The Virago Single Carb Chronicles

                             Single Carb Chronicles

     Over the last several years I’ve seen the KJS single carb manifold (check it out on the web) on several bikes, and got to ride one, also. A Virago Single Carb. I was impressed with the starting, the idle, and the snap when the throttle was cracked open part way.  I thought the performance up in the range was average on the bike I rode, but the owner liked it which counts for a lot. I assumed that a jetting change might produce a little more power in the upper rev range.  Anyway, I wanted to learn more about how these manifolds performed, so I decided to make one myself.  I wanted to know how evenly the cylinders pulled (since with one carb we’ve lost the ability to tune the carbs individually).  I wanted to know whether one cylinder ran richer than the other. And I wanted to know what jetting (and other) changes might be required to make one Hitachi Virago carb run the bike.

I was not really looking for a power increase here.  I think two well-tuned carbs will theoretically beat a single carb in respect to high-end horsepower, since each carb can be more precisely tuned to the cylinder it services, and the motor is clearly getting more air through two carbs than through one.  But the single carb has a nice simplicity to it, is much easier to remove and replace for jetting changes, and hopefully would retain the strong, peppy, usable power of the ’85 1000 Virago that I was working with, and which was probably putting out horsepower and torque in the 50-60 hp/fp range.  It was certainly running well, pulling strong and easily hitting red line.
I tried to find books and tuners who had addressed this kind of project–ones who could share some experience–but couldn’t come up with any.  Understandably KJS won’t go into too much detail on the R&D program, and that’s fair, since they are in business and their research is really what they are selling. I met these guys in Ottawa in ’99.  I know that they modify their carbs extensively, have been doing their “R&D” for several years, and have sold over 500 of their kits.  So they no doubt have a lot of knowledge about how things work.


So where to start?  After getting a couple of astronomical quotes from fabricators to build a manifold from scratch I decided to do part of it myself.  I had the advantage of havng an spare 1000 motor sitting on my engine stand, so using that, I built a model out of ABS sheet and 1 1/2 inch ID PVC plastic pipe– which is right at 40mm, the size of the carb and the size of the intake ports in the heads.  By cutting the PVC pipes into pie-shaped wedges, I could go around corners.         The KJS  caste aluminum manifold has been through three iterations that I know of.  Their first offering stuck straight out like a Harley.  Their second revised model canted back (the route I followed).  For a time they shipped it in two parts, joined by rubber sleeve (similar to radiator hose).  This approach had the advantage of allowing accomodations for any slight differences in the dimensions of Virago engines due to manufacturing/assembly variances (all Virago engines are in theory  exactly the same in these dimensions) and also would allow some “give” in the manifold if  dimensions changed slightly during the heating and cooling cycle.  Their   Plastic


third offering did away with the rubber boot and went back to a single cast aluminum manifold which bolts up to both cylinders and uses silicon for a sealant.

My manifold was to be made of steel tubing, however, which has different expansion qualities than aluminum, so I opted to keep some “give” in the system.  I did this by retaining the front carb holder as part of the front intake path.  This also had the advantage of providing  a vacuum port   which I would need to measure the balance between the cylinders, and service the so-called “pressure sensor” found on all 1000 and 1100 Viragos.  You will note that I placed another vacuum port near the rear cylinder flange for measuring purposes.  My manifold also has a groove in the rear flange to accomodate an O-ring (the same as found in the stock carbholders) so we have a O-ring seal at both cylinder joints.  So—my manifold bolts up solid to the rear cylinder (with an O-ring seal), and plugs into the stock carbholder in the front, as shown–the carbholder bolting up in the 


usual way.  I started out trying to consider optimum mixture flow, equal-as-possible lengths to the two paths, etc., but finally wound up dealing with what I considered to be two overiding considerations; first, mounting the carb in a position and angle where it would clear the tank and engine parts (oil lines, etc.) and second, mounting it so that it would not hit my leg when I was on the bike. Working with alterations to the plastic model, I got an acceptable angle/position on about the third try. It meant mounting the carb farther in than I had originally placed it, but I could now sit on the bike with the carb just scooting past my calf to the rear.  Having abandoned mixture flow considerations, I had no idea how this configuration would effect balance between the cylinders.  I should also note, that gas tanks vary on Viragos, and I can’t say for sure that my carb position would work without interference on all Viragos ever built.

