Do pods and shorties hurt vacuum pressure petcock?

I am wondering if some of my low rpm problems can come from low vacuum pressure in the carbs not pulling hard enough to get enough gas thru the vacuum petcock. Has anyone changed out that Rube Goldberg for a good old on/off/res petcock? If so, which one?

When I've run pods and open exhaust, never had an issue with the vac petcock. But then, on my 550's, the throat of the carbs is relatively small compared to some of the CV carbs. This would probably mean a higher vac at WOT for my manual-slide carbs.

I replaced my original vacuum petcock with a Suzuki repop from z1....had to "re-clock" the two halfs to make output hose line up on my kz, but works well. I have heard good things (aside from high price) about the "Pingle" brand of manual petcocks...(there is also an adaptor required to make it fit our tanks)

No, the vacume to operate the petcock does not change with pods and shorties. Your engine problem is with your shorty exhausts.

baldy110 wrote: No, the vacume to operate the petcock does not change with pods and shorties. Your engine problem is with your shorty exhausts.

That's pretty vague. I believe what he's trying to say is that your shorty exhaust may not be allowing sufficient backpressure which could be causing your low rpm problems.

Any suggestions on replacing the shorties with something that will give some backpressure, aside from buying a rusted out original exhaust off ebay? How does backpressure affect low rpm gas/air mix?

I guess the bike has pod filters, but just in case: do you have a complete stock airbox?
The idling problems could be the pods. Sometimes the rubber of the filters blocks the small carburetor openings. Something you can try is to add extenders between the carburetors & filters.

Mufflers. Search JCWhitney or JPCycles for Dunstall mufflers. Emgo & Helix racing are a couple of brands.

Fuel valve. If the bolt hole spacing is 34 mm, here's a topic: KZRider --- Manual fuel valve option
I prefer the vacuum petcock, so I replace the diaphragm when needed & expect to do so every couple of years or so.

Thanks guys...yes it has pods and I do still have the complete air box. The pods on the carbs don't seem to be blocking any small carb openings. Do you mean that the pods internal rubber is blocking the mouth opening of the carb, making it a smaller opening? Or are you referring to the small needle sized air holes on the inside of the carb mouth, in about 1 inch or so? For extenders , do you mean take the rubbers off the stock air box, attach those to the carb, then attach the pods to the rubbers? Not sure what you mean by extenders and the reference to the old air box. I'll get a rebuild kit for the petcock, I like it also but I don't want it to become my main Main jet and limit fuel flow before it even gets to the carb. It seems that if enough vacuum is not generated, then fuel flow would be inhibited...

BTW, I just drilled out my primary main, the one next to the secondary main (the one under the needle). The primary main has that little hole that feeds fuel to the pilot jet under the plastic cap. I bought a jet kit and the final step is to drill that main out if I still have trouble at low rpms. The kit had a 0.6mm drill to start with, but the jet was already that size, the .6mm pushed right thru it so I drilled it out with the 0.8mm bit. I will be testing this jet mod on Saturday and will update if it got rid of the flubbering when starting off.

There could be small orifices at the intake side that can get blocked by the pod filters.
This may not be a problem w your carbs.

The extenders would be about the same size as the ducts but more substantial to support the filters. Adding them was someone else's suggestion, but I don't know of anyone who has done it. The idea is that doing so will help to stabilize the air flow into the carburetors.

The vacuum petcock allows fuel flow to the carbs by gravity --- the same as a manual fuel valve.
A vac petcock is automatic --- when set to ON or RES, fuel flows only when the engine is running.
To check it: prime the carbs by setting it to PRI for a minute or so, then set it to ON. Disconnect the fuel line & Install a long section of clear tubing onto the petcock & direct the open end into a fuel container. Start the engine --- fuel should flow into the container. Stop the engine --- fuel flow should stop. Do the same test w RES. Next, set it to PRI: fuel should flow --- engine stopped or engine running (note: you may have to re-prime the carbs once or twice).

Thanks for the tip on testing the petcock. I will do that test and also order a rebuild kit just for peace of mind. Once I get the top of the needles (or the bottom of the screw holding it in) shaved to loosen them up in the slide (after I added 2 washers to raise it up) I will be testing the bike with the bigger primary jet, should get to that this weekend. Have a good one!

I assume your talking about a 440 here, in my experience with the small twins, the stock air box is critical for low RPM and idle. the large CV carbs are kinda finicky on the small engine. I had great results with a cheap MAC dual exhaust and the stock air box. the stock pilots were ok but the secondary mains were a bit lean. Drilled them out untill I got good WOT results and then bought the correct size. drilling jets can produce funny results, they are an orifice not a simple profile inside.

bike ran great that way, did 90 on the highway with is pretty Damn good for a lil 440 with my fast ass on it.

Yes its a 440. I got it all sorted out this weekend and it starts and runs like a bike should. Full choke to start, then back it off to 1/2 almost immediately, then take off and ride. In about 1-2 minutes, choke off completely and it runs fantastic. No low rpm hesitation, great idle, smooth as silk right up to top rpm (for me that is about 7K). How high can I safely take the rpm's?

I did three things this weekend; 1) I drilled out the primary main to 0.8 mm as directed with the jet kit. 2) I then put the original air box inlets on the carbs (to make sure the air inlet ports were not at all blocked as mentioned by Martin). I put a pvc 90 degree elbow into that and then put the pods on the other end of the elbow. 3) Filed down the top of the jet needle enough to loosen it in the slide with 2 washers underneath it. (I had previously put in the 102 secondary main jet). With these alterations it was like a miracle change in the way the bike ran. I'm not sure which one had the most effect but I'm not touching anything! I would think that the primary jet size drillout fixed the low rpm starvation that was happening, the extenders made sure the air/fuel mix was as it should be and filing the top of the needle made sure the slides would drop down as quickly as can be (returning to idle rpms was delayed because the needle was too tight in the slide.)

I would say for anyone having low rpm problems who has pods and shorties on, do those alterations, one at a time if you prefer and your problem will go away.