CB200 Float Bowl Height (Not stock carbs) - Help!

3046 Views 7 Replies 4 Participants Last post by TheFreeRenegade, Aug 12, 2019

IainF

Discussion Starter · Mar 23, 2016 (Edited)

Hey guys,

First some background. I bought my 1974 CB200 in August 2014 with 35k kms and no spark. I bought the bike to learn more about motorcycle mechanics and do some wrenching. It's been a journey, but even after many frustrations it's still hella fun to work on. The intention is to do everything myself, OR have someone teach me specific things. This bike is supposed to be primarily a learning tool (which it undoubtably has been). With the exception of this bike and a little bit of basic work on my sport bike, I'm a noob.

The no-spark problem ended up being a simple wiring problem. The existing carbs didn't have any chokes on them (sealed off with rubber and washers), and they were quite gunky, so with the help of my uncle (motorsport mechanic for 20yrs) and his access to suppliers, we found some carbs that fit the mounting measurements and looked identical. They ended up being 16mm instead of the stock 18mm. Mechanics I've talked to told me the little bit of a difference in venturi size shouldn't matter too much, so I've been running with them. The other thing is that they're chinese knock-off keihins. Like I said, they looked identical, but they ended up being a bit taller in dimension. This lead to a throttle cable length problem (which I've resolved), but also some measurement specs are off, i.e. the float bowl height.

So the problem I'm coming to you guys with now is this. While trying to get a good fuel level in the float bowl, it seemed as though the tiniest adjustment (an adjustment I couldn't even see) would result in the float bowl being either completely empty, or quite full. I'm not able to get a float height that results in a fuel level which looks correct to me. I even got out the digital caliber and measured from the arm to the top of the tab on the float, for each trial. I found that at 2.35mm, after 3 minutes with the gas on the bowl was EMPTY, and at 2.34mm the bowl was FULL. Now, I'm not confident whatsoever that my measurements are actually accurate to 0.01mm, but regardless even 0.1mm seems so minuscule that it shouldn't be the difference between full and empty. My left cylinder often doesn't run, and the plug is wet, so I'm assuming its a result of the float bowl being to full and flooding the cylinder. Plus the plugs are very dark so I'm assuming I'm running very rich. I've attached some pictures of my float bowl and carbs. This was the level of the float bowl right after I started the bike and it was running on the right cylinder only. The height from the carb body to the bottom of the float, right when the needle started to seat, was 25mm.

Firstly, do you agree or disagree that this float bowl is too full?
Do you guys have an tricks/tips/suggestions?

I'm at a loss with how to control the level. I've probably adjusted those floats upwards of 50 times over the last year, and have never gotten to a level I liked. I've been aiming for a level basically right at the bottom of the lowest (idle?) jet

Thanks so much guys for helping me out; both in advance for this and in the past with all the other threads on here.
All comments are welcome!

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IainF

Discussion Starter · #2 · Mar 23, 2016

Some more information:

When I first start up the bike, it only runs on the Right cylinder. Then when the engine is warm enough to lean off the choke, I open the throttle a bit, and the left cylinder starts running, and stays running. This process has happened at least the last couple of cold starts (from yesterday and today).
The plug tips are still very black, as shown below. Both look the same.

How long does the engine have to run at a specific condition to change the colour of the plugs? (i.e. if the plugs are black now, then I lean out the mix, how long before the plugs turn a lighter colour?) 10s? 30s? 5min?

Thanks guys.

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66Sprint

· #3 · Mar 23, 2016 (Edited)

I realize it's long distance, but a phone call would expedite this...... ************....Steve

I'll need to know the following.....
What size are the pilot and main jets? ......Too rich may be improper jet sizing....(Plus, it appears you are running resistor plugs)
What diameter is the carb throat AT the juncture to the manifold? (Are the ports matched in alignment and size?...See pic below, will discuss on phone....)
Fuel level in the bowl is set so that at any lean angle encountered in "normal" riding, BOTH jets' tips remain submerged in fuel.....

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IainF

Discussion Starter · #4 · Mar 23, 2016

Thanks Steve,

I'll take the measurements and more pictures, and plan to give you a call on the weekend when I have a bit more dedicated time.

Iain.

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Simo

· #5 · Mar 24, 2016

Talk to Steve,
According to my math 16mm diameter venturi is an area around 20% smaller than the stock 18mm and 35% smaller than the cb175 carbs

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IainF

Discussion Starter · #6 · Mar 24, 2016

So your math is using venturi area approximately equal to a circle I'm assuming? Yeah, I am a bit worried about that. The description for the carbs claimed up to 100cc, so I was putting some (read: a lot, probably) faith in that information (100cc per side/carb).

