Is modifying the vacuum piston and spring of a CV carburetor a good idea?

More about carburetors here: • Constant-velocity carb...
Constant-velocity carburetors were designed to remedy certain deficiencies of mechanical-slide carbs. Accordingly, modifications of some components of a CV carb may, ultimately, hurt drivability and performance. If the butterfly valve is suddenly opened when riding at part throttle, both the velocity and the vacuum in the venturi decrease momentarily. To compensate, the vacuum piston lags behind the butterfly valve (perhaps even drops?) for an instant. The job of the vacuum piston is to keep the velocity of air through the venturi as constant as possible. Modifying the spring and the vacuum piston CAN be beneficial for racing applications if done properly, but for
every-day use, it's best to leave these components alone.
The dimensions of the vacuum port, provided in the video, are for a Harley-Davidson's Keihin CV-40 pumper carb.
In the video, I neglected to mention the importance of the correct fuel level in the float bowls (best float height for full-throttle power at low rpm) for purposes of correctly tuning the carbs. Watch this video series for more info: • Carburetor fuel level ...
constant velocity carburetor
carburador de diafragma
mechanical slide
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[@niconine268]

Great tips right there. Makes sense

[@jamespolucha8790]

That was the best I’ve ever heard for how a cv carb works and why not to modify it, I try to tell people not to do this but I couldn’t explain why, thanks for this great video

[@Zerk_Ziegler]

There are situations where such mods can be beneficial, but if the bike is essentially stock, and good throttle response and fuel economy are the primary targets, it's best to leave the vacuum piston and spring alone.

[@denisthek]

I agree, when I was younger my friends and I knew more than the engineers, I have had modified vehicles and been beaten in drag or roll on races more than once by 100% bikes. It was really good the way you showed how to check the vacuum diaphragm by blowing air into the venturi. Thanks Zerk. Denis Kerechuk

[@Zerk_Ziegler]

Thanks for the comment -- what you describe happens a lot! Also, I hope that it was sufficiently clear in the video that compressed air was being introduced into the vacuum slide/enricher air-intake port, rather than into the venturi.

[@stuartbudd4584]

Thanks for posting this -- I've been buying up second hand Keihin CV carbs and putting them back to stock specification for a couple of years now. They always give a better result in terms of pure rideability when compared to S&S / Mikuni etc. The CV carb is really under rated -- works well on Shovelheads too...

[@Zerk_Ziegler]

It's always best to use Keihin factory parts whenever possible, especially when it comes to float-valve needles.

[@bluestripes1]

not a waste of time, but a classic method of tuning transient fuelling response, lighter spring if switching to less agressive cam, stiffer spring for more aggressive cam, changes the "accel pump" effect. my old SU carburetors were sold from the manufacture with 3 different colored springs you could choose to tune this, and its effects are explained in the SU tuning manual. a stiffer spring rises the slide slower, which makes it richer in transients, this ie needed for big cams with low vacuum tha suffer from reversion. switching to a conservative cam that produces more vacuum that stock would want the lighter spring, this would help keep the engine from going too rich during transients due to high vacuum.

[@Zerk_Ziegler]

Thank you for the comment! If I am not mistaken, in addition to the spring, SUs have an oil-filled damper to stabilize the motion of the vacuum piston. Keihin CVs do not have a damper (the situation is somewhat similar to removing the oil from the front forks and leaving only the spring), so changing the spring rate and/or enlarging the vacuum port more often than not hurts drivability. Also, the instructions, provided with some of the jet kits for the Japanese CV carbs, are nowhere as scientific as the SU tuning manual. They just tell you to replace the spring, drill the slide, and done -- regardless of how (if at all) the engine was modified.

[@joshlucas5907]

Well spoken, thanks for deterring me from drilling my slide and cutting my spring. Lol. I will stick with jetting

[@Zerk_Ziegler]

The worst "mod" of all is when folks try to stretch the spring. At least when the spring is cut, it becomes proportionally stiffer. Stretching the spring takes all predictability out of the equation. A plastically deformed spring is a broken spring.

[@ThomasELeClair]

@@Zerk_Ziegler ,,,,,,,,,,,,,,,,spoken like a design engineer.....................................thankz...................

[@Zerk_Ziegler]

Thank you for watching.

[@ClassicRider]

Thanks for the video - quick question(s): I did a rebuild on my 1979 the other day . The Low Speed Jet had come loose on Carb 3 from a Bike Repair Rebuild. I restored my 79 CB750 that I had stored sine 1988, during shelter in place. I rebuilt the carbs then. Seeking a professional final touch, the Bike Repair Rebuild was supposed to be simply tuning. I discovered that Springs #3 and #4 were longer as if stretched. She seems to run okay, but wonder if even the same spring but stretched, is there any effective difference where the longer on has more spring pressure. Also - where can one find such springs? ?

[@Zerk_Ziegler]

Not a good idea to stretch the springs, in my opinion -- see my response to a comment from @joshlucas5907, below. I don't know if these springs are still available from Honda. Babbitt's Honda Parts online is a good resource. Otherwise, I would try my luck on eBay, Facebook marketplace, Craigslist, etc. 1 month ago

[@ClassicRider]

@@Zerk_Ziegler Hi - Thanks for your reply and suggestions - Agreed - I would not stretch the two remaining, and will look at Babbits and the other suggestions -

[@crazyDIYguy]

I have the CV carb vents protruding from inside my plenum after the air filter in my ram air system. So far the bike performs the best this way. That being said I get wavy acceleration with a slight hesitation before I get positive air pressure, or higher speeds. I'm wondering if I should play with the slides...

