Post by spindlehead on May 4, 2008 13:08:23 GMT -5
Hi SR fans-
I thought I'd share my progress with dialing in a aftermarket VM34-168 carburetor on my '78. I chose the 34mm bore because I'm told that smaller bore carbs deliver better low-end performance. I mostly use this bike around town or on tight-n-twisty canyon roads so it seemed to make sense (I have an XV920 for freeway speeds). She's doing pretty good now so I thought I'd share my journey with current and future tuners of this carb.
My bike is mostly stock except for some Ace bars, the carb and the intake. The stock carb and airbox were removed by some previous owner and replaced with an unknown size Mikuni flatslider and big K&N filter. Now I have the VM34-168 and the big K&N.
For reference:
Stock VM34SS:
#300 main, #25 pilot, P8 needle jet (bleed type), 6FL25 needle, 3.5mm cutaway slide (throttle valve)
VM34-168 out-of-the-box:
#260 main, #35 pilot, Q2 needle jet (159 series primary choke), 6DH4 needle, 2.5-mm cutaway slide (throttle valve)
The out-of-the-box jetting was way too rich on the idle and the needle. With the air screw cranked way out and on the top needle clip, I could function up to about 1/4 throttle but would stall every time I decelerated to a stop. It was bog city at anything above 1/4 twist.
I thought the next logical step would be to match the stock jetting as closely as I can, since the bore size was the same as the stock carb. So I bought a P8 needle jet (159 series), a 6FL25 needle, and a variety of main and pilot sizes. The new needle jet/needle combination was a disaster. It was so rich on the leanest needle clip that the bike could barely start. And that's the only place it would start. This is where I learned that the idle and needle cicuits are not independent of each other as the many manuals and tuning guides teach us. It all works together.
So, took out the skinny 'stock' needle and replaced it with the one that came with the carb... Vroooom!, instant improvement across the board. Interestingly, changing the needle and doing nothing to the pilot jet and air screw changed me from an unstartable condition to a ridable one. After much experimentation, rides, roll-ons, roll-offs, and throttle chops, this is where I am.
#250 main, #27.5 pilot, air screw 2.5 turns out, P8 needle jet, 6DH4 needle on the leanest clip position
It's is still on the rich side on all circuits. Medium brown spark plug from the idle circuit runs and espresso roast on the needle and main. She's smooth and responsive at all throttle positions if I really wind her up. If I ride less aggressively there are boggy stumbling points around 1/2 throttle and again at 3/4, especially going down hill. Dropping down one more pilot jet to a #25 should take care of the idle circuit, I'll need to order P7 (next lower size) needle jet or get a fatter needle to improve on the needle stage. I guess I should try a #240 main also.
I was fairly suprised at the direction the jetting ended up. I thought that I would have to go to richer jetting than stock because of the big free-flowing K&N intake. Going to smaller and smaller main jet was odd feeling. Either the slide cutaway plays a very big role, or the bleed-type needle jet on the stock carb must have very different internal geometry than the primary type on this carb and affects the main jet sizing as well.
Anyway, that's the story. Hope it helps someone who is trying to dial in one of these aftermarket carbs.
carb carby
I thought I'd share my progress with dialing in a aftermarket VM34-168 carburetor on my '78. I chose the 34mm bore because I'm told that smaller bore carbs deliver better low-end performance. I mostly use this bike around town or on tight-n-twisty canyon roads so it seemed to make sense (I have an XV920 for freeway speeds). She's doing pretty good now so I thought I'd share my journey with current and future tuners of this carb.
My bike is mostly stock except for some Ace bars, the carb and the intake. The stock carb and airbox were removed by some previous owner and replaced with an unknown size Mikuni flatslider and big K&N filter. Now I have the VM34-168 and the big K&N.
For reference:
Stock VM34SS:
#300 main, #25 pilot, P8 needle jet (bleed type), 6FL25 needle, 3.5mm cutaway slide (throttle valve)
VM34-168 out-of-the-box:
#260 main, #35 pilot, Q2 needle jet (159 series primary choke), 6DH4 needle, 2.5-mm cutaway slide (throttle valve)
The out-of-the-box jetting was way too rich on the idle and the needle. With the air screw cranked way out and on the top needle clip, I could function up to about 1/4 throttle but would stall every time I decelerated to a stop. It was bog city at anything above 1/4 twist.
I thought the next logical step would be to match the stock jetting as closely as I can, since the bore size was the same as the stock carb. So I bought a P8 needle jet (159 series), a 6FL25 needle, and a variety of main and pilot sizes. The new needle jet/needle combination was a disaster. It was so rich on the leanest needle clip that the bike could barely start. And that's the only place it would start. This is where I learned that the idle and needle cicuits are not independent of each other as the many manuals and tuning guides teach us. It all works together.
So, took out the skinny 'stock' needle and replaced it with the one that came with the carb... Vroooom!, instant improvement across the board. Interestingly, changing the needle and doing nothing to the pilot jet and air screw changed me from an unstartable condition to a ridable one. After much experimentation, rides, roll-ons, roll-offs, and throttle chops, this is where I am.
#250 main, #27.5 pilot, air screw 2.5 turns out, P8 needle jet, 6DH4 needle on the leanest clip position
It's is still on the rich side on all circuits. Medium brown spark plug from the idle circuit runs and espresso roast on the needle and main. She's smooth and responsive at all throttle positions if I really wind her up. If I ride less aggressively there are boggy stumbling points around 1/2 throttle and again at 3/4, especially going down hill. Dropping down one more pilot jet to a #25 should take care of the idle circuit, I'll need to order P7 (next lower size) needle jet or get a fatter needle to improve on the needle stage. I guess I should try a #240 main also.
I was fairly suprised at the direction the jetting ended up. I thought that I would have to go to richer jetting than stock because of the big free-flowing K&N intake. Going to smaller and smaller main jet was odd feeling. Either the slide cutaway plays a very big role, or the bleed-type needle jet on the stock carb must have very different internal geometry than the primary type on this carb and affects the main jet sizing as well.
Anyway, that's the story. Hope it helps someone who is trying to dial in one of these aftermarket carbs.
carb carby
