32mm econojet to 34mm econojet switch - what jets to use?

[cie80]

First time caller so please forgive me if I'm not posting this correctly...  Have a 79 intruder that should have had 32mm econojet carbs stock, am switching to 34mm econojet carbs because they are all I can find (originals were missing when I got the sled).  Should I set the 34s up like factory 32s, leave the stock 34 jets, or something in between?  Does it matter?  Thanks in advance for any help.

[Tory944]

  There isn't much difference in jetting between the 34's and 32's.  My best advice would be to check the jets and get back to us with some numbers...  then we can all help you out as to whether to change them (rich or lean).

[rminier]

X2 what Tory said, not a huge difference. Heck, I remember the hot little liquid 340 that Polaris had in the TXL in the late 70's and then the early 80's TXL Indy came stock with VM-38's. I know that motor what a hot little Revver, but 38's? Wow. The TXL didn't do too badly in cross country racing, for sure.
  If you don't have the stock #s for the stock VM-32 carbs, here they are:
   Pilot      25
   Main     140
   Econo   130 pto, 120 mag
   jet needle 6DH7-3
   Air screw  1.5 turns open

It's interesting to note that the liquid 440 Invader has jetting close to that, even though it uses VM-36 carbs and revs up 800-1000 RPM higher than the Intruder. The only difference being the Invader uses 150 mains, 30 pilots and a 6DH4-3 needle. The power jets are 120's on both carbs for the Invader. A fan cooled motor typically sees the PTO cylinder run a little hotter, since it's further from the fan.
  Any motor, at a given RPM and load, will draw in X amount of air. The VELOCITY of the air thru a larger carb will be slightly less than a smaller one. The increased velocity thru a smaller carb actually creates a better vacuum, signal, draw,...whatever you want to call it, and pull the fuel thru the jets better than the slower air flow thru a larger carb. For this reason, I'm bettin' you will need to go with slightly larger jets in the VM-34's. Probably about 10% as a guess. So, maybe 155 mains, 140-130 econo jets. You can probably try 25 pilots, it ought to idle well with air screws out from 1 to 2 turns open. You can always bump the pilots up to 30's if you find it idles best with air screws less than 1 turn open.
This is all assuming all is well with the motor....compression, good crank seals, and rubber carb boots, etc.
This is all a real shot in the dark on my part...always go rich with the jetting. I don't care if it coughs and hacks and burbles because it's too rich. You can always move back down with the jetting. Running lean really sucks when the pistons start melting

[rminier]

Whoops, I just thought about the stock air box. Did your Intruder still have that? Power jet carbs, Kawi calls them "econo" jet, run best with the stock air box in place. Unfortunately, there were two different sized adapter rings that fit in the air box to match up to the VM 32's used in the 340 Invader and the Intruder, and a larger ring to fit the VM 36's on the 440 Invader. I don't know if the smaller rings for VM 32's could be trimmed a bit to fit 34's. Without the stock air box, we will have to think WAY richer that I first thought.

[cie80]

Yes, I do have the original airbox so assuming I can get them to fit, should be ok there.  The 34s shipped today so I'll let you know what I find inside when I get them.  Thank you guys for your thoughts!

[cie80]

Ok, I got the carbs and the jets are pretty much what was recommended.
Pilot - 30
Main - 150
Needle - 6DH7-3

The power jets are both 120s so am looking for a 130 and a 140 if anyone knows of a source for them.

Also, one of the plastic nipples for a vent line is broken off and I don't believe anyone carries those as a replacement part.

[rminier]

Good start! Send me a PM on here with your address and I will stick the lil' plastic vent connector in an envelope and mail it to you tomorrow. I'm getting a lil' chicken and thinking I might go with a 145 power jet on the pto side and a 135 on mag side. I sure don't want to err on the lean side. As I said, you can always bump jetting down. No need to be less than conservative at first. Dennis Kirk.com is a huge supplier of jets, etc..many others, but they have LOTS of good stuff.