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Discussion Starter · #21 · (Edited)
No vacuum ports back in 1965. Not in the manifolds or flanges.
The FUEL height is is roughly 3mm below the gasket surface and floats are where they are supposed to be. I could try lowering fuel height another 1mm to see if it helps clean it up.

I should have explained my apparent obsession with the pilot jets and bleed holes. Several versions of the Cb160, 175 and 200 used carbs that were very similar in size and design. They all used a #35 or 38 pilot jet, but depending on the application, they had different number and size of air bleed holes. I have sets with 4 pairs of 0.6mm holes and some with three pairs of 0.7mm bleeds and some with 3 pairs of 0.8mm bleeds. There are probably other varieties out there.

They are all the same design and are interchangeable but the 175s had larger bleeds than 160s and that leads me to speculate that the reason was the different way that each motor draws air at idle. 160s had larger carbs than 175s so one might expect that they needed smaller air bleeds to let the carb pick up enough fuel. 175 motors are a little larger and have smaller carbs which potentially could result in higher air speed and more fuel picked up, so they need larger air bleeds to trim the fuel curve.

At idle, one could also argue that cab bore size is irrelevant because the only air going through is under that tiny gap below the slide, but air flow will be more turbulent. What is the reality? I don't know, but that's why the different air bleed trials are next and if that doesn't clean it up enough, fuel level is next.
Being such a small motor, the slightest out of tune amount makes a significant difference. On a large motor or a race bike, idle is not so important. Off idle pick up is critical, but idle? what's that. On a tiny street bike it has to be clean at the bottom end.
 

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Is there anything keeping you from putting a more modern carb on there? I know its significantly more expensive than just figuring out whats wrong with the stock ones... but shiny new bits are shiny
 

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Discussion Starter · #23 ·
And that is a really good question.

I did get a chance to rebuild and modify the auto advance retard or Automatic Timing device to advance timing at idle and remain the same at higher revs and it is so warm here today that I can't tell much. There was a small amount of slack because the springs were stretched so I replaced it with a different unit with heavier springs and heavier weights which i tweaked to get a few extra degrees of static timing.

I need to wait for a cool morning to get more definitive results. if necessary i will also change the fuel level and or the pilot jets.

My son had a great idea. he suggested blocking off a pair of pilot jets to be sure that it stopped the bike form running. His thought was that at idle with a closed throttle if it still runs, then fuel is getting into the airstream from a different circuit.
 

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...
The FUEL height is is roughly 3mm below the gasket surface and floats are where they are supposed to be. I could try lowering fuel height another 1mm to see if it helps clean it up.
...
You could do that by simply raising or lowering one end of the bike by about 30 degrees while it's running at idle. If it stalls out easier with noes down or nose up, you can predict which way to adjust the floats.
... and I think that carburetor setup needs an intake stack, open carbs or cheap pod filters just stuck straight onto the end of the carburetor isn't going to cut it. Look at your stock air filters on that bike, they have a rubber velocity stack built right into the air filter.

Oversize carbs will make it more difficult to tune for lower speeds, that's why trials bikes have small carburetors and world land speed record engines & MX bikes run huge carbs.

You made the ignition advance sooner then the stock setup? That's not going to make it stall less coming off idle, just the opposite.
 

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Discussion Starter · #26 · (Edited)
Thanks for the thoughts. I may not have explained every detail, so let's back up a step or two.

Carbs are stock "PW20" which are larger than on a CB175 or CB200 but stock on this bike. Stock air filters do have a rubber connecting tube but don't have a smooth air inlet aka velocity stack IIRC. I already tried different intake length extenders. Interestingly Honda placed the 160 carbs on very long intake runners. On later bikes they made the intakes much shorter and added length on the filter side to move the carbs closer to the motor and further from the filters. One might speculate that the two changes, smaller carbs and closer to the head, were designed to alleviate exactly the issue I am dealing with. With largish carbs on a small motor, it is very difficult to get jetting right at idle. I used teh smae carbs on a full race motor years ago and they were fine, but that didn't need to idle as long as it picked up well. On our other two CB160 race bikes we use much larger carbs and jetting on one was easy but the other, with 26mm Mikunis took a few dyno sessions and pounds of brass to get it right. I also used CB750 "PW28" crabs on numerous Cb77 race bikes running methanol and gas (not on the same bike at the same time) and could always get then set up in well enough though low speed jetting on methanol always left room for improvement. I had to machoine cutaways and machine different needle profiles and have sets of jets drilled out. That was always a challenge to get them close but those bikes were faster than any of the 350 manx nortons of the day.

The ignition will advance the exactly same rate as stock. I previously used a CB175 (later model) A/R with lighter weights and lighter springs. Now it has a stock A/R but because the points were replaced with a modern DYNA pickup and rotor, the weights had to be trimmed slightly to clear the larger diameter rotor. And yes these motors are quite intolerant to ignition timing. We use fixed timing on our race bikes at anything from 42-50 plus degrees of total advance depending on piston height and combustion chamber shape. This motor has early style low crowns and will work with less total advance. So an extra 5 degrees at idle and stock 45 at full advance should be OK.

BTW it doesn't stall off idle, it picks up OK. It just idles badly and that gets worse when it's hot. Off idle upwards it's fine. Pick up is good. It revs out OK. It's just the wet soggy idle that I am trying to clean up without hurting initial throttle response.
 

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York, PA.

If you were in the South I was going to suggest running it to Hall Brothers. They'll straighten you out in minutes.

You could probably message Rob Hall on Facebook. He'd probably help you out.
 

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... Stock air filters do have a rubber connecting tube but don't have a smooth air inlet aka velocity stack IIRC. ...
Look again, the rubber stack extends into the body of the paper filter and flares slightly near the end when they build them like that, it's one of the reasons the OEM part was more expensive and became obsolete. On later bikes they no longer made the "sloper" and set the cylinders more vertical so everything about the intake shape and design changed. Intake manifold also has to be designed to prevent excess heat and vibration from affecting the carburetor operation.

Race bikes in the period were tuned to run primarily at wide open throttle and high speed the rest of the time but near useless on a street bike in traffic, ignition timing becomes critical. Just as you say, Honda went with smaller vacuum operated carbs when they went to the larger displacement engine. ... or you can sit in traffic blipping the engine like you are sitting on a starting grid. An extra 5 degrees at idle is not a small amount of change. If you want it to plug down in low revs and run steady then retard your ignition timing, that's far from a new concept. If you are going to try and redesign the engine ignition timing, a manual operated ignition advance would be superior to messing with bob weights and springs tensions.
 

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Discussion Starter · #30 · (Edited)
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.

Wheel Tire Fuel tank Automotive fuel system Plant


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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Discussion Starter · #32 ·
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.
 
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