$70 CV carb

concours said:

If you beleive your google-foo, explain how. In depth.

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As altitude increases air gets less dense, that means that what is referred to as 'static air pressure' reduces with altitude! That is how barometric altitude is measured.

Aviation people have complex models of how this works.

Barometric pressure also varies with weather conditions (moisture content). Meteorological people have complex models of how this works.

With lower static pressure, that is at higher altitude, there is less oxygen available in the fuel mix, so we want to add less fuel to compensate.

With higher static pressure, we get the reverse and we need to add more fuel.

So if we assume that changes in static air pressure affect the vacuum pressure acting on the CV carb slide: the fuel mixture is automatically adjusted for altitude!

And the mixture will also be compensated for other static pressure changes, like barometric weather conditions. I had sort of thought that was one of the benefits of CV carbs?

nortonbrian said:

Back in the 1970’s a popular and inexpensive figment was a single carb conversion using a take off carburetor from I think a Harley Sportster. Available really cheap at swap meets. Anyone remember the details?

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In the 70s I had a mk2a , I was skint and doing my apprenticeship
And the bike was my only transport at that time so I fitted a Norton atlas manifold
I cut the back off and re angled it to suit the commando and used one of the amals it was really economical which is what I needed at the time
This started me on the road of running single carbs
I've also had a single 34mm mikuni CV
A 34 VM a 36 VM over the years
But currently on my 750 I run a 40mm TM flat slide on a completely home made manifold which is a league away from the others

SteveA said:

As altitude increases air gets less dense, that means that what is referred to as 'static air pressure' reduces with altitude! That is how barometric altitude is measured.

Aviation people have complex models of how this works.

Barometric pressure also varies with weather conditions (moisture content). Meteorological people have complex models of how this works.

With lower static pressure, that is at higher altitude, there is less oxygen available in the fuel mix, so we want to add less fuel to compensate.

With higher static pressure, we get the reverse and we need to add more fuel.

So if we assume that changes in static air pressure affect the vacuum pressure acting on the CV carb slide: the fuel mixture is automatically adjusted for altitude!

And the mixture will also be compensated for other static pressure changes, like barometric weather conditions. I had sort of thought that was one of the benefits of CV carbs?

Click to expand...

Nope.

The needle lifts in unison with the slide, proportion remains constant.

CV carbs brought a next level of drivability & economy, but did not give any barometric pressure compensation.


"So if we assume that changes in static air pressure affect the vacuum pressure acting on the CV carb slide: the fuel mixture is automatically adjusted for altitude!"

Ambient air pressure does not change the vacuum signal created upstream from the butterfly.

I would have thought air temperature would have a bigger effect on mixture requirements than air pressure. The more important aspect with a Commando is probably throttle response. It surprised me that the heavy crank can be made to work so well. When I built my Seeley 850, I never believed in it until I actually began to race it. When you lower the gearing on a motorcycle, it should accelerate faster. The 850 motor always seems to spin-up at the same rate regardless of the gearing. With wide ratio gears, it is useless. Even the leaner needles in the carbs make a difference.

My 500cc short stroke Triton was totally different. Lowering the gearing on that made it dangerous. It would come out of corners like a rocket and run out of steam towards the ends of the straights, so in the following corners, I was likely to hit someone. The Seeley 850 accelerates slower out of corners but can be ridden faster in corners.

concours said:

Nope.

The needle lifts in unison with the slide, proportion remains constant.

CV carbs brought a next level of drivability & economy, but did not give any barometric pressure compensation.


"So if we assume that changes in static air pressure affect the vacuum pressure acting on the CV carb slide: the fuel mixture is automatically adjusted for altitude!"

Ambient air pressure does not change the vacuum signal created upstream from the butterfly.

Click to expand...

'The needle lifts in unison with the slide, proportion remains constant.'

Good point, I guess I haven't thought about it enough.

