® The truth about throttle shaft seals

There is a pervasive but incorrect notion that your carburetor's throttle shaft seals contribute to performance issues. Actually, they don't. These felt or rubber seals are dust seals only, not vacuum seals. In other words, your carburetors have a designed-in amount of air leakage past the throttle shafts. This is normal and it's the way all Keihin and Mikuni CV carbs are, not just Gold Wing carbs. 1 For this reason, though I sell the felt seals (all four sizes in fact), I am indifferent about their replacement. 2 The sole job of the seals is to slow the entry of abrasive dirt that would hasten the wear of the brass bushings the throttle shafts are mounted in. That's it. They guard the bushings, not the vacuum.

There is a second misunderstanding associated with this one, and that is the idea that spraying flammable aerosol products onto or around carburetors can help detect vacuum leaks. Though like cat videos on YouTube this fallacy virtually defines motorcycle user forums, no thinking mechanic uses anything like that method to check for vacuum leaks. Here's why. A carburetor has half a dozen openings to atmosphere all around it. Any flammable substance sprayed even a short distance -- let alone right at -- the carburetor is going to be picked up by the carburetor and will then affect engine running. Doesn't tell you anything. A professional mechanic uses the historic less air test to find vacuum leaks, not sprayed chemicals. 3

A third tie-in to the subject of throttle shaft seals is the mistaken belief that chemical dip and other immersion methods of carburetor cleaning necessarily hurt the seals. Sure, felt type seals deteriorate over time and become dried up and shrunken, in extreme cases to the point that removing them results in their basically flaking away into dusty bits on the workbench. But that's not the fault of dipping. That's entropy. Again, they're only dust seals, long-term guardians of the throttle shaft bearings.

Instead of all this hand-wringing over supposed effects throttle shaft seals have on performance, it is much wiser to go after the things that really do hurt engine efficiency. There are several. The number one thing to eliminate as a performance problem on vintage Japanese bikes is low cylinder compression. See my articles on this. The fact is, the years do one big thing to Japanese engines: makes 'em "out of breath". I tell all my customers they want 150 psi ("book" for many vintage Japanese engines is 170), but unfortunately 120~140 is very common. It makes little sense to spend your energy on anything else -- ignition, carburetors, whatever-- if your engine develops less than 150 psi. 4

Speaking of ignition, the second thing to check, less important than compression by only the proverbial hair, is ignition timing. Don't overlook it. I'm now over 46 years into this business and have worked for close to a dozen shop owners in addition to having my own business. In all that time and among all those people -- many if not most professionals like myself -- I doubt I have known more than a handful who could service Honda point type ignition properly. While not particularly difficult -- on the order of syncing carbs once you know how -- ignition service on vintage Hondas is nonetheless the most challenging maintenance item bar none for most folks simply because the proper procedure is extremely unintuitive. There are aspects of it that just don't make sense to most people. It's not like any other maintenance task for this reason. Don't take ignition timing too lightly. It is nothing less than the soul of this motorcycle. 5

Get out of your mind that throttle shaft seals affect performance. In fact, the next time someone tells you new seals will perk up your carbs' performance, tell 'em -- well, you know what to tell them. Ride safe.

Notes:
1 Even fuel injection throttle bodies are like this. The only reason worn throttle bodies and the resulting throttle shaft wiggling hurt performance is because the throttle position sensor on the end of the shaft becomes unstable and "confuses" the computer (ECU).

2 Three different throttle shaft sizes and one choke shaft size. These cover all the Honda carburetors that I do most of my business in.

3 See my The 60/40 Rule article.

4 Seriously. Get the most important thing right first. If you own a 30-50 year old motorcycle, you really should also own a quality compression tester.

5 The need for ignition service is almost always blamed on other things, including the carburetor felt seals.


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