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Author Topic: Should I Get Larger Carburetors? - Honda CL200  (Read 587 times)

Offline Classic5

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Should I Get Larger Carburetors? - Honda CL200
« on: Jan 17, 2019, 15:11:38 »
Hey All,

Sorry if this doesn't really belong in the Engines section, but here goes all the same.

I'm continuing my rebuild of a 74' CL200. I currently have stock carbs on it (which I believe are 18mm?), but after doing some reading I've seen some people talk about how the stock CL200 carbs are fine but are really just a little smaller than they might be for peak top end performance. With that in mind, I also had the cylinders bored .5 over. These together make me think that my specific set up could benefit from a non stock set-up.

Do you think that I should pursue a larger carb or should I just jet up?

Thanks!
« Last Edit: Jan 17, 2019, 15:28:44 by Classic5 »

Offline jpmobius

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Re: Should I Get Larger Carburetors? - Honda CL200
« Reply #1 on: Jan 17, 2019, 17:57:47 »
As a generality, most engines will benefit slightly at max output with a slightly larger carb.  The penalty is also general, sacrificing partial throttle performance.  99% of the time a larger carb is a bad idea on a stock engine, which is what you have.  If you significantly increase the displacement, the max usable rpm, the peak power rpm, or the max power output, it may be time to consider a larger carb.  Whether this is a good idea or not can be very engine specific.  In my own experience, motorcycle engines are rarely supplied with carbs that are the limiting factor, and a lot of extra power can be had before the carburetion becomes the bottleneck.  Additionally, tuning (jetting) a carb swap is very often a LOT of trouble even for the very experienced.  For most people, having a bike that drives well is vastly more desirable than one that may go a tiny bit faster but drives like crap otherwise.  Plus, I have seen/ridden quite a few modified bikes that weren't actually faster but seemed so because all the low end was ruined but with the top end still ok (like changing to bigger carbs!) the new contrast creates the illusion of increased power.  The exception to this being a bad idea is where you have an old engine that has a difficult to get original carb, or original parts for it and there is wear or damage that creates the need for a replacement.  Lastly, there are usually control cable and fitment issues.  Most bikes have carbs that are very similar to generic replacements, but that exact fit with no conflicts with the tank, petcock, frame, cables, wiring harness, choke mechanism or throttle mechanism etc. is rare.  Of course, all these things can be overcome, but it can end up being a great deal of work for no discernable gain in performance.

As far as "jetting up", technically there is no such thing.  You may need to re-jet to adjust for changes to the engine, but unless you make significant changes, likely your jetting was ok from the factory.  The slight overbore likely will have insignificant impact on how your carbs are set up - you'll just have to see once you get to riding it.
« Last Edit: Jan 17, 2019, 18:07:43 by jpmobius »
Mobius


On a long enough timeline, the survival rate for everyone drops to zero.

1973 RD350 Yamaha build  http://www.dotheton.com/forum/index.php?topic=66498.0

Offline Nebr_Rex

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Re: Should I Get Larger Carburetors? - Honda CL200
« Reply #2 on: Jan 17, 2019, 20:10:22 »
As a generality, most engines will benefit slightly at max output with a slightly larger carb.  The penalty is also general, sacrificing partial throttle performance.  99% of the time a larger carb is a bad idea on a stock engine, which is what you have.  If you significantly increase the displacement, the max usable rpm, the peak power rpm, or the max power output, it may be time to consider a larger carb.  Whether this is a good idea or not can be very engine specific.  In my own experience, motorcycle engines are rarely supplied with carbs that are the limiting factor, and a lot of extra power can be had before the carburetion becomes the bottleneck.  Additionally, tuning (jetting) a carb swap is very often a LOT of trouble even for the very experienced.  For most people, having a bike that drives well is vastly more desirable than one that may go a tiny bit faster but drives like crap otherwise.  Plus, I have seen/ridden quite a few modified bikes that weren't actually faster but seemed so because all the low end was ruined but with the top end still ok (like changing to bigger carbs!) the new contrast creates the illusion of increased power.  The exception to this being a bad idea is where you have an old engine that has a difficult to get original carb, or original parts for it and there is wear or damage that creates the need for a replacement.  Lastly, there are usually control cable and fitment issues.  Most bikes have carbs that are very similar to generic replacements, but that exact fit with no conflicts with the tank, petcock, frame, cables, wiring harness, choke mechanism or throttle mechanism etc. is rare.  Of course, all these things can be overcome, but it can end up being a great deal of work for no discernable gain in performance.

