Topic: Depression carburators for SOHC

[livefast_dieold]

Dear all,

here in Italy in the local sohc racer community, there is a bit of an ongoing discussion on why on earth the Honda RCB1000 was equipped with depression carburetors. Also, it's our understanding that it's a popular replacement in France (Guy Bertin uses them).

Here a couple of pictures:





I have always been told that dep.carb = slow throttle response and lower performance, but now I'm quite curious if anyone here has different experiences.
I guess that the Honda Endurance team in the 70s didn't use them save a buck...

[MRieck]

 They are a very good carb in wet conditions

[Stev-o]

Is that your race bike?  If yes, can you post a few more pics??


I miei genitori erano italiani e io ho visitato Venezia diversi anni fa, dobbiamo essere passati vicino a casa tua.
Oh come voglio tornare, mi manca davvero il vino, il cibo e le donne !!

[simon#42]

they were fitted to the road bikes they were trying to sell

[livefast_dieold]

Is that your race bike?  If yes, can you post a few more pics??


I miei genitori erano italiani e io ho visitato Venezia diversi anni fa, dobbiamo essere passati vicino a casa tua.
Oh come voglio tornare, mi manca davvero il vino, il cibo e le donne !!

I was born and raised in Vicenza, an hour drive from Venice! :-)
Currently living in Milan, 15 min drive from Yossef

Here some pics of my bike:

[livefast_dieold]

They are a very good carb in wet conditions

which for an endurance racebike could make sense, but then again Guy Bertin used them and a few years ago was kicking asses in France's vintage race scene (well, he is also a former GP rider...)

[MRieck]

They are a very good carb in wet conditions

which for an endurance racebike could make sense, but then again Guy Bertin used them and a few years ago was kicking asses in France's vintage race scene (well, he is also a former GP rider...)

They can be horrible to tune for the street (metal slides) with the all the jets involved BUT the carbs pictured in the first black and white pic are different from the carbs in the color pic. Metal vacuum slides vs rubber diaphragms. Those metal slide carbs sucked when trying to get performance from them. You can get decent performance from diaphragm carbs with easy changes.

[turboguzzi]

ciao ric, outside of italy these are called CV (constant vaccum) carbs, not depression
they all have a butterfly in the middle of the throat so to get the same flow as a smoothbore CR they need to be bigger than slide carbs, 20% bigger according to john robinson's four stroke book.
If i recall right, they were used in racing more because of TT-F racing regulations (production based engines) rather than being better performing, but i might be wrong.
In my GPZ550 i managed to implant CV32s instead fo the original CV27s and make them work quite well, but i heard too that they can be harder to tune compared to slide carbs.
the main nice thing about them is that effort at the throttle grip is super easy compared to CR's as you are just twisting butterfly on shaft and not raising 4 slides!
 

[livefast_dieold]

ciao ric, outside of italy these are called CV (constant vaccum) carbs, not depression
they all have a butterfly in the middle of the throat so to get the same flow as a smoothbore CR they need to be bigger than slide carbs, 20% bigger according to john robinson's four stroke book.
If i recall right, they were used in racing more because of TT-F racing regulations (production based engines) rather than being better performing, but i might be wrong.
In my GPZ550 i managed to implant CV32s instead fo the original CV27s and make them work quite well, but i heard too that they can be harder to tune compared to slide carbs.
the main nice thing about them is that effort at the throttle grip is super easy compared to CR's as you are just twisting butterfly on shaft and not raising 4 slides!
 

whoops :-)
Well they look a bit depressed thought

Mhhh that might be the reason, to avoid fatigue on the rider side.
As for regulations goes, the RCB1000 was far from stock, they even developed a 4 valve head for the sohc engine

[sohc boy]

have used these CV carbs for drag racing off a Honda 900F for quite few yr's with good results ,836cc ,fitted some on street bike as well , good throttle response , fit fiddly jetting em, jet kit fitted ,never had an issue with metal slides , 1100F are larger bore , have some larger one's from a CX 500 on a 1123cc mota 

[slikwilli420]

Pretty rad bike! I see and RC big block behind that distributor as well.

[kmb69]

They can be horrible to tune for the street (metal slides) with the all the jets involved BUT the carbs pictured in the first black and white pic are different from the carbs in the color pic. Metal vacuum slides vs rubber diaphragms. Those metal slide carbs sucked when trying to get performance from them. You can get decent performance from diaphragm carbs with easy changes.

