Drilling out CV40 slider port
Thread Starter
|
Tourer
Joined: Mar 2025
Posts: 250
Likes: 89
From: SWFL
Drilling out CV40 slider port
Drilling out the CV40 carb vacuum slider port to 1/8" became conventional wisdom for faster throttle response. All the kids were doing it. The '96 Dyna I am doing a top end on has it's port drilled to a little over 1/8", not sure exactly how much over. Before I tore it down the carb worked fine like that at 2 -1/2 turns out and a 195 main jet.
But lately, I am seeing respected sources say not to do this, that the stock engineering is sound. It is said to aim for faster response by using a lighter performance spring. Also, if you just must drill it, go no bigger than 7/64" Mine is about 2/64" larger than that.
Since I am basically starting from scratch here my feeling is not to put the over bored slider back in. I have a good clean un-drilled slider on hand. Do I put that one in as it is, or do I drill it to 7/64?
If it matters, the bike is stage 2. I have ordered a performance spring from CV Performance, where I get my Keihin jets and parts.
But lately, I am seeing respected sources say not to do this, that the stock engineering is sound. It is said to aim for faster response by using a lighter performance spring. Also, if you just must drill it, go no bigger than 7/64" Mine is about 2/64" larger than that.
Since I am basically starting from scratch here my feeling is not to put the over bored slider back in. I have a good clean un-drilled slider on hand. Do I put that one in as it is, or do I drill it to 7/64?
If it matters, the bike is stage 2. I have ordered a performance spring from CV Performance, where I get my Keihin jets and parts.
Popular Reply
Apr 2, 2026, 11:07 AM
HDF Community Team
Joined: Dec 2005
Posts: 5,428
Likes: 2,624
From: NC USA
Most of us learned many, many years ago - do not drill the slide and do not cut the spring.
Seasoned HDF Member
Joined: May 2008
Posts: 21,180
Likes: 12,755
From: South Carolina
The spring change won`t make any noticeable difference, stick with the OEM part.
The CV carb is not going to have the throttle response of a Mikuni flat slide carb, no matter what you do to it.
The CV carb is not going to have the throttle response of a Mikuni flat slide carb, no matter what you do to it.
Grand HDF Member
Joined: Aug 2018
Posts: 4,126
Likes: 2,117
From: W. MI.
Leave it stock. I played this game. All I got out of it was a lighter wallet and some useless parts. I think I gave those to my cousin. I have the stock parts back in the bike and it's fine.
Trending Topics
Thread Starter
|
Tourer
Joined: Mar 2025
Posts: 250
Likes: 89
From: SWFL
Bass Player, nice bongo.
Interesting that so far it is do not drill. I have seen drilling to be accepted as the thing to do in several different places and kits. Glad I asked. Not drilling is my inclination.
I always ask about these things, as in if it is such a common accepted "upgrade", why didn't HD switch to it after seeing it being done? Could it be the HD engineers know better?
Nah.
.
Interesting that so far it is do not drill. I have seen drilling to be accepted as the thing to do in several different places and kits. Glad I asked. Not drilling is my inclination.
I always ask about these things, as in if it is such a common accepted "upgrade", why didn't HD switch to it after seeing it being done? Could it be the HD engineers know better?
Nah.
.Last edited by randallissimo; Apr 2, 2026 at 06:16 PM.
HD Forum Stories
The Best of Harley-Davidson for Lifelong Riders
7 Times Harley-Davidson Chucked Tradition Out the Window
Verdad Gallardo
7 Surprising Harley-Davidson Products that Are Not Motorcycles
Verdad Gallardo
8 Best Harley-Davidson Motorcycles Ever
Pouria Savadkouei
10 Worst Harley-Davidson Motorcycles Ever
Pouria Savadkouei
Killer Custom's Jail Break Is The Breakout That Refused to Blend In
Verdad Gallardo
Crazy Bunderbike Build Looks Amazing, But Is It Impossible to Ride?
Verdad Gallardo
Harley-Davidson Reveals Super Cool Cafe Racer Concept
Verdad Gallardo
Engraved Rebellion: Inside Bundnerbike's Glam Rock II
Verdad Gallardo
