23 replies to this topic

[cdn280]

Outlaw Dave, on Jan 29 2007, 08:00 PM, said:

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!

Thats a first ! Now why would it work great on a trials bike, yet not a Honda or KTM ??

[Steve Fracy]

Cuz it's a Beta mate!

[the observer]

cdn280, on Jan 28 2007, 01:40 PM, said:

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.

It may not have a slide. It may have a butterfly.

[paul_thistle]

A CV carb has both a butterfly and a slide. The throttle being connected to the butterfly. The negative pressure in the venturi is vented to the upper side of the slide, whilst atmospheric pressure is vented underneath the slides diaphram to push up as the pressure drops in the venturi. Thereby opening it under heavy load.

The FCR carb is used on most Yamaha YZF's (or All!) and many other brands. I believe it was originally developed for road racers. It has an adjustable accellerator pump that can greatly improve instant throttle response. You can adjust the timing, duration and quantity of fuel sprayed with the accellerator pump. And it has a roller slide that helps keep engine vacuum from "Locking" the slide open.

[CVGMmartin]

I have put a Keihin CV carb with an accelerator pump on my 500 triumph trials bike (I ride in Canadian vintage trials) and it works fine, it idles very well with very clean pick up, probably due to the accelerator pump more than the CV design. It will easily give me instant power to get over large logs (eg 3ft plus). Stiffening up the diaphragm spring very slightly improved it a little and the plug colour is good throughout the range.

[HAM2]

I've often wondered what happens on those 4st (CV) Betas when they drop-off large obstacles or thump into a step...can anybody comment?

[NZRalphy]

Hey the Beta 4T bikes - both the Rev and the Evo - use a CV carb. They run brilliantly. No hiccups or coughs. I haven't had any issues with dirt in the slide/boot. The bike runs great.

You have to take the top off to get to the needle and it is a little challenging to get the top to go onto the carb without causing the boot to be pinched. But it isn't super difficult and the previous suggestions are all good.

[B40RT]

You shiuld try this mod along side your CV carb.

[Stork955]

Gday all, the FCR on the previous page is a standard carb, not C.V. (The difference is when the slide is connected to the throttle itself, which the FCR has, CV's use manifold vacuum as described earlier). EFI is better than a carb as mixture control is more precise. Atomisation of the fuel mixture is still a problem as raw fuel does not burn until it is vaporised and as mentioned, EFI still sprays fuel droplets. This is also true of carbs and in fact carbs don't atomise as well as EFI injectors. Basically manufacturers try to get the drops as small as possible and this is easier with EFI. Fuel wash and oil dilution is the final outcome of all of this but again this has been an issue since day 1 of the internal combustion engine. Its much better these days as a result of using EFI. The other advantage of EFI is the fuel can be delivered to the precise location where it will do the most good -usually onto the head of the inlet valve where it cools the valve, helps atomisation and promotes combustion swirl. The downside of using a mechanical carb on bikes is that inevitably the needle and needle jet wear over time, causing a rich fuel mixture, as they physically touch each other each time there is an intake pulse. Given correct maintenance and no tinkering EFI will never do this.

Cheers,

Stork