Carb Jetting Kits

CARB JETTING KITS

Teaching CV carbs to live with exhausts and air filters

MOTORCYCLE MANUFACturers love constant-velocity carburetors, so much so that almost all streetbikes use them these days.

Reasons why are easy to find. Modern CV carburetors meter fuel precisely, keeping the air/fuel mixture under much tighter control than slide-throttle carbs. That control is vital for good fuel economy and a low level of exhaust emissions. Also, the vacuum-controlled slides in CVs are only influenced by a rider rather than controlled by him: so if he snaps the throttle open at low engine speeds, CV carburetors will compensate, instead of letting the engine shudder and stagger as would equivalently sized slide-throttle carburetors under the same conditions.

But as much as motorcycle manu facturers have grown to like CV car buretors, much of the motorcycle aftermarket has grown to hate themagain, for good reasons. CV carbs aren't as amenable to home tinker ing. requiring more knowledge and effort to recalibrate than the average rider possesses. Motorcycle owners who have modified their engines, ex hausts or airboxes have often failed in attempts to rejet a set of CV carbs to work with the new components. The overall effect has been to discourage the modification of stock engines, which, in turn, reduces the sales of aftermarket hop-up goodies.

Some of the companies most af fected are those making exhaust pipes or replacement air filters. Gen erally. an exhaust-pipe change may require only minor changes in jetting (but not always: pipes that supply far different power characteristics than stock may require far different jet ting). But many people fitting a new exhaust system to their bike also change to individual air filters at the same time, and the removal of the stock airbox can have major effects on carburetion. If, after the modifica tions, their engine ran worse than be fore, both the exhaust system and the air filters shared the blame.

But aftermarket companies are very quick to respond to the needs of their customers, and the last year or so has seen a new product emerge: a jetting kit that recalibrates a particu lar bike's CV carbs to work with indi vidual air filters and/or a high-perfor mance exhaust system. Currently, both K&N and Kerker are offering such kits, developed and manufac tured for them by Dynojet Research. Last year, according to Dynojet, 16,000 of these kits were sold. A few other companies also are offering similarjetting kits, such as the tuning shop Fours N' More, which sells a less-complex alternative to the Dynojet kit. With the growing popu larity of these new products, we de cided to take a detailed look at a cou ple of them.

The guinea pig for our testing was a 1985 Yamaha FJ1100, and we ar ranged for two different tuning set ups. The first was from Kerker, and included a Kerker KX 4-into-i-into2 pipe, individual K&N air filters to replace the complete stock airbox, and a very comprehensive Kerker! Dynojet jetting kit. The second was a combination recommended by John Cordona of Fours N' More: a Super Trapp 4-into-l pipe, the same indi vidual K&N filters, and a Fours N' More jetting kit.

First stop with the FJ was Jerry Branch's dyno. After a series of dyno runs to establish a baseline for the stock FJ, we fitted the Fours N' More kit and the SuperTrapp pipe. Consist ing only of new main and pilot jets, the Fours N' More kit was a snap to install: all that was required was to remove the carburetor assemblies, pop off the float bowls, unscrew the stock jets and screw in the new onesperhaps a 40-minute job for someone not in a hurry. But once we got the bike on the

dyno. there were problems. The modified FJ made more peak power, but it ran so very rich that black smoke puffed out the exhaust. We in stalled smaller jets. but the FJ stayed very rich and never ran cleanly enough to extend the dyno run over the complete range of engine speeds. John Cordona of Fours N' More was present to observe all this, and was more than a bit mystified. He assured us that the kit worked well on the street, and he didn't understand why it would be any different on the dyno.

Next up was the Kerker/Dynojet kit. Its installation was more timeconsuming. simply because of the greater number of parts. Included in the kit are new main jets, new needles (with adjusting clips and optional spacing washers), new main and pilot air-correction jets, a drill to remove the plugs covering the pilot-mixture adjustment screws, and a punch needed to drive the air-correction jets in place. Putting all these parts in the carbs can easily eat up an hour or so, and this is one of Kerker's simpler

STEVE ANDERSON

kits; kits for other models often in clude new slide return springs and/or drills for modifying orifices in the bottom of the slides. But the com plexity paid dividends on the dyno; power was up, and the specific fuel consumption (referred to as BSFC, for brake specific fuel consumption, a measure of engine efficiency given in pounds of fuel required to produce one horsepower for one hour) indi cated that the mixture was leaner

than stock at high speeds~(where pro duction bikes are often too rich), and slightly richer at lower speeds (where production bikes are lean to pass emissions tests). Being curious, we tried the Kerker! Dynojet jetting with the SuperTrapp pipe, as well. The results were much the same as with the Kerker pipe: de cent carburetion with good power. Peak was actually 100.6 horsepower, the highest we saw with the FJ, and 0.6 horsepower higher than with the Kerker pipe.

While the Kerker/Dynojet kit was the clear winner on the dyno, this was probably the least important part of our evaluation. The real test of jet ting, unless you are a racer, is allaround street use, where lowand mid-speed driveability is as impor tant as peak power. And on the street, our experience didn't correlate ex actly with the dyno results. The Kerker/Dynojet kit worked well on the street, providing easy starting and returning 47 mpg during easy riding, which is as good as stock. The modi fied FJ felt powerful, but during roll ons from typical road speeds it had a slight hesitation-not bad, but notice able nonetheless.

Despite its miserable performance on the dyno, the Fours N' More jet ting kit worked fairly well on the street. The FJ didn't start with quite the readiness it did when Kerker equipped, but roll-on response felt much better; when the throttle was opened, the FJ jumped ahead with out any hesitation whatsoever. The only real manifestation of the rich jet ting was the gas mileage, which fell to 40 mpg with the Fours N' More kit.

So in the end, what does it all mean? Either of the jetting kits we tried provided adequate street perfor mance. The Kerker/Dynojet buys that performance through sophistica tion, and its $ 1 55.95 cost (including K&N filters) shows that. Its slight hesitation during roll-ons could probably be tuned-out by raising the carburetor needles haifa notch with the supplied washers, at some cost to fuel economy. Also, Mike Dobeck of Dynojet told us that the FJ 1100 kit has been revised since we acquired our test kit; the latest version is al most all-new, with changes to the slides and their return springs to give better low-speed acceleration. The Fours N' More kit is simpler, relying~ on rich jetting to cover some sins of imprecision. But in the end, it gives usable street performance at a cost of $86, also including K&N filters.

Kerker markets the Kerker/Dyno jet kit as the Stage V kit, which is available through motorcycle deal ers. Dynojet kits are also available from dealers under the K&N brand, or directly from Dynojet Research, 1630 Foothill Drive, Boulder City, Nevada 89005; ([[702] 294-1524). Fours N' More jetting kits are avail able through Fours N' More, 7020 Reseda Blvd., Reseda, California 91335; ([[818] 996-8109).