GL1000 carb thread issues

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GL1000 carb thread issues

Post #1 by mikenixon » Fri Jul 05, 2019 5:41 am

Let's look at it a little more closely. It's important enough.

They may seem similar but the GL1000 and GL1100 carburetors, nearly the same size and bulk, are very different in construction. They're made of very different materials. In fact, a completely stripped, bare GL1000 carburetor casting (single carb body) is so different from a GL1100 casting in metallurgy that it weighs exactly *twice* as much. Yup, twice. The aluminum alloys make the difference. The GL1000 casting, designed in the 1960s -- when it first appeared on Honda's two-cylinder cars -- is rich in zinc. Casting technology in those days was such that many carburetor manufacturers used a freer-pouring zinc-based alloy to compensate for deficiencies in production technology. You can see this ethic in all the carburetors made in the 50s and 60s and it continued into the late 1970s in Keihin carbs.

So what? So what is these carbs, rich in heavy, soft zinc, are extremely vulnerable to thread damage. As are also the similarly-constructed early Honda CB fours. Same material. The metal is so soft the tiny 4mm float bowl threads are at considerable risk of stripping.

But another factor enters in as well. Being a late 50s/early 60s limited production automotive design to begin with, the GL1000 carburetor has one great flaw. Unlike the GL1100 whose float bowl tightens down metal to metal, making it almost impossible to over-tighten, the GL1000's float bowl never completely tightens down. It hovers over a soft rubber sheet gasket, never touching metal to metal. This is a surprisingly archaic design. There is never any tightening, any resistance, especially when using aftermarket gaskets. At least the stock gaskets have stiffening material embedded in the rubber sheet that offers a token squish resistance. The aftermarket gaskets do not and just make things that much worse.

These two things together: buttery soft metal construction and float bowl screws that never ultimately tighten down, add up to disaster. The situation can't be helped. Failure is designed-in. GL1000 float bowl threads will always be a problem, always want to pull out. Everyone who knows these carbs intimately knows this and has had to deal with it.

I handle this three ways. First, I use only the stock style cloth-embedded gaskets. Second, during final assembly I use a special torque-measuring screwdriver that permits only a certain amount of force on the screws that I know from experience is just the right amount. Third, as part of the rebuild I inspect all sixteen float bowl threads. I typically repair half of them on each project using permanent steel thread inserts.

Treat 'em right, gang. Watch those screws. No wood screw "repairs", please! And no oversized screw, drill-out-the-float-bowl-to-fit jobs either! Whew! And plan on, put on your calendar, to one day getting after those float bowl mounting threads and doing what it takes to repair them on these eminently vintage carburetors. That is, if you really care about your machine. These bikes are becoming part of the industry's history, examples of an ethic and a pathos and a kind of time stamp in Honda's technological timeline that is, well, history. And anything remotely like it is rapidly disappearing. Hang onto it. Love it. Preserve it.

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Re: GL1000 carb thread issues

Post #2 by rcmatt007 » Thu Jul 11, 2019 1:33 pm

as always, a great learning experience

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