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Discussion Starter · #1 ·
I'm puzzled.

I've read several times on this board, and taken it as gospel, that the 175 carbs have a 20mm choke, whilst the 200 carbs 'only' have 18mm, and that this is one of the reasons for the 200s parity of horsepower with the higher revving 175.

I've just opened a package, containing a pair of CB200 carbs. To my eye, the carbs are identical. Both have '20' cast on the carb tops. The carb bodies appear identical, and my probing finger confirms my visual opinion, that the choke sizes are IDENTICAL. (Jets and needles obviously differ.) Any explanation for this ?

The one marked difference is the length of the inlet tract. CB200 manifolds are some 10mm shorter than the 175 manifolds and spacers. Which may account for the following observation.

I'd assumed that CB200 and CB175 air filters were identical, and I have fitted CB200 filters to my 175. However, I have noticed that the rubber intake trumpets are slightly kinked and compressed where they slip on the carb bellmouths. This is probably due to that extra 10mm in length on the 175 manifolds.

DSCF2719.JPG
 

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Its not 20mm choke its the ventui diameter( narrowest point inside the carb body)
Both carbs look identical including the 20 stamped on tge cap but the surface area ofthe ventui on 175 carbs is aprox 20mm diameter (its actually more oval than round) and the surface area of the 200 is 18mm diameter.
Because the full area is only exposed at WOT the advantage of the greater flow is only at WOT. Smaller venturi keep the flow speed up (better low end) untill they choke the top end
 

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Discussion Starter · #3 ·
Sorry Simo, but I've looked at these until I'm blue in the face, and I can't make out any difference.

DSCF2721.JPG
 

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Discussion Starter · #4 · (Edited)
Another try, some closeups, front and back. My index finger is an interference fit in both venturis.

c1.JPG c2.JPG

c3.JPG c4.JPG
 

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Discussion Starter · #5 ·
Just back from a bit of googling, and I might have a possible answer, courtesy of postings by Teazer.

If I have read him correctly, the venturis in the sloper carbs are 2mm larger than the ones in the upright engines. So late CB175 and CB200 both have 18mm venturis, in carbs with '20' on the top. Whereas the sloper Cb160 and CB/CL 175 have 20mm venturis.

And what that means for me is that I now have a spare set of carb bodies for my 175 engine, simply swap all the brassware from one to the other ?
 

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Do you use the same finger to messure the bore of the cylinders?
My understanding is both bikes use the same carb body, same float hight, just different venturi and jets. The shorter manifold might be an attempt to put more back into the top end (short manifolds add to the top but cost the bottom torque ) , but the overall lenght head to air filter is the same.
66 sprint has measured the venturi and the numbers were a little over 19mm on the 175 and a little over 17 on the 200.... Its buried in a thread somewhere here on HT
 

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Sensei
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Sloper 175's had 22mm equivalent venturi sizing..... (20 mm slides)
The 175 Verts have 20mm equivalent size venturis...(18 mm slides)
the CB200 has 18 mm equivalent venturi size......(also 18 mm slides)

Equivalent venturi size is calculated using the actual measurements of the OVAL venturi passage (slide removed), NOT by the actual diameter of the slide (as in smooth-bore carbs)

venturi calc 001.jpg

additive area will be equivalent to a circle of X diameter......X = equivalent venturi size
 

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Discussion Starter · #8 · (Edited)
I understand what you are saying, where the venturi is measured, I'm just stating the evidence of my own eyes (and index finger!).

What do you make of the photos that I've posted ? Surely a difference of 2mm would be fairly obvious ?

I don't have an internal micrometer to measure this accurately. The 20mm is the dimension 'w' in your drawing ? If so, I could make a 20mm wide piece of sheet material, see if it fitted, in a horizontal plane, through either carb ?


c3.JPG

I was going to suggest that maybe it was something to do with the UK market, but both my sets of 175 carbs are CL ones, and hence US imports. The CB200 carbs probably are off a UK bike.
 

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Im at work but im guessing W is closer to 16mm, since the venturi apear to be two overlapping circles H is likely to be different between 175 and 200 and even 0.5 mm difference in radis will affect the venturi area
All ive read suggest the below are correct
Cb/cl160 22mm
Cl175 ko sloper carbs are 22mm
Cd/ca/gl 175 22mm
All other cb/cl 175 are 20mm
Cb( cl?) 200 are 18mm
 

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Discussion Starter · #10 ·
Simo, 16mm at point 'W' was spot on, same on both 175 and 200 carbs.

