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Fueling the fires inside the Honda 250-305cc twins…

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UPDATE NOTE: Please see addendum at the bottom of the story containing Honda part numbers and codes for the jet needles.

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While there is a lot of interchangeability within the 250-305cc Honda series of bikes, built in the 1960s, there are specific carburetor setups for each of the three models. In general, the 250cc models had 22mm carburetors and the 305cc versions were equipped with 26mm mixers. The exceptions are:

The 305cc Dreams (equipped with single carburetors) which use the same 22mm carburetor as the 250cc Dreams, with only minor calibration settings. The 250cc models have #115 main jets and a different CA72 needle, whereas the 305cc models have #120 mains and a CA77 specific needle. Honda did have a moment of weirdness where they were buying Mikuni carburetors for the 250s, so there is a separate parts list and calibration for that application.

The Keihin carburetors came in two basic shapes, called either “round bowl” or “square bowl” configurations. As you may infer, the “square bowl” carbs had float bowls with squared off 90 degree corners and straight sides. The shapes of the bowls and bodies dictated appropriately designed floats and float bowls/gaskets. “Round bowl” carburetor floats have rounded out sides, whereas the “square bowl” floats are flat on the outer edges. The basic carburetor shapes have interchangeable parts between the Dreams and the CB/CL models, except the floats, bowl gasket and bowls. For the CA/CB72-77s, the changeover from round bowl to square bowl carburetors seems to have occurred in 1965, when other engine design changes were made. However, all the CL72-77 Scramblers, other than those from the first 1962 CL72 production run, have all featured square bowl carburetors.

The original carburetor float part numbers have changed over the years and some of the later substitutes are from the CB350 twins with a 286 product code, rather than 259 or 268 codes. Some carburetors, seen recently, have had solid plastic floats pirated from other makes/models, which have close dimensions to the original brass units. In the end, a float is a float and as long as the float bodies are pretty consistent in size and shape, then they will work fine and perhaps better in alcohol fuels.

The carburetor castings for all models had the “power jet” casting features as part of the body design, even when the feature was not used. In 1967, Honda “cleaned up” the carburetor bodies, deleting the unnecessary casting details which were not being used. The carburetor bodies of the 26mm styles are physically larger than those of the 22mm because of the slide bore sizes, but the 305 carburetors never had “power jet” systems installed in any application, even though the casting features were included in the larger body sizes.
The main jets are JIS pitch threaded, so only those jets will fit the jet holders correctly. You can tell the JIS thread pitch jets from the later (1968-on) ISO jets because the ISO jets are scribed with a circular groove around the outer circumference of the jet. JIS threaded jets have no such markings, beyond the actual jet size stamping.
There is a difference in the main jet holders where they are cross-drilled for the emulsion tube ends. The Dreams have 4 vertical rows of 3 holes with an extra pair of holes drilled at 90 degrees to those at the base of the jet holder, next to the threads. The CB/CL jet holders have a reversed pattern where the extra hole sets are drilled at the top edge just below where the needle jet is held into the carburetor body, once assembled.

Honda CB77 models were factory equipped with #42/135 jet combos, whereas the CL77s came with #38/130 settings, with the difference in the exhaust systems causing changes in the calibrations setup. The 250cc CB72s had a unique “power jet” fuel system, which added additional mixture volume at ¾ and more throttle settings. A long brass tube dips into the float bowl supply for its fuel source to feed the power jet system. Those carburetors had #35/110 regular jet sizes, plus some added fuel and air correction jets, mounted up high in the carburetor body fed by machined air and fuel passages. The air jets just above the inlets of the carburetor throats required some small tubing connected to the air filter fittings to ensure clean air supplied to the carburetors. The needle jet holder was a CB72 specific part.
The similar-looking CL72 carburetors were not equipped with the power jet system and featured #40/115 jetting. The carburetor bodies are basically identical to those of the CB72 except none of the fuel/air passages are drilled through and the air correction jet location can be used for a spare set of main jets, if you encounter altitude changes requiring a main jet size change. CL72s used Dream jet holders, rather than the CB72 versions.