You must have been having a bad hair day!     

have used these CV carbs for drag racing off a Honda 900F for quite few yr's with good results ,836cc ,fitted some on street bike as well , good throttle response , fit fiddly jetting em, jet kit fitted ,never had an issue with metal slides , 1100F are larger bore , have some larger one's from a CX 500 on a 1123cc mota 

Ditto! I have never had a problem with them since 1965 (CB450 to begin with). 

That is a nice looking drag bike. Need to see you at some of the ManCup events!

[sohc boy]

They are a very good carb in wet conditions

which for an endurance racebike could make sense, but then again Guy Bertin used them and a few years ago was kicking asses in France's vintage race scene (well, he is also a former GP rider...)

They can be horrible to tune for the street (metal slides) with the all the jets involved BUT the carbs pictured in the first black and white pic are different from the carbs in the color pic. Metal vacuum. slides vs rubber diaphragms. Those metal slide carbs sucked when trying to get performance from them. You can get decent performance from diaphragm carbs with easy changes.

....What sort problems did you have with metal slides MReick . slide flutter?

[MRieck]

They are a very good carb in wet conditions

which for an endurance racebike could make sense, but then again Guy Bertin used them and a few years ago was kicking asses in France's vintage race scene (well, he is also a former GP rider...)

They can be horrible to tune for the street (metal slides) with the all the jets involved BUT the carbs pictured in the first black and white pic are different from the carbs in the color pic. Metal vacuum. slides vs rubber diaphragms. Those metal slide carbs sucked when trying to get performance from them. You can get decent performance from diaphragm carbs with easy changes.

....What sort problems did you have with metal slides MReick . slide flutter?

Yes. Wear and vacuum leakage resulting in needle/needle jet wear. I did get some to kick ass BUT I felt part of the formula was running lash at .006 and .008 on the DOHC bikes. It boosted compression and they really revved nice. That design also incorporates multiple jets. Mikuni always did OK with 2. Also....metal slides are really heavy compared to rubber diaphragms especially when combined with plastic slides.

[scottly]

Actually, CV is short for Constant Velocity.
My experience is with the FT 500 "ditch pump" with metal slides. Simply replacing the restrictive stock air-box with a free-flowing filter produced a bad flat spot around 3-4K RPM, and the dyno sniffer read so lean I thought it wasn't working. Increasing the stock 140(?) main jet to about 160 cured the lean bog, but it was still very lean, and was still on the lean side with a 180+ main; at this size, it appeared the needle jet was restricting fuel flow. It seemed the slide was opening too quickly, and at idle the slide would bounce up and down. One thing to note is you can't rely on how far the throttle is twisted to know where the slide is, like a conventional slide carburetor; the position of the slide on a CV carb is regulated by the weight of the piston, the strength of the spring, and the pressure differential across the carb. I gave up and used a VM Mikuni in the end.  

[crazypj]

The only real reason to use CV carbs on a race bike is you can have something that would be 'un-rideable' by 95% of semi skilled riders work very well with only 75% skilled. Until your well up the rev range it doesn't matter where you turn the throttle, the motor takes what IT wants and not what you think it should have. If you read up on 1950's single cylinder 350/500cc racers you'll probably come across something called 'megaphonitis' where rpm has dropped below 'power band' then too much throttle is applied. same principle, you can have whats almost a drag race motor with very narrow power band that becomes ridable
Forget the hype, why do you really think modern bikes use 'fly by wire'? Hmm, maybe something to do with computer control?

[turboguzzi]

Actually, CV is short for Constant Velocity.

indeed, what was i thinking about?

[crazypj]

....................or Constant Vacuum. 
Slides lift depending on vacuum applied to maintain it at constant rate (doesn't actually happen but is often close)
Pleast tell more about how you managed to get CB900/1100 carbs working without stock air-boxes? Leon Moss completely re-built carb bodies (converting them to 2 jet) when Honda UK and American Honda needed them for racing

[Sam Green Racing]

Ric and Yos, I would have thought in long distance endurance racing, the advantage of using CV carbs would be for their better fuel consumption or fuel management capabilities.
 

[crazypj]

Ric and Yos, I would have thought in long distance endurance racing, the advantage of using CV carbs would be for their better fuel consumption or fuel management capabilities.
 
