Pumper carb on KLX 300?

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Jan 2, 2001
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#1
Ok i have a 97 KLX 300 and i love it stock. But everybody says a pumper carb makes the bike so much better. ok so here is my question. I live in the north east and ride endouros and muddie nastie slippery trails. Now tell me if im wrong but dont u want tractable power in these conditions not a ton of wheel spin :think. Where and how does the pumper carb improve the bikes power. I love the tractable smoote power and i can still squiert the back end around corners and i have perfected wheeling over 2 foot logs. Is it worth the money. The bike already does everything i want it to so is it worth it. Or is this mod if u ride in like wide open sand pits. Ive been riding for 7 years and im 12 and im 5'8 and 165 pounds with gear. Any answers would be apreciated. Penguin if u see this i know u have alot of experience with this bike and this mod so please answer.:cool:
 
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#2
A pumper carb, if set up correctly, will give you better low end throttle response and more top end.

A four stroke really hates to deal with lots of fuel at low rpms. The engine only uses 1 batch of fuel for every 4 strokes.

Basically, with a regular, butterfly or CV style carburetor (although the CV & butterfly to a lesser extent than the regular carb) the drill goes like this.

Think of the engine like a pump. Now, the pump is being used to deliver fuel to itself. You open the throttle, which lifts the slide, allowing air to enter the carb - this acuates the pump. Fuel is drawn through the circuits, is mixed with the air and thrown into the head, which delivers it to the cylinder at the right time.

Okay, this process is working really well at high rpm, the fuel and air mix, and whammo! they're channelled quickly through the head, then dumped into the cylinder and ignited.

Now, imagine what's happening at lower rpm's. The "pump" is moving slower, you open up the throttle, the slide lifts, and the pump tries vainly to suck in the fuel. The fuel and air mix, and dribble into the head, where they sit and wait for the intake cycle. At low rpm, this takes forever, and the fuel gets tired of waiting. It falls out of the air - so it's no longer atomized - then the valve opens and the fuel and air dribble into the cylinder. Not a great situation to create that smooth hit of power you asked for when you twisted the throttle!

The CV Carb (like what you have on your KLX) works a little better, because it doesn't use the throttle to control the slide - the throttle position is just a suggestion to the carb - the engine's pumping action is what lifts the slide. So in theory, the carb only delivers as much fuel as the engine can handle. Unfortunately, this can also create some weird carburetion when you let off on the throttle after an extended high-rpm romp (the bike won't slow down, or leans itself out, or surges).

The pumper carb ignores the engine "pump" at lower rpm and generates its own fuel delivery with a pump. This pump squirts the fuel into the air, forcing it to stay mixed better and longer.

This gives you the smoother, stronger throttle response at low rpm's. It also allows you to run a much larger carburetor size than you could have with a "regular" carburetor design. This gives you more top end power.

If you ever experience bog and hit at lower rpm's, the pumper carb will make that go away. If you are someone who spends lots of time at one low throttle position (about 1/4 throttle), you probably won't like the pumper carb, becuase it will have more of a tendency to make the engine "surge".

For my money, a properly set up pumper carb is the best option for a four stroke. But if you get a pumper that's not set up correctly, they are a headache.

If you do purchase a pumper, make sure you get it from a performance shop that knows about how your specific engine works (like Stroker for your kawie). Don't purchase this item from a Carburetor only supplier. It probably won't be set up correctly.

Actually, I'd give the same advice for any aftermarket carb purchase!

Hope I answered your question without talking your ear off (or eyeballs out, as the case may be!).
 

Wolf

doooode
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Jul 31, 2000
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#3
Woohoo, Eureka, Fireworks.......!!!!
I finally understand how my pumper carb works. That was the best explanation I have read yet, and I understood it!!!
Thank you, thank you, thank you:)
 
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Jan 2, 2001
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#4
Thanks dualsporter. This he;ped me decide. Im more of a short shifter so im in it 1/4 throtel range alot. Ans with all the mud here it sounds more like wheel spin instead of a surge of power. So i think my bike will reamain stock for awhile. Thanks again.
CHRIS
 
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Aug 22, 2000
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#5
I finally understand how my pumper carb works. That was the best explanation I have read yet, and I understood it!!!
Yea, I didn't get it either until about a year ago. I'm lucky my boss is such a smart guy - he's able to explain things to me so that I can actually understand them!
 

Boit

Sponsoring Member
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#6
I have a '94 KLX 250 that was modified by Stroker to 338cc's. Their recommendation for the Mikuni pumper carb was nearly right on the money. They only missed the mark slightly with the jetting. As delivered, it was a bit rich and the engine blubbered on the top end. Once I leaned it out one step each on main and pilot jet, the engine came alive. The incredible throttle response is fantastic for tight trails. It's so much easier to ride having confidence in knowing that I can make the bike do anything I want by using the throttle. Third gear wheelies are a snap. . .very handy when coming upon a high speed hump in the trail. By far, this bike has the highest "fun factor" of any machine I've ever ridden. Even more so than my 426.
 
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#7
Their recommendation for the Mikuni pumper carb was nearly right on the money.
Yup, buying it from someone like LR who really knows the KLX makes life much easier when it comes to jetting!
 

penguin

N. Texas SP
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Feb 19, 2000
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#8
for a box stock KLX300 ( no internal engine mods) the 33 mikuni pumper is the best bang for the buck and the easiest of the bunch to get jetted right ( I got mine from Stroker and never touched the jetting). To receive the maximum benefit, you need the oversize header pipe. You do not need the aftermarket muffler, just remove the restrictor from the end of the stocker . The power still remains tractable at low speed, but when you grab a handful of throttle the bike comes on the pipe a lot faster. You have to cut the frame at the right side headstay bracket and fabricate a spacer to put under the back of the fuel tank to make the carb fit, but the 33mm mikuni is the way to go. The 36mm mikuni is getting hard to find and hurts low end response, and the 35mm Keihin FCR hits too hard right off the bottom and would be a handful in slippery conditions. I just sold my 35mm FCR and plan on getting another 33mm Mikuni in the next few months
 
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May 21, 2001
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#9
36mm pumper

I just bought the 36mm from Stroker and they recomended it for my situation. I hope they were not just trying to get rid of one they had on the shelf. He may have recommended the 36mm due to my riding style, type of exhaust or hesitance in wanting to grind my head stays. If I am going to lose low end then that is not the outcome I wanted. Anyone else have any feedback on the 33mm vs. the 36mm?
 

penguin

N. Texas SP
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Feb 19, 2000
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#10
Not to worry, unless you have ridden a KLX with all 3 carbs, you will not notice the difference. The 36mm is a vast improvement over the stock CVK, it is just a little softer off the botom than the 33mm and a bit more softer than the 35mm FCR. The 36mm is a easier install, but I already ground my frame for the FCR so I will be getting a 33mm in the next few months.