CV carbs?

[raging squirrel]

I was wondering if anyone here has ever experimented with constant velocity (cv) carbs on the EA81 engine? Fitting up a set would be simple enough and there are plenty of large displacement motorcycles that used them over the decades. I am seriously considering this in place of the more common Weber swap. Opinions? Anybody already tried it?

[WoodsWagon]

I don't like CV carbs. I'm assuming you're talking about the ones that have a throttle plate and a vacuum diaphragm that controls the slide and needle? They smooth out throttle response, nice for a cruiser bike but not what I'd want on a performance bike.

 

Using a slide carb would be cool, and you could make a manifold so that each carb ran one head. Finding a carb for each head rated for a 450 2 stroke probably wouldn't be too hard.

[flight_of_pain]

Having rebuilt/restored vintage motorcycles for many years, and to make a living, I can only suggest you take the idea of CVs and throw it in the proverbial trash. They tend to not like large pulsing vacuum draws, and on a sube that is what we get.

 

A carb set up for a 2-stroke is also going to be unhappy without changing it completely over to 4-stroke operation.

 

Isaac

[raging squirrel]

Smoothing the air flow was kind of the idea. It can help fuel mileage without hurting horsepower. Acceleration potential is higher with a non CV carb, but in reality, most people can't truly control the throttle well enough to do it. But we are not talking performance bikes, this is a long range cruiser Brat. I had intended to dedicate 1 carb for each pair of cylinders, which should nullify some of the pulsing vacuum draw. The other option was to run all 4 cylinders off of a pair of carbs attached to a common intake plenum. The second option would tend to have less vacuum than a short runner at each head though, and would tune the engine for a higher rpm range, not what I am after.

The most common source for CV carbs in our neck of the woods is actually ATV's. Not sure about converting for 4 stroke use, I didn't realize that they were ever used on 2-strokes. I do know that there is a difference between gravity fed and pressure fed applications. A smaller fuel inlet orifice is required when using a fuel pump. Other than that, a carb doesn't really care or know what it is feeding. Anyone actually try the CV's?

[NorthWet]

...Other than that, a carb doesn't really care or know what it is feeding. Anyone actually try the CV's?

They do care. The metering needles need different tapers to be close to accurate for changes to the cylinder(s) that they feed. Trying to feed one bank from a carb will be like trying to feed a "twin-gle" from one carb: probably get different A/F ratios for each of the pair. Almost any dual-carb arrangement will have this issue, but the inertia of the CV slide will probably aggravate this.

 

My experience was mostly with British and Japanese cars that used multicarb set of SU or Stromberg carbs. Metering needles were the devil to get right when making major changes. All of my other CV and slide throttle experience was one-carb-per cylinder Japanese multis.

[raging squirrel]

Understood. I do realize the differences in needle tapers, etc. Been down that road before. I was referring to the fact that all carbs require vacuum to draw in air and pull fuel through the jets and past the needle. As long as the air flow requirements are sufficient, the carb mixes air and fuel. An O2 meter would greatly aid me in finding that perfect taper.

I got thinking about the pulsing intake/vacuum theory. Kawasaki atv's ran single cyl. 4-strokes with CV's and they worked well. A single gives about as much start and stop to the charge as possible. The common plenum intake with a pair of CV's setup as a progressive two barrel (like the Weber) may provide the most consistent vac and flow. The goal of which is to improve fuel atomization, thereby making the charge easier to ignite and burn more thoroughly (more power from the same amount of fuel).

[NorthWet]

Many bikes used CVs, one per cylinder; just have to get the taper right to deal with the heavy pulsation. Certainly doable, just maybe tedious.

 

The problem that I am envisioning is dealing with 2 pulses in succession from the siamesed intake port, then pausing for 2 beats before the next suck-suck-pause-pause cycle. This would be equivalent to a twingle (4-stroke twin with a 180-degree crank). Might be tough to tune it.

 

Something to consider doing is use an ECU/O2-sensor/duty-solenoids from a carb model to tune the mixture without overthinking the needle taper.

[raging squirrel]

I will be studying this some more, a good friend of mine is an expert with carb tuning and theories. He makes part of his living doing it, so he may have some very useful input. I may try this and the Weber and see which works best. I have given some thought to flat slide carburetion as well since it responds to the pulse so much quicker, but this may not help me with mileage. If raw acceleration was the goal, I feel that flat slides would offer potential. Just one more thing to wonder about...

[raging squirrel]

After discussion with my carb tech buddy, it seems that I was correct in believing that the "pulse issue" was not a big deal. He did, however, point out something that I overlooked. How am I going to play with needle profiles when I can not tell how far open the slide is? A simple response is to tune with the air box, filter, and duct work disconnected. Problem is that this affects air flow and vacuum at the throttle valve. Hmmm...it may get me close enough to be at the "fine tuning" point though. Especially if my filter system is very free flowing. I may have to lean things out a bit to compensate for heavier engine loads afterwards, but the CV system should prevent the need for most of that. Since the throttle valve only opens enough to feed the amount of air the engine can actually use, "over throttling" under heavy loads should not be possible. I will post up some results when the time comes, assuming that I actually go through with this. I will first use a Weber to establish a good baseline for comparison.

[KootenayJK]

IIRC, fuel injection works pretty good for increasing fuel economy without adversely effecting fuel economy. Can be done pretty cheap on these things using components from older VW's, too - except for the whole injector mounting issue. SPFI can be put on, too.

Edited February 4, 2011 by KootenayJK