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Post by marshall on Jun 18, 2006 23:18:01 GMT -5
Forgive me if this is totally off the wall... but I just don't know. Is it possible to fit smaller carbs on a bike? As far as I know, the 1978 Special came with 38mm CV carbs. Would it be possible to put these 36mm carbs on and make it work? ebay link -> tinyurl.com/jtapeAnyone know anybody who's used those? I see they are non-CV type. I also see they are $300+  I'm not sure if I really want to buy them yet, but I would like to know if it's even technically an option. Thanks for any info. |
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Post by tomterrific on Jun 19, 2006 7:18:34 GMT -5
You would be better off with the 34s most people use. Even 34mm is huge for a 325cc cylinder. The stock 38mm CV carbs are the equivalent of approximately 30mm slide carbs.
Others may have a different opinion.
Tom Graham
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Post by grizld1 on Jun 19, 2006 8:00:32 GMT -5
Marshall, I suggest that you take the time to read the many posts on alternative carbs on this board then ask what remains unanswered.
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Post by marshall on Jun 19, 2006 9:20:25 GMT -5
aye,
will do.
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Post by Six-Five-O on Jun 19, 2006 15:51:21 GMT -5
The 34mm Mikuni carbs the the Ebay seller "Sporting For Less" are NOT set up for our XS-650s..The SFL "expert" claims they are, but unless he's has changed the jetting and needles he has beeing installing in the 34s he's selling, then both the jetting and needles are still way off..
Check in with "Burge" and "5twins" as both these guys have tons of real world experience with these carbs and the XS-650 in general..Both can likely get you real close and save you a few bucks and frustration...LOL...
This is not meant as a slam or anything, but there have been several people who have bought off the SPL guy on Ebay and they needed some help getting the so called "ready to roll" 34s straight..
Regards
John
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Post by marshall on Jun 19, 2006 15:58:39 GMT -5
^ thanks. the price really is a killer, I will probably stick it out with my stock carbs and make them work...
...but I think my question has been answered here = )
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Post by blainethemono on Jul 24, 2006 20:44:11 GMT -5
Just FYI, Marshall. Ive got VM36mm Mikunis on my STOCK 650, with STOCK pipes, STOCK ignition, K&N RU-0800 (http://www.knfilters.com/search/product.aspx?Prod=RU-0800) air cleaners and they work just fine...... With MINIMUM jetting required... And just for kicks, the stock main jet for the 1979 38mm CVs is 135 for roughly 324cc each, so YOU do the math on just where to start with carbs 2mm SMALLER that flow gas and air a whole lot better........The K&Ns really help deal with the extra gas too. ROFLMDGAO!!!!!  |
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Post by grizld1 on Jul 25, 2006 9:21:37 GMT -5
The BS38's a decent carb, Marshall. In one form or another it was fitted up to 1980. IMHO the BS34 tin-tops fitted from '80 through '83 were the only real problem units. Now this IS intended as a slam, and a hard one too--SFL's brass is all over the place. He's sent out some VM carbs with pretty close jetting, and some, supposedly configured for unmodified engines, with needle jets too fat to run well in a built-out drag-tuned engine and float valves to match. The boots he supplies with his carbs are junk--blaine (I don't know who you're laughin' it off at), your pic shows a solid pair of Mikuni boots--stock-type for '80-'83 BS34, which fit the VM36 nicely and let you sync the slides with a manometer off the air barbs, which works quicker than pulling the frame covers and filters and checking the lift with a finger. By the way, main jet size is not an isolated indicator of how well an engine breathes--needle jet selection has a big influence on what mains are going to work, and there's a lot of viable combinations. Take a look at the NJ size that goes with the 135 main and refer to the Mikuni tuning manual to find out what that means. Raw unmetered main jet size is not the bottom line on how much fuel's being moved.
I've also set up the VM36 on a stock engine, primarily to baseline before before some engine mods (big pistons, Shell cam, ported head). They worked well. I've also set up stock engines with the VM34. They worked a little better. I've also used the BS34 with a ported head and stock pistons and cam. It was PDQ.
Far as I'm concerned, if you're running the stock airbox, the BS38 is a fine carb. It can be run with pod filters, but slide tracking gets a little touchy, though you probably won't notice that much unless you're really picky. If you're eliminating the airbox, the VM's are a good choice. The BS38 is a good option for late machines set up with BS34's.
