Replacement Generic Carburettor

[Tobias Funke]

i've started a project to replace my old original 400cc dual carburettor with a new, generic, single-barrel one purchased on ebay. 60$ so nobody is complaining ...

unless anyone gives good reasons to not proceed, i'll go through the process of trying to adapt it to my bike, and describe all of it here.

here's some pics of the new unit :

1.jpg 2.jpg 3.jpg 4.jpg

here's a placement on the bike that seems to make sense:

20220902_191404.jpg

there's a handful of basic fabrications needed in order to make it happen :

- two brackets to hold the unit in place
- t-junction to split the output into front and back cylinders
- cutting and adapting the throttle cable to suit
- cutting and adapting the choke cable to suit

[Tobias Funke]

one consideration, the geometry of the pipe might have an effect on the efficiency of the engine.

there's broadly two ways to construct a t-junction :
t-junction configs.png

CFD (in the ANSYS package) shows that a split gives similar result to a lower velocity flow, a rounded split would be smoother, and no split at all produces a lot of turbulence :

t-junction cfds.png
increased turbulence may result in faster and more complete combustion, since the air and fuel may be mixed more intimately.

if time permits, i might even try some different designs and see if they make a difference from an empirical viewpoint.

meantime if anyone has ideas please advise ! here's some interesting references for anyone who may want to find out more :

https://books.google.com.au/books/ab...on&redir_esc=y
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/turbulent-combustion
https://www.sciencedirect.com/science/article/abs/pii/S0142727X13002099

[flyinelvis]

Subscribed.

Popcorn please…

[Tobias Funke]

youre just trying to motivate me, right ?

please bro give reasons .. or as my poppa used to say " just another guy criticising western front, from his house in dzhigda "

[6]

So your plan is to feed both cylinders from the single carb. There are plenty of bikes that work that way, getting even distribution to the cylinders and having room under the tank will be hard. Most bikes that do that have the carb hang out the side (see Harley) because you need a big carb and a straight shot for air to rout to the heads..

That looks like a smaller carb as well. Unsure that would work the best.


Also just introducing turbulence will not increase power. Smooth air flow will definitely yield a better result.. that's one of the big reasons behind intake and head porting. You want straight shots for that air to flow.

Porting used to commonly be porting and polishing but the wisdom started to move to leaving the ruff surface instead of polishing it smooth to help that air fuel mix a bit more, but adding right angles and T's into the system will definitely be a loss.


I am no expert in hydrodynamics, I've just learned and read some stuff.. but there are some guys on here that could definitely claim that tag.. I believe they would agree that design isn't the best.

Also the Hawks front and rear cylinders are jetted differently due to the rear running hotter than the front because of how the coolant flows through the front cylinder first. You would have a lot of trouble achieving that setup.


It's an interesting thoughts, but probably not a great use of time and money.

Check out the fuel injection thread if you want to so something cool with fueling.

[69Falcon]

Although as our carbs age and bodies get damaged, they are a finite resource. And cleaning just one carb is quicker, and if the manifold is permanently connected to the heads it’ll be easier to put the carb on. So I do see som upsides if you can get the fuel flow to the heads good. Depending on size and placement, I wonder if you could run a snorkel to the front like the stock air box does?

[Tobias Funke]

So your plan is to feed both cylinders from the single carb. There are plenty of bikes that work that way, getting even distribution to the cylinders and having room under the tank will be hard. Most bikes that do that have the carb hang out the side (see Harley) because you need a big carb and a straight shot for air to rout to the heads..

yes it sticks up a lot more than the stock carb. but thankfully there's a surprising amount of clearance under the fuel tank.

That looks like a smaller carb as well. Unsure that would work the best.

true, however the seller rated it for 400cc displacement engines.

