butterfly valve

[Paulwhite]

Oh Ok,

I have read the rest of the replies and there are some points to take on board :-

You have adjusted to many things at once (or are trying too) to assume how the motor will react

What you are trying to build is a 'still air box' (very popular in the 70's on grass track/speedway bikes) look here

How we did that back then was to

Set the ignition timing (no need today with cdi) note:- the curve on your cdi box may be wrong & may need alteration or remapping

Fit one known datum ( Exhaust or air box, I suggest the exhaust and leave the airbox off)

check the bikes performance

adjust the carb to suit the current setup

when you feel happy with the performance (no flat spots/plug chops look good, no exhaust bluing etc ) run the bike again for a good while to make sure the engine does not heat seize ( through under jetting, needle adjustments & incorrect ignition timing)

If the engine does seize work out if it was an ignition failure (wrong timing) or a mixture problem & try again

Fit the airbox and note the changes to the engines performance ( with what you are trying to do I would guess the airbox volume should be very close to the Honda original) when you have every thing right just filter the airbox with a piece of ladies stocking/tights ... job done.

Perhaps you are trying to over complex the problem

thanks for the reply i have done an engine check b4 i tackled the airbox and all is fine (as far as i know since the engine is out of the bike), the engine runs nice with the original airbox and mods, the jetting seems to cope well with the polished head and exhaust in all rev ranges (did many a plug chop) as to ignition timing and advance curve iv looked into the engine (the best i can) and found that the 2004 cbr 125rr engine (which is what iv got) was one of the best carb models in terms of advance curve and performance, hence why i chose it to put into the bike. and i got a good engine with 5000 miles, i took it apart and its been run in well, no scuffs or marks below the piston rings, and clean oil, and there known for there reliability. iv tried looking around for any examples of mods done to the engine and cant seem to find anything on removing the airbox, so it seems to be something new for this engine and i want to try and get it right im also going to put a slightly bigger main jet in once iv got the air filter to work just to make sure its not going to run lean at full revs.

iv always had a problem with over-complicating things its just me lol hence why i posted on the forum cant beat some old school knowledge and experience!

and neversaydie thanks for the link i didnt think of a ball valve! defiantly something to consider! and it was a question to which anyone could reply to, but i couldnt fathom why you wanted to know the dimensions .....now i know lol cheers

iv just read the link you sent JimR.(took me a while to get my head in gear lol) and am now thinking a washer with a certain size hole would do a better job. il have to have a play and see what results i get.

Edited December 5, 2010 by Paulwhite

[Cy Welch]

thanks for the reply i have done an engine check b4 i tackled the airbox and all is fine (as far as i know since the engine is out of the bike), the engine runs nice with the original airbox and mods, the jetting seems to cope well with the polished head and exhaust in all rev ranges (did many a plug chop) as to ignition timing and advance curve iv looked into the engine (the best i can) and found that the 2004 cbr 125rr engine (which is what iv got) was one of the best carb models in terms of advance curve and performance, hence why i chose it to put into the bike. and i got a good engine with 5000 miles, i took it apart and its been run in well, no scuffs or marks below the piston rings, and clean oil, and there known for there reliability. iv tried looking around for any examples of mods done to the engine and cant seem to find anything on removing the airbox, so it seems to be something new for this engine and i want to try and get it right im also going to put a slightly bigger main jet in once iv got the air filter to work just to make sure its not going to run lean at full revs.

iv always had a problem with over-complicating things its just me lol hence why i posted on the forum cant beat some old school knowledge and experience!

and neversaydie thanks for the link i didnt think of a ball valve! defiantly something to consider! and it was a question to which anyone could reply to, but i couldnt fathom why you wanted to know the dimensions .....now i know lol cheers

iv just read the link you sent JimR.(took me a while to get my head in gear lol) and am now thinking a washer with a certain size hole would do a better job. il have to have a play and see what results i get.

This does seem to be making things overly complex. When removing an airbox (and LOTS of us XS1100 guys have done that) and switching to pod filters, the new system flows more air. As this causes the carb to work differently it changes the fuel/air mix. Just adjust the jet sizes up to provide more fuel, which is a trial and error type of thing and check the plugs after a full throttle run (clean new plugs) and check the color. If their too white, then you need bigger main jets, if to dark, then smaller. Repeat until you get the right color on the plugs and good performance across throttle settings without flat spots. I'm sure someone out there has changed out the intake on one of those engines and has some idea of how much of a change in jets is needed, but if not, I'd start by going up two jet sizes and see how it runs. Gotta put a load on it to test it though, and you will probably have to go up on the idle jet size too, unless you can compensate with the mix adjustment screw.

Trying to get a specific amount of restriction is easy as everyone has said, just create a box or something over the filter and make a hole, and make it bigger till it right (if you don't want to rejet). A good starting point would be to try a hole the same size as the inlet on the stock airbox for the engine, that is likely what's causing most of the restriction with the stock setup.

Just remember, you can't tune the carbs (other than idle) out of the frame/bike, cause to tell what it's doing requires a load.