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Post by blainethemono on Jul 25, 2006 14:41:17 GMT -5
Griz.... Im also running an 82 Honda 900F with VM33 Smoothbores, Kerker header and pods. They run a 125 main, so I used that as a reference point when picking a baseline for my 650. She idles great, and once warmed up, can take a quick on throttle from 2 grand, w/o stumbling. Right now Im running a 130 main with the std setup Sudco needles and air jets. Ill do a broader plug test this weekend, but for the moment I feel Im pretty much locked and loaded. |
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Post by grizld1 on Jul 25, 2006 23:07:53 GMT -5
That's an interesting setup. A lot of NJ/MJ combinations work; your Sudco standard Q5 NJ is allowing you to use an unusually small MJ, and the AJ, which most of us don't use, is also richening your midrange. I'd like to hear the results of your throttle chops.
My most recent build, still a little too fresh for very hard running, uses 77.5 mm Venolia pistons, Shell #1 cam, ported head, Mac 2-2 with restrictors, VM36 carbs. The NJ's a P6, MJ 190, needle 6DP1 at middle notch, pilot 25, no AJ--pretty typical setup for the VM36 on an XS650. Fuel consumption on the last few runs at highway speeds of 60-75 mph under break-in drill (constant changes of speed and load) has been around 52 mpg. Solid idle, good transitions and response, very ridable and tractable. Should be able to start chops later this week.
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Post by blainethemono on Jul 28, 2006 15:54:29 GMT -5
Man, you have it pegged griz..... A lil carbon around the ring of the plug. Electrode dark, but no black smoke from the exhaust. Im intrigued by the larger mj, with a P6 needle. Maybe a smaller aj would be in order for my set-up, to lean out that mid-range a bit. I do have a 17.5 I can try. I might have to experiment with that. SUDCO says you can vacuum gauge the idle and air screw settings, but mine are all over the place @ idle. Any thoughts on this? Im going to try a plug run Saturday. Ill fill you in later....
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Post by grizld1 on Jul 28, 2006 16:08:42 GMT -5
What kind of gauge are you using, Blaine? If it's a dial-type, you may have to damp it down some, till the needles are just pulsing. If you're getting a lot of jumping around from a mercury, ATF, or other fluid-tube manometer, you may need to put restrictors in the line, just before the columns, to calm things down--again, you want just a little pulsing. Set idle synch with the throttle stops and lift synch with the carb-top adjustors. A smaller AJ will richen your mix--it works by restricting air admitted to the pilot circuit. Your Sudco standard AJ--that's the way-small jet in the back of the intake bell--is a 2.0, which is as lean as they come. Try removing it--that's the only step leaner you have. You mention a 17.5 pilot--if your air screws give you max. idle speed at 2.5 turns or less, you don't need to reduce pilot size. If you have to turn 'em out more than that, a step down is in order. The VM's are very touchy about pilot selection--often a one-step change in needle position forces a compensating pilot jet change.
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Post by blainethemono on Jul 29, 2006 0:00:46 GMT -5
Excellent advise Griz.....Thanx a lot..... My gauge obviously will not work on these carbs. Its a V-8 based unit. I have carb stix, so Ill use those.... Ok, I have some more work to do. Its not quite "MINIMAL jetting required". Dropping the needle was my next step. Logical and fairly straight forward. Me bad, I interchanged the terms on aj & pilot....
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Post by grizld1 on Jul 29, 2006 9:15:20 GMT -5
I'm interested to see if you can clean up the midrange with those big NJ's. Gave my unit its first brief full-throttle run yesterday--clean to 7K (high as I wanted to go), with a little breakup rolling off to 7/8. Smoke ring a tad thick--a little rich. Gonna leave it alone for the mo', since it's way close and I'm installing straight-through glass baffles this weekend anyway; I'm thinking MJ will wind up at 200 or 210.
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Post by blainethemono on Jul 30, 2006 21:20:22 GMT -5
Well Griz, lowering the needle just KILLED my off idle response & power thru the upper ranges, with either a 130 or 135 main. She runs great with a 130 main and the 6FJ6 needle at 3rd notch position, but after awhile the plugs start to carbon up. She pulls right off of idle with no stumbling and pulls hard to redline. Im thinking of trying a leaner needle jet, going from a Q-5 to a Q-4 or 3... The taper of the needle seems to be perfect, its just needs a lil leaning out thru the entire range... Since she hasnt fouled a plug in 40 miles of riding, a .010mm change might be more in order then a .050mm change. Of course, trying a 6DH4 Jet needle might be easier...
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I'm not sure if I really want to buy them yet, but I would like to know if it's even technically an option.