For the CL-77 Scramblers, a different needle and jet setup are the only differences between the CB and CL carburetor sets; otherwise the carbs can be interchanged back and forth between the two models. There is a simple rule for setting float levels on these carburetors: 22mm carburetors have 26.5mm float levels and 26mm carburetors have 22.5mm float levels.

In recent years, the introduction of alcohol-based fuels has caused several problems with these vintage carburetors, including swelling of the rubber parts, etching of the carburetor bodies and some of the brass components, plus the need to re-jet the carburetors to compensate for the change in fluid viscosity and fuel energy. After tuning a stock CB77 to factory specifications, it became apparent that the engines were running somewhat lean at higher power settings. For the 305 Super Hawk, a change from the stock #135 to #140 main jets cured the lean surging and helped smooth out both fuel delivery as well as increase power outputs markedly. Other vintage models have responded similarly, so don’t be afraid to increase the main jet size one step to find a solution to performance and general running problems, when everything else has been set to proper specifications and performance lags. Sometimes, it pays to add more fuel to the fire…

For 40-50 year-old carburetors, more issues arise in that the mounting flanges tend to warp, causing air leaks, as well as the tendency to cause the slide bores to become out-of-round. When this occurs, the carburetor slides will start sticking in the bores, causing unwanted acceleration or high idle conditions. It is important to check and change the o-rings for the carburetor bodies, as well as those which are installed in the fiber insulators. Another source of an air leak can come from degradation of the insulators because of the alcohol fuels in combination with heat and age, which causes the narrow o-ring channels break off making a large vacuum leak.

Addendum:

Slides, needles and needle jets

Carburetors for the CB77 and CL77 are interchangeable, with the only difference being the taper of the "Jet Needle." The difference between the CL72 and CB72 is a different needle, a 3.0mm slide and the power jet circuit, featured on the CB72s only. The part # for the CB77 needle is 16151-275-004 vs. 16151-278-004 for the CL77. The model applications are generally stamped either on the upper edge of the carburetor-mounting flange and/or sometimes on the
choke lever arms. The CB77 needles are stamped K24231; CL needles K24304; CA needles K22401; and CB72 needles K22402. Most of the major parts of the carburetor are not available separately, but the "banjo" fittings for the fuel lines are similar to the 17625-250-000 Joint; 90168-250-010 Bolt; and 17791-250-000 Gasket (4) from the C70 fuel petcock.

The carburetor insulator spacers are specifically designed for each application. The CB77 insulators are generally marked 25-26 (mm) on top. CB72 insulators are marked 22-24mm. The holes are tapered to maintain air velocity through the intake system. Honda Dream insulators came in two specific types, using different sized o-rings to seal against the cylinder heads.

Be sure to use only the correct-sized O-rings when you are replacing them, as oversized (in thickness) O-rings will often swell up with today’s fuels and break through the thin outer edge of the insulator, resulting in an unwanted air leak. The insulators for CBs and CLs are the same, depending upon displacement size.

Keihin carb needle information:

Model - Honda P/N - Needle mark

CA72 16151-259-004 K22401
CB72 16151-268-004 K22402
CL72 16151-273-004 K22302
CA77 16151-266-004 K23401
CL77 16151-278-004 K24304
CB77 16151-275-004 K24231

The needle jets for CA72, CB72 and CL72 are all the same part number (from the Dreams). The needle jet part, for the CB/CL77s, is unique to the 26mm carburetors.

Carburetor slides for the 22mm carbs are all chrome-plated brass units. Honda switched from the chrome slides on the 26mm carburetors to an anodized alloy slide, probably due to the tendency for the chrome ones to wear the chrome off the slides and/or to stick in the bores more readily. The newer alloy slides seem to fit a little more loosely and don’t show the same traits of slide sticking, unless the carburetor bores are badly warped. The alloy slides will fit round and square bowl carburetor bodies equally.

Bill “MrHonda” Silver

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