Carburetor Basics

or

Better Running through Better Breathing

by
Jim Steinborn for the Honda ST

Edited (with permission) for the Vmax by Terry Hayden

What follows are my (our) experiences in airbox and carburetor jet modifications for better aspiration at high engine speeds.


If any of the pictures overlap, you need to widen your browser window.

Warning

Even with some pointers, changing your carburetor jetting can be a trying experience. Fortunately, all that is outlined can be undone if it doesn't work out to your satisfaction. In any event, it will probably take quite a bit of fiddling to get everything just so. However, the results should justify the effort. In any event, please read through this article completely before deciding to do this yourself.

To improve the breathing and fuelling of the Vmax, it is recommended to open up the intake a little . This procedure involves taking off the "Y" piece, installing a free flow filter and putting in an aftermarket needle. (Note: If you trim the airbox inlet to match the filter the carbs you'll need to spend some time on the pilot circuits to get them to suit your particular area). An ideal situation would be to spend some time on a dynamometer with an exhaust gas analyzer to get the mixture set correctly across the entire powerband. The result will be close to a 5% horsepower increase accompanied of course by an increase in intake noise on full throttle (wwaaaaahhh).

Comparing a "Dyno Tuned' piped and kitted Vmax" to the stock one is interesting. Maxers are reporting that the modifications resulted in a stronger midrange as well as a much more impressive high-rpm performance. The stock Vmax seems to start running out of breath above 8,500 rpm. At high engine speeds, it appears that the airbox (not to mention the exhaust) is just too restrictive to allow easy breathing (one of the reasons for the restrictive airbox from the factory has to do with reducing intake noise to meet EPA standards).

Carburetor Basics

The last thing I intend is to insult the intelligence of anyone reading this, but many of us aren't very conversant with the inner mysteries of constant vacuum (CV) carburetors. I think that the rest of the re-jetting procedure will be more interesting with an overview of CV basics. The October, 1995 issue of Motorcyclist magazine has a good article on this subject. Much of this information has been gleaned from this article and from the factory Service Manuals. Please keep in mind that the illustrations on this page show a side draft carburator while the Vmax has down drafts (feel free to visualize the diagrams on their side).

As outlined in other articles on this site, the carburetors on the Vmax belong to the family of 'constant velocity' (CV) carburetors. As the name implies, the diameter of the main bore changes in proportion to the volume of the incoming air so that the velocity remains constant. This gives a small diameter at idle and low speeds, so that the incoming air has sufficient velocity to draw fuel up from the float chamber. At large throttle openings, the bore is wide open, allowing unrestricted airflow to maximize power output at high engine speeds.

On the side of the Vmax carburetor, there is a vacuum chamber containing a diaphragm attached to a piston (the vacuum piston). The vacuum piston resides in a bore that is perpendicular to and intersects the main bore of the carburetor. At idle, the piston is pushed most of the way down by a spring, almost completely closing off the main bore. As the throttle butterfly valve is opened, the airflow in the main bore exerts a negative pressure on the lower section of the vacuum piston. At this point, air is drawn from the vacuum chamber through a hole in the bottom of the piston, overcoming the spring pressure and causing the piston to rise.

In the float chamber, the main jet controls the amount of fuel sent to a tube called the needle jet. The needle jet opens into the main bore of the carburetor and allows the fuel into the intake manifold by means of the negative pressure formed by the intake air rushing through the venturi. This is the very same principle witnessed by blowing across the top of a soda straw and drawing up the liquid.

The piston carries the 'jet needle' that fits into the needle jet (aren't you just loving this?). The jet needle is straight for about 1/3 of its length; the rest is tapered. At idle and low speeds, the piston is nearly all the way down, pushing the needle into the needle jet most of the way. In this position, the straight, large-diameter part of the needle (the root) fills up most of the space inside the needle jet tube, restricting the fuel flow to a narrow annular space around the needle. As the piston rises with increased engine speed, the needle is withdrawn from the jet. Because the needle is tapered, the annular space through which the fuel can travel increases, allowing more fuel to match the increased airflow.
 
 
 

 
 
 
 
 
 
 
 
 


From idle up to about 1/8 throttle (i.e. moderate speed cruising), the major source of fuel is the pilot (slow) circuit. This includes the pilot jet and the pilot screw adjustment. On the Vmax's carburetors, turning the pilot screw out results in a richer mixture at small throttle openings.

