Mixture adjustment

[Jurrien]

Hi,

I've got a stock Skorpion with the airfilterbox completely removed and K&N filters directly attached to the original carbs. The sparkplug is a bit white so it needs more fuel. The jet in the right carb (the CV carb) is a 165 so I think there is no dynojet kit installed. If I hold the carbs upsidedown, the CV-throttle valve slides to open so I think the spring is removed?

My question is: what should I adjust to create a ritcher mixture?

[DAVID THOMPSON]

i have a traveller and i replaced the oem air box back to original on the bike
and spent an other year fixing
all the other mods that made mine almost un ridable

dave

[bikeseamus]

First of all, replace the spring in the carb. No spring allows the slide to open too easily, leaning the mixture. Raising the float level will richen the mixture at all RPMs. Increasing the diameter in the mains or primary jets will richen the mixture as well. Start by replacing the secondary slide spring. It may be all that is necessary. Go from there.

[Bill Jurgenson]

While I agree that the spring needs to be in place, it is new to me that removing it leans the mixture.
As in any slider carb, the slider pulls the metering needle up out of the needle jet, allowing more fuel to be "sucked" out of the needle jet (i. e. richer) and not less. So removing the spring lets the vacuum actuated slide open very soon. The purpose of a CV (constant velocity) carb is to keep gasflow over needle jet as constant as possible by opening the slide only as high as the intake vacuum which lifts the slide can lift it regardless of how much the twistgrip actuated butterfly valve is opened. The needle is tapered to change the cross section of the needle jet as the needle rises. e. g. a Dynojet kit needle has a shorter taper and longer straight part at the end so that the needle jet is completely opened sooner. Another modification is to very slightly enlargen the venturi hole in the bottom of the slider so that it rises sooner. The Dynojet kit includes a twist drill to do this. The Tunebike kit has venturi tube to be installed into that slider hole so that the slider can react more quickly at the same vacuum pressure as without it. Having installed and used both kits, I cannot tell (seat of pants) that either really improve anything. The spacer ring of the Tunebike kit does improve the carb.
Anyhow, needle up = more gas = richer, needle down = less gas = leaner.
OTOH slider down = higher gas flow = "vacuum" = more gas above the needle jet which, however is metered down by the needle. That is the purpose of the different dynojet needle. Removing the spring drops the gasflow rate at the needle jet for the given intake vacuum.
The main jet has no metering function until the needle jet is completely open. Setting the float level higher has virtually no effect as long as the main jet is always submerged. Too high and the carb floods, too low and the level drops below the main at full gas or perhaps when the bike leans tho normally at speed the centrifugal force prevents this.
So increasing the main has no effect except at full throttle AND maximum intake vacuum.
Increasing the left main has more effect in general and increasing the idle jet at low rpm.
More important is tonset the lag of the secondary carb. Open the twistgrip and place a 6mm (1/4") under the left slide and let the grip loose. Now adjust the linkage to just begin to open the right side.
The OEM carb is very rich to start with. A larger secondary main, e. g. 170 or even 175 only makes sense for all out racing where the stock carb is mandatory.

[bikeseamus]

Bill
As a rule, needle up equals more fuel molecules is mostly right. The exception being when the intake velocity is low enough not to fully utilize the venturi. I am sure you know this, as you are a knowledgable fellow of some experience.

If his secondary slide is opening too early at low RPMs then his intake velocity is too low to properly utilize the carb design.

No question, a flow bench and an EGA would answer this question beyond a doubt. What I was suggesting is that his fuel metering has gone haywire as a result of the loss of the secondary fuel modulation. His secondary slide is very likely not only opening too quickly at low RPMs, but it is likely fluttering as well.... not a good idea.

Increasing the jet diameter will richen the mixture, but raising the float level will most certainly also richen the mixture at all RPMs. Not having the stock airbox in place is another joker he has in his deck. You have more experience in running MZs without airboxes, but I am guessing it will lean the mixture a bit, all other things being equal.

As you also know, how he rides the bike is another large factor. Back when I was a service manager, I would have to honestly ask people.. "How do you ride this bike? It can be tuned any number of ways, but you need to be honest with me when you tell me what you are looking for here and when you shift and your riding habits make a big difference"

I was surprised to find out how many people told me " I never let the tachometer go above 5000 RPMs because I don't want to hurt the engine"
Wellnow.... hmmmmmmm. It was a real learning experience.

In any case, I want to thank you for your Ignitech advice and recommendations. They are good people. The TDM CDI box was put on my MZ by the previous owner, who also hogged out the jets with a drill. As you said, I went back to stock jetting and it is still not running lean. I upgraded to an iridium spark plug and the mixture is just fine at all RPMs.

I have kept the stock muffler because it makes for quiet touring, but the Ignitech CDI box should improve low RPM tractability and just about finish off the performance modifications. I will say here that the brembo rear master cylinder from a GSXR 750 works better than the stocker did. Thanks again for your tech advice. Jim