Once I had the design down to my satisfaction I went to my welder and grabbed a piece of the 1 1/2 steel tubing we were going to use.  I then picked up some steel plate to make the mounting flanges.  Then it was on to my machinist who fashioned the flanges–two flat ones for the  rear cylinder and carb mount, and a ring flange to simulate the lip of the carb, to pop into the front carb holder.  The ID of all these pieces was machined so that they fit over the steel tubing with a snug slip fit.

With these parts in hand, I mounted the rear flange and the carbholder onto my spare engine.  I then made a fixture (which I bolted to the engine) to hold the carb mounting flange in the right position.  I then loaded the engine on its stand into into my pickup and drove to my welder.  The engine stand has wheels, so I just rolled the whole thing into his shop and said, “O.K. Dennis, there are the three ends in the just the right positions.  So work your magic and connect them up with some steel tubing–and here is a plastic model to guide you.”  And that is how the manifold was built.  Dennis has a selection of radius curves in this tubing, and was able to get away with just two pieces!


     In choosing a carb, I decided to stick with one of the Hitachis that was on the bike.  Both were  in good shape, and the price was right since I already had it.  I considered using a Mikuni TM40 pumper (accelerator pump) model, but that’s a $300 pop.  Whatever carb you choose, remember that you’ll need to design at least one flange on the manifold to fit the carbholder for that carb.  Note also that if you chose to split a pair of stock Hitachis there maybe no turning back, since you’ll be tempted to destroy the linkage between them in the process.  Also look forward to some fiddling with the choke to get a workable way of actuating it.  In my case this turned out to be a little lever on the carb itself which I just depress and hold a while for cold starts.

My Hitachis already had Dynojet kits in them.  I was running the needles at the recommended setting (3 notches from the top).  Dynojet jets are designated as “DJ” for these applications. I was running 118’s, two steps leaner than the 122’s provided in the kit for a stock exhaust, and four steps leaner than the 126’s provided for aftermarket exhaust.  My friend and noted tuner, Magik Woo, found that the engine needed 118’s.

I decided to stick with the Supertrapp two-into-one slip-on pipe I was using.  This adds another variable since the Supertrapp is adjustible for back pressure.  More disks and you get more noise and less back pressure.  Fewer disks and its somwhat quieter with more back pressure.  Directions say that 6-8 disks approximate stock back pressure.  Turned out I was running 15 for reasons lost in the mists of time.  More on this when we get into tuning.

For an air cleaner I just wanted to keep it simple and get something on there.  I cut down one of the stock air boots, made a metal insert out of a piece of drain pipe, and mounted a UNI universal foam filter.  It slants back out of the way so my right leg can still hug the tank. With no air box this filter flows a lot freer than stock. Going to a single carb adds plenty of restriction–one 40mm hole to feed two cylinders, as opposed to two 40mm holes–so the more air the merrier I was told.

Finally there were a couple of little details.  I made a little air filter for the the carb venting pipe  and modified it to fit up inside the frame air box (not used any more) to position it in nice quiet air. There was also the question of the engine breather pipe.  For now I just left it hooked up to the frame air box, and since this is now open, it breathes into the air. A more permanent solution needs to be devised. In the stock setup, fumes from this breather mix with the intake air, and get burned along with the fuel.   Mvc-010f   Mvc-013f



     Since writing this article, I’ve discovered Factory Pro’s tuning instructions for CV carbs. I might have gone about it differently a second time.  So consider this the story of a neophyte struggling with the tuning issues–without the help of a dyno and a choice of needles.  But in the end, she does run pretty good, and the cost was reasonable.        I’ll try to summarize the results of my first efforts to tune this carb.  I put fuel to it, actuated the choke and pressed the starter.  The bike started right up.  After a little warm-up the engine idled with a loping Harley type idle.  I was able to take a CO reading on the front cylinder and this dialed in fine (3-4% CO) at around 1 1/2 turns out.  This indicated that the stock pilot jet would work. The bike would rev but was clearly starving for fuel above 3000 rpm.  I installed a 126 jet (all jets used are Dynojet jets) and went for a ride.  The bike was very crisp off idle (pretty much on the pilot jet) and took off smartly.  (Common wisdom says that a single carb will give you increased low end, and reduced high end.)  At about 4000 rpm the engine started to misbehave and fade.  I made 2 more runs with  a 128, and then a 132 main.  Each time things got better to where I achieved pretty good performance up to around 4500 rpm..

