Jump to content
Sign in to follow this  

boring out a 4-str carb?

Recommended Posts

A friend of mine asked if boring out a 4-str carb got the same or similar results as boring out a 2-str carb. He just picked up his 05 CRF450 and can get the carb bored out for free, so he was asking.

I told him I thought so, but I am not sure. Has anyone done this, and if so, what kind of results did you get?


Share this post

Link to post
Share on other sites

Unless your name is Ricky, Chad or Kevin you are not riding your bike to it potential anyway.

Leave it alone and learn to ride it.

To answer your question:

Smaller bore carbs tend to give better low end response, but restrict high rpm running.

It's the other way for Big bores.

Share this post

Link to post
Share on other sites

i used to think as above that a 39mm might give away to gain at the top if bored to 41mm. however a top tuner-dale lineweaver-says that the 39mm carb was made for only 250-300cc multi cylinders and therefore a bigger carb would be good.

my info is that the body is used for the 37, 39 and 41mm carb. if you bore it out would you let us know here how it went?

i'm not sure what you'll do with the central 'loop' of aluminium that the slide goes over. it must be fragile?

if you do bore the carb you should do it off centre so that you take nothing from the base of the carn and only 2mm from the 'roof' and 1mm from both left and right.

it'll be interesting to see how you clamp the carb.

jetting will likely go up +5 to +8 as a safety precaution.

i'd expect just 1 bhp advantage as i believe the ports will be restrictive and not the carb. dyno before and after? gotta be worth it if you're doing it properly.



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:

Sign in to follow this  

  • Similar Content

    • By SNORE125
      Anyone else have 130 main and 38 pilot jets already in the carb I took the wifes ttr 230 carb apart to add the bigger jets and this bike from 05 that I bought new already had them in it? I was reading that they came with a 125 main and a 36 pilot.
    • By Charles Joseph Miron
      Hello all.  Need a helping hand here..  
      I bought an older TT225 and am not about to spend almost $600 on a new carburetor for it. So I'm hoping someone can help me out with an idea of what (other than original carburetor) style, type and brand I should go with..  I'm also having a bit of trouble finding info on this bike.  It's a 1986 Yamaha TT225.  
      Thank you in advanced
    • By Br0ck
      Hey guys, I went on vacation all summer and let my bike sit (2002 CR250). When I got back I ripped it apart and cleaned carburetor and put it back together and it started fine and ran alright, but idled really high. I adjusted the idle screw and realized it was a ridiculous amount of turns out. I can't remember exactly how many but id say 6+ turns out. My killswitch was a POS and didn't work anymore so I replaced it with an oem replacement. My last ride I crashed the bike pretty bad and I was off the bike for 2 weeks. I can't get the bike to start ever since I got installed the new killswitch. I think I wired it right I just replaced one wire at a time and it looked pretty straight forward. I verified it by testing spark while holding it in I was getting no spark but with it in running position it would spark. 
      Anyway I am just looking for some base settings for the Mikuni TMX to get my bike running again, my carburetor has a 35 Pilot and 380 Main jet which appear to be stock. I am located in Edmonton, AB, Canada which is about 2500ft and usually ride between 10-25 celcius.
      Thanks for any replies. 
    • By Goon Rides
      Hey guys... so I have a problem... I took the bike at Cal City, broke it in, and while I was doing so, the bike was bogging in first gear (Yes, first gear). Also it doesn't idle at all. I have to keep blipping it otherwise it'll stall. So I adjusted the screw circled in image 1. Well I was listening to the sound of the of the bike, yet the rpm's weren't increasing, and it was still stalling. So then, I unscrewed the screw in image 2, well I found out that it was fully twisted to the right. So then, I twisted it to the left, still nothing; it was still stalling... where's the idle screw? I am sure that the screw in image 1 is for the air/fuel mixture screw, and the screw in image 2 is the idle screw, just broken? I don't know, any and every help would be appreciated.

    • By Zach7018
      So Ive decided its time to pull apart my favorite machine and freshen it up, and I'm looking to make some improvements in the process. I already have a 3x3 mod and a JD jet kit done and it seemed to have changed the world. The stock bike just plain sucked and now its a lot better. I never expect 450 race bike power here but I know there is a little more and I'm wondering if the juice is worth the squeeze. I already plan to add a header and exhaust, an FCR39 carb and stage 1 hotcams. After some research it really seems like those places are where the gains actually come from, so would it be worth it to throw a 434 big bore kit at this bike or just stick with what I already mentioned and a new stock bore top end??? Also my rides are often solo and pretty isolated,how much of a reliability headache can I expect with these mods?