What does the diaphragm actually do?

I've been pondering this for a couple weeks now and can't find any real answers.

I'm wondering what the actual purpose of the diaphragm/cylinder is, and when does it do it's sliding.
I know that it raises the needle to allow a greater volume of fuel to flow from the main jet.
Does the blocking of the venturi control airflow only? Also, when is the whole assembly raised by vacuum? Is it only under load, is it rpm based? Does the whole travel of the assembly move linearly or is it some other kinda of variable movement?

Lots of questions I know you guys can shed some light on the operation of it for me!

Thanx!

Kev

 

As intake velocity increases, the resultant vacuum is transferred to the chamber above the diaphragm and it then lifts in response to this vacuum.

The diaphragm lifts the slide, and "opens" the intake port as it lifts the needle as well, supplying not only fuel from the main jet but more airflow as well.

The REAL purpose is to keep us mere humans from direct control of the carburetors, for fear we might pollute excessively. We're only allowed to open the butterflies and "request" more intake charge; the carbs control the degree to which our measly request is granted.

 

is this kind of carburetor (with the diaphram and slide) used exclusively on the yamaha or YICS system of motors?

do honda/suzuki/kawasaki et al use the same type carb?

 

So it's the low pressure created by the venturi that lifts the slide? If that's the case then the slide position must be varied by both RPM and butterfly position?

or am I way off lol

 

It's of type CV or Constant Velocity Carb. It's all i've seen used on the bikes i've worked on (and that's not too many) only other type i've seen is slide controlled throttle (don't know the name of this type) but it's when the throttle cable does the actual raising of the slide and needle. Mostly seen in small cc motors (quads, dirt bikes etc)

 

If your wife/girlfriend is pregnant, then it has failed



common someone had to say it

 

LOL!

 

Have to admit it crossed my mind as I read the subject.

 

Nowadays, pretty much all larger displacement carbureted motorcycles (except racebikes) use them. They're the "cleanest" of the carburetor types (since WE don't get to control the fuel) from an emissions standpoint.

They're not used on smaller displacement motors where emissions aren't a major issue.

 

Here'a good primer on CV carb theory: CV Carbs

 

Mann you guys are too quick.. I was coming in to lay that one down, but I see I was to late, the hole was made... *Drum roll

 

Thanx Stereo! That does help answer lots!

 

I've got a better idea on what to look for for tuning Ruby. I set my pilots for 4 turns out, did a plug chop from 80 km/h (about 5 mins at that speed) and had WHITE plugs.

When i got home i let her idle for about 5 mins, haven't checked plugs from that idle yet, but i did bring the colortune home from my buddies, so i'll get her warmed up and colortune when it gets darker out tonight, See if me changing the pilots does anything at all to the idle mixture.

I'll report back when i'm done tonight.

I'm starting to wonder if i don't have plugged air passages or something, and my slide isn't seeing the depression it needs to raise itself. Ruby acts like she's flooding at idle and lean as hell while running at speed.

 

Well, no..... the real purpose of the CV carb is to give the engine the correct fuelling regardless of how ham-fisted the rider is.

 

I'm uploading a video right now, will be about 30 mins, so here's the low down as a preview.

Bike runs awesome except for if i idle for more than like 10 seconds, i have to open the throttle pretty good to get it out of bog mode, almost like it's drowning itself.

The colortune doesn't really show this, the only thing it shows that's odd, is if i crack it from idle it shows rich, and comes back down to idle, orange (rich) and then the rpm's come back up to normal, along with the colour.

Let me know your thoughts after i get the vid posted and take a look. Let me know if what i'm seeing is normal or if there's something odd.

