carbs - replacing slide with CV

Just a quick query.

If you were to replace slide carbs with CV carbs would you be looking
for similar size or larger?

Any experiences swapping (either direction) between slide & CV?

 

Laverda triple. Maybe a Laverda twin as well.

Never thought of that

Will probably stick with the stock airbox - no point in going to
civilised carbs then sticking pods on.

 

Larger. The problem with slide valve carbs was that they polluted the
atmosphere when the rider opened the throttle suddenly. The engine
would bog as it ingested too much gasoline, and then the rider would
roll off the throttle and the fuel would be kicked out of the exhaust
pipe, unburned.

CV carbs take direct control of the mixture strength out of the
rider's control, so a larger diameter venturi can be used to get good
power at higher engine RPM.

30mm slide carbs were typically replaced with 34~36mm CV carbs.

As I recall, early GT750 Water Buffalos came with 30mm slide type
carbs, and later GT750's had 36~38mm CV carbs, maybe even 40mm carbs.

Each cylinder was 250cc.

If you want to educate yourself on carbs, google for the Sudco
International site and find the Mikuni booklet.

 

That's a good point. While the CV carb may run too lean at larger
throttle openings with individual filters, it will run too rich at
small throttle settings,

But the carbs do have idle mixture screws that can be adjusted after
the EPA anti-tamper plugs are drilled out.

The best website for learning how to tune CV carbs is at www.factorypro.com

Don't bother with Dyno Jet, those guys are in business to make a lot
of money off the ignorance of riders who think they can just slap a
kit into their carburetor and it will magically work better, even if
they don't know how to tune it.

 

Mark's reply makes the most sense. By and large, manufacturers went to
CVs for a variety of reasons. They're more forgiving of brutal
treatment like whacking open the throttle from low revs. They pollute
less. Crucially, they're lighter on the throttle wrist, and if you're
fiddling with a Laverda I'd guess this is a major consideration. Along
with a Guzzi Le Mans Mark Four (with the 40mm pumper Dell 'Orto slide
carbs), the old Laverda triples I rode had the heaviest throttles of
any bike I can recall.

Again, there are good and there are poor CV carbs. Those early Honda
ones with the steel pistons rather than rubber diaphragms I'd put in
the first category: I don't think anyone else really used them, and
even Honda gave up on them after a while.

For a Lav triple, I'd start looking at something around the 32-34mm
size, but that's really just a guess. I'm trying to think of an old
bike that had (roughly) 330cc cylinders and used CV carbs: there
weren't many. Yamaha XS650? Or look for a set from a more modern
1200cc four.

Never done a mod like it myself. I'll be interested to hear how you
get on. Out of interest, why? To make for a lighter throttle?

 

Hmm, the throttle on the SFC isn't particularly heavy although it does
need two handfuls to get it wide open.

One bloke who's already done it used 3 GSXR11 carbs. He did say that if
he was doing it again he'd use Hinkley triple carbs. Mind you, he's just
stuck fuel injection on his RGA so I think that tells you something
about him.

Not so much to lighten the throttle, more for improved fuel economy &
more power - the dellorto pumpers are great at WFO but not so good
otherwise.

A few of the Laverda owners have gone over to 36mm flatslides (either
Mikunis or Keihins) and they've all reported improvements in both power
(not so much at the top but a much better curve) and economy. One Jota
owner gained 4bhp at the top end, more torque & a smoother curve in the
mid range and was then delighted to find his fuel consumption had gone
from an average of 33 to 40mpg.

 

Improved fuel economy. Better power. Getting both sounds unlikely but
it's the results reported by the people I know who've stuck more modern
carbs on their triples - I've seen the dyno charts & I've ridden with
some of them enough to know that gas stops have got further apart since
the carb change.

I suspect the only reason Laverda stuck with Dellorto pumpers as long as
they did was because they couldn't afford anything more modern.

Stock has a lot going for it but it's a bike for riding so modernising
where appropriate is useful - already done the brakes, I've got modern
switches to wire in & CVKs are well cheap on fleabay. A proper ignition
to replace the Bosch horror isn't so cheap :-(

Don't think Laverda had when they were building triples. I'd love to fit
a bigger airbox in but then I'd have to find someplace else to stick the
battery

 

I like to think I've got a fair idea but there's always someone who
knows more than me.

Most of the Laverda owners seem to have gone for 36mm flatslides but
they're a pricey option so I'm contemplating CVs

I got given a bank of GSXR CVKs a year or two back but I suspect they're
a little small at 34mm. Standard carbs were PHF32 but Laverda sold an
upgrade of PHF36s so the bike is designed to run 36mm flatslides. Guess
I'm looking at 38 or 40 mm CVKs.

Time to hit google to see what ran those sizes.

Wonder if the 34mm CVKs would suit a 500 Laverda twin? Back to google
for that as well...