Also just introducing turbulence will not increase power. Smooth air flow will definitely yield a better result.. that's one of the big reasons behind intake and head porting. You want straight shots for that air to flow.

restriction to airflows causing pressure drop would definitely reduce power. yet increased turbulence will increase it **if** the fuel/air mix homogeny was deficient. its hard for me to say if the proposed changes would have a substantial effect, however i doubt the 'natural' configuration causes a big pressure drop

Also the Hawks front and rear cylinders are jetted differently due to the rear running hotter than the front because of how the coolant flows through the front cylinder first. You would have a lot of trouble achieving that setup.

thanks for the information bro.. if thats the case then i'll look at improving the design of the cooling system. i think adjusting fuel/air mix is a silly way to even temperatures. im very surprised that 30cm of piping makes any difference to heat removal capacity !

It's an interesting thoughts, but probably not a great use of time and money.

dont spend money and buy, spend time and learn.

Check out the fuel injection thread if you want to so something cool with fueling.

thanks but i dont like efi. it was only introduced to make servicing of cars faster and less demanding of skill

Although as our carbs age and bodies get damaged, they are a finite resource. And cleaning just one carb is quicker, and if the manifold is permanently connected to the heads it’ll be easier to put the carb on. So I do see som upsides if you can get the fuel flow to the heads good. Depending on size and placement, I wonder if you could run a snorkel to the front like the stock air box does?

i think that carb bodies last indefinitely but valves, jets and gaskets are replaced periodically, as we all know.

you guys would be surpised at what is possible. a few weeks ago i made 80mm servo-actuated throttle bodies for a twincharger, using only common tools. i saved 300$ per unit !

ofc you can run the air from anywhere you like almost. i dont think it makes any difference to the operation of the machine..

[6]

yes it sticks up a lot more than the stock carb. but thankfully there's a surprising amount of clearance under the fuel tank.



true, however the seller rated it for 400cc displacement engines.



restriction to airflows causing pressure drop would definitely reduce power. yet increased turbulence will increase it **if** the fuel/air mix homogeny was deficient. its hard for me to say if the proposed changes would have a substantial effect, however i doubt the 'natural' configuration causes a big pressure drop



thanks for the information bro.. if thats the case then i'll look at improving the design of the cooling system. i think adjusting fuel/air mix is a silly way to even temperatures. im very surprised that 30cm of piping makes any difference to heat removal capacity !



dont spend money and buy, spend time and learn.



thanks but i dont like efi. it was only introduced to make servicing of cars faster and less demanding of skill



i think that carb bodies last indefinitely but valves, jets and gaskets are replaced periodically, as we all know.

you guys would be surpised at what is possible. a few weeks ago i made 80mm servo-actuated throttle bodies for a twincharger, using only common tools. i saved 300$ per unit !

ofc you can run the air from anywhere you like almost. i dont think it makes any difference to the operation of the machine..

I'd advise to first learn why things are done the way that are before trying to improve or modify the systems wholesale.

There is a lot wrong up above. I don't have time to address it all right noe. And again some are things that show a lack of fundamental knowledge.

We see this now and again. You seem to be bright with presumably some education behind you, but don't assume you are the first with your background to look into these things.

The Honda engineers in the 80s where pretty bright and pretty good with carbs. As where and are the racers, and parts companies that came in after.

And don't trust an eBay seller of Chinese carbs to tell you what will work based on a line of text like "up to 400cc"

[bk94si]

thanks but i dont like efi. it was only introduced to make servicing of cars faster and less demanding of skill

I like carbs myself but I am pretty sure this is not true. EFI is more efficient and provides more power.

[squirrelman]

EFI is more efficient and provides more power.

yes, as long as multiple sensors, wires, computers and injectors are working right. EFI was mandated primarily to reduce and control excess emissions and to improve fuel mileage.

[rpcraft]

I'm just hear for the cheap entertainment of watching someone reinvent the wheel at this point...

[squirrelman]

I'm just hear for the cheap entertainment of watching someone reinvent the wheel at this point...

don't get him started on THAT !

[rpcraft]

Well I didn't mean it literally but at this point.... Yeah....

[ParcNHawk]

Getting your bike running perfectly doesn't mean you know how to tune a carb. And knowing all the fluid dynamics in the world doesn't mean that you can tune your bike perfectly.

But

Somewhere in the middle we can all learn and we all grow.

[joel]