® Carburetors part 11


Honda’s idle drop procedure
Honda’s idle drop procedure is hugely misunderstood and misappropriated on the powersports interweb. In fact, it’s a perfect example of how the ‘net takes something that had relevance within a specific context, and tries to make it valid outside that unique space--with the expectable huge fail. It’s simply a lack of knowledge. Honda’s idle drop procedure isn’t valid outside its unique context and it was never meant to be. No one on the inside of the industry makes that mistake. Even history has shown the ethos of the idle drop procedure to be political and not practical. Honda’s idle drop procedure was developed to allow less well equipped motorcycle dealers to approximate a procedure that Honda’s car dealers were, at Honda’s insistence of course, then using. This was the propane enrichment technique. Honda had their car dealers adjust the carburetor while feeding propane into it, supplementing the normal gasoline intake. The tech got the best idle with this setup, then of course he removed the propane bottle before giving the car back to the customer. The engine went from two simultaneous fuel sources to just one. What do you suppose was the result? An abnormally lean idle, of course. And that was the emissions-driven objective, which everyone knows was austere and unrealistic. Similarly, correct pilot screw adjustment on vintage Honda motorcycles is well known to range up to twice the published factory spec on some models. The motorcycle-specific idle drop procedure is the same technique as the automotive-specific propane enrichment, only it uses an electronic tachometer instead of a propane setup. Just as with the propane method, the mechanic gets the best idle possible as he adjusts the pilot screws, then before being done with the job he purposely “effs” it up, turning the pilots until the tach shows a certain amount of idle speed reduction. Don’t think of this as tuning. It is not. I have heard of folks doing a sort of half of an idle drop procedure—only without the drop. And I suppose that is okay, though kind of an odd approach that leaves me wondering. At the end of the day, Honda’s idle drop procedure is an emissions adjustment, not a tuning one.

Pilot screw adjustment
On a related note, many in the powersports media seem to think it is illegal to adjust the idle mixture (pilot) screw on 1978 and later emissions-spec bikes. While this meme played well in magazines at the outset of the emissions era, it is worn out now and simply not true. Even emissions-era Hondas can and should be adjusted, and Honda supplied the information and even the replacement parts and tools with which to perform this adjustment. One of the things that makes me sad is how many carburetors I get in for rebuild whose pilot screws have apparently never been adjusted. The limiting caps are still in place. Thirty to forty years--even up to fifty--and no proper service. The adjustment should have been made by the dealer at the time the bike was sold to the original purchaser, or at least during any one of the half-dozen opportunities that occurred after. Even more depressing is that some of these carburetors have performance jet kits in them. It is a bitter irony--sealed pilot screws combined with aftermarket jet kits. A mockery of logic that affects this career mechanic deeply.

Negotiable specifications
Inexperienced DIY-ers seem confused about the difference between negotiable and non-negotiable factory specifications. I once knew a professional mechanic who would, for special customers, put a dial indicator on the top of the valve adjusting screw to more exactly get the valve clearances to spec. This is insane. You need to understand the implications. The average person could be forgiven for misunderstanding when a published adjustment specification--due to its role in the big picture of maintenance--is inviolate and when it is not. But a professional should know better. Valve clearances are not sacrosanct. You will note that on bikes made since the 1980s many of Honda’s valve clearances are specified in a range. They are not mathematically or in any engineering sense linked to the fuel or other systems in such a manner that exact specification is crucial. Tight valves are bad, of course, both due to compression loss and valve cooling--but not due to any significant tuning consideration. On the other hand, a little extra valve clearance is known by every career motorcycle mechanic to be a good thing all the way around--in relation to tuning and valve life both. But ad-hoc reinterpretation of piston-to-cylinder clearances, a specification that by contrast literally defines engine condition and directly influences engine longevity, is not a good thing to do. Yet many reverse these. They treat the negotiable as non-negotiable, and vice versa.

Twisted throttle shafts
When carburetors sit with unstabilized fuel in them, the fuel begins gelling in just three weeks. Varnish forms all through the carburetors. This eventually causes the float valves to gunk up and fail to seal, and if the petcock is left open, the carburetors can flood with fuel. So now there is a layer of fossil gum all across the carb assembly internally. On some vintage Hondas it can accumulate to the level of the throttle shafts. As this material ages it turns to resin, about as hard and as tenacious as the chewing gum stuck underneath your desk in junior high. Now the throttle shafts are literally encased in epoxy. Along comes the unsuspecting purchaser of this dormant motorcycle. He attempts to make the engine run, and finding the throttle very hard to turn, simply bears down harder. The throttle shafts, all of them encased in resin, tenaciously resist movement. But eventually, sufficiently brutal force results in those shafts rotating perceptibly. But in the process they twist. These soft brass shafts, even more delicate because they're slitted to hold the throttle butterflies, distort easily. They offer very little resistance. Certain models of old Hondas consistently present this malady to the carburetor rebuilder.

Part 12


Last updated January 2025
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