As you can see, there are a huge number of variables involved in proper fuel delivery:

  1. From zero to 1/8 throttle, the pilot circuit is the major contributor, influenced somewhat by the root diameter of the jet needle.
  2. In the lower midrange, the starting point of the needle taper is crucial.
  3. In the upper midrange, the degree (steepness) of the needle taper is significant.
  4. In the high speed range, the main jet is the biggest factor.

This doesn't even consider the tension of the vacuum piston spring or float height.

The Torque Monster

The Vmax V-4 engine is engineered for a very strong and broad torque curve. This is done at the expense of ultimate top-end power. Case in point: with a little less engine displacement, the ZX-9R outputs 129.57 @ 10750 rpm at the rear wheel compared to the stock Vmax's 110 (or so) hp at 8,500 rpm. On the other hand, the ZX-9R makes 68.46 of torque @ 9250 rpm compared to the Vmax's 80+ lb-ft at 6,000 rpm. I'm not trying to imply that the ZX-9R is in any way deficient in the torque department (for a sportbike), but its engine has been designed for maximum top-end power at the (relative) expense of low end torque.

One of the ways to engineer for a generous torque curve is to design in a high intake velocity. This is done on the Vmax by using narrow airbox intake tracts and smaller carbs. All of this helps to fatten up the midrange torque curve, but does not increase the top-end power much, if any at all.

One disadvantage of this approach is that it limits breathing at high engine speeds. There is only so much air that can be easily drawn down a narrow passage. V-boost helps by allowing a single cylinder to draw from two carbs at speeds above 5850 rpm. But, the engine still starts to run out of air above 8500. As a result of all this, one cannot make huge increases in the horsepower of this engine without headwork and installing flatslide carbs or doing a major amount of work such as big bore rebuild. One can, however, make more incremental changes. A jet kit and pipe are the best bang for the buck. The modifications below deal with putting in a kit and modifying the airbox. When finished, the limiting factor is no longer the airbox inlet but primarily the induction system itself.

Legalese

You must be aware that any modification to the fuel delivery or emissions system of your motorcycle renders it illegal for operation on public roads. Being a law-abiding citizen, after these modifications, we naturally restrict our Vmax's to race tracks ; ) ...In addition, modifying your motorcycle is inherently risky and you need to use your own judgment on the advisability of any portion of this procedure (no smoking, please). In short, if it seems like a bad idea, don't do it.

Let's do it!

Since we may not all have unlimited access to a dyno let's try to replicate a dynotune results using tried and true measures done by those who do...

Needed Materials:

Airbox

The airbox inlets on the Vmax are about 2" by 1.5". That's pretty small, even for a torque-monster engine with tiny (35mm) carburetors. That's why installing a less restrictive filter doesn't effect the mixture too much: the airbox inlet is the limiting factor.

To reduce the restriction of the stock airbox, simply remove the "Y" piece or shim it up 3/16" or whatever will fit under the dummy cover.

Carburetors

This is the time-consuming part.

  1. Selecting jets.
  2. Let's talk about the main jets first since if you decide to stick with the stock sizes you can leave the carb rack on the bike.

    According to the FACTORY instructions, you select the proper main jet by trying out different size jets and select the one that gives the highest top speed. Unfortunately, trying to ascertain the top speed of the Vmax over and over is not very practical, however, here we can rely on the work that others have done. Many have tried: 152.5 (stock), 155 and 157.5 and ended back with 152.5. Therefore unless you are in an area with very cool sea level air, I would recommend starting with stock main jets. More on this later. If you are sticking the stock main jets, skip to the "needles" section".. If not, continue by...

    Removing the carburetors.

    This is difficult to do the first time, as these items are all stuck together pretty well. On occasion, I have run the engine a bit to warm up the rubber connecting tubes, allowing easier removal. Note: with the carbs removed, check to see how the carb boots align with the manifolds. If the manifolds stick quite a bit into the airflow path, now might be a good time to pull them off as well to do some porting and matching work...

