OK, one for the Gurus I think.

My old 1981 Yamaha XS400 cruiser, my "project" bike keeps me puzzled and confounded but we are getting there, soon it will just be cosmetic work to do (I hope). Hopefully the Alien electrics and clutch adjustments are OK now.

The bike has been running well but consuming a lot more fuel than it should be, possible up to double. A little while ago a couple of mechanics that I know who are old bike enthusiasts (to them 1981 is fairly new!) both test rode it and said rather than playing around too much with the carbs to try drilling a couple of 3/8th holes in the air-box near the regular intake, they recon this is a common cure to a common problem with older bikes. The air filters are good.

OK, I thought, if it caused a problem, it's a plastic aire box so I can just tape over the holes so I did it. I started the bike with choke as it was cold and rode off, the spitting and backfiring was something like "shock-and-awe" over Bagdad. Oooops I thought but when I finished with choke she ran like a dream and seems to be running a lot better. The erratic idle between 1k and 3k is much improved, not eleminated but a lot better. I have not yet got a result on miles-per-gallon yet as I have not had chance to run her out to reserve yet.

Before I yeild to the temptation of slightly increasing the size of the holes, does anyone know what is going on here and have they heard of this "cure" before, past experience leads me to trust these guys and what they say.

Does this mean unfiltered air is entering the engine?

No, but the air intake prior to filtration is a little larger hence once running at a higher speed it will suck more (filtered) air into the engine.

Running without an air filter is simply not an option, too silly for words.

I guess you're just increasing the flow rate of the air box.  Same idea as using a lunch box or pod filters.  If you were rich before, adding more air to the equation brings you closer to stoch, and you just run better.

It's just a guess...

Quote from: Cal Price on April 02, 2006, 04:26:04 AM
OK, one for the Gurus I think....said rather than playing around too much with the carbs to try drilling a couple of 3/8th holes in the air-box near the regular intake...does anyone know what is going on here and have they heard of this "cure" before, past experience leads me to trust these guys and what they say.

I did a little googling on the subject of drilling holes in airboxes...there were a lot of hits and most applied to automobiles. Regardless, I agree that it provides a less restricted intake for the carbs. In this case, non filtered.

This leads me to wonder if the designers used the surface area of the piston as a guide for the entry area of the airbox? Is there a magic value afterwhich no benefit is obtained? The intake size of the carbs is a variable. Another limiting part is the size of the intake valve port also. All the air/fuel has to pass this restriction on its way to the combustion chamber. Hmmmm. It does sound like a little voodoo is involved.

Either that or some serious fluid mechanic theory. Changing volumes and their rates of change. Where's the Bernoulli brothers when you need them? They might find this interesting. 

It is a shame we don't have an iris for playing with the intake area. Sounds like the variable venturi setup on some high end bikes.

Dgyver, Bob B, werase90, JamesG....our racer friends may have some good input here Cal.

N3

If the extra holes purpase is to let more air into the airbox then its similar to what people do to cars.

An increased opening into an airbox usually reduces air flow restriction, allowing for higher horse power (as long as air to fuel mixtures are the same) at the price of increased induction noise.