VM34's @ 1/8- 1/4, too rich or too lean?

[barncat]

finished experimenting with exhaust cans. settled on 17" x 1.750" ID Thrush perf core glasspacks, which sound good and perform great on custom 2 into 2 1.500" headers. fresh top end rebuild '73 motor with porting.

using VM34's and have a rather vexing hesitation at approx 1/8 to 1/4 throttle under load (not when just blipping throttle). kick starts great, idles great, pulls very hard after 1/4 all the way to WOT. bog is also noticeable in 4th and 5th gear when soft pedaling it at low throttle opening.

25 pilots, air screws out 1 1/4 turns, P-6 NJ, 6F9 needles at 2nd from top, 190 mains, velocity stack screen temporarily over carbs, 2.0 slide cutaway.

thought i'd consult those who've tuned a few of these motors. don't know if that narrow dead spot is too rich or too lean.

my guess is go to 2.5 slides and/or 22.5 pilots with further air screw adjustment...

 

[grizld1]

#2.5 is the factory installed generic slide for the VM34, and it works very well on the XS650; the #2 slides may be too rich. Slides are high dollar items, though, and I've often ended up with 22.5 PJ's. Your air screw setting, if correct, doesn't indicate a need for leaner PJ's, but they might work for you. The one hard and fast rule in carb tuning is this: one change at a time. You might try the 22.5's and see what happens; if they do the job you might save some money, if not, you won't be out much. You might consider mounting the air filters you plan to use before you get any deeper into the tuning process.

 

[racerdave]

Can you try turning on one of the enrichners {choke} when you feel the hesitation?

 

[barncat]

racerdave- with my particular enrichener setup, reaching down to engage while moving is not really an option... plus it immediately runs rough on kickover with "chokes" on and usually starts without. not a bad idea though.

grizld1- i have 22.5 pilots on order as of last night. was not lucky enough to have same lying around. there goes another 5 days... yes, because of your tech info and others i know that 2.5 slides are preferable. i have a surface plate and height gauge and am debating whether to Dremel mod them- but that is of course an irreversible decision. sounds like you share my thinking that this is likely a too rich condition.

yeah, need to get final air filters on. had some cheapo pods on that were not helping. don't really like the look of Uni foam hence the delay in deciding.

i've tuned VM's on a number of CB twins, a SS750F, a couple of TX500's (one of my favorite motors even though they seep oil at the head). i could be wrong, but the XS650 seems more sensitive to subtle tweaking- from intake through exhaust. this is perhaps due to the 360* crank? a brief experiment with straight pipes was a complete waste of time...

funny note: since i didn't research these motors before buying several and first pulled the cylinders off and spun the crank, it was like, WTF? is this thing broken? i'd probably like a rephased motor better, but that's not possible on these early ones of course...

 

[barncat]

also, just to be crystal clear, a 2.5 slide for example, means exactly 2.5mm from the flat base of the slide to the apex of the cutaway? i've not had occasion to modify any before...

i have a another pair on hand here that say 1.5 but actually measure 2.5mm. modded by a PO?

 

[racerdave]

This may help

 

[TwoManyXS1Bs]

also, just to be crystal clear, a 2.5 slide for example, means exactly 2.5mm from the flat base of the slide to the apex of the cutaway? i've not had occasion to modify any before...

i have a another pair on hand here that say 1.5 but actually measure 2.5mm. modded by a PO?

Here ya go:

http://www.xs650.com/forum/showpost.php?p=286391&postcount=41

From this 'fun' thread:

http://www.xs650.com/forum/showthread.php?t=6570&highlight=cutaway

 

[barncat]

glad i asked! victim of common misconception out there, and confusing designation system.

i just pulled and measured the 2.0 slides prior to reading these posts, and they didn't spec in mm either so i figured something was up. also, as a machinist, the "lay" or machining marks of the curved edge of the 1.5 slides appeared to be factory, and not hand modified, so that didn't jive with a 1.5mm cutaway.

thanks to all for assistance thus far. i did just recheck the cam chain which was a tad loose despite all the valve/ignition etc... stuff done with rebuild. will get to the bottom of this. it's very close- always tweaking with first hard road tests.

 

[barncat]

while i'm deep into slide cutaways today... FWIW just discovered that there is no curvature at all- it's an optical illusion. duh. just a flat plane.

 

[TwoManyXS1Bs]

... as a machinist...

Are you considering modifying the cutaways?

 

[barncat]

probably. the angle changes as well along with the cutaway height. the tuning in that very specific range is not responding and 2.5's are the known optimal setup.

sadly i sold my 2 milling machines a few years back when i thought i was moving to CA. this would be a 5 second end mill cut to make but with at least 1/2 hour setup- if you had the right diameter collet.

 

[barncat]

turns out my initial tuning guess was correct. certainly helps that grizld1 and others have provided ballpark VM jetting for these motors- plus this website is probably the best i've ever used for search. valuable relevant info pops up right away. kudos to administrators.

i did hand mod the slides from 2.0 to 2.5's. turned out great. used a height gauge to scribe light .156" lines on them then taped some 120 grit paper to a surface plate and used careful draw strokes to remove the brass. we'll call it Manual Abrasive Machining

went to the 22.5 pilots with screws just 5/8 turns out. may be a tad lean- will see how that setting holds up with more miles. also, for those of us who don't tune motors full time- don't forget to leave a touch of slack in your throttle cables when syncing so the slides will drop as far onto the stops as necessary if you get a runaway idle condition...

made a brief foray into Tuning Hell yesterday when several incorrect assumptions were made re fried reg/rec, capacitor, plugs, wires, etc (it's PMA). turns out the timing was too retarded which as many of you know will immediately cause motor to run like sh*t.

at any rate, these motors are surprisingly smooth when everything is dialed in.

 

[grizld1]

Congrats--cutting slides down for a richer cutaway is pretty common practice and easily done, but cutting the radius for a leaner mix is a whole different ballgame. (Warning, kids--the guy has some serious skills, don't try this stuff at home.)

Sounds like your PJ's are about right; don't be skittish about turning the air screws in as tight as 1/2 turn out if you need to. If you don't hear misfires (exhaust popping) under engine braking, the PJ's are rich enough.