All carburettors are affected by ambient air pressure, with lower ambient pressure the density of the air flowing through the carb is lower. Lower oxygen density means you need more air to maintain performance. Superchargers used to be popular on big aero engines.

Velocity is also a factor......so this chap said! https://en.wikipedia.org/wiki/Bernoulli's_principle

acotrel said:

I would have thought air temperature would have a bigger effect on mixture requirements than air pressure. The more important aspect with a Commando is probably throttle response. It surprised me that the heavy crank can be made to work so well. When I built my Seeley 850, I never believed in it until I actually began to race it. When you lower the gearing on a motorcycle, it should accelerate faster. The 850 motor always seems to spin-up at the same rate regardless of the gearing. With wide ratio gears, it is useless. Even the leaner needles in the carbs make a difference.

Click to expand...

Those who use dynos for tuning like to record ambient conditions for each run for comparisons. They record temperature and pressure.

The Rotax DPM system i mentioned uses flat slide carbs (NON-cv) and varied fuel mixture by having closed air circuit on all bowl vents, and connected to a common chamber. The computer uses temp and pressure sensor, and calculates the needed pressure in the chamber. Then, an small pintle valve provides precisely metered vacuum to slightly reduce the pressure in the chamber & bowls to slightly below ambient, thereby leaning the mixture to the calculated amount.

It was a good stepping stone before direct injected two-strokes.
I put 25,000 snow miles on those systems.

I have had these CV carbs fitted for 3 yrs now and can require a bit of trial and error, mainly with the needle, I start by fitting the richest main and pilot jets supplied , buying and fitting an adjustable low speed air screw and removing material from the aluminium needle by grabbing in my drill and using fine emery. I try to remove about .005 along the taper. It is interesting as only a few thou can make a big difference mainly in warm up time, ie cold start response. One bike my 828 Triumph, Norton cranked big bore responds fairly quickly. A Norton 850 fitted with the Norvil starter takes 3-400 yards down the road until it responds nicely. Just a couple of thou on the needle. My brothers chopper takes even longer and I intend to take a bit more off the needle, it too is a Norton stroked big bore 828. Before I fitted Shropshire Cycle supplied starters, ( these things work great) to both Triumphs mentioned it would be about 3-4-5 kicks to fire off with the CV, about the same with the Mikuni cold start, previously fitted. I tried some Sportster 88 needles and they needed some sanding to prevent prolonged warm up. On all these bikes the carb has no sign of 8 stroking due to over richness. Rollon is great and clean, no hesitation. There can be cold spit back if the stock needle is used. On this Norton and one I have just fitted to my wifes Nevawozz Norton I have had to remove metal from the frame brace to clear the carb top just as the factory did when playing with SU fitment. Another Norton coming along and a pre-unit Triumph I am ready to fire are both CV recipients. The 62 Trumpet is also Norton stroked and decompressed, only a kicker so may not be ridden much with rooted knee. I used MAP 38mm manifold and rubber for the Norton and welded up a std single carb manifold with a stub to suit for the Triumphs. As they are pumper carbs the start is best with 3-4 pumps and FULL enrichment on the pull cable or it will be a longer process. Set the idle and it does just that. I do have another one on an 850 Triumph bobber, the rollon is a hoot, way better than the previous Dellorto.

With my 850 motor on methanol, half a thou on the needles is the difference between fast and slow. If using petrol, it would be one quarter of a thou - not possible with a normal carburetor. A thou off the needle diameter is huge.
I use methanol at standard compression. My needle jets were 0.117 thou. and I could not induce the cough by lowering the needles. I made two more jets at 0.1165, and got the cough. When I raised the needles one notch, the bike became extremely fast. Methanol jets usually flow twice as much as petrol jets. With petrol, that half a thou difference would become a quarter of a thou.
With a CV carb you might have two things happening, but the needle would still need to be correct.
With a road bike, it does not matter. You are not seeking the fastest acceleration. You would never notice the difference. Reliability is more important.