The various comments and comparison numbers on compression test results had me wondering, so I decided to do a little testing.

(On my stock, 1990 Evo, 32k on the clock, CV carb.)

Filter cover and filter on, throttle at idle: 70 pounds

Filter cover and filter on, throttle at 100%: 130 pounds

Filter and cover off, throttle at idle: 70 pounds

Filter and cover off, throttle at 100%: 135 pounds

Filter and cover off, throttle at 100%, hold the slide up: 150 pounds

Ok. I feel better now.

 

I am surprised that jump is as big as it is holding the slide up.

 

Wow that is a really big difference.

 

Why doesn't the throttle hold it up all the way. Is it out of adjustment?

 

The slide on a CV is operated by vacuum pressure. There is no mechanical linkage to it.

This vid is pretty good at showing how it works.

 

Not ever having one, that kinda slipped my mind. So next question is then, Why would you run one if you couldn't get maximum lift to the slide especially when you just proved the performance difference in cfm intake (15 psi is a lot)......

 

You could probably write long papers on that (and I'm sure lots of guys have), but in practical terms, I think it comes down to a few things:

- CV carbs give more accurate AFR over the entire power range, which translates into better mileage.

- S&S style carbs give snappier throttle response. Although the accelerator pump helps, a CV carb has a certain delay as the slide moves and the main jet opens up.

- CV carbs self-adjust for altitude variations (at least to a degree)

- S&S style carbs are simpler and quite a bit easier to get set up

Kind of a "pick-em" situation, really. If I was building a bike from scratch, I'd probably get an S&S. Since I've already got a perfectly good CV, I don't see any reason to change.

(THC, that's your cue!!)

 

To funny I had done the same tests ahh 2-3 years ago after fixing a blown/leaking head gasket. Went and ck'd notes in service manual.

Had the same numbers as you mention here. Was told I should have rebuilt the top end. Everything had mic'd out good and I honed the cylinders and slapped it back together.

Have been asked a few times "Ok what have you got done to that EVO it runs that good ?"

WP