[/quote

Ric and Yos, I would have thought in long distance endurance racing, the advantage of using CV carbs would be for their better fuel consumption or fuel management capabilities.
 

After modifying CV carbs for years they only offer better fuel management in multi jet form (3 jet for Kei-Hin) Pilot system fed from primary main jet means total low rpm fuel flow can be worked out and 'corrected' secondary main jet for high rpm can be fixed but the MAJOR problem is the transition between primary and secondary circuits as too much fuel is delivered at transition point.
It took me a lot of time and effort to figure out what was actually happening as there is no research available and the 'normal assumption' is a lean mixture not over rich. It's possible to get lean plug reading when doing 'plug chops' if they are done in 'normal' way and even a 'fixed throttle' position won't work with CV carbs as cruising airflow is way different to 'acceleration' airflow (the 'mark twist-grip is a total waste of time as you never have any idea where slide is (even at max throttle, max rpm) The mods I've been doing to' 3 jet' CV cabs can also have been a major help on 'conventional' 2 jet Mikuni carbs, increasing mid range HP and 'low rpm' driveability. I've tried the same mods on Mikuni VM carbs but results have been variable

[01Thomas]

I was in Milan last week! Not sure when I'll get there again as I normally go to Turin.

I was born and raised in Vicenza, an hour drive from Venice! :-)
Currently living in Milan, 15 min drive from Yossef

[sohc boy]

....................or Constant Vacuum. 
Slides lift depending on vacuum applied to maintain it at constant rate (doesn't actually happen but is often close)
Pleast tell more about how you managed to get CB900/1100 carbs working without stock air-boxes? Leon Moss completely re-built carb bodies (converting them to 2 jet) when Honda UK and American Honda needed them for racing

...... mine were modified in 90's with a U.K. jet kit (dont recall brand ) by some mate's had the local bike shop that road raced 900 1100F+R's in 80's,i drag raced few 900 1100F's with CV's ,90's  i'd have to pull em apart to refresh my memory, air jet in the mouth of carb , passage blocked off , an spacer under slide, will pull apart +take pic's if want   

[Sam Green Racing]

crazypj, this was what I meant by fuel management.
your quote (Slides lift depending on vacuum applied to maintain it at constant rate (doesn't actually happen but is often close).
I was referring fuel management with regards to the rider.
There are a lot of corners to come out of where the twistgrip has to be re-opened and with normal carbs, open it to soon and to much and you are on your arse at the side of the track.
With the CV carbs, as long as you don't open it to soon, it don't matter if you open it to far as the carb will only give the motor what the motor wants and that makes for better consistency (lap times) and better fuel consumption over a lot of laps as in an endurance race.
 

[POPS 911]

That black and white picture showed carbs with DIAL-A-JETS on top of the 4 carbs = a small flat blade screwdriver can tune those carbs rich or lean. Fast by GAST flat slides can come with built in power jets and your race pack read out will tell you up or down. Don Plesser could sell you 4 jets in a pack in many sizes for your carbs and if you brought your OWN dyno to the track tuning was dead on the money [ MSP]

POPS BK911
PS. DIXIE INTERNATIONAL in Columbus had many sets for sale and I bought a few. Went to the guys house [OSU Engineer ] who invented these sets and he sold me a dual set for a 920 XVR Yamaha that came very lean from Japan for EPA regulations. Never had a lean problem after that fix.

[crazypj]

....................or Constant Vacuum. 
Slides lift depending on vacuum applied to maintain it at constant rate (doesn't actually happen but is often close)
Pleast tell more about how you managed to get CB900/1100 carbs working without stock air-boxes? Leon Moss completely re-built carb bodies (converting them to 2 jet) when Honda UK and American Honda needed them for racing

...... mine were modified in 90's with a U.K. jet kit (dont recall brand ) by some mate's had the local bike shop that road raced 900 1100F+R's in 80's,i drag raced few 900 1100F's with CV's ,90's  i'd have to pull em apart to refresh my memory, air jet in the mouth of carb , passage blocked off , an spacer under slide, will pull apart +take pic's if want   

It may have been a Dynojet kit but in UK more likely a LEDAR set up as Leon Moss did the carbs for Honda UK and also for the 'Superbike' series (I'm from Britain and ran dealership workshops there for several; years)