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Post by grizld1 on Jul 30, 2006 23:05:33 GMT -5
Blaine, those NJ's are expensive brass. I'd really advise you to review some of the VM36 threads (there's plenty there) and have a look at what's been working well. I finished dial-in on my build yesterday--MJ was a tad fat, but operation cleaned up nicely (good, thin smoke ring, clean roll-offs from full to 7/8 and full to 3/4) by leaning things out a touch with a baffle change. You'll get better results with fatter mains and much smaller NJ's. I once got things to run close to decent with a Q-0 just fooling around, but the midrange wouldn't quite clean up without starving at full throttle, and fuel consumption was ridiculous. The P-6 NJ is what most of us are using, though I once set up VM34's on a slightly modified mill with P-4's with decent results but a bit of a low-throttle flat spot. P-6 NJ's and a needle change likely would have cured it, but a Ford van terminated the process (and damn near terminated me).
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Post by blainethemono on Aug 1, 2006 11:56:05 GMT -5
Well Griz, thats what started my WHOLE discussion. I started a thread where I specifically asked about jetting for 36mm. And nobody had anything. They all said 34mm, even though I know of folks running 36mm very well. Im letting you know Im running RICH, and you are telling me to go RICHER still??!!! Everything I know about carb tuning says Im should go LEANER. Maybe its lost incommunication, But "FATTER" to me means larger mj opening , and I dont need more rich.... Oh well, Im going back to what I know. Thanx very much for your input. Im so close to having her dialed in that a smaller needle jet seems in order, and its cheap @ twice the price. Ill check out the 36mm threads.  |
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Post by grizld1 on Aug 1, 2006 13:03:42 GMT -5
No, Blaine, I'm not recommending a richer overall mix. There's feedback between the two components; the same MJ will "act" much fatter with a bigger NJ, and all things being equal a reduction in NJ size will call for larger mains. See how things work with P-6 NJ's and 180 mains if you're around 400 feet above sea level--a little less if high, maybe 180 or 190 on the ocean.
I'm sorry I missed your thread; a "sticky" on this might be in order.
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Post by blainethemono on Aug 1, 2006 23:13:29 GMT -5
Ill give it a shot Griz. I ordered a couple of thicker needles from Sudco....Not to expensive, and an easy try..... Since I didnt buy the carbs from Sudco, they wouldnt give jetting suggestions, but from what I told them, they said it was the next logical choice to try. Im in Seattle, so we might be close to that elevation. Again, thanx for the input... I agree, a "sticky" would be in order. Something to dispell the myth, once and for all, that 34mm carbs are the only way to go with the XS650 engine
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Post by grizld1 on Aug 2, 2006 8:31:20 GMT -5
There's no myth to be dispelled, mah man. 650motorcycles.com sells VM36 kits baselined for the stocker and recommends them highly, and it's hard to find a more credible authority than that. Nobody who knows these bikes is going to express disrespect for Mr. Bertaut's view on the subject! Is the VM34 preferred for the stock engine? That's the general consensus, and a view held by many who are equally respected and deserving of it. In general, the VM36 tends to be the carb of choice where the engine's been built out some or some building's planned. None of this is carved in stone, preferences vary even among expert tuners, and I don't know that anyone's said otherwise. The VM34 has seen successful competition use.
Maybe a sub-board on alternative carbs could make things more accessible. There's a good post, some months old, giving baseline jetting for the VM36, and quite a few on preferred (and dispreferred) vendors. I've done several posts cautioning guys on how expensive brass gets when you buy carbs cheap from a bad source.
A little math here. Each increment of 1 number step on Mikuni NJ's is .025 mm (needle jets are produced only in increments of 2 number steps, or .050 mm., except for 4, 5, and 6.) Each letter series starts with 0 and ends with 8, going to 0 on the next letter. The Q-5's are 9 steps, or .225 mm., bigger than the P-6, which is usually workable--in inches, that's almost 9 'thou, and That Is A Lot!
Here's a baseline for you. It's not a recipe, it's not a prescription, it's what's worked for one guy on one stock motor at one time in one place, and your particular setup will take some tweaking, so have alternate pilots and mains on hand. Needle: 6DP1-3. NJ:P-6. PJ:22.5. AJ: NONE. TV: 2.5. MJ:180.
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Post by grizld1 on Aug 2, 2006 9:44:10 GMT -5
Blaine, I just reviewed your original post and found that I did not miss it. I referred you to a thread with sound information and to two fine sources of baselined carbs, and I don't know how else I could have helped you. I certainly didn't tell you that the VM36 wasn't a workable carb. Hang with it--those carbs will make you real happy when they're dialed in.
