Motorcycle Repair: 1972 Honda CB175

Question
Bill,

   I am rebuilding the two carbs on a 72 honda cb175 what are the factory settings for the adjustment screws and the jets? Does it matter how far the jets are turned in or do you just seat them to the bottom? Thanks so much.

Answer
Jon, Let's be clear on what is what and what goes where....  All the jets are located inside the float bowl. These include the small idle jets and the main jets/holder. On the outside, you have idle speed screws, located in the middle of the carb body and the idle mixture screws which are more forward. The idle speed screws just raise the carburetor slides when you turn the screws inwards. The forward idle mixture screws are turned out 1 to 1.5 turns, depending upon which series of carburetors you have on the bike. Look for a series of letters, like: CBC, CLC, 306A, 306B, etc, and should be marked on the forward edges of the carburetor intake extension flanges. The float level is set at 21mm which is where the float tang just nudges the end of the float valve needle, which has a spring-loaded tip and is not to be compressed during measuring.

Start with the mixture screws turned out about 1.25 turns and each of the speed screws just touching the slides, then turned in about 1/8 turn. BE VERY CAREFUL to note the positions of the throttle slides, as they are left and right side specific. When installed, the carburetor slide cutaways at the bottom edges of the slides are to be pointed outwards, towards the inlet/air filters. If you put them in backwards, the engine won't idle and will foul spark plugs very quickly.

The "jets" are just screwed in gently, but firmly so they don't fall out during operation. They are not "adjustable" to be turned in or outwards.

http://www.cmsnl.com/honda-cb175-super-sport-175-k6-1972-usa_model422/partslist/ for description of the parts installed.

In order for the carburetor to function properly, you need to have two good cylinders with proper compression readings AND the ignition timing must be adjusted correctly, so the points just open at the F mark alignment on the rotor. Ensure that the spark advancer is able to advance and retard easily.

Bill Silver