CV Carb Tuning 101- basic misconceptions and mis-information

[crazypj]

I was going to put this in the velocity stack thread but realised I was going to be around the 'half page rule' ;D so decided to start a new thread.

There are major differences in jetting for road and track, all this applies to street motorcycles although it is applicable to track bikes.
I have a LOT of experience re-jetting loads of different bikes over a 40+ yr period,(and still ongoing)
when looking for max power, the jetting can be (and usually is) completely different to jetting for max driveability

For CV carbs

If you like mediocre performance or just don't have a clue what your losing, ignore everything I'm saying and find another thread

On later model CV carbs (after 77~78) the best thing to do is keep stock airbox, unless it's a bike where a lot of information is available (usually GSX-R, Mikuni, flat slides, etc)

70~77 can be made to work and WILL need re-jetting for top end performance plus get rid of mid range richness when airbox is removed.

Mid range WILL be rich with stock jetting and pods

Top end WILL be lean, particularly Kei-Hin 3 jet carbs, (Kei-Hin are more difficult to tune than Mikuni)

Major misconception is where main jet starts working.

It HAS AN EFFECT from almost closed throttle (around 1/8 throttle and up)

If you don't believe me, simply drill main jet out to about 2.5mm (3/32") and see what happens. (pretty much everyone working on this old stuff should have a few spare main-jets they are not going to use again)
It is obviously too rich for anything over half throttle, BUT, it also make lower throttle positions rich as the needle/needle jet doesn't close off fuel flow and will act as a 'surface discharge' carb

Mid range will be affected not just by how air enters carbs removing stock airbox intakes but also by different pressure because of airbox removal (usually around 4,500 ~ 7,500 rpm)
higher pressure (close to atmospheric, no restriction from airbox to cause lower pressure) affects how slide lifts and also how air pressure 'pushes' on fuel in float chamber

All for now, I'm tired ???

 

[djelliott]

Thanks for posting this PJ. I've been thinking about switching back from the VM30's to the stockers. I think for the kind of riding I mostly do that they would be much better. At best I've had the bike running pretty good with the VM 30's but never great. I ride a lot of busy streets with a lot of stop and go traffic and feel the CV carbs will handle that much better. The Mikuni's are usually fine when I'm caning the hell out of the bike but in traffic, not so much. Either way at least I'll have both options. Fortunately I also have a ton of brand new Kei-hin jets in various sizes so it won't really cost me anything to experiment.

 

[smoketriples]

4 strokes.....for 2 strokes, pods are not a good idea. Two strokes regurgitate back fuel during the cycle and the metal backed pods cause real issues.

 

[djelliott]

4 strokes.....for 2 strokes, pods are not a good idea. Two strokes regurgitate back fuel during the cycle and the metal backed pods cause real issues.

True. I work on vintage scooters here and there and sometimes you need to "burp the airbox" because after awhile oil builds up in there.

 

[teazer]

Pods on two strokes work fine if they are jetted right. RD Yamahas are particularly susceptible to the metal end cap syndrome. Fit pods with foam ends and all is fine once they are jetted appropriately. And the trick with RDs is a taller needle jet to change the fuel slope.

To PJ's point about overlap between circuits: That's true of all carbs, but CV carbs don't respond the way that traditional VM carbs do because the slide is only fully lifted at high load/engine speed. On a VM slide carb we talk about fueling at say mid throttle meaning the slide is half way up and the needle and needle jet are the control point. With CV carbs at half throttle the slide could be anywhere from closed to wide open depending on engine speed and load.

Slides rise with the relative pressure difference between the intake and under the slide. Remove the airbox and the ambient air above the slide is not the same as it was with an airbox, so the slide lifts at a different rate. And it's different at different loads, so the carburation is way different.

On a CV carb, basically a main jet has more or less the same relative effect at all throttle openings. Bigger main jets make it richer all the way through the rev range even when that's not what the motor needs. That's one reason why the mid range is now too rich even when the top end is right.

 

[crazypj]

Response to Rocan in 'Velocity stacks tuning'

it will run with stock jetting, sure. But it will run lean. Most people dick around and don't have any idea what there doing, hence why you see so many bikes running like crap even after rejetting.

truth is, the only sure fire way to get a good mixture after making changes to stock is to get it on a dyno with an exhaust gas analyzer. thats not even mentioning that most bikes will have exhaust changes and intake changes before they even run good as a stock bike...

I'm not picking on Rocan as he's probably never had a dyno run done.

Things people should be aware of on a dyno run

A dyno run probably won't 'fix' anything without a second session to verify jetting changes/modifications worked (or got closer to where you want to be)

A dyno will only get you about 95% to where you need to be, when you know how to interpret results/print out from 5 band gas analyser
Dyno doesn't mimic real world loads or airflow, badly set up dyno room will give false readings and usually leads to lean jetting.
Dyno operator also has an influence on results, many don't know how to set carbs (you can make FI adjustments through computer on FI bikes without removing anything other than seat to connect bike to computer)
Friend of mine in Kentucky would usually find 15~20 bhp on HD's after customers bolted on in mis-matched parts (exhaust, carb, ignition, etc from different manufactures who all 'promised' X amount increase)

Quite often though dyno runs are only for 'bragging rights' (my Busa has more HP/torque than your Busa type of thing)
Customers are usually more interested in max HP and not driveability :

It will give you a rel good baseline though and 2~3 dyno runs will save around 40~50 hrs or more of 'road testing' (assuming you can find a road close to home, with slight uphill and relatively high speed limit and low traffic volume ;D )
I was real lucky when I did my 378 motor, could still get dyno runs done in work (not allowed to run any 'outside' bikes on dyno for last 2 yrs )

It's expensive to get dyno runs done in a shop but may be worth it for the time it saves to get bike running real close to optimum instead of 'just' running
You may get lucky and find a well set up dyno with balanced airflow and fans, then you can get to around 98~99% of jetting sorted which is good as it's ever going to get due to 'environmental variations' (temp, humidity, altitude, etc)

 

[dean owens]

On later model CV carbs (after 77~78) the best thing to do is keep stock airbox, unless it's a bike where a lot of information is available (usually GSX-R, Mikuni, flat slides, etc)

would you consider the cx500 has having enough information available? seems there have been a number of people run pods on them with success. i'm working on a cx500 build and would like to go with pods. if not, are there other carbs i can use? how difficult is that?

 

[Rocan]

thanks for picking on my PJ

but truth is your absolutely right. but I'm not completely wrong either. the point I was trying to make was the point you were saying, in that a dyno is just another tool. heck, you yourself in all your wisdom and mastery got your carbs damn near close, but dyno showed you were rich (if i remember correctly). as you said, dyno allows you to look at the results and say to yourself "oh, I need to TRY and richen the mixture between X and Y, and lean it out between Y and Z.

 

[smoketriples]

Pods on two strokes work fine if they are jetted right. RD Yamahas are particularly susceptible to the metal end cap syndrome. Fit pods with foam ends and all is fine once they are jetted appropriately. And the trick with RDs is a taller needle jet to change the fuel slope.

Foam backed is the key, and not cheap like the metal end cap versions. With CVs on a two stroke I'm just as happy with a stock airbox and good oiled foam filter. VMs I'd consider pods.