Honda CB175 Motorcycle Tuneup Specsby John Willis
Honda's CB175 was a small-bore version of Honda's famed CB line of small street bikes, manufactured between 1968 to 1973. Like all CBs, the CB175 was extremely reliable and needed little maintenance, though smaller CB engines were a little finicky. Because the CB175 had only 175ccs of combustion chamber volume and just two small carburetors, keeping a CB175 engine in top form required -- and still requires -- a little more attention than a larger Honda would. Tuning is similar to that of all CBs, specifically the other CB and CB-derivative inline twins of the late '60s and early '70s. One of the best tuning tools you can have is a good aftermarket manual. CB-specific online forums can fill in information gaps in the manuals.
Tuning old motorcycles involves three main parts: carburetor tuning, ignition timing and valve timing. Carburetors, especially in smaller motorcycles, are the most likely place for you to return your bike's performance to nearly new. While you're there, check your whole throttle and throttle cable assembly. With the bike turned off, open the throttle fully, then let go, making sure it doesn't bind. If it does, lubricate the cable. Next, make sure your carburetors can breathe. That means they must have a clean air filter. If your bike has aftermarket pod filters, clean them thoroughly with carb cleaner, let them dry and replace them. Replace the air filter (unless it's a K&N; filter, which you must clean rather than replace). If your bike ran well before the tuneup and you haven't changed locations (to a higher altitude or different humidity,) you likely don't need to change jets. The air screw should be turned 1-1/4 inch out, and the carb float level should be at 21mm. If you go all the way through your tuneup steps and the engine's still not running well, inspect the slides to make sure they are mechanically synchronized. If they are, consider a carburetor rebuild kit. You can order them through a motorcycle dealer.
The next high-return tuneup step is setting your timing. Unlike modern motorcycles, the CB175 did not use electronic ignition; it had points. You'll need a timing light. Set the point assembly to the stock advance. While you're dealing with the ignition, take a look at the spark plugs. You can always clean them up, but if they're especially dirty or if there's any pitting or corrosion, replace them. The gap should be .024 inch to .028 inch.
A twin overhead cam engine, the CB175 have two valves per cylinder. Over time, especially with high-revving engines, parts will wear in your cam assembly. With a feeler gauge, check your valve clearances: .002 inch on intake and exhaust.
You've tune the three main things that are going to return good performance. Now make sure everything else is working well. Inspect your tires for tread wear, cracked sidewalls and proper inflation. Inspect your brake drum pads and replace them if they're worn. Inspect your front fork seals. You may need to replace them if there is leakage. If they feel like they're functioning well, you might just clean them thoroughly and reinspect after a little use. Look at your chain and sprockets. Check for obstructions, lubrication and wear. Now, check your fluids: motor oil and brake fluid. If the bike has been sitting for a long time (months) and you've done all of the above tasks but your CB175 still won't start easily and run well, replace the old fuel with fresh fuel.
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