  3. Changing the main jets. Invert the carburetor assembly. Remove the four brass bolts that are on the inner lower side of the float bowls. Using a screwdriver that fits very precisely, unseat the main jet. Use a spare jet needle or toothpick to unscrew the main jet completely and remove it through the opening. Replace your new jet by doing the opposite. (if the jets are too big you have to pop the bowls)

    Regardless, if you have an older bike, now would be a good time to pop one of the float bowls to see if it needs cleaning. If so, repeat for the other three carburetors.

  4. Needles. The top of the Factory needle has 5 grooves in it. These receive a spring metal 'e-clip' (supplied). The grooves allow you to adjust the height of the needle: the higher the needle, the sooner the taper takes effect. The grooves are numbered from the top, so #1 is the lowest position and #5 holds the needle up the highest. I settled on #3. More on this later, as well.
  1. Changing the needles. On the side of the carburetor, you will find a round, shiny cover. This is the vacuum chamber cover. Remove the four screws holding it on. These screws are sometimes very tight. ( If one of the screws is a tamper proof torx screw with a nib in the centre then you will either have to beg borrow or buy the special bit or use a pair of vise grips to loosen it.) My FACTORY kit came with replacement recessed hex head screws to replace these; I recommend using them. When removing the cover, keep an eye on the spring: it is about 3/4" in diameter and 5" or 6" long. Pull the spring out of the piston, which is attached to the black rubber vacuum diaphragm. Stick two fingers into the piston and spread them apart, pressing against the inside walls of the piston. Withdraw the piston from its bore. The edge of the diaphragm may be stuck down: pull it up gently using the recessed tab.
  1. Look down inside the piston and you'll find a plastic screw that can be removed with a medium screwdriver. Turn the diaphragm upside down and push gently on the needle. First a spring, a washer, and the needle assembly will come out.

    Install the e-clip into the appropriate groove in the new Factory needle. Reinstall the white retaining disk under the clip and the small washer on top. Drop the assembly through the hole in the bottom of the piston. Make sure that the tit on the white disk seats properly in the small hole. Drop in the small spring and refasten with the plastic screw. (as a tip, use a small piece of tape on the end of your screwdriver to hold the screw on the driver to assist with installation). Make sure the needle has a bit of room for play. Put the piston/diaphragm back into its bore, aligning the tab in the diaphragm edge with the groove in the vacuum chamber body.

    If carbs were removed, reinstall the carburetor assembly onto the engine, pushing the carb flanges back into the connecting tubes and reversing the procedure for removing them. Make sure that the connecting tube band clamps are nice and snug (if there are any leaks there, all bets are off). Re-attach the airbox and install the K&N filter. Screw down the air cleaner lid. If the carb assembly was taken apart you will have to resync before tuning for your kit.

Tests and Adjustments

The FACTORY kit came with pretty good instructions regarding adjustments. Here is a brief summary.

Hopefully, the engine will start :^) Do not warm up the engine! Lean problems improve and rich problems get worse as the engine warms up, so starting the test with a cold engine can be beneficial. Starting cold, you can watch the progression of symptoms up to operating temperature. The engine does need to be at operating temperature, however, to be sure that a lean or rich problem definitely exists.

Plse keep in mind that the diagrams are not from the Vmax..go here for Vmax

  1. Pilot circuit (zero to 1/8 throttle)-
  2. The mixture is lean if:

    Correction: turn the pilot screws OUT 1/2 turn or so. If the pilot screws end up turned out very much more than three and one half turns total (stock is about 2 1/2 turns), you should use larger pilot jets (stock is #37.5).

    Correction: turn pilot screws IN 1/2 turn.

  3. Full Throttle - lower midrange
  4. Adjust the needle clip so that the engine will accept full throttle between about 2750 and 3250 rpm.
  5. Full Throttle - upper midrange
  6. Set the needle height for the best power between about 4000 and 6000rpm. If the engine pulls better when cold but 'soft' when at operating temperature, it is too rich in the midrange and the needle should be lowered.

Conclusion

This procedure results in a somewhat stronger midrange (not that it was lacking to begin with) and a much more satisfying rush from about 8000rpm to redline. Your fuel mileage will decrease slightly (3-5 mpg), depending on the final adjustment of the pilot mixture. I rather like the increased intake noise, especially when riding around town: I can hear it, but no one else really notices above the roar of the exhaust ; )

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