Hi Team, Disclaimer: This is the first time I've done this mod and I will test it on my drag bike as soon as possible and report back on how it went. This mod does NOT use the standard airbox or any filtering (racing use only). Okay, while waiting for something to dry, I had a spare few hours, so I did the following... Why did I do it? Well, I've noticed that the standard airbox on a ZXR250A and C model doesn't quite line up the inbuilt velocity stacks with the carburetor venturi area. This means there is often a 1mm-2mm distortion and me being me, I wanted the velocity stack to perfectly lineup to create a constant cylindrical shape for the incoming air. What to do? Well, first thing is to remove the velocity stacks from the airbox body. This requires cutting up a standard airbox lower section. After cutting out each individual velocity stack I had to enlarge the bolt holes as these are part of the problem. Cutting the velocity stacks out won't align them to the individual carbs unless you drill the holes. I went 1-2mm wider as per the following image...top hole has been drilled out; bottom hole is original size. I then mounted the individual velocity stacks to each carb. Using some of Blair's lovely green seals to make sure everything is sealed nice and tight. The result is exactly what I was looking for... Finally, the carbs look pretty good and they should be fine for the drag bike. It won't go any faster, but at least I know I've helped the bike's induction be as efficient as possible (I think).
Yes, I agree, it is odd that they don't line up 100%, but I guess with mass manufacturing comes some level of acceptable tolerance/deviation. Yes, I will run them straight. I will place a small custom filter around the end of the rubber inlet as this tube connects to the side fairing to direct incoming air into the standard airbox. I run the rubber tube as it fills the cavity where the standard airbox used to be with fresh air, but it will need a filter with these carbs as it might suck in a bit of dust (on the return road etc). My current filter/airbox is a hybrid of the standard airbox and looks a bit like a hood scoop pointing backwards and it is a sealed affair so I haven't had to address additional filtering through the rubber inlet. I will post a pic of the hybrid airbox/filter when I get a chance.
Grey, Pics for you below. Airbox is cut in half and crankcase breather hose is plumbed into bottom of modified box. Looks terrible, but actually works really well and is super easy to remove and replace. The original airbox is a huge PITA to work with. Second photo is of the carbs that are on the bike now. Notice how badly they are aligned to the velocity stacks in the bottom part of the airbox. I will run these carbs for the next couple of drag meets and then try the newer carbs for a test after that. Will keep this thread posted with updates. Cheers Frank
Looks mean like that ..... but certainly if u are racing it u need to make it easy to work on. I wonder how much the 'lip' of the carby unsettles the air flow? ... if at all? Good luck with it ..... Ill be watching the progress
That's a good question, to which I have no factual answer. I guess when the engine is doing 19,000rpm you want zero things to negatively effect the combustion process. My thinking is that moving the velocity stack to be perfectly inline with the venturi area can't hurt. Do we have any scientists on this forum?
Hi Frank, Just out of curiosity- regarding your airbox modification, with the rear and lower section of the airbox removed, how well do the two remaining bolts seal the base and lid, and do you think it would be a successful endeavour to seal an aluminium collet in between the two halves in the place of your corflute? Regards, Bryn
Hi Bryn, Seal is pretty tight as there is a bolt at the front of the airbox and the two that run through the airbox push the top part into the grooved area on the outside of the standard airbox. The back of the airbox is filter material with meshing behind it. The mesh is cut to size and tabs left on the perimeter to hold the filter in place and to give the tape something to grab when I seal it. I haven't done the tape for a about a year in the photo, so it looks much cleaner and better sealed now that I have replaced it. There's no corflute used. Cheers Frank
My apologies, a bad case of "smll screenitis" got a better idea now, have you found any change in torque or throttle responce?
Not really Bryn. The filter material I used is about the same as the normal filter. I would assume if I used hi-flow material it would maybe rev a little quicker as it would breath better. I have another set of carbs I plan to run soon. These I will run without an airbox, so will be able to compare throttle response better then.
The 'lip' (step?) where your velocity stacks meet the carb mouth would be a little restricting at full throttle as the air would 'bump' off the step unsettling the flow and acting as if the intake area was of a smaller diameter. Try to get as smooth a flow as possible.
