23 replies to this topic

[Dixie]

Well here we go

pros and cons of cv carbs???

in my experience (limited i know)dont take the top off the carb

the diaphram goes in about the size of the carb top,but after fuel has been under it sweels to about twice it size and they where about 50 quid ten years ago,so when you trapped it putting it back in and split the thing it cost loads,

will the new beta carb need regular cleaning?? i bet it will.

i did have a few little tricks for re assy,but will save them for later..

[Marky G]

That made me laugh, I had the exact same thing happen to me on a KLX 650, took the carb top off to see if the boot was split and I could not get it back in for the life of me....Had to spend £75 notes on a genuine Kawcker one


DONT TAKE THE CARB TOP OFF!

[Neil King]

Mark, the trick to getting the CV diaphram back in: Put a thin layer of oil along the lip of the diaphram (just rub an oily index finger around it), put the diaphram in the carb and as close fitting in the groove as possible as you are compressing the spring with the cap, apply a little pressure on the cap and wiggle it around on to of the diaphram and you will feel it slip ino the groove, hold on tight untill you get the screws in and snugged up.

[PA.]

I was in this bird today and low and behold if there was not a diapham in there.

Attached Images

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[paul_thistle]

To install the diaphragm, put two fingers in the back of the carb and hold the throttle slide all the way open. Then push the diaphragm down around the valve. In this position it should fit in the groove, if not lower it until it does.Then put the spring in and install the cap while continuing to hold the throttle valve open. Don't release until you have tightened a couple of screws.

[Ridgrunr]

My 1985 Honda V65 Sabre has CV carbs. 4 of them, Keihins, on that V4 engine. I had a little mid range flat spot at about 4000 when I first got it, but shimmed the needles 2mm which fixed that. Thing still runs like new at 54,000 miles, with not a burble or hesitation up to an indicated 152 mph. I've had it from sea level to to the top of Mt Evans in Colorado at 14,000 ft. You could tell there was less air at 14,000 feet because even that 1100cc engine doesn't produce the power at 14,000 ft that it does at sea level, but it always runs perfect, and I've never opened up the carbs since I shimmed the needles.

My 2006 Kawasaki Brute Force 650i quad also has CV carbs, two mikunis. It also runs perfectly from sea level to the 10,000 ft I've had it on the Paiute trail. I've never touched the jetting, it is stock. I live at 5300 ft MSL.

So for my experience, the CV carbs have been terrific. I think they are better for all around performance than conventional throttle slide carbs, and start easier too. I do have to use the choke when cold, but as soon as the machines are running, I turn the choke off or they will start to burble.

I guess the down side is the rubber CV boot. You have to be careful not to tear them when working on the carbs, as I've heard of some problems over the years with torn boots.

Kawasaki seems to be the last hold out however on the CV carb thing on their quads. Almost every other brand has made or is making the transition to EFI, which is mostly EPA driven.

My brother in law has EFI on his 2006 Can-Am 650. It's a tad faster than my Kawasaki, and uses about .5 gallons less gas through the 5.4 gallons of gas the quads hold, so the EFI is a probably little more efficient than the CV carbs, but not by much.

[HAM2]

PA., on Dec 2 2006, 09:56 AM, said:

I was in this bird today and low and behold if there was not a diapham in there.

Got a bit excited there, I thought you were going to post a pic of a woman

[seandellear]

Not sure how well she would run on a CV though, especially if she had a split boot! fnar fnar lol

[DonD]

You may notice that they don't use CV carbs on serious dirt bikes. The reason is that the slide in the carb is effected by bumps. The vacuum is not enough to hold the slide up when you hit a bump and so the carb will shut down momentarily. I learned this lesson quite painfully some 25 years ago when I put a CV carb on an OSSA. It ran beautifully around the parking lot but coughed dramatically on the first small rock face I tried. Cost me one fingernail! Oh yeh, I remember that.

[sideup]

Don D,

Anybody correct me if I am wrong, but isn't a Keihin FCR-MX 39, used on 2006 KTM off road bikes a CV type carb?
If it is they must work pretty good. I don't want to start a brand war, but the KTM makes pretty serious off-road bikes.
Allot can happen in 25 years.

[MalibuDon]

I don't believe the Keihin FCR-MX 39 is a CV carb. It's a flat-slide carb, AFAIK.

Everyone can judge for themselves:

http://www.keihin-us...ducts/fcrmx.php


Ultimate off-road performance.

Keihin has developed an FCR carburetor designed for the specialized needs of the off-road racer. The FCR-MX carburetor has many new features to take motocross and ATV performance to the next level.

FCR-MX Features

* Over-flow drain joint and tube.
* Air cut valve (ACV) for de-acceleration enrichment.
* Hot re-start knob.
* Quad vents.
* Throttle cable enclosure.
* High capacity float chamber.
* Float chamber baffle for rough terrain.


Click on the PDF for an exploded view of the carb. If there's a CV setup, I can't find it. However, I am not a Carb Doctor, and I don't play one on TV. I leave stuff like this to the experts!

[sideup]

MD
Thanks for the picture. My information was from a KtM owner, and trusted tech guy.
Sure looks like the throttle valve (19) (slide) is free floating on the rollers. Sure can't see where a throttle cable is connected to it.
Hard to tell from a picture, as you said, we need someone who knows how it works.

[Ridgrunr]

I don't claim to be any sort of fuel engineer, but there is an interesting article in my new Cycle World mag that came in the mail yesterday comparing carbs vrs EFI and the merits of both. Yes EFI does inject instant fuel into the cylinder on demand, but the problem with EFI is that that the fuel that it injects directly into the cylinder is still very much in large droplet form and not the small molecule form needed for really efficient combustion. This is called "cracking".

So what basically happens in EFI systems is that part of the fuel is never burned, because the droplets are so big, then the piston rings pull that raw unburned fuel down into the crank. Over time what this does in a 4 stoke engine is to slowly dilute the oil with gas, reducing it's ability to lubricate, while also significantly raising the volume in the crank/transmission with gas diluted oil. This isn't a good thing.

So maybe EFI is not the cure all in a Trials application unless they can figure out how to put in a catalyst of some sort to better "crack" the fuel mixture which will also add to the complexity and weight of the system.

The new carbs being designed today are becoming more efficient at breaking those fuel droplets down so they can be more easily and efficiently burned. In small displacement low RPM applications like Trials motors, this is a prime directive for them to run decently. Maybe they'll make them easier to start too. The key I guess is in finding and using the right carb for the application. We'll just have to wait and see if anyone figures it out.

Edited by Ridgrunr, 23 January 2007 - 06:52 PM.

[cdn280]

CV carbs have no place on a trials bike. They were developed only to make smooth power and make the emissions people happy. On KTM's, the LC4 is the only engine to have a CV carb in the model range, (i have one) and the rest are pumper FCR's.
The spring that supports the floating throttle slide is controlled by engine vacuum only, so there is a slight lag between opening the throttle and the vacuum and slide to do its thing. (get sucked more open) Its the reason so many lean jetted 4T's with CV carbs bog when whacking the throttle open. Hitting big bumps causes the slide to move unintentionally and also can cause bogging.
Again... not good for jumping/hopping.

[Outlaw Dave]

Hey Cdn 280 - Better let your fellow Ontarian and Trials Expert Jordan Szoke know this as he just did a demo on the Beta 4T at the Vancouver show - He was so impressed he immediately ordered a bike.
Also the Beta factory would likely need to know this, plus Steve Saunders etc - because they are obviously
unaware of this!