DadBenny's JAK Update
by: Dave Benson

A lot has happened with this project since last time. The original objective was to extract as much performance from the V-Star 1100 for the least expense possible. It was also my desire not to modify any stock components, so that if it ever became necessary, the bike could be returned to stock condition. Also, the needed parts must be readily accessible to any ISRA member who wanted to follow along and modify their bike the same way. So far we have succeeded: The bike is an absolute animal; no stock parts were altered other than straight removal; any added parts were common items that we can easily acquire; and finally, the cost will be well under $100.00. We'll take a break from the boring technical explanations this month and get right to the components and how they were made and assembled. Next month will include all the lab data and dyno results, along with as much theory as you can stomach.

The Filters

Filters Installed

The filters were selected to fit the existing hardware conditions. The carburetors are connected to the factory airbox via two rubber 90-degree ducts, which clamped onto the carb flange. It made sense to leave these ducts in place and adapt the K&N filters to them under the tank where they would fit best. The Inside diameter at the upper end of these ducts is 2 5/8 inches. The inside diameter of the ducts themselves is 2 inches. So, to keep the induction tract relatively constant from the filters to the carburetor horn and to maintain a nice smooth flow, the inside diameter needed to be maintained at 2 inches, the K&N filter needed to be one with a 2 1/8 inch clamp on flange. The filters also needed to fit under the tank, so the height was limited to about 3 inches. Finally, the filter with the largest filtration area needed to be found that fit those requirements.

K&N makes a filter that has an offset 2 1/8" flange I.D. It is an oval 3x4 inches in width and stands 2 3/4 inches tall. They can be found in the Dennis Kirk catalogue under part number 30-1570 for $31.99 each. I'm sure they can be sourced from other suppliers by the dimensions and description. Also needed would be a crankcase vent filter (Dennnis Kirk part number 30-982 for $19.99). This is a simple pleated filter to clamp on to the 1/2" vent hose, which was attached to the airbox.

Filter Adapters

Filter Adapter

The next problem is the means of adapting two ducts with a 2 5/8" female flange to two filters with 2 1/8" female flanges. Since the filters and the existing duct were both female, a 2 58 male to 2 1/8 male adapter needed to be made. . I shopped around Home Depot and Pep Boys Automotive supply looking for ideas. I was considering PVC tubing and bushings, but couldn't come up with the exactly correct dimensions to satisfy all the conditions -- remember, we wanted to do this with readily accessible parts and tools. One day, I was at Pep Boys quietly fondling the tail pipe adapters, when the perfect solution came to mind. There is a muffler adapter, which has an inside diameter of 2 inches and an outside diameter of 2 1/8 inches. There also is an adapter with a 2 1/2" I.D. and a 2 5/8" OD. All that would be needed was something to fill the space between the 2 1/2" ID of one pipe and the 2 1/8" OD of the other piece and I would have the basic elements of a 2 1/8" to 2 5/8" male to male adapter to make the connection.

I was considering filling the 3/16" space with silicone rubber, which would probably have worked fine, but I finally came up with a solution, which I thought was easier. One day I was in the plumbing department at Home Depot playing around with various plumbing fittings and came upon a rubber Fernco adapter. Those are the rubber bits that clamp onto PVC pipe to make a repair connection. The 1 1/2" Fernco connector is made to connect two lengths of 1 1/2" PVC tubing and it ends up being approx 2" ID and about 1/4" thick. This was the solution I chose for my prototype, although silicone rubber caulking would probably be fine also.

Filter Adapters

I first cut two lengths of the 2 1/2"ID muffler adapter to 1/2" lengths. It is difficult to cut these accurately and squarely using a hack saw, so I marked the pipe and cut it slightly longer. I then used my bench grinder to grind them down to the correct length marks. The 2" ID pipe was cut to two lengths of 1" by the same method. The short pieces were filed and cleaned of any burrs and loose filings.

The Fernco coupling was cut in half at the mid length point to make two short rubber hose elements (see attached picture). I soaked the rubber fittings in boiling hot water to make it more pliable and stretched it over one of the 1" long pieces of 2" ID pipe. Then I force fit the short piece of 2 1/2" pipe over the outside of this rubber piece. It helps to have soaked the rubber in hot water again to keep it pliable. After a lot of poking and pushing with the tip of a screwdriver blade, I was able to force the larger diameter piece of pipe over the rubber spacer. Finally, I trimmed away the excess rubber using a sharp utility knife. I now had my two adaptors, which allowed me to fit the K&Ns to the rubber inlet duct.