As far as "jetting up", technically there is no such thing.  You may need to re-jet to adjust for changes to the engine, but unless you make significant changes, likely your jetting was ok from the factory.  The slight overbore likely will have insignificant impact on how your carbs are set up - you'll just have to see once you get to riding it.


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Offline Classic5

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Re: Should I Get Larger Carburetors? - Honda CL200
« Reply #3 on: Jan 21, 2019, 11:19:06 »
Hey jpmobius,

Thanks for the great insight! I really appreciate the information and the peace of mind that knowing that I probably currently have the right tools to get the most out of my current set up.

Just for my own knowledge, can you expand a little on what you meant when you said, "extra power can be had before the carburetion becomes the bottleneck."? I'm always looking to learn more.

Thanks!

Offline teazer

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Re: Should I Get Larger Carburetors? - Honda CL200
« Reply #4 on: Jan 21, 2019, 14:31:17 »
What he means I think is that on many engines the bottleneck is porting or cam design or combustion chamber shape or whatever that limit power more than the carb size.

Another thing to think about is the overall shape of the inlet port from the intake through to the valve.  For best performance all sections of that inlet tract have to be the correct size relative to each other.  And to add to that complication, bigger carbs tend to help top end and hurt low end because the gas velocity is too low and fuel atomization is poor and cylinder filling is also poor.

Honda fitted larger carbs to the earlier CB160 and as the capacity increased to 17cc and then to 200cc, they fitted smaller carbs to make it easier to ride. Early Honda carbs had an oval shaped bore ( more like a figure 8 but who's counting).  The bores were tall and thin so that at small throttle openings it was like having a small carb and with the carb wide open it was like a larger bore carb. That's how CB77 carbs are that we fitted to a CB160 race bike.  We once ran a apir of 160 racers side by side and the 200cc version with stock 160 carbs was just easier to ride at lower speeds and coming out of corners and the 180cc with big carbs would walk away once they were wide open at higher revs. Same cam and porting mods on both. 

So if you are riding hard all the time, go with bigger carbs.  If you are going for coffee or commuting stay with stock or CB160 carbs.

We run 26mm Mikunis on a CB200 head on our 250 race bike and CB77 (26mm) on the 180cc motor.  Hint:26mm Mikunis are larger than 26mm Keihins because they are measured differently.

