Carburetor 4-Way Crossover Equalization Manifold

The Carburetor 4-Way Crossover Equalization Manifold is a simple assembly of hoses, T-connectors, and hose clamps which is placed over the existing vacuum spigots between the carburetors and engine. These spigots are normally capped with rubber covers which are removed only when performing a carburetor synchronization.

Credit for introducing this idea goes to the creative Mr. Bill Jinks of the Owners Association.


The idea is to connect the carburetor outputs/engine intakes for the purpose of:

  • Balancing out the vacuum impulse demands between the carburetors.
  • Equalizing the vacuum variances of each individual cylinder's intake strokes.
  • Normalizing any air flow variances between the carburetors.
  • Slightly increasing the amount of fuel/air charge available by providing each cylinder access to all four carburetors thereby increasing the effective size of each carb.

Another theory, from MikeGTX: As each pair of pistons (1&4 and 2&3) rise and fall together, one is on the induction stroke and the other on the firing stroke. So, one set of inlet valves only is open at a time. The mod connects two inlet stubs together, so as the induction stroke of one cylinder takes place, it is able to draw through the link pipe as well as it's own carb. My theory is that the additional flow through the vacuum stub creates a bit of swirl in the inlet air flow itself. This could help to hold larger fuel droplets in the charge, rather than them falling out onto the inlet tract throat. Tuners sometimes modify the heads to induce this kind of swirl a bit further downstream as a means of helping to fill the combustion chamber more efficiently.

I am admittedly not a carburetion theory expert so I can't attest to the validity of these ideas. Preliminary testing with clear crossover tubing did reveal the presence of raw fuel droplets forming in the hoses indicating true crossover functionality.


There is anecdotal evidence from those who have tried this that the following benefits are real:

  • Easier cold starting.
  • Better mileage.
  • Reduced vibrations and smoother running.
  • Improved low rpm performance and responsiveness.

I cannot attest to the first two items above because my cold starting has never been a problem and I have not done any fuel mileage comparisons. However, I can vouch for the reduced vibrations and the improved low end response.

I had a 6500 rpm buzz made quite noticeable by the poor vibration isolation characteristics of the solid rubber feet on the Fatory Corbin seat. After installing the Carburetor 4-Way Crossover Equalization Manifold this buzz was reduced to a slight tingle. After installing the manifold, snapping the throttle open quickly from a steady 2500 rpm (about as small a throttle opening as I can get) in 1st gear results in a lifting of the front wheel.

There has been an actual before and after dyno test done on a bike with stock jetting and no benefits were found as measured by power and torque curves. Benefits in terms of smoothness and throttle response are difficult to quantify but, theoretically, the manifold may produce some improvements in those areas. Nevertheless, the manifold will not perform at its best for stock jetted bikes and is not recommended in that application.

Dynamometer results of a rejetted bike with the manifold attached has confirmed an increase in torque averaging 4 to 5 lb-ft up to about 4000 rpm with a 7 lb-ft boost at 2700 rpm.


To make the Carburetor 4-Way Crossover Equalization Manifold, you need:

  • Two feet of high-quality flame resistant fuel hose, 1/4" inside diameter. A recommended product is "Gates LOL Plus 4LOLC+ Flame Resistant Nitril Hose." I found fuel hose at Orchard Supply Hardware marked SAE J30R7. 1/4 inch hose is used rather than a tighter spigot fit 3/16" to provide a higher flow rate for the crossover of vacuum pressure and air/fuel charge. As long as a supple material is used, the hose clamps should provide a suitable seal at the joints.
  • Two one-piece barbed 1/4" Brass 'T' Fittings. Do not use piece together fittings. Also, do not use plastic vacuum T's, they are not fuel rated and will not hold up to the heat.
  • Ten 5/16" - 5/8" Stainless Steel Hose Clamps. A Marine Supply store will carry fully stainless clamps. Be wary of those where the worm screw portion is plated low quality junk.

The cost of these items should be about:

  1. Fuel Line - $1 to $2 per foot
  2. Brass T's - $2 to $3 each
  3. Hose Clamps - $0.80 to $1.50 each

The total cost of materials should be about $20.


To make the Carburetor 4-Way Crossover Equalization Manifold:

  1. Cut two end sections of fuel hose 4 3/4" in length
  2. Cut two center downtubes 2" in length.
  3. Cut a center tube section such that there is a 3 1/8" center to center distance on the brass T's. For me this was a tube length of 2 3/4". Depending on the style of T's you get this length may vary slightly.

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First, remove the carburetor synchronization spigot vacuum caps.

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Installation then is simple. Just mount the manifold as shown in the photos below. If is easiest to mount and tighten down the center two manifold downtubes first then attach the outer hoses. Use a socket head on the end of a screwdriver attachment rather than a flat head screwdriver to tighten the hose clamps. A flat head screwdriver won't get enough bite to tighten the clamps as much as they should. I originally left small air leaks at the spigot joints due to hose clamps that were not tightened enough. You must really clamp those down. Route the coolant line and clutch cable as shown.

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If you still have the Air Induction System (AIS) with the hose that connects to the spigot on carburetor number 4 (the rightmost carb), you may do either of:

  1. Simply leave the AIS hose disconnected. You may wish to plug the end of the hose to prevent dirt or grime from accumulating inside. The bike will not have the reduced exhaust emissions that result from the AIS so be aware of this if you are subject to periodic emissions tesing in your area.
  2. Construct a slightly more complicated manifold using an additional 'T' fitting and section of hose such that the AIS hose is 'spliced' into the manifold tubing thereby preserving the functionality of the AIS as well as the manifold on carburetor number 4.


Bill Jinks now recommends an simpler manifold as an alternative. Use two lengths of hose to connect cylinders 1 to 4 and 2 to 3 separately. The benefits from this setup may be nearly equal to the more complex four-way manifold. It also has the benefit of eliminating all the connections made necessary by the 'T' connectors and the additional potential of air leaks.

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Last Updated: 08-29-2003

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