Pumping it up!

[The Pastor]

to check the breather hole, i could just ride with the filler cap open?

I just tried that, and it didnt change a thing.

Also would the float level change when you play around with the jetting?

[Motu]

I doubt if a 250 would suck enough fuel to flow faster than the fuel lines and tap...I think you have another problem.

To run alcohol on standard carbs you needed bigger hoses and fittings to cope with the flow - maybe you should look into an alky set up.My Blue Magnum's can run petrol or Alky,they have 10mm hose fittings.

[The Pastor]

if anyone is wondering, i pulled the plugs and they are ok

[Squiggles]

Does it still bog mid revs?

[The Pastor]

Does it still bog mid revs?

it only bogs on low revs - 7000 and under

actually mid revs are all ive got

7-14000rpm is my usable range - which is good for road riding. But obviously I want the top end

[The Pastor]

ive been having a chat to a mate who knows everything there is to know about carbs and cbr250's

he reckons i need trumpets on my carbs to reduce the pressure at the top end.

I will have a go and report back! - he said the reason he needed the fuel pump was for long periods of time at wot in 6th.

[pete376403]

Wow, thats a theory I haven't heard before.

I always thought that bell mouths (trumpets) on carbs were to provide a smooth transition for air entering the carb, to avoid the shock wave that a sharp edged step (ie the carb opening) would cause, which effectively reduces the throat size.

When running extreme overlap cams there will often be blow back through the carb and the bell mouth assists in capture of the fuel/air "fog" so it can be drawn into the engine, rather than wasted. The length of the bell mouth is also used to assist in the ram effect, but this tends to have greatest effect at one particular rev range. Variable lengths can be used to broaden the effective rev range.

Thats what I thought anyway

[The Pastor]

sorry i wasnt clear on my post before

he said the trumpets reduce pressure inside the carb (lower than atsomopheric pressure) therefore sucking in more petrol.

Time will tell how it works!

[motorbyclist]

are your 250 honda electrics going to be reliably up to the task?

are you sure this isn't a kink in the fuel line, blocked filter or petcock issue - my pocketrocket died at high revs simply because the fuel line went uphill at one point and was holding an air bubble!

the issue could be a carburation one - you've removed the airbox and opened the jets to compensate... unfortunately you've gone and undone what honda engineers spent a lot of time getting right. perhaps the intake cam timing etc requires a certain vacuum at the intake end to function smoothly, as does the exhaust end need the right backpressure and pipe length to work at it's optimum for the exhaust timing


if all that is ok and the petcock will still work, then the pump will need to have a means of pumping fuel back into the tank (via a relief/bypass valve), or be able to reliably stop/start and maintain flow while it isn't running

most importantly you must be sure that the bike will be able to start!


my suggestion would be to run the bike on a 600ml coke bottle of petrol or similar to ensure the problem is not the float valve (by eliminating the petcock/filter) - then fill the tank to the brim to ensure that an increase in fuel pressure will infact provide a solution


then as a final solution, go nick the working fuel system from another honda or whatever fits rather than pissing around designing a fuel pumping system yourself

he said the trumpets reduce pressure inside the carb (lower than atsomopheric pressure) therefore sucking in more petrol.

how exactly?

they improve the flow rate by the methods described above - which would indicate a pressure closer to atmospheric than without them

fuel is (primarily) drawn into the carburetor from the bowls by the venturi effect

[pete376403]

sorry i wasnt clear on my post before

he said the trumpets reduce pressure inside the carb (lower than atsomopheric pressure) therefore sucking in more petrol.

Time will tell how it works!

Well thats not quite how it works either. The only thing that makes air (and with it the fuel) enter the carb is atmospheric pressure, assuming a non turbo/super charged engine. What happens in a carb depends a lot on Bernoullis Principle (Google it) Basically this says when air has to increase speed, it will reduce pressure.* The carb has a restriction (venturi) near or at the point fuel is introduced to the air stream. The restriction will cause the airspeed to accelerate. Now the fuel is at atmospheric pressure also - the air pressing down on the top of the fuel in the float bowl provides this. The pressure in the fuel system up to the needle valve is irrelevant, because the needle valve is maintaining a level of fuel in the bowl,then closing when this level is achieved. So you have fuel at atmospheric pressure, and air, because it is speeding up, at less than atmo pressure. This causes fuel to rise up the jet holder and into the venturi, where is mixes with the air, to pass into the engine.

You don't want the reduction in pressure to occur anywhere but in the venturi. Bell mouth/trumpets are not going to achieve this, but they will smooth the airflow into the carb, and they can be used to alter the tuned length of the intake.
And they look quite cool.
*the opposite is true, when air speed slows,it increases in pressure. this occurs in, for example, turbocharger compressors - the air comes off the turbine blades at high speed, enters the scroll housing where it is slowed down and the rise in pressure results

http://en.wikipedia.org/wiki/Bernoul...ld_application

[The Pastor]

yeah thats pretty much what he said. trumpets increase the speed (by reducing the diameter) which lowers the pressure....

or something!

- andrew, i was going to nick one of a mc19 cbr250 but they are prone to death. I might not need a fuel pump but we will see how it goes, i'll re asemble the bike today, sunburn willing!

[Ixion]

You could of course quite easily check this theory about the float bowls running dry.

Take the bike to whatever revs it is it dies at, let the engine die, then (if your tap is not vac operated) turn off the petrol , pull in the clutch hit the kill switch . Roll to a stop, hop off and undo the carb drain plugs. If the bowls are empty, no petrol comes out. Theory proven. Otherwise, otherwise.

[Squiggles]

You could of course quite easily check this theory about the float bowls running dry.

Take the bike to whatever revs it is it dies at, let the engine die, then (if your tap is not vac operated) turn off the petrol , pull in the clutch hit the kill switch . Roll to a stop, hop off and undo the carb drain plugs. If the bowls are empty, no petrol comes out. Theory proven. Otherwise, otherwise.

There be no need for sense in this thread! The man wants to burn money so lets just support his efforts rather than talk of floats and adjusting their levels...

[The Pastor]

I've attached the trumpets and went for a ride, its pretty sweet now.

[The Pastor]

so i guess in the end it wasnt draining the carbs, just running really lean.