Forum

[Tomo]

Norm, I have not touched the jet yet, its still the standard one. Based on past experience and reading through the notes on Mikarb, when an exhaust or air filter is changed it usually requires the jet size to be increased. If not then the mixture can be too weak resulting in rough running/spluttering, in the 3/4 to full throttle range. If below that maybe just a case of raising the needle a notch or two. I think I need first to take the bike out again and see exactly at what speed it starts to play up, before I do anything though.

[PeteF]

Trouble is Tomo, you changed from a 60s style (which is pretty much straight through) to one with baffles in. If anything this should make the engine run too rich. As I said before, what does a plug chop say (from the speed it's spluttering)

[papasmurf]

Tomo, I really do suggest checking you don't have any of the problems explained in the document I referenced first.

[Revband]

A carb only mixes air and petrol, the density of the mixture is controlled by the amount of air passing through the carb (bore size) and the amount of petrol passing through the jets (jet size) this mixture is controlled by the pilot jet the needle jet and the main jet, anything which restricts airflow into the carb (different filter etcetra) will require adjustment to the carb to correct mixture, anything which happens after the carb (valves/cams, piston, exhaust) will not affect the mixture. Ie if the airflow increases due to a free flow exhaust the petrol flow will increase in proportion to the airflow increase. In over fifty years tuning motorcycles for road use and also for racing I have found that this has always held true. obviously when racing it is more important to ensure that the fuel air mixture is as correct as possible, but the fuel air mixture will never be perfect all the time with a carb, because atmosphric pressure makes a difference to the mixture, at low atmosphric pressure the air is less dense so high atmopheric pressure will give a different fuel air mixture to low atmospheric pressure.

[Revband]

Note to self, must learn to proof read before posting. please excuse the mis spelling.

[mauri]

[PeteF]

Revband,
That's interesting but a bit counter-intuitive.
Thinking about it, your probably right. You certainly have more experience of these things than me.
Except!
What if the carb is now undersized for the airflow of the de-restricted engine?
Just a thought.

[Revband]

It's fairly simple really Pete, the speed of the airflow through the carb will increase to meet the demands of the engine, as the airflow increases it draws more petrol through the jet's.

[wilf]

For what it's worth my '05 bullet 500 has standard air filter and shorter type silencer,(looks 60's style) as fitted by the previous owner and runs fine on 117.5 main jet. Can't recall what the pilot jet is or needle position.

[PeteF]

Understand that Rev but surely, if you demand more and more airflow, there will come a point when the carb is too small to efficienty pass enough air. The carb will then be the bottle neck in the gas flow.