Reasons why not to drill your carburetor vacuum slides (and avoid buying jet kits with drill bits)

Some jet kits supply a drill bit with instructions to enlarge a hole or put more holes thru the bottom of the vacuum slide. The gimmicky idea here is that extra holes will cause the slide to respond quicker, make more power, and like all that spam you get from women you don't even know, add 3 inches to your length and girth. This is not a good idea and here is why.

Keihin put a precision hole in the slide that is sized with respect to the venturi diameter to make the carburetor respond in a stable linear manner to your throttle input based upon the load on the engine. In addition to raising the needle to meter the fuel, the slide position controls the airflow into the motor, both of which are a function of engine load. Drilling out the slide changes the flowrate and pressure-balance across the diaphragm that determines the slide position, so the response is no longer linear and may go into an unstable flutter mode.

The result of drilling causes two major problems in driveability and engine response to throttle position:

When the throttle is opened the needle is now pulled too high, which dumps excess fuel that the motor can't use because the airflow and fuel don't meet the engine load requirement. Then to correct this problem a smaller main jet and/or a step-tapered needle is supplied in the kit in an attempt to cut back on the excess part-throttle fuel yet supply the necessary WOT fuel.

When the throttle is closed the needle now drops too quickly and the deceleration-enrichment circuit can no longer function, which causes backfiring and lean-pops in the exhaust. So to correct this problem a larger pilot jet is supplied with the kit and the idle mixture screw is turned open to provide the extra fuel needed on overrun. But this dumps excess fuel all the time--which makes the bike run rich at the low end, fouls the plugs, ruins your fuel mileage and does nothing to make more power.

One thing leads to another--so one stupid thing (drilling) has now created two new issues that are patched with kludges resulting in lower efficiency, and poor driveability and performance.

Unless you are turbocharging, supercharging, changing cams or making some other major mechanical modification to the motor that increases the maximum airflow requirement to a point that exceeds the venturi rating, then the stock carburetor has plenty of adjustment potential to meet the demand without resorting to gimmicks using drill bits...

 

Thanks for sharing, Most of my bike are fuel injection
Toe does have the Super Charger..........

 

Gee you should have told me this 20+ years ago when I did mine. lol

Depends on how good a tuner you are and what modifications you have done to your engine/bike. Even the factories have to compromise on some of their engineering because of EPA, density altitude of area, etc. So some things can be modified for the better. I agree if you go to far with drilling, things could get screwed up. As most don't know much about carbs , their circuits, over all how they actually work and how to tune them.

Now if someone designs a setup the way you described then they screwed up big time. May be the original kits were closer to being right on compared to today's. At any rate a methodical tuner will not just go in and start drilling and making multiple modifications unless they have previous good data/experience to back it up.

I guess if your a novice at this stuff then beware. I get at least 40mpg arond town (unless hammering all the time), 45+ at highway, excellent throttle response in all rpm ranges and loads for the last 20 plus years. So it can be done and work well if you do it right. That's my experience with several bikes with drilled slides.

 

I disagree.
The people selling you the kit are experts who have tested the product. You keep leaving out one of the most important aspects of the slide. It has a spring to help it snap closed in case the throttle cable breaks.
And the hole does not go through the bottom of the slide it goes through the top of the slide. The slide is a upside down bucket. When that bucket tries to go down with the help of spring force there is air trapped under it. By increasing the size of the hole in the bucket (slide) It helps make the slide close quicker and gives better throttle response.

You get jets to increase the fuel flow because of free flowing intake components and better exhaust flow out of the engine.

The reason the kit costs so much for seemingly very little is that there is a coupon inside the kit that gives you $100 off when of if you get your bike dyno tuned.
The problem is that the dynotune people who have bought into this whole system will charge you up to $300 if they have to do any adjustments to your carbs in the process.

That is why is better to just buy the jets and find out the proper size drill for the slides if you do not have the money for the dynotuning.
So if you are upset that you have paid a lot and e results are not that good think about all of that.
A lot of varibles to consider. Type of air intake and filters, type of exhausts and their restriction, float postion, strength of spring in the slides, position of the needle in the slide, size of idle jet, etc., etc,.,.

One more thing....How far above or below sea level?
Temperature of outside air.

See what I mean.

Oh and ... That hole you drilled. It can always be filled in and re drilled.

 

This time i agree with Don. Some jetkits require stock springs to be stretched or include modified springs, so rate of slide action might also need mods such as drilled holes.

Don't think added hole size is needed after testing and sorry you did it ?? JBWeld to the rescue !!

I think it's been well established that, although jet kits may improve some bikes, on VFRs simple jet-juggling and 2 or 3 washers on the main jet needle, along with careful, accurate tuning of idle mixture screws will do whatever a jetkit can with fewer chances for tuning glitches.

 

on VFRs simple jet-juggling and 2 or 3 washers on the main jet needle, along with careful, accurate tuning of idle mixture screws will do whatever a jetkit can with fewer chances for tuning glitches.
This has been my experience on my '97. Amazing how much difference having the baffle in or out of my Staintune makes in how it runs at this altitude (5500+ ft.)

 

I Can Chime in with a four letter word CARB I mean California Air Resource Board. They have been killing performance in Carbs for Years. Most of the places where a manufacture tells you to drill out something Chances are it was blocked off by CARB to meet air qualities. It is something that us California riders been dealing with for years. So get out the drill press and line up the vice and start drilling.

 

What CARB forgets is that when motorcycles and cars are tested that it is just for THAT altitude.
Many Honda motorcycles only have one main jet change for when they are being sold in Colorado or 49 states and they do not change the needle position which is 90% of most driving conditions. CARB should do its homework before making rules.
When you see that all the stock motorcycles only have two different main jet sizes for the entire USA You know there is tuning to be done. Needles and slow jets are also open for tuning.

Now all these new motorcycles have exhaust systems that look like transformer toys that cost hundreds or thousands of dollars and even have foot rests built into the muffler. Crazy!

 

I agree completely.
The VF1000R That i had required two .030 washers on each needle with free flowing intake filters.
Slow jet #38 changed to #40 . The main jet could be anything from 120 to 150. But how often would you be at full throttle?

 

I never said I agree with CARB Just saying that is one of the reasons you would need to Drill your/our Carbs

 

Personally, I've only found drilling the slides to cause problems. I agree with the original post - the stock carbs can be tuned without drilling the slides. I always keep the slides stock and use the stock springs. Adjustable needles are nice and I will use the ones from the Factory Pro kit sometimes, otherwise just the ol' washer trick as mentioned above.

 

Well Hell Whew! I almost jumped on a Dyno jet kit , but before I did I took a deep look into what was going on for what gains, found out later through a infamous HRC tuner that there was only one real kit that was worth a dang, and it was by HRC , the kits where around back in the days, but today, ha, so I left well enough alone, I've heard to many wish I hadn't done that stories.