Cafe Racer Forum banner
1 - 15 of 15 Posts

·
Registered
Joined
·
2 Posts
Discussion Starter · #1 ·
Hello everyone, I've been working on 74 cb 360 that I got from a guy in boxes, what a prodject but any ways I was wondering what size jets I should be using, carbs are # 745b, will be running pod filters, 2 into 1 exhaust, just got done rebuilding the top end, I put in Charlie's place ignition, m unit blue , thanks

Tire Wheel Fuel tank Automotive fuel system Vehicle
Tire Wheel Fuel tank Automotive fuel system Vehicle
 

·
Registered
Joined
·
9,010 Posts
Same answer: If you run pods with no intake stacks it won't make any difference what jets you use, it still won't run right until you put the intake stacks back on the carbs, then you should be able to run the stock original jetting. You're not putting any additional oxygen into the engine by fitting pods, that's a misconception, you are in fact reducing the oxygen and reducing the carburetors ability to atomize fuel into the more turbulent air intake. (you can't fix that situation by just dumping more fuel into the mix, put the intake stacks back on and put the air filter on that, the size of your valves and piston displacement has not changed.)
 

·
Registered
Joined
·
1,473 Posts
then find someone with a dyno so you dont have to haul ass up and down the street 100 times
 

·
Registered
Joined
·
9,010 Posts
I know a place that does dyno testing, it will only cost him about 400$ per run and reveal how little power a CB360 produces.
... and it will still run like garbage if you don't funnel the air into the carburetors. That's what an air intake does, in the simplest terms, it funnels the air into your carburetor and it works because you can indeed compress air by passing it through a funnel.

You know how much difference your exhaust system change makes on the carburetor jetting? Give you a hint; it's a 4 cycle engine, the intake vacuum pressure used to operate the carburetor is produced by the movement of the piston on its intake stroke and the exhaust stroke does not happen at the same time as the intake stroke.

Pods will do two things for you, they will make the engine induction sound much louder and on average make the bike considerably less fuel efficient. Tiny pod filter on the crankcase vent will clog with oil and water eventually messing up your engine.
 

·
Registered
Joined
·
2,117 Posts
That's what an air intake does, in the simplest terms, it funnels the air into your carburetor and it works because you can indeed compress air by passing it through a funnel.
This raises so many questions.

How you can compress air by drawing it through a funnel?

How do carbs that have their float bowls vented to atmosphere work with velocity stacks that compress the intake charge?

How do venturis work if funnels compress air?
 

·
Registered
Joined
·
9,010 Posts
This raises so many questions. <- Yes, like why is the sky blue :unsure:

How you can compress air by drawing it through a funnel? <- very very fast actually and by means of a vacuum created by the intake cycle inside a 4-cycle engine.

How do carbs that have their float bowls vented to atmosphere work with velocity stacks that compress the intake charge? <- your engine is a piston driven air pump, it creates a significant vacuum, far more vacuum then the positive atmospheric pressure in the float bowl or in the air box, that engine vacuum draws air past the venturi and draws fuel up through an emulsion tube to begin the aeration and atomization of the liquid fuel into the intake air stream. Gravity is why the atmosphere has positive air pressure, unlike the vacuum of space or the vacuum inside your head when the piston is dropping in a cylindrical barrel, creating a void if not for the intake valve being open and allowing the fuel air mix (roughly 14.5 parts air to 1 part gasoline is close to optimum) so yes, your internal combustion engine will NOT operate in the vacuum void of outer space.

How do venturis work if funnels compress air? <- Ever seen a fly sprayer, a fuel vanturi works just like a fly sprayer and it ain't no funnel. When you draw air through a funnel (which is nothing like a fuel venturi) it compresses air (reduces the spaces between the molecules to be exact) more molecules = more oxygen = more complete and rapid oxidation of fuel air mix which = more fire.
but you already know that, I was posting it for the noobie that discarded the important motorcycle parts that Mr. Honda put on there to make it work.
 

·
Registered
Joined
·
2,117 Posts
In fact I don't "already know that" - because it isn't true.

Well you got the first part right; very very fast is the only way. The Mach number of the flow has to be over 0.3 before air is considered compressible in fluid dynamic calculations.

But aside from that:

Air does not compress as the flow area decreases in a funnel. It's velocity increases and it's pressure decreases. Don't take my word for it, Google "Bernoulli's principle".

The engine creates more vacuum than atmospheric? That's not physically possible. Atmospheric is relative to absolute zero, you can't draw a vacuum lower than that. Again, don't take my word for it. Google what the vacuum is in space.

A venturi works just like a fly sprayer? That's not an answer. Hint: Bernoulli's principle... and funnels work for creating this too. It doesn't have to be a parabolic shape. I do wonder if you truly know how it works because if you do, how do you think the flow can be compressed while at the same time have lower pressure?

You obviously know a lot of things about bikes and can offer helpful info, but then you go off the deep end tossing out things as fact when they don't make sense according to the most basic fluid dynamic principles.
 

·
Administrator
Joined
·
543 Posts
I think the intent Trials has in mind is laminar flow. The venturi speeds up the air and this creates a vacuum to pull fuel from the bowls. Well, in reality the atmospheric vent is what pushes the fuel into the vacuum created, but anyway...Tumbling air does a poor job of this and an orderly laminar flow actually allows more air to get through. The air isn't actually compressed to get more in, it is just stacked neatly so more fits.
 

·
Registered
Joined
·
9,010 Posts
I think the intent Trials has in mind is laminar flow. The venturi speeds up the air and this creates a vacuum to pull fuel from the bowls. Well, in reality the atmospheric vent is what pushes the fuel into the vacuum created, but anyway...Tumbling air does a poor job of this and an orderly laminar flow actually allows more air to get through. The air isn't actually compressed to get more in, it is just stacked neatly so more fits.
(y) Tinsnips you get it, so why can't the rest :unsure: unless they are just playing devils advocate
 

·
Registered
Joined
·
9,010 Posts
For those not old enough to know what a fly sprayer is:

it's the thing yer daddy used to spray DDT in your bedroom with and here you have a device that works very much like a carburetor works, it atomizes liquid fuel by means of an air pump and how well that functions depends on the design of the pump. Just like your carburetor, If you leave any parts off it won't work so good.
 

·
Registered
Joined
·
2,117 Posts
(y) Tinsnips you get it, so why can't the rest :unsure: unless they are just playing devils advocate
So...

After claiming drawing air though a funnel compresses it, and the engine creates vacuum much greater than atmospheric... both of which are patently false and easily disproven with the most basic fluid dynamic principles...

You think I'm the one who doesn't get it?

(That was a rhetorical question by the way)
 

·
Registered
Joined
·
9,010 Posts
Dude, what I'm saying is his motorcycle will work as good as it did before if he just puts the intake snorkels back on the carburetor mouth and he won't need to drop large coin to buy flatsides or make excuses why it isn't tuned right, where do you stand on that point because the rest of your discussion is moot.
 

·
Registered
Joined
·
1,969 Posts
Head Jaw Carnivore Fang Terrestrial animal
 

·
Registered
Joined
·
9,010 Posts
Here you go JJ360 this will work, although in reality this setup has less filter surface area then the stock air filter design, so it's a downgrade modification

if you want to know the science behind it :geek: figure it out for yourself.
 
1 - 15 of 15 Posts
Top