VM26SS on a KZ650 JETTING

Help!! Kz people, I pulled a set of vm24ss off a 79 kz650 due to some issues. They're jetted for pods and a 4 into 1 at a 117.5 main instead of the factory 90 size. I grabbed a set of vm26ss which have 110 main jets. Here's my question, what's the jetting suggestion for this, will the 26-versus-24mm affect the need for bigger jets as potentially more air coming in? I know the engine is only going to pull in what it pulls in, but am I over thinking? The VM24SS seemed to run fine but they got damaged during o-ring replacement. I was thinking 120's but that seems huge but the 117.5 do as well. I'm just wondering about compensating for the 26mm versus 24mm thanks in advance    

Stock for Vm24 on a 650 in the uk is 15 pilot and 102.5 main. If you were using (good) pods and 4 : 1 exhaust you would leave the same but fit air correctors
Stock on vm26 on a 900 in the uk is 17.5 pilot and 115 main. So a lot bigger than 650/24 combo. 
Vm24 are more than enough for 650s so I would stick with those or you'll be fighting mixture issues and guessing at jet sizes for a while. You'll be creating issues for no reason.
Vm24 on  650.
Vm24 dia = 452mm sq x 4 = 1808mm sq div by 652cc = 2.77mm sq per cc.
Vm26 on a 900. (stock) 
Vm26 dia = 531mm sq x 4 = 2124mm sq div by 902cc = 2.35mm sq per cc.
Vm26 on a 1000(stock)
Vm26 dia = 531mm sq x 4 = 2124mm sq div by 1015 = 2.09mm sq per cc. 

So if 2.09mm sq per cc is good enough on a Z1000 then 2.77mm sq per cc is over carbed on a 650.

The 117.5's in the vm24's were miles to big for pods or anything else and you would have been losing top end power.
Should have been one size up from stock on the main, all other settings stock and fit air correctors. 
As for the VM26's you should start with one size up from stock on the main leaving all other jetting and settings stock as a base line and if using pods fit air correctors and go from there with your plug chops or dyno.
Use only good quality free flowing pods such as K&N,APE, S&B, Ramair or Unifilter and not cheap EMGO, DNA or other Chinese junk or you are wasting your time. 

 

Just trying to think this through from an engineering viewpoint.  The function of carbs is to mix air and fuel at a nominal rate of 15/1 air to fuel, with a richer ratio at idle and a leaner ratio at highway speeds. They do this by drawing fuel from the bowls, which requires a negative pressure difference between to bowls and the mixing chamber, in a way that is similar to sucking soda up a straw from a glass.  The negative throat pressure is created by running the air through the carb throat at high velocity, so that the pressure in the throat is lower than in the bowl.  (Bernoulli principle). This is what draws fuel up the jets into the throat.

Engineers have to design carbs to function across a range of engine rpms, hence the different jets and internals.  Carbs that are too small will limit the engine at high rpm's.  If they are too large they will be impossible to tune at lower rpms.  (Back in those days engineers were also thinking about emission standards, but that's another issue.). Are VM26 carbs too large for an unmodified 650 engine?  I don't have enough experience to say yes or no, but I know that the engineers decided on VM24 for solid reasons.  Safe to say that simply slapping a set of VM26 carbs on a 650 engine will likely result in some unpleasant surprises and a lot of work to get them to function effectively.  I think the main advantage of VM26 carbs on a 650 engine would be at higher rpm's, with little difference at lower rpm's.  At all engine rpm's however the carbs need to be "tuned" or "balanced" to provide the correct amount of fuel and mixing to allow the engine to function optimally.

The key to successfully making the change would rest with finding the right pilot jets, needles and needle jets and finally the right main jets.  You should spend some time looking at the differences between these components on your VM24's and 26's to understand how they might impact engine performance.

To repeat...Carbs function by drawing fuel from the bowls, which requires a negative pressure difference between to bowls and the mixing chamber, in a way that is similar to sucking soda up a straw from a glass.  The negative throat pressure is created by running the air through the carb throat at high velocity, so that the pressure in the throat is lower than in the bowl.  (Bernoulli principle). This is what draws fuel up the jets into the throat.

Carbs with larger throat (VM26) will have lower velocity in the throat, hence less negative pressure with respect to the bowls.  This means less fuel draw up the jets at similar engine rpm's.  It may also result in less mixing once fuel reaches the throat, but the effect of less mixing would probably disappear at higher revs.  You might be able to compensate for the larger throat at idle by lowering the slides a bit when you synch them.  This should increase air velocity through the throat.

Assuming the engine is unmodified you will want more or less the same air and fuel volumes at low rpm's.  At higher rpm's it will be the needles and needle jets that limit fuel delivery.  Up until maximum rpm's it is the differential between the two that limits fuel into the throats.  At maximum rpm it will likely be the needle jet that governs maximum fuel delivery, as a VM26 needle jet will probably have a smaller diameter orifice than a main jet.

The model of the VM26 carbs will have some impact on how you tune them. 

Just some random general thoughts.  Keep us informed on your progress.

wrote:

The key to successfully making the change would rest with finding the right pilot jets, needles and needle jets and finally the right main jets.  You should spend some time looking at the differences between these components on your VM24's and 26's to understand how they might impact engine performance.

To repeat...Carbs function by drawing fuel from the bowls, which requires a negative pressure difference between to bowls and the mixing chamber, in a way that is similar to sucking soda up a straw from a glass.  The negative throat pressure is created by running the air through the carb throat at high velocity, so that the pressure in the throat is lower than in the bowl.  (Bernoulli principle). This is what draws fuel up the jets into the throat.

Carbs with larger throat (VM26) will have lower velocity in the throat, hence less negative pressure with respect to the bowls.  This means less fuel draw up the jets at similar engine rpm's.  It may also result in less mixing once fuel reaches the throat, but the effect of less mixing would probably disappear at higher revs.  You might be able to compensate for the larger throat at idle by lowering the slides a bit when you synch them.  This should increase air velocity through the throat.


 

Some good nuggets of wisdom here.  Thanks for posting...

There is a long thread here currently where a member keeps fouling plugs on his 900.  Stock jetting, mains, are way richer than 1000cc bikes running the same VM26's.  Reason being, less suction with the smaller engine means the mains need to be larger.

There are guys here running VM26's on a 650, but I agree that the smaller carbs will be easier to tune, and there will be fewer compromises.

When talking about damaging the carbs while removing O-rings, what specifically happened?  Sometimes there are solutions.