Jets
Honda's streetbike carburetors contain jets which of course size down the fuel tubes to a precise amount so that the carburetor can be factory-assembled for different target markets. The DOHC 450 for example from year to year shipped with a surprisingly large range of main jets due to this carburetor's extreme sensitivity to altitude and other envrionmental factors. Most of Honda's jets are marked in increments of 100th mm. Thus a "75" jet is 75/100 mm or 0.75mm. Honda's racing carburetors use a completely different scheme in which the jets are not marked by diameter but by flow, that is, how quickly a test liquid passes through the jet in a given amount of time. The diameter and flow systems have no interchangability, and to make the jets visually distinct Honda's racing jets are made with a hex-shaped head. The rationale behind flow-rated jets is percentages can more easily be applied to them in racing preparation to make fast, intelligent changes in jetting, since flow is already in view. The diameter-rated jets don't as readily allow this because diameter and flow are not directly proportional the way area and flow are.

Honda's push-in jets as used in the 350 and DOHC 450 twins (and the GL1000 and smaller SOHC fours) have two characteristics which trip people up. One, they are counterbored on both ends. The deeper counterbore should face the fuel. The other interesting characteristic is the push-in jet has two machined grooves. One of these grooves holds the spring that retains the jet, the other receives a sealing o-ring. The two grooves look the same but are actually slightly different and aftermarket jet makers often get this wrong with the result that the retaining clip will not fit properly onto the jet. The biggest problem however is finding the correct o-ring. None of the aftermarket carb parts sources have the right size--even the ones already mounted to aftermarket jets are incorrect--and Honda never sold the o-ring separately. There is however one company--San Jose, California based K&L--an aftermarkt company that supplies the correct size, sourced-from-Keihin o-ring. Contact www.newmotorcycleparts.net, a K&L retailer, to get these proprietarily-dimensioned o-rings. For the 350 twin these are 1.1mm thick and 4.2mm i.d. for the larger secondary main jet—part number 18-4609, and 1.1 x 3.2 for the smaller primary main jet—part number 18-4604. K&L also retails these o-rings through Amazon.

Adjustments
Adjusting the pilot (idle mixture) screw has always been a challenge for riders. The factory of course published recommended settings but these are initial and subject to conditions. The experienced mechanic gets throttle sync very close first, then adjusts the pilot screw by ear, noting the highest idle obtainable with the adjustment. He will in addition use throttle response to fine-tune the last 1/8 turn of the screw. Even better is to use an exhaust gas analyzer (EGA). Though Honda never published carbon monoxide (CO) specs, 3 percent CO at 400-800 PPM HC on the EGA is correct for 1960s-1970s Hondas. Though Honda did at one time advocate a pilot screw adjusting method called the "idle drop" procedure, this technique is for emissions only. It is not a tuning adjustment. It does not put the bike in the best tune.

Professional mechanics view carburetor synchronization differently from the rest of the world. While the general public regards it in the same terms as the wash they give their ten-year-old car once a year out of guilt--far from that, carburetor adjustment and synchronization is actually the capstone of the engine part of the preventative maintenance tetralogy that includes compression, valves, and ignition, a part of the complete routine that Honda calls for every 2500-3000 miles on vintage models. You must have everything else perfect before synchronizing. If for example you have fuel varnish built up in your carburetor's circuits from infrequent riding, or valves that haven't been adjusted in years, these are problems. Syncing won't make up for dirty carbs or tight valves. Syncing is always the last step in a list of tuning tasks, and it is never a substitute for any one or all of them.

Honda has rarely advocated synchronizing carburetors at any rpm other than idle. Synchronization accomplishes just one thing: a smooth, steady idle. That in turn can result in easier starting and a satisfyingly crisp throttle response. That's it--idle and slightly off-idle. Sync will have no effect on open-throttle performance. It's all about slow-speed civility. However, that by itself is important enough in a streetbike, which spends most of its time in the carburetor's lower ranges and in accelerating and decelerating. Carburetor synchronization is eighty percent nuance. The why of syncing is fairly scientific. But the how, the hands-on part, is anything but scientific. Synchronization is not merely aligning the individual throttles. Whether using vacum gauges or doing it by ear and feel, equal airflow is the goal, and airflow is subject to more than merely the position of each throttle plate (or slide). If one cylinder for example has 10 psi less compression or slightly different ignition timing, or runs at a slightly different temperature, or one of the carbs has some varnish that affects fuel discharge--then equal airflow won't be a matter of simple throttle position. To compensate, a variable cylinder's throttle, though your ear or your gauges won't show it because you have dutifully made the carbs feel or read all the same, will actually have been made slightly out of mechanical sync in the process of achieving perfect running, dynamic sync. That's right. The throttles don't have to be perfectly matched to achieve exactly the same airflow, and sometimes they must actually not be to get perfect sync. If this is hard to accept, if syncing to you has never meant anything but getting the throttles aligned, then consider that modern fuel injected bikes' throttles--which on many inline-four sportbikes are permanently fixed all to one shaft--still require periodic synchronization. Think about why a system that is manufactured with perfect physical sync would even need regular resynching, and the fact that they are synced by adjusting throttle body mounted air bleeds, and the fact that just removing and replacing a brace of carburetors makes them have to be resynced, and you will better undertand the dynamics at work in carburetor synchronization.