Ed Gouldsmith recently posted that he tried the silicone sealant to fill the space between the inner and outer adapter elements. He reports that the process was easier than using the Fernco adapter, so I would recommend his way as the best and easiest. What Ed did was place the inner and outer metal adapter parts on a piece of waxed paper. He then filled the 3/16 space with silicone rubber sealant. The next day, the adaptors lifted off the waxed paper quite easily and Ed reports that they work very well.

When everything was clamped together, JAK was born. None of the parts used were expensive nor were they hard to find. I imagine that the same or similar parts can be found anywhere in the world at common hardware and automotive suppliers in the sizes mentioned or in nearly identical metric equivalents.

Main Jets

Dyno Sheet

Before recently putting the carbs back on the bike, the #145 main jets were installed. So far, these have seemed to give very solid performance, and will suffice as an excellent starting point to take to the dyno for final tweaking. I so far have tried the #130 mains and found them to be too lean at the high end of the range. When I road tested the bike with the 130 jets, the engine started misfiring well before the rev limiter kicked in. It is easy to identify this as a lean condition by running it under a load or under heavy acceleration until the engine begins misfiring. At this point, if the throttle is quickly closed slightly and a brief surge of power is felt. Then the indication is that the engine is set up too lean. This is exactly what happened.

The next set of main jets I tried was the #135 jets. The increase in performance was quite remarkable, but there still was evidence of a lean condition at a higher rpm, but it occurred just before the rev limiter took over. It was encouraging that we were getting closer to the ideal point but still larger jets needed to be tried. I finished this session with a good high-speed blast and pulled the plugs for examination. The color was nearly correct, but a little too clean, also indicating a lean mixture.

Next up were the #140 main jets. With these jets, the performance was very strong all through the range all the way to the rev limiter, which was reached alarmingly fast. To be honest, the first time I hit the rev limiter, I thought I had the wrong jet size but when I glanced down at the speedometer and saw 105 and I was in fourth gear, I knew it was the rev limiter. I kicked it into fifth gear and tried for the rev limiter again. The bike accelerated very strongly and still had a lot to give when I had to slow down at about 115 because of traffic. I was also freezing my ass of and it was getting dark and near the freezing point. A patch of black ice at those speeds would have ruined my day so I returned home.

The next main jet I tried was the #145 jet. I couldn't perceive any difference in performance over the 140 jets so I decided to stay with the 145s for the next phase, which was to start experimenting with the needle sizes and settings. It seemed that the #145 is very close to the ideal, so any further jet size changes will have to wait until I get on the dyno. Warmer weather would help also.

Note on jet sizes: The Mikuni numbering system is in tenths of a millimeter. For example, a #90 jet is .90 millimeters. A #100 jet is 1.00 mm and so on. My #145 jet has a 1.45 mm hole drilled in the center of it. Mikuni makes jets in increments of 2.5, so the series would be 90, 92.5, 95, 97.5 etc. Dynojet manufactures and numbers their jets the same way, except their increments are in units of 2. So their series would be 90, 92, 94, etc. Main jets can be purchased over the counter or by ordering them through your dealer. If they don't know what type to order, tell them that you need the "small slotted" type in the size necessary. They only cost about $3.00 each. I have heard that Dynojet claims to have some sort of cryptic numbering system, which is unrelated to the Mikuni system. This is simply not true. A Mikuni #145 jet is exactly the same size and physical appearance as a DJ145 jet. Certain other manufacturers do not stamp sizes on theirs. That's their business and their choice if they want to confound the race tuners into buying someone else's product. Enough said.

Needles

Needle Schematic

The mid-range is controlled by the throttle slide, which is controlled by venturi vacuum. At the beginning of this article, I promised to stick to the nuts and bolts details and leave the theory and technical stuff for next month. I will do that. For now, all we need to understand that a vacuum controls the airflow from lower mid range to high midrange actuated diaphragm which lifts the throttle slide in response to engine speed. As the airflow increases, so must the metered amount of fuel. This is accomplished by a tapered needle which is attached to the bottom of the slide and which fits into a jet at the bottom of the carburetor barrel. As the slide is lifted by the vacuum diaphragm, the needle is lifted by the slide. Since the needle is tapered, more fuel is admitted to the mixture as it is lifted. More about that will be discussed next month.

