Carb slide modification

8 replies to this topic
  • iceraceminiA

Posted March 08, 2011 - 11:58 AM


Has anyone ever modified a carburator slide. Meaning cutting or filing the arched cut-out for a rich condition. Im trying to balance my stock carb
with a 412cc Big Bore kit from Powroll.

Working on a 1974 Honda xl 350 Carb is a 32mm CV Keihin (Piston Valve)


  • michael hasek

Posted March 08, 2011 - 12:20 PM


I had to do it on a MR250 once . There wasn't a slide available so I cut it ...Carefully ! It ran perfect after. Be careful to de burr the edge after. You might try the carb section for more advice .


Posted March 08, 2011 - 12:37 PM


If the part is available, buy it. You'll never come close to accomplishing what you need to do with a file or a dremel or anything short of a Bridgeport Mill and an experienced machininst. The time you end up saving will be worth much more than the cost!
Also...are you sure you can't cure the rich condition with some jetting changes and air fuel screw adjustments?

  • scalejockey

Posted March 08, 2011 - 01:53 PM


Yes i have...The slide cut away comes into play at just above idle to 1\4 throttle. But..beings you've got a CV carb this might not help.Try a stiffer spring first. The cv can be really tricky,,the slide will open only with the proper air\air box side volume and vacuum. There might be an air jet in your carb below the diaphram that you could "adjust".
Good luck..

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  • rayivers

Posted March 08, 2011 - 04:23 PM


I've done several as well - two of them by just taping a flat piece of sandpaper to the benchtop and carefully sanding the cutaway down half a millimeter, at a slightly steeper angle than stock. The one that came out best was done by my machinist on a Bridgeport, as mentioned - he even made up a slide holder for the vise. He also did a .5mm richer one by machining the flat part of the bottom, with the needle clip moved down a notch to compensate.

In every case the component actually causing the rich condition turned out to be a worn needle jet - in your carb this is P/N 16012-356-004 - but the leaner slide further improved performance in the summer. It doesn't appear that Honda specified any leaner or richer slides for your carb.


  • scalejockey

Posted March 08, 2011 - 06:56 PM


Good call ray.. the jet needle\needle jet can wear.
Iv'e seen this more often in the high miles larger in-line fours,also sinking floats.

Years ago i had access to a machine shop and had lots of fun distroying carbs and motors:smirk: But i learned a lot:banana:

  • iceraceminiA

Posted March 09, 2011 - 11:15 PM


Really appreciate all the great input.
I finally talked to an old timer honda mechanic and he suggested finding some new old parts for my carb. He explained that with the big bore kit installed those bikes vibrated quite a bit. The needle jet and seat would oval out just enough to not meter fuel properly. I think Ray is on to somthing here. I found a keyster carb kit on ebay that I will try. If that don't work I may get an identical carb off a savage yard and try the slide milling. I don't really want to file my slide down if at all possible.

You guys rock with great info.

  • rayivers

Posted March 10, 2011 - 03:48 PM


There's one other thing you might want to try, especially if the Keyster needle works OK as a spare; building out the old needle's straight section (this attacks the worn needle jet problem from the other direction).

If the needle is sanded / cleaned up and a thin, even coat of solder applied to the straight section above the taper, then chucked into a drill and the solder turned down perfectly flat, it should work a lot like replacing the needle and jet. You'll probably start out super lean and have to turn it down a few times to get it right, and the solder will surely wear down after a while. A thinner, harder layer would be better - maybe epoxy paint or some kind of metallic coating, spun under a heat lamp to dry?

I've never tried this as I always lucked out and found a NOS needle/jet first, but if I absolutely couldn't find one, I would definitely give it a shot before cutting the slide. Apparently others have also had success with filling and redrilling the needle jet, but I'd need a small drill press and a set of jet drills to attempt this myself.


  • bombdigitty

Posted March 11, 2011 - 08:11 AM


Ive seen a product for sleds to be uses on round slide Mikuni's. There called UFO's

There plastic discs that mount in the bottom of the slide to help with air flow. Maybe these would help?


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