I measured this using a strip of card, which I kept trimming until it was a sliding fit at point w in the 175 carbs. I then tried it in the 200 carbs, with the same result.

I can see that point w is the narrowest point, formed at the intersection of two machined holes through the carb body. The machined holes must have the same radius, otherwise the top to bottom measurement, 'h', would differ. So, I need to measure 'h' on both carbs.

And I get 22mm on both carbs.

Apologies if I seem to be banging on about this, but on the evidence in front of me it seems that CB200 owners might be disappointed if they bought a set of 175 carbs as a performance upgrade. I include myself in that statement.
 

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Sensei
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Since you guys are delving deeper into this,...Note corrections: Do NOT measure with throttle stop screw in place.....
You want micrometer accurate measurements as the SQUARED difference between 15.95mm and 16.05mm is a lot......
Also, the CB200 carb has "bulges" where the circles overlap (green in drawing)....These areas must be subtracted from the totals......

corrected venturi drawing 001.jpg
 

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Discussion Starter · #14 · (Edited)
My CL175 carbs also have those bulges. I've reviewed the video, and found this text.

carbcomp.JPG

And I still can't find a measurable difference between the carbs here. In those measurements stated above, I think that 21.99 and 21.69 are close enough to be the same measurement as my crudely measured 22mm.

The significant measurement is the 16.98 width for the CB175 carb, vs the 15.81mm for the CB200 carb. In my case, I'm getting 16mm for both carbs, which is close enough to the 15.81 measurement.

Is it possible that CL175 carbs differ from CB175 carbs in this dimension ? Jetting is certainly different.
 

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Id imagine checking the parts numbers would show the 175 bodies as being the same but to be honest ive never checked, but I cant see any other practical way of checking outside buying alot of carbs and jewelery wax
Are the air jets in the throat of the carbs 200 different from the 175? (as all the parts swap your 200 carbs might already be 175 bodies... PO, or mechanic might have aleady messed with them)
 

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In carburation, VERY small differences make very large changes.....You can't just "crudely measure". or say "close enough"......

Numerically, replacing a 100 jet with a 110 jet SEEMS to only be a 10% increase in sizing....calculated, it's a 21% increase in size.....
Replacing the 100 jet with a 130 jet is not a 30% increase, it's a 69% increase in opening area......
This is the difference an additional 0.30 mm (1.00mm to 1.30 mm) can make.....

Your "insignificant difference" between 21.99 and 21.69 (the same 0.30mm) has similar consequences.....Proportionally smaller (% wise) but still significant....

The carbs have different part numbers....... -338- vs -235-
 

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Discussion Starter · #17 ·
I'm pretty sure that these are original CB200 carbs.

Here's a photo of lettering on mounting flange, not present on 175 carb.

DSCF2729.JPG

Also, different air jets, CB200 on left

DSCF2727.JPG
 

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Discussion Starter · #18 ·
I'm sure that I couldn't measure a difference of 0.3mm with the tools shown in the video. I'd put that difference down to experimental error.

I was being disparaging towards myself when I said 'crudely measured'. My point was that if I take a piece of material, measuring xmm, and that has the same fit in both carbs, then both venturis measure the same. The crude measurement is the 'xmm', which I found to be 16mm.
 

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Youre right in that a piece of material xmm fiting correctly in two places would give x as the same messurement, after all thats how we use feeler gauges. But patricks method also seems sound (plasticining ) and the variences of 0.3 mm shouldnt be discounted, overbores go up in 0.25 mm increaments which is smaller than the measure youre suggesting is variance on a nominal size. All that aside I dont have any 200 carbs only 175's so I cant messure the difference myself.
 

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Discussion Starter · #20 ·
Thanks Simo and Sprint,

Looks as though we've pretty much exhausted this topic. Maybe I ought to look out for a UK CB175 carb body, see if that differs from the CL175 one.

Another thought, at the back of the FSM it quotes the CL175K7 power output as 16bhp at 9500rpm. Could that be a clue ?

And finally, if the difference IS as imperceptible as it apparently seems, would a few minutes with a Dremel sort it out ? Not that there's any way I'd actually do that.

Cheers,

Richard
 
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