I then decided I had better back up consider my other variables.  I pulled the disks out of my Supertrapp and found that I had been running 15 as noted ealier.  I reduced these to 8, and went for a ride.  Major improvement.  The bike would pull to red line (although not too smartly.)  I did a chop test on the plugs at around 5000 rpms, and found them to be dead white and maybe a little lean.  At this point I added another helper which I should have done at the very beginning.  I taped my throttle and marked the CLOSED, WOT, 1/4, 1/2, and 3/4 points.

At this point, however, a major problem surfaced.. After each run, I would come home and check my plugs–and they would show very even color.  However, after my last run I puttered around town to show my new creation to the local Yamaha dealer, my machinist, and my welder.  When I checked my plugs back in my garage, the back one was normal, but the front one was black.  Since we don’t have two carbs any more, there is no simple adjustment to correct this.

While I had no idea what KJS does to their carbs, they do specify plugs two heat ranges higher than stock (I am running stock NGK BP7ES plugs.)  The heat range of a plug needs to relate to the heat range of the cylinder.  You want to plug temperature to be high enough to where combustion residue is burned off, but not so high as to burn the electrodes up and /or cause pre-ignition.  If the cylinder runs cooler, then a hotter plug will be required to burn off combustion residue and keep itself clean.  But in my book, a hotter plug should not be used to “solve” a condition where the cylinder is simply running too rich.  Why this rich condition is happening, and at what rpm range, is still a mystery.  My (apparant) ability to set the pilot circuit properly in the beginning, adds to the puzzle. We will struggle hard to get to the bottom of this.


After thinking things over, I decided to go back to where I was, that is, go back up to 15 disks in my exhaust.  I did this, and for my next run, raise the needle a notch, and bumped the main jet up to 136.  The test run was very informative.  The engine was clearly running rich at cruising  throttle openings in the 2000 to 2500 rpm range.  Over that it would straighten out pretty well.  Again I had the black plug.

For my next run I lowered the needle back down (lean) and installed a 138 jet.  Rich running at lower revs disappeared, upper mid range acceleration was still a little rough, but I had a clean pull at WOT for the first time–although not quite as strong as I had with two carbs.  At this point I can say I’ve got a pretty nice running bike.  Low end acceleration is really good.  It may not be as strong as a two carber if you you twist it hard from the git-go.  But the bike gets peppy with so little throttle that it puts a smile on your face.  Just crack the throttle and she’s off.  It will cruise smoothly at any legal speed and you can see 75 and beyond anytime you want without a fuss.  Crank open hard at 4500 rpm and you know something is a little off–not sure which way (rich or lean)–I suspect lean.  Hold it open and it’ll go through the wobbles and start to pull evenly to red line, so the main jet seems to getting in the ball park.

Future plans?  For my next run I’m going to a 140 main.  While I’m mostly on the needle in my problem range, I feel that more main may help.  Longer range, once I’m convinced I have done about all I can do on the road, I’ll scrape up my pennies and pay for a dyno run.  Depending on results, I do have the further option of maybe finding a different needle with a  taper that might give me a little richer upper mid range (if that’s what I need).  I did buy a couple of BP6ES plugs (one range hotter) but I’ve pretty well solved my black plug problem so haven’t used them


     Well, now, for the two of you who are still following this project, I have a report to make. The 140 jet did the trick, and I can now say that this system is pretty well dialed in. The bike has a  funky low end (a good to go at very little throttle) and now pulls pretty clean up to red line. Hiccups in the 4000-6000 rpm range just about gone.  Overall, I would say power maybe down a bit from the two carb set-up (if you seriously hammer) but the feel you get out of the bike is a lively, repsonsive Virago 1000 which puts a smile on you face and can flirt you with traffic court any time you ask it to.

The bigger main has clearly richened up things when you are on still on the needle at higher throuttle openings.  The only slight downer is that the front plug now goes deep chocolate when running at low rpms around town.  But not black or sooty and nowhere near any danger of fouling..  The the bike runs and cruises smoothly at all rpm ranges.  Maybe I’ll go to a # 6 plug in the front cylinder, or maybe I’ll spring for some platinum or iridium plugs (platinum NGKs are gone) for a little added insurance.