Since the first phase of this project was to find the ideal main jet size, the needle was started out on the lowest clip setting which gives the highest needle position and hence the richer mixture. In order to isolate the main jet, which is mounted in series with the needle jet, the needle jet orifice needed to be as large as possible. When the needle is at it's highest position, it must provide at least as much clearance as the main jet, or the main jet size would not be getting tested. In other words, when the needle is fully lifted by the slide, the orifice must be at least as large as the size of the main jet for any testing of the main jet to be effective.

The factory needle in USA bikes is required by law to have only one needle clip slot. The smog sniffers in Washington who want to prevent any tampering with the ecologically correct mixture settings mandate this. There are a few ways around this, two of which we will discuss here. One solution is to buy a Cobra jet kit, which includes Dynojet main jets and a needle with 6 slots for changing the clip position. The attached picture shows the two needles side by side. It is difficult to notice, but the Mikuni needle is actually thinner at the tip. Another solution is to shim the factory needle by means of small washers used as shims under the needle clip. This effectively raises the needle the same as moving the clip to a lower position. I ended up using small rubber o-rings 1/16 thick with a 1/8" id. The o-ring slipped on to the needle shaft and under the clip made perfect shims. I started out with two per needle, which lifted them about 1/8". I used Danco #61 O-rings form Home Depot for the shims.

Filter Adapters

I measured the needles with a vernier caliper to study the relative tapers of the Dynojet vs. the stock Mikuni needles. Notice the dimensions noted in the attached drawing. What I found was that the Dynojet needle began tapering at the point where it would be located in the needle jet at closed throttle and continued its straight taper to the tip which is where it would be located at full throttle. The Mikuni needle started a more gradual taper from about the 1/4 to 1/2 throttle position. From the half throttle position to the tip (full throttle position) the taper becomes more pronounced and ends up being thinner at the tip than the Dynojet needle. What I determined was that the stock needle, if shimmed to the equivalent position to approximately slot 3 on the DJ needle, it gives a richer mixture from abut 3/4 throttle to full throttle position than the DJ needle does. The Mikuni stock needle at the equivalent position would show a weaker mixture at the very low end of the range. Since I'm developing this project from the top of the range (main jet) and working down to the bottom of the operating range (pilot jet and mixture screws) I decided to make a test run with the stock needles shimmed to the highest position possible and work down from there.

My most recent run was with the #145 jets still in place and the stock needles shimmed with 4 small brass washers to lift the needle to a higher position. I ran the bike under the full range of conditions. Low speed roll-ons, high-speed roll-ons, and maximum performance accelerations from a stop. I found no decrease in performance at the top end and a very slight indication of a lean condition at the very bottom of the range. Low speed roll-ons actually seemed better. With the Dynojet needle on slot six, I tried top gear roll-ons from 30 mph. The engine would sputter and lurch up to about 35 mph when it smoothed out and accelerated nicely. It actually seemed rich rather than lean at that point. With the factory needle shimmed as it was, the full throttle roll on in top gear at 30 mph was a bit smoother and the transition to higher speed running seemed about the same. The highway high-speed roll-ons seemed the same if not better with the shimmed Mikuni needle than with the Dynojet needle.

My next step is to do a long distance run for a mileage test. My last test was with the #145 jets and the DJ needle on slot 6. I got 40 miles per gallon on that ride. I figure this would be an effective test of the midrange mixture because normal highway cruising uses mostly the midrange and would give me a rough gauge of the needle efficiency. If we find that the stock needles with shims are better or equal to the Dynojet needle, we can eliminate about $76.00 bucks from the budget.

Summary

As the Frankenstar monster sits right now down in the garage, the only items which have been added have been the two K&N filters, a crankcase breather filter, and about 10 bucks worth of hardware items. All the unnecessary stock items have been removed intact and could be re-installed if necessary. I have a Cobra jet kit sitting on my bench and right now I am using... uh... nothing. Hmmm.

So far, all of our stated objectives have been met. Noticeable performance gains, low cost (under $100), readily available parts, and no permanent modification to the bike. I am at the point right now where I am watching the weather reports in eager anticipation of another productive testing session. Life is good.

Next month, I hope to have a comprehensive set of dyno reports and a final set of numbers and specifications to share with everyone. See you then.