best CV carbs

[special-k]

We have a 73 650 with the twin throttle cables. We put on a set of carbs (twin cable type also) They have a round venturi as opposed to the original style that had a hump on the floor of the carb. We thought they came of of a tx750 but now I'm not sure. In any event they worked fine with a 2-1 header and K&N filters and seemed to be faster?. I know everyone says you need foam filters but I ran both and couldn't feel a difference. Does any one have an opinion/proof of one type of OEM CV carb being any better than another? Thanks.

 

[5twins]

The later linked models are easier to sync and hold that sync longer. The newer the carb set, the more modern they are and they generally just work better and run smoother. Starting in '78, the needles were spring loaded. This really helps smooth the low end and midrange response.

If you check the needles in the carbs you have now, you'll see they're retained in the slide with a large plastic disc or washer. Hold that washer down in the slide with your finger, grasp the needle and check it's up-down play. They generally have near 1/2mm of play. Now, changing the needle clip move it 1mm. So, you have a needle able to bounce up and down through half a clip setting while in operation. The spring loaded needles maintain their position more precisely which I think is what makes them operate smoother.

 

[special-k]

So was it 78 that they went to the linked carbs? I'm assuming the spring loaded needles are only available on linked ones? Thanks for the info. I have an old set of 36mm vm 's from a xs650 /750 flattracker that I'm debating on installing but I have to get intakes for them and they are both left side carbs so adjusting them will be a pain. They were also set up 2-2 pipes with megaphones so I'm not sure if the jetting will be close with my 2-1. I'm trying to find out if it worth it to go the vm's. The bike will only be used on the ice and a few vet/vintage 1/2 miles for fun. Thanks for the info.

 

[5twins]

The linked carbs started with the '76-'77 set but that set still had the old style "loose" needle mount. For what you're doing, none of this is probably going to matter much. The old carb sets work fine if they're in good shape, they're just not quite as "civilized". But like I said, a bit smoother low end and midrange probably isn't of much concern in your application.

I guess I should ask where you want best power. Do you want better midrange or top end? The '76-'77 set makes the best midrange power. Anything newer than that will make better top end because they will work with larger mains than the '76-'77 set will.

 

[special-k]

For dirt track midrange is important because it gets a good drive out of the corner. It needs to be smooth to keep from breaking the rear tire loose excessively with some overrev on the top to compensate for gearing issues and changes in track conditions. Not too much to ask is it