[donkey]

I'm looking for a better/bigger carb. The more simple and cheapest options are Keihin 30mm Flat-Slide and Mikuni VM 34mm, but who is better?
http://www.royalenfieldusa.com/30mm-flatslide-motorcycle-performance-carburetor-p-7042.html
http://www.royalenfieldusa.com/mikuni-34mm-carburetor-500cc-only-p-506.html

[HRAB]

I am really fond of the flat slide carburetor. It is about 1/3 the physical size of the Mikuni 34 and performed well on our Electra test bike. The performance was most noteable in the midrange where the HP curve was raised and made more linear. Peak hp increased, and the torque curve was flattened further into the RPM range.

I have not tested the Mikuni 34 on a dyno.

I've attached a scan of the dyno runs comparing the stock vs modified bike.
jim

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[LotusSevenMan]

Useful power hike etc while maintaining reliability. Like it!

[baird4444]

My first thought was why??

but after seeing the dyno sheet I want one!! Max H.P. at 2 grand??
I gotta say that I've pretty much been a believer in staying stock and not wasting money to gain 1/2 horse here and there but those #'s are good if I'm reading them right. I hope CMW has a good stock of these...   
do ya keep jets in stock?  Have you had enough experience to be able to tell me that if I'm running a 27.5 pilot and 130 main then i need a ##  for the same set up??
Mike

[mwoulfe]

Question about the 30mm flatslide. It comes with #25 and #40 pilot jet. I'm running a 30 pilot and 135 main in my stock mikarb, and it's the perfect combination for easy starting and a nice tan plug. I tried the 40 pilot and ended up with a black,fouled spark plug. 25 has to be too lean. I ordered a set of tuning jets from royal Enfield and they sent me....another 25 and another 40!!! Where do you find jets for this carb in the 30's range? Has anyone dialed in a flatslide with a stock engine and brit-style muffler? What size jets did you end up with?  Thanks...woulfe

[baird4444]

I just installed mine yesterday. It came fitted with the 38 slow jet and 140 needle jet. The extra slow jet was a 125. It took me a while to get the air screw dialed in...   I'm still not sure if to screw in is lean or rich, any body??
Plug had a lot of carbon and engine would not rev past half throttle, bout 60mph. This morning I went down to the 135 main and have full throttle. It pops thru carb when idling down some times, to lean or to rich?
 Plug is still black but I'm getting closer. My next step will be to work the air screw when I figure out which way to go.

The choke is a pain in the arse...  It's a bitch to reach thru to pull. I'm guessing that it doesn't enrich quite as much as the old....  just higher revs... might be better once I get used to it.

YES, it does give more power at the bottom end of the revs. The motor doesn't labor as much at speed either. Running 60 the motor seems like 50... 
I really think I'm going to like this-
   
My old set up was a 130 main and 27.5 pilot. I've got a K&N air filter and my Bazooka silencer has been drilled out with a 9/16's
                      - Mike

[cyrusb]

Baird4444 The pilot screw can be either, it depends on what side of the slide its on. If its upstream(aircleaner side) you are metering air, so its "in" for richer mixtures. Downstream of the slide, your metering fuel, so its "in" for leaner mixtures.

[ace.cafe]

A flat-slide carb will flow a bit better than a round slide.
It will do this over the entire range, and will help.
The better intake charge speed will provide some torque increase.
The larger venturi diameter will only help at throttle openings that provide a larger intake area than the smaller carb could provide at max throttle. Primarily top end.

To be fair, that dyno chart pdf filename mentions the carb and classic exaust, so I''m taking that to mean the comparison on the chart is between a stock bike, and a bike with flatslide carb and improved classic exhaust system installed.

Please note that the exhaust system would be responsible for some of that small lower end bump in the torque curve, as well as some of the rest of it.
I don't think that chart reflects the carb only. I suppose it is possible for it to reflect the carb alone, but I think it is the combination of the carb and exhaust.

To properly read that chart, the two uppermost curves are the torque curves being compared, and the low lowermost curves are the hp curves being compared.
To find which ones pair up together for comparison purposes, each pair of hp and torque curves will intersect at 5250 rpm.  So, one of the hp curves near the bottom will intersect with one of the torque curves that start up near the top, and they will intersect at 5250 rpm, and that's one pair of curves that goes together.  The other pair of curves that go together also intersect at 5250 rpm. Torque and hp will always intersect at 5250 rpm on a proper dyno chart.

So, as you can see there is not alot of low end difference in either torque or hp, until approximately 3000 rpm and up, where the carb and exhaust improvements are mostly notable on that chart.
Max hp is definitely not happening at 2k rpm, as suggested above.
But, the torque curve is definitely extended nicely into the higher rpms, and is almost table-flat out past 4500 rpm, which is very nice. That extended torque curve is what's hauling up the hp curve higher along with it, since hp is a function of torque x rpm.

However, I do think that a flat-slide carb is a good choice when available, over a round slide. They were designed to be an improvement to the round slide type, and it has been pretty well proven for years that they actually are a bit better.