Jump to content

Mikuni TM33 pumper carb question

Recommended Posts

Hi All,

I have the Bill Blue Mikuni TM33/34 pumper carb installed on my KLX250SF which I use solely as a city commuter.  I called Bill Blue to ask if it's possible to alter the pump timing so I'm not using as much gas going from stoplight to stoplight.  I'm currently getting 40mpg which Bill says is normal for the pumper and I believe him.  My preference would be to delay the pump timing to half throttle to give a boost when I need it and get better mileage cruising from block to block.  Bill says the TM33 doesn't work that way and the pump timing comes on at closed to 1/4 throttle to compensate for the delay that is typical of a CV carb due to the delay in engine vacuum getting gas into the carb, etc.  This all makes sense, so I asked him if I could just disable the pump and he said it wouldn't work right.  I do understand that the pump is what provides the fantastic throttle response.  

My question is how does a pumper carb differ from a CV carb aside from the pump and why wouldn't the pumper perform without the pump?  I would imagine you would need to make some jetting changes to compensate for the lack of pump, but what are the differences in the carb design that would make them not funciton properly without the pump?  

I'm planning to add the 351 big bore in the relative near future and I'm trying to decide if I should just go back to the stock carb and have it jetted accordingly or jet my TM33 to suit and disable the pump.  When I originally purchased the bike, the previous owner installed a Megabomb and Powercore 4 but I don't believe he made enough jetting changes because the bike still took about 5 minutes to warm up.  I also had the backfire screen cut out of the airbox, a crankcase breather installed, and the smog equipment removed.  I'd never tried a pumper before and since the bike needed a rejet, and I had to replace that horrificly obnoxious Powercore 4 with a Q4, I thought I'd give a pumper a try.  The bike now only takes about a minute to warm up.  

Any advice would be most appreciated.  Thanks!!!

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with: