I don't understand how velocity stacks or Pods actually work?

[Guest]

Granted, I'm new to the motorcycle world, but when i look at the stock air box as well as the type of carburetors on my 1982 GS750E (vacuum actuated - not mechanically actuated), I don't understand how people are getting pods or stacks to work. How are they creating enough vacuum without idling the engine ridiculously high?? Someone please help me understand. Thanks!!

[Gorminrider]

a3', there are idle passages that BYPASS the main jets+vacuum needle...your bike runs on these at idle and they continue to contribute gas as the revs go up, but since they supply a fixed amount that is not exactly adjustable while riding (idle mix screws), their proportionate contribution is much less versus the main jets.

The so-called choke on cv carbs is another bypass, but it is metered by the rider. So-called, because our cv carbs don't need a butterfly valve to "choke" the air off and thereby force the engine to draw from special fixed jets. Ours meter the gas instead.


actually come to think of it, don't all carbs , motorcycles or not ? have various ways to keep the motor happening independent of the main jet. The circumstance of running slowly seems to require it.

[a3tripod]

So, if i understand you correctly, the idle passages are sized such that the engine idle will be stable at low RPMs, regardless of the intake changes (e.g. pods or stacks). As RPM increases and the threshold of the idle passage is exceeded/not useful, there should theoretically be enough vacuum at that point? Am i way off?

[hannibal]

Vacuum is not created by the air intake, it's created by the down stroke of the engine's pistons. So a change from airbox to pods just allows the motor to breathe easier.

The pilot circuit of the carb supplies fuel at idle and low throttle. If you modify the intake and exhaust to the extent the motor draws a lot more air, you will need to increase the size of the pilot jet to supply more fuel to match. The pilot system is designed to allow some flexibility, but it can't properly handle large changes without alter its jet size.

[a3tripod]

Thanks Hannibal. We intend to change the exhaust and intake which would obviously be less restrictive on both ends - as such were going to also make use of a Dynojet S3 kit to compensate. I know I'm a total noob here, which is the whole reason why I'm diving right in (to learn)! Thanks to all who have been supportive thus far.

[hannibal]

You typically increase main jets and possibly needle jets and/or needles when adding pods and exhaust. I've seen a guide that says when you add a, b, and c, you increase the main jet by x number of sizes. If you increase the mains by a certain number of sizes, you'll also need to increase the pilot jet by a size to allow enough fuel at idle and low throttle.

If you're willing to learn, you're in the right place. I've learned tons of info over the last 6 months. This is one of the best auto or bike forums I've seen!

[don]

I have 3 GS's. All of them have a DJ kit in them. My 1100 has stock airbox and filter with open exhaust and uses the stage-1 kit, my 550 has open intake with stock exhaust and uses the stage-3 kit, my 450 has open intake and open exhaust and uses the stage-3 kit. In all cases all I did was read and follow Dyna-Jet's directions then do some minimal fine tuning. All of these bikes have the stock pilot jets and all idle/start just like they are supposed to.

Hope it was helpful,
Don