I'm still really hoping I can make these work as sourcing new ones or parts for my stock ones would be a pain/pricey, plus I liked the idea of having new, clean carbs. And I already paid for these (though they were fairly inexpensive). But alas, newer is not always better. I'll see what Steve has to say.

Just out of curiosity, why are CB175 carbs bigger for a smaller engine?

Thanks,
Iain.

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66Sprint

· #7 · Mar 24, 2016 (Edited)

The 175 has more HP......The 200 is more "tractable" and has "gentler/smoother power delivery" so is a better "beginner bike"....
(Same mistake they made with the 500T to replace the "hotter" 450)

The factor is R squared x Pi.... The following is based on the "equivalent venturi size" as the openings are actually oval, not round...
Yours With D=16, R=8......=64 Pi
CB200 With D=18, R=9......=81 Pi
CB175 With D=20, R=10... =100 PI
.....64/81=.790 or 21% smaller portal...=26.58% increase in air velocity
As air velocity increases, so does the 'pull" on the liquid fuel through the jets.....
Since the idle jetting is adjustable to some extent, the stock 38 should still work,(although I'd try a #35) but as the slide opens,due to the increased velocity, the main jet "feeds" like a larger jet due to the increased "pull" on it....Just from the raw numbers, you are going to want to reduce that by ~ the 20% in size as well, and tweek it from there.....

This is why the CB200 (18mm carb and 81 Pi) runs an 88 main while the CB175 (20mm carb and 100 Pi) runs a 98 main....Yet both have 38 pilots....
Calculating main jet open areas gives .1936 Pi sq MM for the 88, and the 98 has .2401 Pi sq mm......
( 1936/2401= 80.63%, or approximately the difference in throat area proportions as 80.63% is close to the 81% throat area difference)
Based on rough calculations, you will need a 38 pilot and a 70 or 72.5 main, (I'd get 35 pilots as well, and 70, 72.5, and 75 jets, a box of plugs, and work from richer to leaner).....

Are these the carbs you purchased? http://wincycles.com/Carburetor/c7/p1657/Honda-CB175-Carburetor-1969-1973/product_info.html
I have been told they are bolt-on and run..... Have not tried myself, but considering a set......
A pair of carbs for only about $20 more than you will be spending on jets and plugs to maybe get yours right......Something to consider......

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TheFreeRenegade

· #8 · Aug 12, 2019

66Sprint said:
The 175 has more HP......The 200 is more "tractable" and has "gentler/smoother power delivery" so is a better "beginner bike"....
(Same mistake they made with the 500T to replace the "hotter" 450)

The factor is R squared x Pi.... The following is based on the "equivalent venturi size" as the openings are actually oval, not round...
Yours With D=16, R=8......=64 Pi
CB200 With D=18, R=9......=81 Pi
CB175 With D=20, R=10... =100 PI
.....64/81=.790 or 21% smaller portal...=26.58% increase in air velocity
As air velocity increases, so does the 'pull" on the liquid fuel through the jets.....
Since the idle jetting is adjustable to some extent, the stock 38 should still work,(although I'd try a #35) but as the slide opens,due to the increased velocity, the main jet "feeds" like a larger jet due to the increased "pull" on it....Just from the raw numbers, you are going to want to reduce that by ~ the 20% in size as well, and tweek it from there.....

This is why the CB200 (18mm carb and 81 Pi) runs an 88 main while the CB175 (20mm carb and 100 Pi) runs a 98 main....Yet both have 38 pilots....
Calculating main jet open areas gives .1936 Pi sq MM for the 88, and the 98 has .2401 Pi sq mm......
( 1936/2401= 80.63%, or approximately the difference in throat area proportions as 80.63% is close to the 81% throat area difference)
Based on rough calculations, you will need a 38 pilot and a 70 or 72.5 main, (I'd get 35 pilots as well, and 70, 72.5, and 75 jets, a box of plugs, and work from richer to leaner).....

Are these the carbs you purchased? 1969-1973 Honda CB175 Carburetor Carb
I have been told they are bolt-on and run..... Have not tried myself, but considering a set......
A pair of carbs for only about $20 more than you will be spending on jets and plugs to maybe get yours right......Something to consider......

Hi Steve,
Did you ever end up purchasing a set of the carbs by wincycles.com? While performing my 1st carb clean, I noticed some pretty severe marring at the narrow end of the venturi on my original carbs. I'm not sure if this is having a negative effect on my performance, but would have peace of mind having a brand new set. I just wasn't sure if they would be a quality product, and if they would present the same problems as experienced by Iain.
Thanks,
Hayden

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