[@Zerk_Ziegler]

Before you do, you may want to check the price of replacement ones ...

[@crazyDIYguy]

@@Zerk_Ziegler I have two extra racks from two different identical parts bikes. I'm just ripping my hair out with this ram air system and you seem smart. I'm just going to have to get the wideband 02 on it and read my AFR inevitably.

[@Zerk_Ziegler]

@@crazyDIYguy Using a wideband O2 sensor to read AFR is always a good idea. It does help to approach jetting in a methodical manner -- see, e.g., www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html. You can also use an acceleration meter, like one from SPLLab, to fine-tune the results.

[@user-xh6yu8jw1z]

The vacuum pistonon my 2000 Heritage has 6 hole. 1 for the needle and 5 for vacuum. I found it on line; however, now I don't know where it was at. Anyway I changed the piston to a 2 hole stock piston, now the bike doesn't want to run when I open the throttle. It will run when i open the enricher.

[@Zerk_Ziegler]

Sounds like there is an intake leak. I have a couple of videos on the subject you might find helpful.

[@petev.9357]

I agree, leave the slide and spring alone. If for no other reason each slide is about $150.😲

[@Zerk_Ziegler]

Ha! I still remember the sticker shock when I priced replacement vacuum pistons for a Kawasaki Ninja back in the day. But, for some reason, Harley-Davidson slides for Keihin carbs are like, $50.

[@southern_merican]

What would cause mine to bounce up and down and never open all the way? 250cc

[@Zerk_Ziegler]

Did you check if the rubber diaphragm has a tear in it?

[@southern_merican]

@@Zerk_Ziegler yes I checked /removed and cleaned everything ,plus sonic cleaned twice to make sure. but is it supposed to "bounce"?

[@southern_merican]

it is a 2005 baja wilderness 250, so it's old I might just be asking too much,lol

[@Zerk_Ziegler]

@@southern_merican Are you saying the vacuum piston is bouncing when the engine is running at constant throttle? Is the engine running OK or no? Have you checked compression?

[@southern_merican]

@@Zerk_Ziegler yes "piston" with needle in the MIDDLE of the 5 clip positions. it's all the way down when idling and when I go full throttle for sec, it comes up to halfway and bounces,never opening up all the way. the butterfly does open fully tho. main jet is 96 and tube directly under it is a 85. is that a problem? it's how I got it. also compression is 130psi.. yes engine runs ok. but starts losing power after 3rd gear

[@choppergirl]

These carbs get clogged up and fail so often and cause everybody a world of grief, I wish I could just change everything I have to fuel injectors and a fuel map. I don't know why we are still using 1880's technology in 2024.

[@Zerk_Ziegler]

Personally, I've had very little trouble with carbs. Motorcycle carburetors are stone simple (compared to those on cars) and pretty darn reliable. Because of gasoline formulations these days, it does help to use marine-grade fuel stabilizer at every fill up. When upgrading or rebuilding, it's best to stay away from aftermarket kits and parts -- I use factory components whenever possible (especially float-valve needles). A good-quality fuel filter is always a good idea. FI is nice, but has its own issues: fuel pumps go bad, injectors clog, and fuel fitting leak.

[@choppergirl]

@@Zerk_ZieglerI cleaned and rebuilt an 1985 CMX250 carb I've been having problems with for 20 years now... on and off the bike, on and off, on and off, on and off. Tab breaking off float, float height wrong, idle jet getting clogged, you name it, who knows. Waiting on an air valve from aliexpress that was impossible to find, my last guess as to why it's idling fine on choke now, but bogging down when I turn the throttle, until I get pass the stumbling point and then it runs fine. With choke full on. Which I guess, yeah, is still not fine. You know, I thought I could get around all of this by buying a whole new carb off of ebay, which is exactly what I did. They sent me a dual carb, and I have a single carb. Apparently on the same model bike at some point, they switched from single to dual, and most of them are dual, but I got the super rare parts for it are unobanium single carb version made in the first model year 😞 The week before this, it was a vacuum operated fuel petcock valve that had a recall on it on my Honda Shadow I narrowed down and traced all the way backwards to the safety recall sheet from 15 years ago...

[@ClassicRider]

Being? - Martin Heidegger ? btw: not a Babbits nor Ebay -

[@Zerk_Ziegler]

I think in this case Being is just a matter of Time :) These parts will pop up eventually ...

[@timothywhieldon1971]

modifying things like this are a HUGE waste of time and $

[@Zerk_Ziegler]

In most cases. Yet, a whole segment of the aftermarket powersports industry emerged in this area.

[@timothywhieldon1971]

@@Zerk_Ziegler because people think it goes faster... but it does not. but dyno vs reality of running lean or rich and lack of power. nearly ALL carburated engines after the 60's were basically as good as you can get them and ONLY tuning needed to be done from high altitude or exhaust etc.... You know what im talking about!

[@Zerk_Ziegler]

Another neat thing about constant-velocity carbs is that they automatically compensate for a fair amount of altitude change.

[@timothywhieldon1971]

@@Zerk_Ziegler at first i was intimidated with carbs and getting them tuned and jetted correctly but in the end i was over complicating things and now they are EASY to work on.