Monday, April 10, 2023

Flaps up, Flaps down, Flaps gone! Honda’s carb choke problem

If you have been around vintage Hondas for a while, you may have seen the destruction that occurs when the little spring-loaded choke plate flaps break away and get sucked into the intake port. The results are usually either a piece jamming the intake valve open, where it then gets whacked by the piston on the upstroke or the part gets all the way past the valve and into the combustion chamber where it bangs around denting up the chamber and piston crown or worse…

                                                                  Honda CB/CL/SL choke flaps

Honda’s vision of reducing the amount of unnecessary hydrocarbons that are created when the engines are put on full choke during a cold start. Original choke plates were solid doors that shut off 99% of the air coming into the engine when fully closed. Keihin engineers decided to create a spring-loaded flapper door that was mounted in the middle of the choke plate so that even with full choke, there was a way for air to pass through and keep the plugs from fuel fouling and reduce the unburned fuel that occurs when the airway is totally obscured.

                                                             Honda CB550 choke flaps

Honda put these little flapper doors on many models from the late 1960s and well into the 1980s. The tension springs are very tiny and the little flaps that they control can easily overwhelm the spring tension after a few miles/years. Generally what happens is that the spring tip breaks off, allowing the little flap to be subject to the whims of the intake air which is moving at a fairly fast rate. The flappers were scaled to the sizes of the carburetors, so the little 90s got tiny ones and the 350-450s got much large versions.

If you are looking at buying a 1960-80s Honda with the choke flappers on the carburetor(s), then it is wise to have the air filters removed to inspect the condition of the flap. If it missing or damaged, look for signs that either the compression is low or there are indications that the engine has been rebuilt on the top end. Pull the spark plug (s) out and look for signs of damage. What you might find is a bike that “hasn’t run” for a while and may or may not even turn over depending upon the amount of damage done. If the bike was trolling around town when the flap lets go, then sometimes it will quit right away before too many engine cycles occur. If it happens on the freeway at 7k rpms, the destruction happens quickly and is usually deadly to the engine components and hopefully not to the driver.

Looking up Honda carb choke flaps on eBay, for instance, will bring up a number of results for the various types depending on the size of the carburetor throat. They became sturdier in design and probably in materials towards the end of the carburetor run. Honda began to switch from flat plates to enricheners using little plungers, some with calibrated needles to sidestep the flapper problem once and for all.

                                                                    Honda CB750 choke flaps

Choke enrichener

Bill Silver

aka MrHonda