Offline jpmobius

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Re: Should I Get Larger Carburetors? - Honda CL200
« Reply #5 on: Jan 21, 2019, 14:38:46 »
Sure.  The amount of power you get from any engine is a function of how much oxygen you can get into the combustion chamber.  It is the limiting factor because you are restricted by the available oxygen in the atmosphere (unless you artificially add oxygen).  Generally, it is a straightforward task to supply the corresponding amount of fuel, so considering how to get more "air" in somewhat simplifies how to do it.  There are several ways, and the principles usually overlap.  The simplest to understand (not necessarily the easiest to physically implement) is to make the mechanical pumping action better.  In a 4 cycle engine this means things like improving the flow of gasses in the intake and exhaust, fooling around with the cam to hold the valves open longer, or making the piston larger to make a bigger "pump".  This concept will only take you so far, because the pumping action of a piston engine is not continuous, but in fact massively intermittent.  On the human scale, it seems like air is continuously sucked into the engine, but this is only so because the intake "pulses" occur so rapidly.  This intermittentcy is another potential source of adding air.  As valves open and close, they very abruptly start and stop very high velocity gas (air/fuel mixture on the intake side, exhaust gases on the exhaust), and when they do, a shock wave is created that travels along the pipe in which the gases are traveling.  This can have a very positive or negative effect, either pulling gasses along or pushing back against the flow.  These shock waves travel at the speed of sound, and are unaffected by the speed of the gases, but very much affected by their density and temperature.  When a wave reaches and exits the end of a pipe, an inverse wave is created and travels through the gases back down the same pipe.  These waves pass through each other, also without ill effect, and if you can get one to arrive at just the right time, say a positive wave arriving just as the intake valve closes, this wave will "push" intake mixture through the closing valve.  In a high performance application, you probably designed your cam shaft to hold the intake valve open longer than mechanically needed, so that inhaled intake mixture would actually be pushed out the valve into the intake tract as the piston ascends on its compression stroke.  If you get that shock wave to arrive at just the right time, it can be counted on to stuff that escaping intake charge back into the combustion chamber along with some extra it is carrying along, effectively supercharging the pressure of the combustion chamber (at the bottom of the stroke) to above atmospheric.  In other words, adding more oxygen than would be possible with just a simple mechanical pump.  This effect works in the exhaust as well, where you would like negative waves arriving when the exhaust valve opens to suck the spent charge out of the chamber, and a positive to arrive before the valve closes to stuff any new intake charge rushing past the valve back into the combustion chamber - sort of a reverse supercharging.  This is principally why a good set of "headers" adds power, simply reducing the exhausts restriction does not help much unless the OEM exhaust is really bad.  Otherwise, simply removing the exhaust altogether would work.  Another thing to consider is air temperature and density (and humidity).  Cool, dry air contains more oxygen, so sourcing the engine air from a cool location is a good idea.
Obviously a lot of this is pretty complicated to both figure out and actually implement, which is why re-engineering an engine for more power and also getting it to run well under all conditions is challenging.  Extremely challenging with new engines, as very smart people have likely found a better balance of functionality vs. max output that most of us can.  With an old engine, you still have a pretty good shot though.  Start with the easy and basic stuff first, and do one thing at a time so you can back track if things don't work out.  Typically, one would start with the intake and exhaust.  This is practical on an old engine, as sound volume is very important on a stock bike, and a compromise not allowable when designed.  If you are ok with a lot more noise, often there is power to be had in exchange.  After that most folks start looking at redesigning the camshaft(s), intake and exhaust port profiles and compression ratios and head design.  If you can, it is a LOT easier to simply make the engine bigger by increasing the displacement.  You suck in more air with more oxygen, and generally keep all the complicated stuff the same.  At some point, you can get all these modifications to work well, and the amount of air the engine can suck in will exceed what will flow through the carburetor easily.  Make the carb bigger, and the problem goes away - but there are a few things to remember.  The first is that carburetors need very high air speed through them to atomize fuel sufficiently.  This is easy in a small carb (for the engine) as the big air volume through the small carb bore results in high air speed.  As the carb gets bigger, the air speed goes down and the atomization becomes poor.  This is important for two reasons.  Actually one, but it is at least two parts.  Your engine can NOT use liquid fuel.  The fuel must be changed into a gas before it will chemically react with the oxygen.  So you need the tiniest drops possible, or they can not be used.  Because there is a crazy short time span in which the drops can evaporate, the drop size is super important.  You need really good atomization all the time which generally means a smaller bore than would be optimal for max power.  So you get the tiny drops, it can all evaporate, and combine with all the available oxygen.  This evaporation is a key element in the process, not just because it is required to burn, but because it is critical for cooling (the part 2!).  When a liquid turns into a gas, it changes phase.  Making this phase change requires substantial energy which is provided as heat from the engine.  Without this physics phenomenon, our engines would not be possible - no amount of external water cooling would be enough.  In fact, this is the reason that it is virtually impossible to run engines as lean as the chemical reaction ratio would suggest:  The extra fuel is needed for cooling, and the better atomization, the more phase change, and the less fuel wasted.
« Last Edit: Jan 21, 2019, 14:40:39 by jpmobius »
Mobius


On a long enough timeline, the survival rate for everyone drops to zero.

