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VM34/VM36 AFTERMARKET CARBURETORS

Post 1. Introduction, Selection, Mounting, Cables

Post 2. Baseline Jetting, Startup Adjustments (Mixture and Sync)

Post 3. Tuning
....General
....Midrange
....Main jet selection

Introduction

Fitting VM-series round slide carburetors is a popular modification. Unfortunately, many inexperienced owners buy the carbs with unrealistic expectations. Mounting VM's is a custom modification, and there is no such thing as a bolt-on pretuned custom carburetor. The most any vendor can do is to supply a close baseline from which the carbs can be tuned to the individual engine and environment. There are no shortcuts.

If you expect, as one vendor's site claims, that new VM's will correct problems you are unable to resolve with your OEM carburetors, you will almost certainly be disappointed, and might be better off having your carbs remanufactured to spec. (check www.motorcyclecarbs.com). If you have experience tuning motorcycle carbs or are patient and enjoy detail work, problem solving, and learning as you go, VM's are a relatively economical option that can deliver enhanced performance and tunability.

Bear in mind that OEM CV carbs compensate to a significant degree for atmospheric changes (elevation, humidity, and ambient temperature) and the VM's do not, and are higher maintenance systems. Also, unless you're moving the battery and fabricating a custom airbox, you'll be committed to using pod filters, fabric or foam, which block up when wet (and no, stock frame covers won't prevent this). Finally, bear this in mind. The number of shops able and willing to work on vintage systems is dwindling rapidly. If you're not confident in your ability to complete the project, case your area for professional help. Bear in mind that while dyno tuning is costly, tuning and diagnostics from the saddle is extremely time consuming and even more costly.

These are the only downsides. Properly tuned, the VM carbs will deliver fine performance and mileage--my current engine, cammed, ported and bored (700cc), delivers 48 mpg in hard running with VM36's--except when the pods get wet!

It isn't my intention to rewrite the Sudco Mikuni Tuning Manual. Anyone who buys these carbs and is unwilling to spend $10 for that book gets no sympathy from me. The OEM carb threads were written for beginners, and included quite a few details that are not in print elsewhere. But for the VM's there's a detailed manual available and much of the material on carburetor anatomy in the OEM section applies, so I will not address float level settings, location of components, etc. in detail. If you buy the carbs, buy the book. If it doesn't make sense to you, study till it does.

Selection

Every now and then I come across a thread (not at 650 Garage, I'm happy to say!) full of vehement argument on VM34 vs. VM36 carbs.

The experts are divided in their views, so we can dismiss the notion that amateurs have the last word to say on the issue in spite of the frequent ranting. In theory, the larger venturi of the VM36 should cost you a bit of low end power in exchange for a little more in the high rpm range; but master tuner Michael Morse reminds us that many successful 750 cc. race engines use the 34's, and claims that any gain in power achieved with the 36's would theoretically occur well past redline and would thus contribute nothing in actual use. At the same time, master tuner Bob Bertaut appears to favor the VM36 even on otherwise stock engines.

I've used both carbs successfully on both stock and performance engines, and in my experience performance differences between them are strictly marginal. Significant differences in performance and fuel consumption will be due more to engine condition, maintenance or tuning issues than to venturi diameter.

Mounting

The VM34 can be mounted either in aftermarket rubber boots from Mikuni or on billet aluminum mounts from Shell Racing, which set the carburetors back a bit for some performance gain in the lower rpm range. The VM36 can be mounted in either OEM boots for the BS34 (yes, you read that right; the OD of the VM36 spigot is only slightly larger than the OD of the BS34, and while it's a tight fit, it works well) or on Shell Racing mounts.

There's a bit more involved than simply bolting up the mounts and popping the carbs on. As a first step, fit the carbs to the mounts on the bench, remove the slides, and check for any up-steps where the spigot meets the mount. If you find a step, smooth it away by working the mount with a dremel or similar tool. Then hang the mounts and check for any upsteps where the mounts meet the ports after you've positioned the mounts for best fit. Work the ports slightly to match.

Cables

The usual setup consists of a primary cable from the twistgrip running to a junction tube in which the primary pulls a plate which joins it to secondary cables which pull the slides. It's also possible to use the two-cable throttle from early XS650's, eliminating the junction tube.

Some builders let the junction tube hang loose under the top tube of the frame. I prefer to secure it to the top of the tube under the tank with crossed pull ties.

Length is always an issue; even from the best and most experienced sources, a pre-made cable can only be an approximation. Bear in mind that the carb-top cable adjustors are used to set lift synch; if they must be turned out to eliminate cable slack, the cable assembly is too long. If the cable is overlength, there are two solutions.

