Has anybody drilled the slide hole a bit larger in an attempt to get the slide to open a bit quicker?  I have got my wife's carb spot on and while it now has great throttle feel and response to large inputs it is just plain lazy compared to a slider carb.  My brother has a reworked CV carb (by Wood) on his 100-inch HD and it opens nice and quick.  I know there must be more we can get out of this carb.

I realize there are likely equal arguments with respect to drill or not drill and with respect to the slide spring and I don't want to start a huge debate.  I simply would like some feedback if you have tried it...

batman wrote on 05/13/19 at 13:08:14:


Thank you so much for feedback!  I have played with the TEV valve and got mixed results.  I tried less spring rate, spacers, etc. and in the end the difference was small.  A spacer seemed to help but when it got cold the next week it got worse so I ended up right where I started.  With a weaker spring the carb pulls from the TEV circuit at low engine speeds and runs too rich so that is not an option.  At this point I am thinking exhaust leak up front...

Right now the carb just needs to open a little faster so I will try opening the bleed hole one numbered drill bit size at a time...

batman wrote on 05/13/19 at 13:52:42:


Great.  The engine pulls very strong but does not have nice snappy response.  My wife loves the bike but complains about slow response.  I think increasing the bleed hole a very small amount is the key.

My SRX600 secondary CV carb has the slide bleed hole increased along with a new spring and response is instant -- Way better than it was stock.  Compared to stock XL600 stock SRX600 was a joke.

DragBikeMike wrote on 05/13/19 at 15:22:11:



Thank you so much for all the good information Mike.  I will look into it early tomorrow morning.  Agreed on momentary lean condition.  Dial-A-Jet can help with such conditions and I have a link to a dyno test that shows the same somewhere...  Here it is:  http://www.valkyrieforum.com/bbs/index.php?topic=25860.0

I have a Dial-A-Jet in stock so I think I may give it a try with a smaller main at some point if time permits...

I don't know how many of us are old enough to have ridden a 4 stroke motorcycle before the CV carb was common.....and can remember the "cough" that occurred when you opened the cable operated slide too quickly.  If you were at low rpm and cracked the throttle open, the vacuum in the carb throat would instantly drop and no fuel would be picked up from the float bowl....so the engine would gasp and stop running until you closed the throttle and allowed the vacuum to return.  On these motorcycles it was necessary to roll the throttle on at a speed that the engine could handle - rather than open it instantly.

The CV carb was developed to prevent this situation - the hand throttle operates the butterfly - and the slide responds in to the engine vacuum and rises as the vacuum increases.  If you crack the throttle open too quickly - the slide will not rise until the engine vacuum is building and this helps to provide smooth engine operation.  (On British cars the slide has an oil filled piston in the center of the slide to keep the slide from moving too quickly or bouncing around - American cars instead developed the accelerator pump to shoot fuel into the intake and compensate for the lean condition that exists when you crack the throttle open).

When you increase the size of the holes in the vacuum slide to allow more air to flow - it allows the slide to move up/down faster.  If you go too far - you will allow the slide to raise too quickly and could get into a situation where the mixture goes too lean and the engine accelerates poorly - or the slide begins to bounce up/down in response to the pulses of the intake vacuum (put a vacuum gauge on the intake and watch the needle bounce up and down to prove the intake vacuum is not steady).  The stock vacuum operated petcock has a small orifice to dampen the vacuum pulses to keep the diaphragm from moving around).

So....I am posting this just to caution you to go slowly and not go too big on the holes.  Raising the slide too quickly could cause you to have inconsistent fuel/air mixtures - and could result in the slide moving around a lot during steady cruising (vacuum pulses could cause the vacuum on top of the slide to change too quickly).

Maybe you should start by enlarging the 2 existing holes a small amount, then drilling a 3rd hole if you want to go farther - you could epoxy the 3rd hole closed if you end up going too far.

This is a great thread with lots of great information.  Personally, I just slap the clutch lever and get all of the throttle response I need.    

I suggest you test it first by removing a screw.  It will allow you to determine if you like the results before you remove any material from the slide.  If you aren’t happy with the performance you can simply reinstall the screw.  No harm no foul.

I will be so bold to suggest that you could leave one screw out permanently.  The remaining screw and plate are captivated within the slide.  In the event the screw comes loose, nothin is goin through the engine.  It will simply shut you down.  Not a pleasant experience but I think it’s highly unlikely.