But you can't get all the power back at altitude.

The topic of improving airflow by making changes to the intake system has arisen before. However, I have seen no documentation on this forum of any measurable effects. One or two people have posted dyno charts, some of which have vanished. As far as I know, no one actually posted a dyno run of, say, a stock bike followed by a dyno run on the exact same bike with the air filter removed. Or for that matter, with the entire airbox removed. At least one of the posted dyno runs did not have the exhaust gas analyzer connected, so it was impossible to see whether the mixture changed with engine RPM, an effect I would expect if there is airflow restriction. Some of the posted runs have had incorrect RPM measurements, which have led to incorrect torque and/or horsepower measurements. In short, all we seem to have are "seat of the pants" measurements, and we all are susceptible to the placebo effect. If it's noisier, it must be more powerful.

By the way, unless the high-altitude airbox openings are reversible, the leaned high-altitude mixture will remain to plague the engine on the return to low altitude. Airplane carburetors have a pilot-operated mixture control, which changes gasoline flow to compensate for air density (altitude) changes. If I remember correctly, my old Trail 90 had a carburetor with an altitude control on it - a two position control for above or below 6000 feet. It made no difference that I could tell, but I never had the chance to take it over 6000 feet.

As a matter of interest, it seems to me it would be possible to convert a CV carburetor to non-CV operation. Block the air passage from the venturi to the top of the diaphragm, open the top to atmosphere, figure out a way to connect the throttle control to the slide, and block the butterfly wide open. Especially at first, I think I would want to remember carefully where the kill switch is.

Agreed.. to get back to an optimal mixture as you return to a low altitude, it would be necessary to re-cork the airbox to match your jetting.  This has been thought of and little adjustable airbox doors have been invented.

I would not take any stock in dyno runs where the only change was airbox on/airbox off..  you have to re-balance both the airbox and jetting.  The "airbox off" run could easily yield less power because now the mixture is too lean, and without the EGA connected, who knows which side of optimal the mixture is on?  The point was that you can compensate for high altitude by making the airbox less restrictive, keeping the mixture more or less the same as you had at low altitude.  

Bubba, congratulations on getting your bike running right.  Mine has a little burble-pop on deceleration too.  Think of that and a little flamethrowing as not bugs but features

Maybe you guys need something like this: http://www.thunderproducts.com/tpi_valve.htm
Lancer mentions it as an optional component of his performance carb kit.
EDIT: I see Boule'tard beat me to the punch.

Quote:


That's pretty much creating a ghetto VM carb.  I say ghetto because I've read that the angled bottom of the slide on a VM is actually an important detail (for off-idle mix I believe).  Also, doesn't the CV slide not fully block the throat?  That would make it hard to idle.  

I may have to do that little experiment...next spring. Right now we have 4 inches of snow down here at 5280, up about 5000 more feet there's going to be snow till spring...