carburetor differance

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shovelhead
Posts: 8
Joined: Mon Sep 01, 2014 12:19 pm

carburetor differance

Postby shovelhead » Mon Sep 08, 2014 12:49 am

Is a cv carburetor better than the carburetors with mechanically operated slides? I have both; should I stick to what came with the engine? Everything is in pieces but I think I have carbs from a 82 custom and a 80 night hawk

Mainerider
Posts: 332
Joined: Thu Sep 02, 2010 8:45 pm

Re: carburetor differance

Postby Mainerider » Mon Sep 08, 2014 6:55 am

Do you have that backwards? Did you mean an '82 Night Hawk and an '80 Custom?

The head on the '81 and '82 SOHC 650 with CV carbs is different than the head on the '79 and '80 mechanical slide carb model; the intake spacing is different so without a lot of modification and headache the carbs are not interchangeable. In most areas, the CV carbs are far superior to the mechanical version; better throttle response with less bogging down when snapped open quickly, and a much, much lighter throttle action ( no heavy throttle spring needed on a CV carb) are just a couple of the advantages.
The only real advantage to the mechanical slide carb is that, all other things being equal, the mechanical will move more air because even when fully open and horizontal, the CV carb's butterfly throttle plate is still a slight impediment to air flow passing through the carb throat.
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shovelhead
Posts: 8
Joined: Mon Sep 01, 2014 12:19 pm

Re: carburetor differance

Postby shovelhead » Mon Sep 08, 2014 12:36 pm

You have answered a lot of questions. Thank you. I thought the mechanical slide carburetor might be a poor man's Mikuni. I am also an old school mechanic and the slides and diaphrams on these carburetors are a complete mystery to me.

MEHall
Posts: 403
Joined: Mon Nov 15, 2010 8:25 am
Location: Sweden

Re: carburetor differance

Postby MEHall » Tue Sep 09, 2014 9:26 am

I can only say if you want faster response in throttle you go with the mechanical carburetor but the CV carburetor is more forgiving! :wink:



8)
Riding bikes? I ride where ever i want and whenever i want. :wink:

Online manual: http://cosky0.tripod.com/

Mainerider
Posts: 332
Joined: Thu Sep 02, 2010 8:45 pm

Re: carburetor differance

Postby Mainerider » Tue Sep 09, 2014 1:28 pm

Actually, that is true only at higher engine speeds; that ties back to my point about flowing more air due to less obstruction in the carb throat. Mechanical slide carbs were often preferred in racing applications due to the added air flow capability at WOT.

For real world applications...
One of the problems with mechanical slide carbs ( and one of many reasons why they were rapidly replaced in large part by CV carbs) is that a large grab of throttle at lower engine speeds will cause a temporary leaning of the air/fuel mixture, which often resulted in a bogging-down condition until the fuel flow caught up with the added air flow. This issue was addressed in several ways, such as with the use of accelerator pumps to add a short burst of fuel enrichment to the mix, but the problem was never fully resolved.

With CV carbs, the air pressure remains constant regardless of throttle position, so a ham-fisted rider is essentially removed from the air/fuel equation and can not easily affect the air/fuel ratio.

Another major advantage to CV carbs is they allowed the bike designers to package the bikes in a more compact manner because CV carbs have a much lower height; as bikes became more complex, finding space for everything was a challenge. Unlike mechanical carbs, with CVs there is no need to make the carbs rack tall in order to accommodate the distance the slides have to travel vertically as they are mechanically pulled open by the throttle linkage assembly.

Tied in to this point is the additional benefit of not having to use a heavy return throttle spring to overcome the engine vacuum effect on the slides; any one who has ridden both a slide carb 650 and a CV carb 650 can attest to how much easier (and less tiring) the throttle action is on the CV model.
Last edited by Mainerider on Tue Sep 09, 2014 1:35 pm, edited 1 time in total.
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shovelhead
Posts: 8
Joined: Mon Sep 01, 2014 12:19 pm

Re: carburetor differance

Postby shovelhead » Wed Sep 10, 2014 2:19 pm

Thank you for the detailed reply. This old dog will need to learn some new tricks. I have several old Asian bikes and if I am going to keep them I will need to learn how things work. Is there any advantage to diaphragm versus piston operated slides?

Mainerider
Posts: 332
Joined: Thu Sep 02, 2010 8:45 pm

Re: carburetor differance

Postby Mainerider » Wed Sep 10, 2014 6:15 pm

If you are going to be delving into the carbs on these older bikes, I would highly recommend this book by John Robinson:

Haynes Motorcycle Fuel Systems TechBook

It is about $35 through Amazon and it is worth every single penny and then some. He explains the history and evolution of motorcycle carbs ( and fuel injection) and thoroughly explains what all of the components do and how they function. If you have any interest at all in really understanding carburetors you will have a hard time putting this book down; very well-written and a good read.
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RebelYankee
Posts: 17
Joined: Tue Jul 09, 2013 12:37 pm

Re: carburetor differance

Postby RebelYankee » Fri Oct 03, 2014 7:40 pm

I just changed out my 1980 piston carbs with cv's from a 1982. I will tell you that you will have to change the head (already mentioned), but you will need to change the airbox as well since at high RPM's, the vacuum from the airbox helps pull in fuel to the bowls via a hose (connects to the bowl vent tubes). This hose is not present on the 1980 airbox.

Also will need new push and pull cables.

As far as performance, I haven't noticed a difference. But, the CV carbs are so much easier on the wrist, especially when cruising. They are also much easier to adjust and get running properly. I am happy with the change!

Jamie


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