 

http://www.youtube.com/watch?v=m--xJuhB51s

The point that it dies, i hit the choke just to see what it would do

 

Dunno if it will help you or not... My 750 had this exact problem, even if I turned the pilot mixtures way to the lean side. Dropping float level did nothing, either. Then I stuffed the rubber dust boots on the enrichment plungers with silicone grease (they looked pretty loose), and the problem went away.
I'm still not exactly sure how to explain it. My enrichment plungers were not leaky... But the air that seeped through the dust boots could have entered the carbs via the enrichment air passages. :?

 

I'd check tht but my choke is a little different than XJ's, i don't have the plungers with the fork that pulls them up, ihave a sliding rod that uncovers the holes internally in the carb (i think that's how it works)

 

NM i do have plungers they're just at a different angle, here's what mine look like (not my carbs but the same thing i have)

http://i16.tinypic.com/432dtlw.jpg

 

Quite true. Old-school cable pulls the slide valve carbs are nearly impossible to set up so that you can suddenly grab a handful without bogging; you have to "feed" the engine more throttle as it gains RPM. With the CVs, they do the same thing for you.

Here's something to ponder: WOT with CV carbs is more like WOB until (or unless) the diaphragms respond. YOU crank a handful, you get Wide Open Butterflies; the diaphragm has to pull the slide up for Wide Open Throttle.

Explain some tuning mysteries maybe?

 

There's definitely more scope for older CV carbs to misbehave more often than the old style cable pull slides. You've got diaphrams that can split, diaphram springs that can sag and far more seals / O rings that can perish.

My old XT500 has a cable pull slide in the carb. It's got 1 seal and 2 O rings in the ENTIRE carb. That's it 8O



But you can't crack the throttle open - you've got to wind it on......

 

Ever installed a jet kit? A lot of jet kits come with a drill bit to enlarge the hole(s) in the slide - resulting in a quicker throttle response.

 

bump for opinions while we watch hockey

should i lean out my idle and maybe check my air passages for slide?

 

The type of diaphramless carb, where the rider has DIRECT control of both butterfly opening angle AND needle position is called a:

Flat slide carb.

If, in the same application, both a CV and FS carb are properly set up, the FS will provide MUCH crisper throttle response. This can be read to mean a quicker, snappier bike. Bikes that can't, or have trouble, wheelie-ing with a CV setup will often have no trouble when switched to FS.

As Fitz has already stated, the EPA is happier with your CV setup.

 

Hi all,
I think this has to do with this topic so I'm posting a related question I hope.
1- 4into1 exhaust(2to1 + 2to1 = 4to1 outlet hole size)

2- pod air filters

3- stage 3 dinojet kit installed with 122 mains, adjustable needle clips set at one position lower than suggested to richen mid range

4- vacume holes in slides dilled out 1/32" larger

5- new throttle shaft seals

6- carbs are clean (carb cleaning 101 clean)

7- polished slide bores (had to after reading ricks post on that :lol: )

8- floats at correct level

9- pilot air screws out 3 1/4 turns

OK so we all know what was done
My question finally. My bike flutters from 5k to 6k thats it, runs perfect everywhere else it does it in 2nd through 5th gear rpms clime to fast in 1st to really notice it. Its also running lean but I'm starting to get some color in the plugs. The flutter is only at 1/2 to WOT rolling on from 3k. Does it sound like I need to go larger with the pilot jet, too large of transition between pilot and main?
I'm thinking resync ,recheck floats, and check for vacume leaks again. I'm goin to drop the needle clip down one more position and run it tonight to see if that helps. It almost feels like the slide is bouncing in the carb from 5k to 6k I bought a bore scope one of those cameras mounted on a 2' fiberoptic lead maybe I can stick that in one of my carbs to watch the slide while driving, better find a deserted road first though :lol:
Well that's it any ideas or theories would be greatly apprieciated
Thanks in advance,
James

 

Another idea/question. What would be the pluses or minuses of enlarging the hole in the slide by a little. I'm thinkin it would help the slide raise faster and/or keep it raised for a longer amount of time because the depression can transfer to the top of the hat faster.

Thoughts?