So in summary, we can say that we took one of the stock carbs on the bike and

*Gave it as much air as we could on the intake side
*Gave it low (half way to straight pipe) backpressure on the exhaust side
*Left the fuel level in the float bowl unchanged
*Found that the pilot (idle) circuit adjusted well, indicating the pilot jet was O.K. in size
*Found that the Dynojet needle worked at roughly the same position it was in with two carbs, and
*Increased the main jet by 11 steps from 118 to 140 in the Dynojet system

And wound up with pretty satsifactory results.  This says that gas flow behavior issues, while no doubt present, weren’t serious enough to sink the tuning process on the relatively low compression, understressed engine..  In short, the bike runs good.

What’s next?   We’ll want to just ride it a while and see if we really have and like it.  We’ll want to check gas mileage.  We may spring for some guages and measure head temperature.  And we’ll no doubt go for Dyno run to get a better handle on torque, horsepower, and rich/lean running under various conditions.  All this may not happen until the Spring.


Just a couple of new inputs and thoughts.

I finally got around to speaking with Supertrapp about the number of disks I was running.  I’m at 15, and Supertrapp told me that 22 equals a straight pipe, which is what I wanted to know.  The more disks, the lower the back pressure.  I ordered 7 more disks and so some additional experimentation with yet less back pressure will now be possible.  I personally don’t like loud pipes, and the more disks you run, the louder the Supertrapp sounds, but we’ll see.

Also, Supertrapp has apparantly discontinued their pipe for Virago, which is a downer for the few of us who really like them.

I plan to try some larger jets also.  I have a 142 lying around and that will be next.  It seems to me that making this thing work at the top end is a function of as little back pressure as possible and a big jet, whereas lower end and midrange seem good at “normal” pilot and needle settings, but remembering that air intake and exhaust are much more free flowing than stock.

One point I don’t think I’ve covered is the choke.  My solution here was to build a little lever onto the bracket which pulls the choke plunger up.  I abandoned the choke lever on the handle bar, and simply activate the choke manually when starting the engine.  I’ve looked at different ways to handle the choke, but this seems the simplest.  I plan to make a little catch bar which will be able to hold the choke open at full and half settings.  I’ll post a picture of this gizmo when I’ve fabbed it.


I finally got around to taking my Yamaha out on a serious ride.  I am running a 142 main now, and after my last test run the plugs looked, well,  is “perfect”  too strong a word?  And the front plug was so close to the rear in light grey color that I am concluding  the “rich front plug” problem has gone away –for reasons I can’t fully explain.  Anyway, I filled up with Chevron Supreme and then did 57 miles of freeway and back roads riding.  I did not hammer the bike, rode 60-65 on the freeway, slower on the back roads.  After doing the run, I hit the same Chevron station and filled up a again with 1.18 gallons of gas.  That works out to right at 49 miles to the gallon!!  About as high as you’ll ever see with a Virago.  The engine ran cool.  I could touch the side covers and cylinders (and was out of the Kaiser burn ward in a day!  Just kdding!)

The bike has a really satisflying low end response, and good torque and pulling power up to 3-4000 rpm.  At higher RPM’s (as I’ve said) it is not quite as strong as a well tuned 2-carb set up, but is still fine for my riding style.  I don’t see 5000 rpm very often   The overall impression is a peppy, responsive, and fun motor.

One slightly sour note on the choke.  I did most of this project in the summer time, and the choke did its job to start the bike.  In colder weather I find that the choke circuit can’t supply enough fuel to get the engine going.  It will start and die immediately.  Cutting the air to the carb helps considerably to richen things up and doing that I can start the bike.  Once warm, the problem goes away.  This may not be an easy one to solve.

The dyno run and further development will have to wait a while due to a “broke” condition that plagues me from time to time.

UPDATE 7/14/2007 (CURRENT)

     You’ll note that the above activity occured in 2003/4 and it is now 2007.  I can now report that the starting/idling problems are solved (choke fuel intake tube enlarged a bit, and pilot jet (clogged!) thoroughly cleaned.  So in addition to starting fine, the engine has smoothed out a bit more  and the bike runs well through the full range as far as I’m concerned.  What’s left? A more elegant solution to the choke lever, and some cosmetics (I’m not too good at those).  And maybe a trip to Factory Pro for tuning at the professional level.


KJS is offering an updated kit featuring an aftermarket Mikuni flat slide pumper carb.  A Virago Single Carb. Someone who installed one wrote me and said it worked really well.  Check out the KJS website for details.


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