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Post by blainethemono on Aug 6, 2006 23:23:55 GMT -5
Thanx for the support Griz. I must be loosing it over the years... I thought I had the math right, but a simple equation shows you are on the money. A 180 is where I should be tuning @. My 900 has a 125 mj for 225c each. The 650 needs to supply 320+cc each. Boy, was I a bit off... :0 She is a bit fat w/130 and the stock needles, but I understand the benefits of the P6 scenario. Sometimes I need a 2 by 4 to the head to get the picture. Ill get the parts underway & keep ya posted.. |
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Post by grizld1 on Aug 7, 2006 9:07:02 GMT -5
Have a look at the different combinations of NJ's and MJ's factory-installed in the BS38's over the years--you'll notice the setups with fatter MJ specify leaner NJ, and conversely (different carb design; Z-series NJ's would make a serious mess in a VM carbie on a 650 twin!) Within reason, there's a range of combinations that'll work.
There are quite a few members with much more technical background and tuning experience than Yrs. Truly, and I hope they'll jump in here; but my seat-of-the-pants procedure for the NJ pretty simple. If needle position/selection and jet changes (MJ and PJ) won't clean up a fat midrange without starving in the first 1/8 and last 1/8 throttle ranges, reduce NJ size and come back up on pilots and mains. If a lean condition in the last 1/8 seems really stubborn, you're screwing in huge MJ's and not seeing much change as you go up, needle changes aren't helping, and you get low-throttle flat spots that just won't tune out, step up the NJ and reduce MJ size.
But all that having been said, the 159 P-6 NJ will give you fine results in a stock-to-mildly-built XS650 engine when everything else is tweaked in. Enjoy!
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Post by blainethemono on Aug 12, 2006 21:49:06 GMT -5
Well Griz, so far Im pretty impressed with the setup...Ill give it some more riding time Sunday, but so far, she looks good to go..
Thanx again....
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Post by blainethemono on Aug 14, 2006 22:23:03 GMT -5
Hay Griz....prolly the 1st time in her life, she hit 90mph today!!
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Post by grizld1 on Aug 15, 2006 23:55:00 GMT -5
There's more to be had where that came from--keep tuning and have fun!
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Post by blainethemono on Aug 21, 2006 22:22:40 GMT -5
Thanx.....My mpg is hovering around 36-38 right now. Pretty dismal from the stock 50-55. Ive ordered a leaner pilot jet. #22.5 & larger main set, #190. If I understood your past post, you guys have removed the air jet, ( aj = none), @ my present config, my bike hates it. Sputters, hacks and backfires. Cant even roll the throttle off of idle w/o her dying, & thats with a completely hot engine... Oh well, thats what carb tuning is all about... |
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jack
Junior Member
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Post by jack on Aug 22, 2006 17:38:46 GMT -5
Sounds like ya still running lean. The Vms and TMs have enough air speed off idle or the initial take off, to develop enough vacuum to begin fuel through the needle jet.Try dropping the c-clip a notch at a time to richen up the mixture.
When tuning my TM flat slides I went as low as 15 on the pilot with a 1000cc motor,motor ran perfect on take off,etc,except the motor sorta developed a cough and would die,went to 17.7 a no more coughing but it did stumble and spit when cold but once she warmed up,the motor pulled like a freight train,move the c clip to richen the mixture,solved the problem.............Jack
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Post by grizld1 on Aug 23, 2006 9:30:11 GMT -5
If removing the AJ's has your pilot circuit too lean, try lifting the needles a notch, as Jack suggests. Then reset your air screws and let me know where they are when you set 'em for highest idle.
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Post by blainethemono on Aug 28, 2006 10:50:36 GMT -5
Well, I removed the AJs, and installed the 22.5 pilots. The bottom end is much more responsive now. Still takes forever for her to warm up though. She will hack and cough for several miles before she will take an off idle throttle w/o complaint. Midrange and top end seem good. Idle seems ok, but she will drop down to just over a grand @ stop lights, and sometimes die. Air screws are out about 3/4s of a turn. If I adjust my idle screws to max idle speed she wont come down smoothly after a throttle blip. Im close though, real close. Needles are @ center clip. I think you guys are right, though. The final piece will be richening up the needle. One of the real enjoyable things about all this, is the learning experience, and satisfaction of blazing a small trail for others, who need answers to certain questions about carb tuning. You guys have been great... Stop on over to my digs sometime, and say Hello..... www.cb1100f.net |
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