Specific throttle position issues - help me re-jet please!

[lumberjack]

I bought a 1979 XS650 special from a local fellow, bike has some goodies in it (PMA kit, new clutch etc). He installed the XS performance pod filters, and gutted the factory mufflers (end caps are on there, but I removed them today... seems to have run a tad better)

Anyway, I know I will need to rejet, because his claimed "jetting" seems off. (even though he rebuilt the carbs)

Here's the setup:
Stock bore/engine Xs650
BS38's
XS pods
Factory gutted pipes (may as well say straight pipes, at this point)

I haven't had a chance to rip open the carbs fully, but I know the MAIN's are 140s

My issue:

- It used to idle just fine on choke (it would hold high RPM, then run and idle fine). Suddenly, the choke does nothing really, and the bike will actually run better with the choke off, right off cold (about 30 seconds after start, actually)
- I have moments where it runs better on the choke, but will die if I come to an idle stop. (so acceleration is great on choke, but stop light? dead, turn choke off or risk kicking it in traffic).
- From idle to about 3K rpm, it's a slow steady roll on, but lacks MAJOR kick.. just feels WEAK.
- From 3-5K I get a (often enough) spurts and sneezes, occasional coughs and cut outs.
(you know the bike is trying to pull * wheezes* keeps pulling etc)
- From 5500 to redline, pulls like a beast. which leads me to believe the MAINS are FINE. I think!
- LOTS of POP POP POP backfire on downshift and quite a bit on Decel.
- The left carb is smoking at stop lights, once the bike is running hot (fully warmed up); There is smoke coming out of the filter...
- There is some sort of "leak" on the left carb, that I can't pinpoint. Essentially, if it's running, and I put my bare hand beside the carb, I can feel a slight pulsating leak, as though someone was intermittently blowing on my hand - Not sure what it is, but I can't find it. Carb was rebuilt, maybe something isn't tight. The leak is definitely up TOP (towards the diaphragm cover plate, petcock side of the carb)

What I would also like:

WHAT ARE YOUR SPECS!

Seriously, if anyone is running pod filters and straight pipes, what does your jetting/needle clip position look like?

I feel that the needles are on the wrong clip and the pilot is wrong, for my bike. Mains seem OK, based on high rpm performance.

Any advice?

THANKS!

PS if anyone wants to donate some VM34's to a poor kid, I'll take them haha (jokes)

 

[TwoManyXS1Bs]

Lumberjack, you may want to take a step back and rethink this. When an engine's behaviour changes, it's usually not because the jetting magically became incorrect. Plus, trying to do fine-tune on a machine that may have other issues is like trying to evaluate shock absorbers while a tire is flat.

Your first task should be to completely disassemble and clean the carbs and fuel system, making note of all parts used in the carbs, jet #'s, needle, and such. Inspect the diaphrams.

Once the carbs/fuel system are in top shape, then meaningful tuning can commence...

 

[5twins]

First off, those air filters are crap. They cause all sorts of tuning issues and you'll never get it to run perfectly using them. If you want pods, get the UNI foam ones. They work OK. That being said, the usual set-up for the '78-'79 carb set with mods like yours is 1 to 3 up on the mains, 1 to 2 up on the pilots, and the needles leaned one step. The 140 mains are 2 steps up from the stock 135s and sound about right from your description. I'll bet the last guy didn't change anything else though. Tuning these carbs usually requires different pilots and needle settings, along with the larger mains.

 

[lumberjack]

Lumberjack, you may want to take a step back and rethink this. When an engine's behaviour changes, it's usually not because the jetting magically became incorrect. Plus, trying to do fine-tune on a machine that may have other issues is like trying to evaluate shock absorbers while a tire is flat.

Your first task should be to completely disassemble and clean the carbs and fuel system, making note of all parts used in the carbs, jet #'s, needle, and such. Inspect the diaphrams.

Once the carbs/fuel system are in top shape, then meaningful tuning can commence...

I really think I went abut explaining it wrong, haha. I know that jets won't suddenly be "different", but other factors that were changed may NOT jive well with the jetting in place. I FORGOT TO NOTE: I had an exhaust leak, which required both exhaust donut/ring gaskets (at the head) to be replaced. That definitely changed how the bike runs, due to the exhaust being sealed up properly. My best guess is the last guy thought the leak is normal, and tuned for that.

I guess what I meant was that after the tightening down of manifolds, replacement of gaskets etc, the performance got weirder/worse.

 

[lumberjack]

First off, those air filters are crap. They cause all sorts of tuning issues and you'll never get it to run perfectly using them. If you want pods, get the UNI foam ones. They work OK. That being said, the usual set-up for the '78-'79 carb set with mods like yours is 1 to 3 up on the mains, 1 to 2 up on the pilots, and the needles leaned one step. The 140 mains are 2 steps up from the stock 135s and sound about right from your description. I'll bet the last guy didn't change anything else though. Tuning these carbs usually requires different pilots and needle settings, along with the larger mains.

See, the thing is he gave me a baggy of jets and needles and whatnot. He had 137.5, 140 mains, and 35 pilots in there... but I didn't check which pilots are in the carb (that's a task for this week).

I guess I will open it up, and see what I am dealing with. The mains seem fine, not touching those... but the rest..

One question: Why would I want to LEAN the needles? (as in, move the clip UP I assume, along the shaft, away from the needle tip). When I tuned my V-star 650, I had pods and exhaust, and I tuned the jets for richer settings to make up for the increased air flow, which included shimming the needles to make them RICHER, no?

or am I understanding this wrong... which way am I moving the clip on the needle? down towards the needles tapered point, or up towards the butt end of the shaft?

 

[XSLeo]

Have you read this, www.amckayltd.com/carbguide,pdf It may answer some of your questions.
The jetting overlaps the next up or down. As in bigger mains overlap into the needle and needle jet, this richens the mix, often a bit to rich. So you lean the needle by moving the clip up one groove, this drops the needle and leans the mix.
Leo

 

[5twins]

Yes, as Leo said, the larger mains and pilots bleed over into the upper and lower midrange, making it too rich. Your V-Star is a much newer bike with much leaner carbs from the factory. That's why you had to richen everything.

 

[chizler62]

"Move the clip up...away from the needle tip" will richen.
The clip grooves are numbered. #1 is the top groove and #5 is the bottom nearest the ground while assembled.
#1 leanest
#5 richest

IMO It sounds as if you have a lean misfire and a possible needle/seat and float issue with left carb.
Pipes get yellow or blue?
I'd disassemble carbs. Check and record all settings/jets. Ensure proper float heights(check each side of float on each carb possible twisting) pay close attention to needle valve and seat assembly. Dab common grease on needle tip this will ensure floats don't stick.
clean and blow liquid thru each circuit. Air isn't sufficient is ensure there aren't any restrictions.
sync carbs.
Do an idle drop. Adjusting the fuel screws will indicate if you have the right pilot jet or not. THIS STEP IMO IS THE SINGLE BIGGEST MISTAKE PEOPLE MAKE TRYING TO JET. The pilot jet is functional 100% of the time.

Then on to the needles, keep in mind that when you suddenly close the throttle you cut off more air than fuel giving you those deceleration pops.Typically.

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