Sloper 175 (one year only) had the same low ports as a 160 (CB96) and 125 (CB93) but a slightly deeper dome combustion chamber. When Honda introduced the upright/vertical 175 motor they made a lot of changes. The most important was that they raised the inlet ports to get a larger short turn radius and that improved gas flow. They then tilted the inlet manifold face upwards to get a similar downdraft look to the intakes that was generally accepted as a performance design. Why they took that opportunity to move the carbs forward is anyone's guess, but a more vertical cylinder is taller than a sloper so they probably did it to avoid the carbs hitting the fuel tank. Or they did it because rotating the cylinder up caused the carbs to be too far back, so they moved them. All that series of motors had relatively long inlets.

When we built our first CB160 race bike I have a couple of CB160 and 175 heads that I has sawn though the ports to better see what was going on. We also had a few different heads on the flow bench. We were able to get enough flow out of a 160 head to support our HP targets but it's easier with a 175 or 200 head. Others took rubber molds of the intake ports to compare them which saves heads but doesn't reveal where there is insufficient metal until you break through. We had a bunch of spare motors so it made sense to cut up some that were in bad condition.

On our race bikes we pay a lot of attention to part throttle running to get them to pull cleanly out of corners. With a wide ratio 4 or 5 speed transmission they need to pull well at all throttle openings. For sure wide open down a straightaway was most important but getting around and out of corners was also important. my first vintage/classic race wasn't until 1978 so i can't talk to much earlier times.

The FSM for a 125/160 twin specs out the ignition curve as having a 6 degree range, so we are still within stock spec but at the more advanced end of the range. There low compression motors are really not very sensitive to ignition timing but this particular motor doesn't want to run cleanly at idle so I am trying anything and everything.

The different weights and springs we on a different A/R. I had a similar CB175 unti on there and it had different springs and weights to the stock CB160 unit marked CB93. What I had to alter were the "stops" where they rest on the rotor because I changed to a Dyna rotor wich is thicker there, so I ground them back to allow almost the stock static timing leaving enough metal to grind more if it need to retard it slightly at idle.

No vacuum operated carbs on early Hondas until the Black Bomber and none ever fitted to the 125-160-175-200 line. Maybe they did in the eighties or later on a single carb CD175 or TwinStar - I didn't bother to check.

 

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That is a sweet looking ride, are those dry foam air filters or oiled foam air filters?

 

Thanks. They are UNI foam (over a spring). I also tried a pair of K&N oiled conical filters and they are no better.

I was given the bike, so i felt sort of obligated to do something with it. And everything had to be rebuilt or machined or replaced. The PO had already ground off the side cover mounts and battery box mounts, so going back to stock filters isn't really an option. Motors are simple. We had plenty of spare parts and new parts to build one. Rear fender took forever to shorten and leaving the rolled edge so it looks stock but is 4 inches shorter. Forks i think were a spare set of CL175 in better condition that the ones on the bike and I like the polished look. All teh eelctrics were updated with solid state R/R from Matt at SparckMoto.com LED lights all round. Added LED indicators and a front brake switch (not standard in 65). Alloy rims of course and all new bearings, seals and custom rearsets based off a pair of Dunstall levers that had new "arms" shaped and welded in then chromed.

AGM battery sits in a shallow trough under the seat hump.

Somewhere here I have a pair of "Shemozzles" from a Rocket 3 project but only two out of three, and it might be possible to modify them to fit these carbs and then slide filters over the outside. Or perhaps a pair of Polini velocity stacks might work.

I tired clipons and ace (clubman's) bars but the bike is so small, I went back to bars with a slight rise which are way more comfortable.