1973 RD350 Yamaha build  http://www.dotheton.com/forum/index.php?topic=66498.0

Offline MiniatureNinja

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Re: Should I Get Larger Carburetors? - Honda CL200
« Reply #6 on: Jan 21, 2019, 14:51:39 »
something people need to realize: a larger bore carburetor DOES NOT equal more air into the engine.

it can, when you're talking stroked engines that suck a lot of air. but what is important on our smaller, higher revving engines; is velocity
moving the air through at a faster pace does equal more air. you can do that with a smaller bore, less restrictive intake system. start by match porting the head, cleaning up casting, retaining stock length of air intake and use laminar stacks.

no need for a larger carb on such a small engine
'75 Honda CB360 - thread

Offline teazer

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Re: Should I Get Larger Carburetors? - Honda CL200
« Reply #7 on: Jan 21, 2019, 16:44:43 »
Just to add to all of that data overload, there something else.  JPMobius mentioned that it's gaseous fuel; that burns and he's right.  In fact it's the gas on the surface of each bubble of liquid fuel that burns. Better atomization changes a few large bubbles into lots of smaller bubbles and by some trick of math, more small bubbles have greater surface area than the same volume of large spheres, so more fuel burns.

We had that problem on our CB200 head with two stroke Mikuni carbs.  Air to fuel ratio looked fine, but we had way too much unburned fuel in the exhaust.  Changing to 4 stroke type emulsion tubes (needle jets) made a huge difference to cleaning up the exhaust and worked better. Small carbs do create higher velocity air flow and better atomization at all engine speeds and throttle positions than big carbs.

The trick is to match all of that for your motor the way you ride it.  As it happens, a CB200 will work better with slightly larger carbs, but to get the best out of it, you need a different camshaft, higher compression and a few other changes.  Check out what Texstar did with their CB200's for more ideas - just to keep the brain working.

Way back in the seventies we had to create tables of mean gas velocity through the carb the hard way - no PCs back then.  Now I can enter data into a spreadsheet and immediately determine an appropriate size carb.  And there is software around to analyze flow and pulse patterns and to simulate performance.  Progress is amazing.
« Last Edit: Jan 21, 2019, 16:49:05 by teazer »

Offline Classic5

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Re: Should I Get Larger Carburetors? - Honda CL200
« Reply #8 on: Jan 21, 2019, 18:06:09 »
Hey All,

Thank you for the amazing information! Way more than I would have ever hoped for! The idea about tuning the lengths of the intake and exhaust to maximize the pressure waves is fascinating. If I were to remove the silencer from the end of the stock exhaust to open up the airway, would I be able to use a short length of pipe slid into each exhaust to slowly change the overall length by hand and then notice when I've reached a length that maximizes the effect of harnessing that shock wave? Would the frequency of the wave change with change in RPM?(AKA, making it difficult to hand tune)

Also, is there a way to know if my current carb set-up is doing a good job at atomizing the fuel outside of ensuring that my jets and emulsion tubes are clean?

And lastly, does anyone have any recommendations to achieve a good laminar air intake? I've heard about velocity stacks, but with their lack of air filters I would be hesitant to use them for a run around town bike.


Offline teazer

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Re: Should I Get Larger Carburetors? - Honda CL200
« Reply #9 on: Jan 21, 2019, 18:42:25 »
The problem I have with street bikes is knowing where to stop.  I keep looking for more.  More revs, more HP, so I sometimes go too far. To be honest, the best way to get more HP out of a 200 is to buy a 250 if you follow my drift.