One of our members (I think it was Slide) threaded a clutch cable adjustor into his throttle housing to take up the slack. The other solution is to shorten the cable. Open the junction tube and free the primary cable. Make sure that the primary cable end is brass (cast cable ends are not removable). Wrap several turns of copper wire above the cable end for a heat shield and heat the cable end with a mini torch, pulling gently as you heat it. Cut the cable and sheath to the length required and refit the cable end, fraying the cable slightly behind it; a probe stuck into the end of the cable works well for this. Melt 60/40 solder in a small vessel (your stovetop will work fine for a heat source). Step the heat up slowly till you find the point where the solder flows. Flux the cable and end with Ruby Fluid and dip into the solder until the end is covered. Hold 20 seconds or so, remove, rinse thoroughly with water, clean up with a file, and test.

For those who want to try making their own cables, all the materials (and some really trick tools as well) are available from www.flandersco.com. The site's well worth a visit.

Clearance between secondary cables and petcocks is often a problem. Replacing vacuum petcocks with the more compact manual units from Mike's XS helps, but even so it's usually necessary to lift the back of the tank. 45 degree carb top adjustors help; they're available from 650 Central, 650 Motorcycles.com, motorcyclecarbs.com, and Sudco.

Post 2. Initial Setup.

Baseline Jetting

Carburetors are delivered from Mikuni with generic jetting in place. These are available at a very low price; components required to jet for specific applications must be purchased separately, and account for much of the price differences among vendors. Sporting for Less, for example, claims to sell carbs prejetted for the XS650, and sells them very cheaply; but they send out only a few viably baselined carbs (a handful of happy campers is kept around to cast doubt on those who complain), and many that have received no modifications except changes of main and pilot jets. The generic needle jets and needles in particular work very poorly in the XS650.

The first step in setup is to strip the carbs, wash them with gasoline to remove any debris (metal shavings, etc.) left by the manufacturer, check float level, and inventory jets. Note that the float pin is a snap-in type; follow removal procedure given above for BS34's. The tiny brass air jet in the back of the intake bell should be removed if present; it's not used in our application. The generic slides usually work very well. The following baseline has provided excellent results; but remember, it's a baseline, not a recipe, and your individual build may call for mains outside this range.

Needle jet: 159 series, P5 or P6

Needle: 6F9

Pilot Jet: VM22/210 series, sizes 20-25

Main Jet: 4/042 series, sizes 180-200

Start with 3 pilot sizes: 20, 22.5, and 25; if most of your riding is done at high altitude (over 2500 ft.), reduce the range 1 step to 17.5, 20, and 22.5 and for mains, 170-190. Start out with jets in the middle of the ranges and the clip in the middle groove of the needle. These are baseline guides, not a recipe; your engine and situation may call for jetting outside these ranges.

Startup Adjustments: Pilot Mixture and Synchronizing

Before attempting carb adjustments, be sure that cam chain, valve lash, and ignition adjustments are correct, and that ignition components are in good condition. Prep. work on these systems will eliminate a lot of guesswork later.

Start with the mixture screws turned out 3/4 turn (note that the screws function in reverse from BS-series carbs; turning clockwise reduces air and richens, turning CCW adds air and leans). The large adjustment screws on the carb bodies control idle sync, blocking the slides from dropping to full closure. Bob Bertaut recommends setting idle sync by using the smooth ends of drill bits as gauges, inserting them from the air intake end of the carb to the slide cutaway and increasing or decreasing size to achieve the desired idle speed (go to 650 motorcycles.com, then click on performance parts and then VM36 carbs). Try a 5/32" bit for an initial setting, move the screws equal amounts until idle speed is around 1100, then equalize settings with the drill bit method or attach a manometer.

The slides must also lift off the stops at the same time. To set the lift sync, loosen the locknuts on the carb to adjustors. With air filters removed, open the throttle and feel for the point at which one of the slides clears the venturi, then feel for the slide in the other carb. Set the adjustors so that both clear at the same time. Then fine-adjust by setting a thumb against

one slide and a finger against the other and turning the throttle until the slack is taken up. You should feel slight movement in the slides as the cables tension. Turn the adjustors until both slides respond at exactly the same time. Once the engine is running, I prefer to follow up with a manometer.

Do rough adjustments on the mixture screws as the engine warms--whatever it takes to keep it running. Once a stable idle is achieved, set mixture screws by the dead cylinder method described for OEM carbs. Use the adjustor on the primary cable to control rpm's and conserve synchronization. If mix screws must be backed out 2 turns or more, increase pilot jet size 1 step and reset. If they must be screwed in less than 1/2 turn, decrease 1 size and reset. This is strictly a temporary selection--don't ditch the other pilots!