CV Carb Mods

[FLHS93]

I have written to you before about installing SE heads and a EV59 cam in my bike this spring. The main problem I have is dialing in the carb. I have sorted it out to a 50 slow, 200 main and a 88 sportster needle with a .110" hole drilled in the slide. The carb works great except that I have no idle mixture. Because of the big cam I had to increase the amount the idle speed screw is turned to get 950 rpm which uncovered the first hole of the slow speed jet. While working on the carb one day I forgot to plug the vacuum hose for my Hypercharger and when I started the bike up it was stumbling at idle and I discovered I had idle mixture back. The hose is .187" dia. and I came to the conclusion That I will have to drill a hole in the throttle plate to bleed enough air to be able to close the plate far enough to close off the slow jet at idle. My guess is .125" to start. I have had no luck in getting any info on this. Do you agree with my theory or seen this problem? The carb is a stock 40. Thanks.

[Mike]

You'll see a similar hole in the Woods carbs. The hole size varies depending on application, cam and motor size. Start small and go up being prepared to replace the plate if needed (or solder the hole and start over, we used to do something similar, same theory on round slide Mikuni's). The other option is to go down on slow jet but then off idle, low part throttle areas will go lean. Woods also drills the air passages in his carbs to help this problem. Not somthing I would attempt for lack of knowledge about sizes and affects.
Mike

[FLHS93]

I did my research and asked questions and finally got a decent answer from a Holley carb book. Started at 1/16" hole drilled halfway between the throttle shaft and outside edge and in the center on the bottom half of the butterfly. I had adjusted the screw on the idle speed and set it to just cover the low speed holes. I used that as a baseline. The first hole was too small but it helped. I took the carb back off and drilled it to 3/32" with a hand drill. It worked. The bike now idles much smoother and low speed flatspot that I felt was gone. I also put a shim under the needle and put in a 205 main. The bike feels like it has 10hp more and easier to drive at low speeds. Did I mention it gets 38mpg? Hello dyno!

[Mike]

Great, thanks for the update.
Mike

[FLHS93]

I have not gotten to the dyno yet, but there were more changes. After I fixed the idle mixture I noticed a flat spot from 3k to 4k rpm then wake up and take off. I put a shim under the needle which worked for 2 up but was still lean solo. I was told to cut 2 coils off the slide spring. I found out that with the lower vacuum of a big cam the spring is making the slide open less and run lean. After I did this the flat spot was gone and now ran rich. I took out the shim and it runs great and the mileage is still 38mpg. It isn't as hot rod sounding but drives way better. Spanking my friends for fun is priceless.