Thread: stock air box mod

this is an attempt on a otherwise messed up air box from tweety to see if it can flow with my E.G.88CC bike with a 19mm carb and ported stock intake.i love the stock airbox look.more to follow.its in the beginning stage.and limited cutting tools.lol. have some body work and paint yet.lol.i hope it works.

Shouldn't be necessary to enlarge the inlet by much, if any. The two main restrictions are the rubber duct and the thick foam element. The inlet rubber piece isn't strictly necessary, but it does keep out water (if you ride in wet conditions) and muffles induction noise. Once the smallest point on the inlet side of canister is at least 2mm larger than the carb, it's no longer a restriction.

You can probably hog-out the OEM rubber boot. Since you've already performed metal surgery on the canister, a slick solution would be to fab a metal spigot & weld it on to provide a mounting point for a larger ID rubber hose.

Fitting a low-restriction foam element inside that canister is more of a challenge. The stocker is thick and it needs to be. It lacks surface area and structural rigidity needed to seal against the inside of the canister. The ideal would be a cylindrical foam element that's about .75" smaller (length & width) than the ID of the airbox assembly, with a rubber tube outlet that seals against the outlet to the carb. Think of a UNI filter with the carb fitting forming a T-shape profile and both ends sealed like a beer can. A much easier approach might be to thin out a rectangular area .75" smaller than the perimeter of the foam, using hardware screen (shaped like a shoebox lid) to support the now-raised lip of the foam and maintaining postive seal. The foam will have to be cleaned more frequently, but ought to flow relatively freely.

Im going to try to cut and epoxy a square k&n filter to stick in mine with some larger rubber hose going up into the frame from the back of the airbox. Your going flow more for sure,cant wait to see how it turns out!

I love the way the stock setup muffles the intake noise so it doesn't compete with the exhaust. I'm not a big fan of angling the carb. I think it takes away from the looks and sound of the CT. This is a great project and I'm sure there are others who are also looking for ways to increase flow while maintaining the stock appearance.

The problem is a simple matter of real estate...there's not enough of it on a CT to allow big upgrades on the induction side with the stock airbox in place. I've never been overly fond of induction noise associated with an open-element air cleaner asm., and find it more annoying with each passing year. That's the price you pay if you want to run anything capable of keeping up with 50-55mph traffic. Aesthetics are subjective, performance is objective. Unless you're happy with 45mph max speed, form must follow function in this instance. Even if you could source a pair of "mushroom" filters from UNI, K&N, or whomever, that would fit inside the airbox, connected to a "T" fitting, that stock carb is tiny compared to anything even remotely large enough to feed a 110cc+ motor. Air filter dimensions aside, if the float bowl won't clear the cylinder and/or the throttle cap is too close to the underside of the frame, the only choices become offsetting the carb to one side, cutting & welding or running an expensive downdraft carb.

Honda tried to solve the problem on the ST90 with the external air box. Kevin.

Originally Posted by kziolk

Honda tried to solve the problem on the ST90 with the external air box. Kevin.

Exactly! And that's what has to be done when significantly more power gets bolted into a CT70. The angled intake just happens to be the most workable solution on the smaller bike. Yes, the ST90 frame is only a little bit larger. When space is this tight, even a small increase in available room can make a big difference,