Performance bikes played with straight exhaust lengths on a Rotax single years ago and spent days on a dyno trying different lengths before they settled on the optimum but I din't think it added a lot of HP. My preference is usually for a mild camshaft, high compression and smallish ports and carbs.  Years ago I raced a Honda CB72 punched out to 350cc and running specially modified pistons and oversized valves and tried every cam on teh market.

After blowing up a couple of times and trying different things, I came back to the highest compression I could use, stock valves and appropriately oversized carbs.  What seemed to make the most difference was  raising compression and getting a decent squish band but that took a fair amount of machining to the head and to pistons. We have even tried welding up combustion chambers and changing the shape to get more compression, but simply decking the head on a 200 and setting squish to say 35-40 thou is a good place to start.

There's not much on a 200 that could be swapped in from a 160 or 175, but you could get Megacycle to grind a cam if the budget is wide and deep.  If not, get 1mm off the head at your local auto machine shop. If it needs new pistons, look hard at a set of 57mm racing pistons, but that is not cheap.

Heads can be ported but don't remove much metal, just clean up the ports and pay attention to the exhaust ports which are restrictive.

Back to your questions.  Velocity stacks inside filters are the way to go if you can get them to fit. If not leave the intake stock.  Those motors seem to like long long pipes and long megaphones rather that long straight pipes or short pipes.  You could also try a 2 into 1 pipe set up. We run that on our race bikes.

But whatever else you do, don't take anything one of us says as either the absolute truth or the right answer for you an your bike.  none of us has tested every possible combination or idea and what we are looking for may not be what you want.  Get a copy of the A G Bell book and see what others have done.  Keep searching and learning.

BTW, the best way to add performance is by adding lightness as Colin Chapman used to say.

Offline jpmobius

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Re: Should I Get Larger Carburetors? - Honda CL200
« Reply #10 on: Jan 21, 2019, 18:51:34 »

But whatever else you do, don't take anything one of us says as either the absolute truth or the right answer for you an your bike.  none of us has tested every possible combination or idea and what we are looking for may not be what you want.  Get a copy of the A G Bell book and see what others have done.  Keep searching and learning.

BTW, the best way to add performance is by adding lightness as Colin Chapman used to say.
Mobius


On a long enough timeline, the survival rate for everyone drops to zero.

1973 RD350 Yamaha build  http://www.dotheton.com/forum/index.php?topic=66498.0

Offline Classic5

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Re: Should I Get Larger Carburetors? - Honda CL200
« Reply #11 on: Jan 22, 2019, 16:26:04 »
Hey All,

Thanks for the all the great tips! I was thinking about starting simple and pursuing the laminar flow on my stock carburetors, and found a wide variety of sizes and shapes and prices of velocity stacks available out there. Is there any rhyme or reason to picking one over the other? What makes one stack better than another?

Offline teazer

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Re: Should I Get Larger Carburetors? - Honda CL200
« Reply #12 on: Jan 22, 2019, 17:25:28 »
http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=268543

https://www.thesamba.com/vw/forum/viewtopic.php?t=643596

I think that the chart in that second post is from work down by David Vizard.   I'm pretty sure that A.G. Bell has info on shapes and lengths too.

Offline Turnturtle

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Re: Should I Get Larger Carburetors? - Honda CL200
« Reply #13 on: Jan 22, 2019, 17:58:21 »
There is plenty of theory and some useful info in this article by Gordon Blair.
http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf

Offline teazer

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Re: Should I Get Larger Carburetors? - Honda CL200
« Reply #14 on: Jan 22, 2019, 22:20:54 »
And just to complicate things B;lair and Bell don't agree on the formulae, or put another way, the formulae do not always generate the same answers.

And in the real world, that is OK because the differences are small and are really only important at one rev range.