Single best jetting advice I've found

[ann0yed]

This is the best PILOT jetting solution I've come across for my '79 Special with:
XS Performance Stainless Steel Headpipes - 1.75" (45mm.) OD
XS Performance Reverse Cone Megaphone Mufflers
XS Performance torque inserts
XS Performance pod filters

The advice:

"These carbs are *really* easy to run rich. I would stick with stock jetting, to be completely honest. *maybe* a step up on the pilot, and *maybe* two on the main, for totally open intake/pipes. But i'd have a hard time justifying that for most people who run open pipes, since they rarely sit at 6k+." -Sundie

I just spend a couple months trying to find the right sized PILOT JET. I read SO MANY jetting posts, many of which suggested I go up, one, two, three sizes and higher. I recall reading somewhere someone saying they had a 45 pilot (and not even for open pipes AND pod filters - just one of the two) and I was like "Dang, how could that guy run a pilot so huge?!?!", well, IT CAN HAPPEN TO YOU TOO! Naturally I kept going up and up and eventually got to a size 45 and that was the breaking point for me ; I had a NIGHTMARE ride to work and back today through a sizeable chunk of Manhattan, AWFUL.

Following the advice I'm passing along here I decided to go back a few steps to my 30 pilot (one step up from stock) and when I did it (without taking the carbs off the bike, which I've never done before and which I feel like a JACKASS about now) and with a turn of the mixture screws IT RUNS GREAT NOW! Its so easy to be lured into "bigger equals better" and it isn't always the case.

Oh and in case anyone is wondering, I'm two sizes up from stock on the MAIN and that seems to work for me. Is it right? I don't know, I might go down a size there too eventually. My main concern was the pilot. DONE.

I hope this helps someone out there! It MUST!

-jonathan

 

[CDNTX650]

both times i have tried to go way bigger here it has sucked too. On my 78 i ran stock jetting with pods and baffled aftermarket pipes. Ran great. Plugs were a touch dark.

My cb500/4 was the same. Mac-4 to 1 with stock airbox and i never had to touch the jetting. Plugs were a touch dark with stock jetting too. Must be our elevation here.

 

[5twins]

This or any bike really does need some jet tweaking for mods but as noted, it is usually only minor. I've modded and re-jetted practically every bike I've owned and it was pretty much the same - minor jetting changes were needed. From all the stuff I was reading about the 650s, I thought maybe they were different. The two biggest info sources (MikesXS & the Minton Mods) called for mains in the mid 140s. I thought that was a rather large increase (4 to 9 sizes depending on model year) and I was right. I can see Mike suggesting these just to cover his butt against customer claims about seized motors. Minton I'm not too sure about. Probably had something to do with his article deadline and not enough time for proper testing.

The carbs on these bikes are much more complicated than one would think for a lowly old design twin. The jetting specs for the 38s changed no fewer than 4 times during the 10 years they were used. The slide design was changed in '78 which required the use of a longer 5 series needle. The pilot jet type changed in '76 which required a newly designed float bowl to use it. Float settings varied slightly over the years as did mix screw styles and recommended settings. One needs to be aware of all these specs for his particular carb set if he's re-jetting for mods and wants to be successful at it.

 

[Travis]

Good stuff.... I've fallen for that trap too.. Went way to big on the mains on a GS500 I was working on and fouled plugs.. Was super rich from 3/4 to full throttle and would bog. Went back to a few sizes over stock and it ran great and the plugs looked good too.

 

[Sundie]

Hey dude, thanks! Always nice to be appreciated.

The main thing i find people having problems with CV carbs is talking about "throttle position", which is misleading. Your main/needle/needle jet have only a nodding aquaintance with throttle position. Until you're over 6k *and* the butterflies are fully open for more than a couple of seconds, the slides are not fully open. Until that point, the engine is not sucking enough air to overcome the spring. And at cruise, they're only up about an eighth of an inch.

I've also found that carbs that required upjetting before being dunked in the ultrasonic cleaner go back to stock after.

Just my 2p.

 

[bloodthirstysystem]

even with stock setups, i have read that going up one size on the pilots will fix the leaness of these bikes.

 

[Sundie]

even with stock setups, i have read that going up one size on the pilots will fix the leaness of these bikes.

That all depends on the carb and conditions. for bs34's, 99% of the time i jet them stock with cdn needles on the middle/cdn needle jets, turn the idle adjust out to 1.5, balance them at idle, and call it a day. And that's for pods and open exhaust.

If you're going up a size in the pilot on BS34's, and you still have the US needle/needle jet, you're fixing the wrong problem, IMHO. And one day when you're stuck on a hot day in stop and go traffic, your bike will start running craptacularly.