. I do have the carb dialed in very precisely for my home area, 1700', and it's on the "clean" side of lean. I hated the CVK34. When I'd go to Colorado and ride over Imogene Pass and similar high altitude passes, the bike fueled horribly. Even with camp sessions of jetting leaner, the bike always had a small window of what I'd call optimum performance.

When I got the pumper carb I figured I'd be in for at least as many jetting sessions when going to the mountains. Surprise...my home altitude setup worked great. On Imogene...over 13,000'...and others I'd get some rich "burbling" shortly before and after the summit, but it would clean out quickly. Below 12,000' I never notice anything amiss. I don't get a bunch of soot on the muffler tip. I'll be the first to say I'm a little shocked. I figured bigger carb and a pumper component would make life harder at altitude. A quick, small twist of the idle **** for a short time at high altitude is all I've ever done.

Now, the mileage. I have an Acerbis 3.7-gallon tank. I have hit 200 miles on a couple of huge rides with this tank. Checking exact mileage at the pump or at camp with a 2.5 gallon graduated opaque plastic fuel jug, I've gotten as high as 64 mpg with high 50's the most common. It's nearly impossible to get this bike to fall out of the 50's. On one ride in the Needles section of Canyonlands NP south of Moab, I got a permit and did the ride in Horse Canyon...used to be open to anyone but requires permit now. That ride is nearly all in a deep sand wash, and I must have hit it in dry conditions. I had to keep the engine pegged in lower gears the whole ride. I could never quite get on top of the sand for more than 50 yards or so. I've never encountered sand like that. It was brutal. I kinda feared for my engine and clutch actually. Anyway, the end result back at camp was 47 mpg. That's the only time this bike ever got into the 40's.

 

Thanks TNC, this is great feedback. The TM36-68 is not the correct carb for a 250cc jug, rather the TM33 seems to be the best unit. Good to hear your mileage didn't suffer much. It gives me hope.

I would probably consider a 300cc kit with the TM36-68 given your mileage figures. You say it's a factory piston and cylinder. Are these the KLR 300 parts? Did it mount right up to your 250 without issue?

 

Yes, the factory KLX300 cylinder and piston are a direct bolt-on for the KLX250.

 

Wow, the OEM parts for a 300cc swap are 'spensive! A check on the Kawasaki parts page quickly escalated to near $800 doing some quick estimating.

 

Yes. If I hadn't worked part time at the dealership, I probably wouldn't have gone this route. Obviously the quality of the cylinder and piston are great, but I would choose another supplier if having to pay full cost. I do like that my compression ratio didn't change with the OEM 300 setup. This makes my bike less sensitive to fuel quality. If I recall most other aftermarket setups go from the stock 11:1 to 12:1. Doesn't sound like much, but some have reported having to use at least an 88 or 89 octane fuel when conditions cause you to have to lug the engine.