VanceHines jetting

How many sizes up (main/pilot) should I go with a 4-1 V+H system. I have a stock airbox and stock filter. And I plan on keeping the baffle as it is for now. I havent pulled the carbs to see whats there yet, but would going up two main jet sizes be a good start or is more required with a 4-1? And is staying on the lean side a safe bet?

happy spring!

bike: 1982 kz1000m

2 mains for the 4-1, 1 main for the pod filters. for every 3 mains you go up then go up 1 pilot jet

You don't need to rejet for using the V&H 4 into 1 unless you modify the airbox. Give it a try on the stock jetting.

i read this and thought it was a good way to start.
go up 2 mains for pods and 2 for 4-1 pipe, =4 mains,
then -1 main mains. if you go up 3 mains you have to go up one pilot jet. this is where i got the info but the site is down right now. http://motorcycle%20jetting_rules_of_thumb_w43c551.cfm/

Just made a suggestion based on my experience. Do whatever you like... when you read stuff, do you question where they came up with their facts/figures? These rule of thumb jetting things are fairly ridiculous. They rely on a "formula" but don't take into consideration that each carb assembly is jetted differently based on carb internals. For instance, CV34s have two basic jetting strategies... one is rich mid-range with leaner top end and the other is fairly lean mid-range with richer top end. If you use the same formula on both, one of the bikes won't make it past 5K rpm. Also, when you rejet the most common CV34 (lean mid-range and richer top), you go from about #120 mains to #140 mains (secondary main) which is EIGHT sizes larger on the main jet. Never saw a formula that recommended this. Same for BS34... you go from a 122.5 to 127.5 (on most assemblies) to a #140 main. I have never found ANY formula very useful.

On the VM mechanical slide side, the formulas don't work all that well either. If you add pods and a 4 into 1 with a 1976 KZ900 using VM26SC carbs, you can go up ONE SIZE on the main jets and stock pilots will be fine... On the 78 KZ1000 using VM26SS you need to go up two sizes in pilot jet and five sizes or so on the main jet to make the bike run right. How did a formula apply here? It didn't.

i can't get to the link, i think the site is down or gone. they were, if i remember just a basic starting point for mikuni carbs. with my bike it came with 102.5 mains and a 15 pilot. i'm running 117.5 mains and a 20 pilot, so it didn't work for me either, i think it's a place to start though. i ended up going up 6 mains and 2 pilots. bike runs well. no hard feelings. i mean it does make sense that you will normally have to re-jet when upgrading exhaust or pods, but i guess it depends on what carbs you have.

No, most of the time when you change exhaust systems on a vintage bike, the stock jetting is fine. Most aftermarket systems don't flow any better than the stock system did. There may be exceptions but not Mac, V&H or Kerker...

I think that jetting is mostly trial and error (plus an EGA installed on my test bike) for EACH carb assembly and what you learn for a particular carb assembly doesn't have a bunch to do with any other. I have pretty much learned what will work and won't work based on some experience in the area. The most widely distributed formula came from an old XS1100 site and it said go up X jet sizes for a pipe, go up X sizes for pods, etc. The formula worked poorly for the XS1100 but it is tempting to "formula" things as it makes things simple I guess. Anyway, no hard feelings... the matter is worth discussing.

"Most aftermarket systems don't flow any better than the stock system did." thats just what i was thinking.

thanks for the replies and the healthy discussion. I like hearing thedifferent ideas. I'll run it as is for now. But keep your eys out for this post to resurface in about a month if things dont work out. We'll know who was right

Wiredgeorge - you deserve a medal.i dont know how many times your posts have helped me out in a pinch. keep it up man

And Vach - thanks for the jet specs on your ride, they may come in handy

I had a 900 bored to 1015 with K&N pods and a V&H pipe(baffle in place) 117.5 mains and 17.5 pilots that thing would pull teeth from the bottom all the way to redline and then some.