98 Big Twin Evo with a mild cam, 40K miles, Edelbrock Performer cylinder heads, Edelbrock intake manifold, K&N filter, Dyna 2000 ignition, Screamin' Eagle 2-1 exhaust, Screamin' Eagle plug wires, stock 40mm CV carb.

This weekend I rebuilt my stock 40mm Keihin carburetor with the CV-40 Master Kit Rev 1 Yost Power Tube kit. Here's my experience and my impressions.

CARBURETOR REMOVAL
Removing the carb is fairly straightforward:
-- remove the air filter and air filter backing plate
-- stuff a rag under the carb to catch dripping fuel and remove the fuel line from the carb
-- disconnect the choke **** from the mounting plate (to allow easier removal of the enrichener valve assembly)
-- fully loosen one of the throttle cable adjusters
-- open the clamshell throttle cover and disconnect both throttle cables from the wheel (don't lose the brass cable ends, which fall off easily!)
-- work the throttle off of the rubber spigot by gently twisting and pulling
-- disconnect vacuum hose to VOES and Evap Canister
-- unscrew enrichener valve from carb (easy with a 9/16" open end)
-- disconnect throttle cables from carb (make not of routing and of which cable goes to which position on the cable wheel)

CARBURETOR OVERHAUL
I won't go step-by-step on this, but trust me: disassembly, cleaning and reassembly are very simple with basic hand tools and a SHOP MANUAL. If you don't own one, get one: The Yost kit does not include complete carb overhaul instructions and there are a few minor gotchas that could bite you in the butt if you don't read the manual. for example, the float pin must be removed from one side only, and it's a press fit into one of the two pedestals (the pedestal with the rounded top). There's a little arrow on the body of the carb next to the rounded pedestal that shows the direction of removal. You'll need a small punch of some sort to remove the pin. I sacrificed a hex wrench by snapping off the short leg of it and used the long leg as a punch. Other than that, the only remotely unique tool you'll need is a flat-blade screwdriver with a thin shaft to remove the slow speed jet. Follow the instructions in the shop manual for disassemly, cleaning and inspection, and assembly, and augment with the Yost instructions. Read the Yost instructions first!

CV-40 MASTER KIT OVERVIEW
The kit includes the following:
-- emulsion tube (it replaces what is called the 'needle jet holder' in the shop manual)
-- Yost power tube (press fitted into the emulsion tube)
-- grooved jet needles (a blue one and a red one, with a snap ring that slips into the groove of the needle you choose. More later)
-- 1/8" drill bit
-- 7/64" drill bit
-- compression spring
-- 165, 175, 185, 195 main jets
-- washers (actually, they're brass spacers that Yost, for some reason, calls washers)

Note: The CV-40 kit does not include slow speed jets. If you buy online from Yost, they'll provide the one you want. I didn't know this when I purchased my kit from San Francisco Choppers, so I had to stop at my local dealership and buy the jet I wanted.

YOST INSTALLATION
Before you install this kit, I recomment that you send email to customerservice@yostperformance.com and describe your application and desired results ("all the power I can squeeze out of this thing" ... "the best trade-off between performance and power" ... "all the fuel mileage I can get" ... or whatever. Be sure to describe the make and model of your bike, engine mods done, your riding style, etc. You'll receive recommendations for which color needle jet to use (and where to place the snap ring on it), the best main jet, the best slow speed jet, and guidance on which size holes to drill in the slide with the provided drill bits.

Installation of the kit is simply a matter of substituting a few Yost parts for the factory ones, per the instructions provided in the Yost kit and the guidance provided via email. The only "work" you'll do on yo

 

Very good review.

I think it was the Yost that woke up your bike, cleaning an air filter helps, but not that much.

I had one installed on my bike 2001 FXD, stock cv carb after I changed mufflers to Cycle Shacks and put a AN Big Sucker AC on. At the time I wasn't too familiar with the inside workings of a carb and jetting so I paid the $ to local indy. After being there while they dyno tuned it I think I could have installed it without a problem (I had already drilled out the cap for the Idle Mixture Adjustment Screw).

It made a difference on my bike also, quick throttle response and no more surging when cruising at 30 mph in the city. Mileage is 47 mpg on the highway.

 

Wish I could get mileage like that but I only get 39/40 on the highway, but I have a dynojet kit.

 

I should have said running 55-65 mph on two lane highways. Tend to stay away from 4 lane+ interstate highways unless I need to get somewhere quick. Running 70-75 mph the mileage drops down to low 40's, so your not too far off.

 

maybe if I had a dyno tune Id do better.

 

Anyone know the difference between the red and blu needle?