Cv Carb Question

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Cooie
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Cv Carb Question

#1

Post by Cooie »

Hi guys, i have researched many posts on the CV Carb topic but can't find an answer so sorry if it has already been discussed.
Did Harley use both 38 and 40 mm Keihin CV carbs on their later models.
I am looking to fit a CV carb and want to know which is the best unit.
I am looking at a model # 27421-99C - I have searched this model and it looks like it is a 38 mm.
Any help appreciated.
old.wrench
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Re: Cv Carb Question

#2

Post by old.wrench »

Cooie,
Not sure about the 99-C variant, my experience is with the the 99-A which is 40mm. Properly jetted and setup, it works great on the Panhead motors. A Keihin CV was used on the Buell Blast, maybe it was the 38mm?
Regards,
Geo.
Cooie
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Re: Cv Carb Question

#3

Post by Cooie »

Thanks for the info, I have read many article re jetting. Further research has suggested the Twin cams used a 38mm CV and the Evos used a 40mm CV. I have always liked the CV carbs for kick only bikes, I will continue my search for a 40mm
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Re: Cv Carb Question

#4

Post by Andygears »

If you have the 38mm unit, I would think it could be made to run properly on a semi-stock panhead. Usually a slightly smaller carb gives crisper response but could be somewhat restrictive at high RPMs. Do you run high RPMs? Or is your motor stroked or have a radical cam? Ported or with short drag pipes?
Too large a carb is harder to tune to a smooth idle, especially on a pan with restrictive intake ports and long runners. It's harder to get correct velocity at low RPMs. The CV aspect will help with this as a vacumn slide keeps the airflow low till vacumn builds and raises the slide.
Just like a big camshaft sometimes more is not better.

Just my 2 cents

Andygears
Cooie
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Re: Cv Carb Question

#5

Post by Cooie »

Thanks Andy, I'm not looking for big revs, I have a 59 - STS shovel design heads, ported and polished and running a crane cam aimed at torque as I am running a hack. Low to mid range is important on my bike.
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Re: Cv Carb Question

#6

Post by nmaineron »

We used to run 38MM Mikuni round slide back in the day.They were on a lot of snow sleds and were plentiful in this area.they worked well on Sportsters after they got the right jets and throttle cable.
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Re: Cv Carb Question

#7

Post by Dave_R »

Just to share an opinion.....

For torque, I would go with the smaller throat. I ran a 40mm Mikuni for a long time. When the slider became too worn out to even hold a idle and also leaked a lot, I replaced it. I found a 45mm Mikuni at a swap meet, and tried it for about a month. It really ran like crap (high idle, weak torque, and high end sputtering), and could never get it dialed in. So, looking for another replacement, I came across a 34mm. Just bolting that on I found a dramatic increase in smoothness and torque, and had plenty of high end when I called for it. I also noticed an increase in the gas mileage. Pan is 0.070 over with 8.5:1 pistons, and a Sifton 412 cam. That was about 8 years ago and still going.

So, seems to me that the difference between 38mm and 40mm is kind of small, so either would work, but I would lean toward the 38 given the choice. I'm not sure if it really makes a lot of difference in a pan, due to the flow of the ports, manifold design etc., etc.

The smaller throat gives you more velocity which is what helps torque. Unless you run a bored out monster stroker... but that would be another issue.

- Dave
Cooie
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Re: Cv Carb Question

#8

Post by Cooie »

Thanks for your input Dave, what you say makes sense as does Andygears.
I think you guys are right, I will give the 38 a go as I need low to midrange more so than top end.
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