Carb question

[TektroG]

This is a question relating to jet sizes and power limits, because I am currently trying to register my bike abroad in a country in which power is limited to 106 bhp.

I have the official Suzuki manual and have it on good authority that the limitation for my bike is in the carbs rather than exhaust or CDI (from Suzuki dealership). Bike is an RF900 and in the carb spec it lists the size of the main air jets. Cylinders 1 & 4 are 0.8mm and 2 & 3 are 0.9mm, however it lists the main jet type as No.112.5 On the exploded diagram it identifies main, starter and pilot jet but no such item as a main air jet. Does the size correspond just to the size of a hole through which air enters the carb?

On the spec for the country I am moving to it lists main air jet sizes as 0.7mm and 0.8mm respectively. Is it likely that this is the means by which the power is limited? Carb body and all other specs are the same.

[guydewdney]

the air jet is the one as you look into the carb from the air filter end, usually at the bottom of the cone, down a small tube.

it allows air in to the emulsion tube area, creating the mist of fuel / air.

you cant (sensibly) restrict power using fuel jets - reduce the jet size = lean engine = burnt valves. increase = rich engine = flooded or sooted up.

Restrictions are done by:-
1 resticting the amount the slide can move up (eg a fat washer thing in the top diaphram)

2 restricting the air flow in the inlet rubber (like a lip inside the tube, or a washer arrangement)

3) exhaust / ecu etc restrictions - which you say isnt applicable.

[TektroG]

Fair enough but given all else is the same why the different jet sizes?

[Shifty]

Presumably, in the same way that most forms of tuning for more power basically involve getting more fuel through in one way or another, then restricitng the amount of fuel flow (by smaller jets or otherwise) should have the effect of restricting the power.

[TektroG]

Hold on that was my first assumption, but guy's answer suggests not, so now I am confused. Have we not just gone round in a circle here?

What I really am trying to get at is can I just buy 2 new main jets of 0.7mm and swap the 0.8mm ones or do I need to buy a second hand set of carbs? All changes will be undone once bike is successfully registered anyway.

[trikerdrew]

http://www.bat-motorcycles.co.uk/

and or

http://www.nrp-carbs.co.uk/

I hate carbs.They are the devils work :)

Drew

[Blackjack]

Bottom line is that you need to burn a given amount of fuel/air mixture to make a given amount of horsepower.

The air/fuel mixture needs to be in the ratio of around 14 parts air to 1 part fuel.

If you restrict the power output by making the inlet smaller (which is effectively what happens with slide limiter or washers), the air tends to travel faster.

Faster travelling air means lower pressure air.

Lower pressure means it sucks more fuel through a given jet size.

Therefore you need a smaller jet to maintain the correct fuel air mixture.

[TektroG]

Its a venturi, correct?

So a smaller aperture for air to enter through will be matched by a smaller main jet size, thus limiting overall power?

[pyro]

How do they measure the power?
Cos you could just put a big fibre washer in the top of the carbs to restrict the opening of it?

[Shifty]

Hold on that was my first assumption, but guy's answer suggests not, ?

'Spose restricting the jetting would only have a relatively small effect on power now I come to think about it that way. Apologies for the potential red herring.

[TektroG]

The saga continues - apparently Suzuki furnish me with an official list of mods that I need to do to the bike so I may end up buying a whole set of carbs then ebaying them later. If it proves all far too expensive I'll bring it back to the UK and flog it, but I wanted to hang onto it as it is a minter and really low mileage.