Which Pumper Carb to buy?

Sup guys, I was on fourstrokeworks.com and was lookin at their pumper carbs, but I was wondering which is the best for my application

I have a KLX300 with a big gun full system and an Uni filter, plus the free mods. They offer a Mikuni 36mm TM Pumper Carb w/o choke, and they offer Keihin FCR Pumper Carburetors in 35mm and 41mm w/ or w/o a choke.

1st, Is the choke necessary with the pumper, 2nd whats the best throat diameter for a stock 300? with a big bore too, in case i get one? Oh, and which company typically has a better product?

Also, how much HP and throttle responce does a new carb actually add? how noticeable is it?

And what else do you need to install one ie, new throttle, new cables, etc...?


General consensis on the pumpers for the 300:

Get a hot start if you get a pumper.

33 mikuni for tight trails and especially if you are not going to do a big bore.

36 mikuni for big bore or if you ride WFO a lot. A little flat off the bottom.

35 FCR Keihen is perhaps the best all around, but can be very finiky to jet properly. And it's expensive.

I don't think a 41 FCR would be good for the KLX. Too big.

Yup, I agree with what Bill said.

Don't let your ego get in the way of getting the 33mm. Consider where you do most of your riding. I've ridden two bikes , back to back, one with the 33 and the other with the 36mm. The 33 is more precise in the low to mid range. The 36 comes on stronger from mid to WOT. The 36mm is on my bike. If I was to do it over again, I would have bought the 33mm.

You might want to check out www.pro-flo.com for carbs. But, when comparing prices, check to see what extras you get. Pro-flo is carb only.

Ride on


I have heard of a few guys buying the 33's and then boring them out to a 34-35 saying it's a good compromise. Borsy

The 36mm is on my bike. If I was to do it over again, I would have bought the 33mm.

I'm glad to hear you say that as last week I ordered the 33mm from Pro-flo. For $248 shipped it's significantly cheaper than anything else. I rarely, if ever, go wide open so it should be the right choice for my KLX300.

I've got the 33mm Mikuni with a choke. It starts right up, hot or cold (no hot start, and I live in Arizona). When I first bought the bike, hadn't found the choke and it was very hard to start cold. Once the choke was discovered, it's been great. On hot starts, it kicks right over unless the bike is on a steep incline, in my case usually downhill. I do follow the factory recommended starting drill religiously.

I had the Keihin pumper on my KLX along with the free mods and Big Gun pipe. There was no choke or hot start, but never seemed to have a problem with that. SoCal weather is pretty stable, so can't speak for hotter or colder climates, though.

As for power, it was just more everywhere. No drastic change in the powerband but pulled better from the bottom on up. When I sold the KLX, I put the stock stuff back on and couldn't believe what a difference. It felt like a minibike compared to with the carb and pipe.

Jetting was not a problem, but always had a slight idle inconsistency. It wasn't enough to keep pulling the carb off to chase it, and never noticed it when riding. Probably a little lean on the pilot, but left it alone.

Using the KLX250 manifold, it was a straight bolt on, which was nice too.


When buying the pumper, do you need to buy a new throttle cable?

When buying the pumper, do you need to buy a new throttle cable?

I have the 36mm mikuni pumper and was able to use one of the stock cables. Iit is a little tight though.

Ride on


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