Carburetor Information
U.S. CB750 Models 1969-1978
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Carb Buying Tips
With as many "surprises" as I came across it's obvious there are things to watch out for when buying a rack of carbs for a project. These carbs are better than 30 years old, and most at one time in their lives have been in the possession of Billy Joe Bob Bighammer. This guy has no concept of "that's just wrong", and his fixes and adaptations prove it. Here's a few tips to make sure you're not hunting down a parts rack to fix the "just needs cleaning" rack in chunks on your bench.

Corrosion: If you're looking at a set on eBay, Craigs List, or other online source as the seller for pics of the inside of the bowls. I can't count how many racks looked decent from the outside only to find they had an entire tank of gas rot in them leaving little behind in the way of aluminum casting. Beside the obvious missing "appendages" such as float towers or emulsification bosses, corrosion can also deteriorate threads. Even mild deterioration can allow fuel to weep past the threads around the idle jet and emulsfication tube, and alter the fuel mixture from one carb to another.

Missing or mis-matched parts: Floats should all be the same and match the casting numbers.Early carbs were designed for brass floats, and later the composite foam. Although you CAN use either, some deviation from the standard 26mm float level might be necessary to keep the transitions from one circuit to another smooth. I've come across a number of racks that were missing the emulsification tubes.

Damage: Besides the obvious, stripped threads, cracks in the emulsification bosses, idle jet bore, or float towers are all going pretty much rain on the parade. Cracked, broken, or incomplete choke butterflys is another common one. Sliders heavily worn from open V-stacks or no air cleaner at all, damage at the base of the sliders where they meet the carb casting means having to hunt down more or putting up with poor idle and low end performance. If the sliders show "sanding" from dirt chances are the slider bore in the casting will also be worn and the slider fit sloppily affecting low end and possibly hanging or lagging when throttling down.

Claims to ignore completely: "They came off a running bike". Yeah, okay, like WHEN? 6 years ago before the bike was pushed off in a field to rot? Before the fire? Before it was used to prop up the barbecue pit or before the chickens started roosting on it?

"The bike ran good before I took them off". The term "good" is subjective. If it hits on at least two cylinders good old Billy Joe Bob Bighammer gets wood and thinks "Dayum, this har motorsickle souns good as hoot owl fukkin a bobkat".

"I don't know anything about them, I got them in a batch of parts". Translated: These things are junk I wouldn't use on my ex-wifes bike, or, I looked inside 'em and they're trashed so I closed 'em up and dipped em in freshly changed oil so you wouldn't know I saw they were screwed.

Yeah, there are probably some honest claims along those lines, but when you're throwing in the neighborhood of $50 or more of your hard earned money at a rack it's better to question than to blindly accept the claims of someone you have probably never and may never meet. You also have to assume they are likely going to need a rebuild so the cost of kits comes into consideration.


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