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CV Carburetor - Interesing Article

  • Thread starter Thread starter cberkeley
  • Start date Start date
Wonderful! I've been looking for something like this since I got my first bike. Thanks so much.
 
Does anyone know how accurately CV carbs compensate for altitude???????

Does anyone know how accurately CV carbs compensate for altitude???????

Just moved to Colorado, have two bikes with CV carbs, one with VM. Trying to decide which one to keep. As far as the altitude compensation bit, wondering how well it works. A bit cold up high now to go experiment with it up at 10,000 feet or so. So how accurate is the mixture controlled through changes in altitude?

Is it perfect?
Fairly close?
Sort of OK a little bit sometimes?
slightly better than VM carbs?
Or totally bogus, more of a sales gimmick?

Cletus, thanks for posting that article, it's a good one.
 
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"Does anyone know how accurately CV carbs compensate for altitude?"


CV carbs primarily control the speed of the air and/or mixture thru the carb--hence the term "constant velocity". So my thought is that unless thin/light air flows at a different speed than, say, sea level air, the CV carbs aren't going to cure a lean condition at high altitude. Someone who knows Physics should weigh in on this point.

Maybe this information will give a clue: Suzuki produced a Service Bulletin in May of 1981, in which they addressed the operation of motorcycles at altitudes above 4000 feet--but it was all about the fact that current Suzuki models were EPA certified at altitudes less than 4000 feet altitude. The Bulletin came with a list of 1980 and '81 Suzuki motorcycles and showed, in most cases, a Low Altitude main jet and a High Altitude main jet change for each model. Usually, the High Altitude Main jet number was 2.5 less than the Low Altitude number.

For example, for the 80' & '81 850s (which had CV carbs), the Low Altitude main jet was specified at #115. The High Altitude main jet was #112.5.

Changing to the High Altitude jet was supposed "to restore the emission characteristics to the low altitude levels". And then there was this: "These adjustments generally will not affect driveability or fuel economy".

My guess is that the smaller jet probably did help driveability some, but who knows, maybe Suzuki was driven only by a desire to comply with EPA and take no chances.

Hope this helps some.

Tomcat
 
I didn't read the whole thing, but at the end, he's not right about it being "folly" to try to get accurate plug reads simply because we use lead free gas.
He doesn't mention chop testing for the three circuits. If you just cruise into the driveway in his example, yes, the reads will be inaccurate and not a good test of the overall jetting.
Plugs reads are still a great way to see how your engine's running, lead free gas or not, bikes, cars, new, old, etc. But you must chop test correctly and understand what you're testing for.
 
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