locosaki
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| posted on 2/2/09 at 10:36 PM |
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Flat side carbs
Is there much to gain in removing the standard cv carbs and replacing them with fcr carbs ??
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Memphis Twin
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| posted on 3/2/09 at 12:12 AM |
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Hell yeah!
Instant throttle response due to accelerator pumps. No butterflies to get in the way of airflow. You need a sensitive right foot though....
CVs are just updated SUs really.
Actually they're more like Strombergs...
[Edited on 3/2/09 by Memphis Twin]
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idl1975
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| posted on 3/2/09 at 09:45 AM |
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quote:
Originally posted by locosaki
Is there much to gain in removing the standard cv carbs and replacing them with fcr carbs ??
Yes. But FCRs or TDMRs are costly bits of kit. More power and better throttle response in their power band.
However, fuelling is absolutely critical (and notoriously difficult to perfect), and as MT suggests, as they are non-CV, they don't manage load
and airflow for you. Whack open the throttle at low revs, mixture velocity drops off massively and then you get fuel drop-out.
Once you get them into the powerband, the advantage is very evident. Assuming you're using a bike engine, they're worth doing, but make
sure they will work with your gearing. I.e. if you want the engine to pull from very low down, you may not be able to get the fuelling right. If you
just want more midrange to top end, they're not a bad investment.
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minitici
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| posted on 3/2/09 at 10:06 AM |
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I've got a set of FCR39's with ZX10, ZZR1100 spacing.
These carbs have been fitted with Flow Commander System.
The carbs are complete with the matching inlet rubbers.
These carbs were fitted to a ZZR1100 fitted with an 1166cc big bore kit so would need re-jetted for the ZX10.
If you are interested PM me
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Norledge
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| posted on 3/2/09 at 10:37 AM |
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Sorry for the stupid question... What are flat side carbs?? 
www.built4track.co.uk - cheap track days and the like!
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idl1975
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| posted on 3/2/09 at 11:59 AM |
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quote:
Originally posted by Norledge
Sorry for the stupid question... What are flat side carbs??
CV carbs have a vacuum regulated slide across the venturi together with a butterfly valve actuated directly by throttle cables. With throttle wound
to the stops, the butterfly is open, with the throttle released it snaps "shut" (not quite, for obvious reasons). The carb slide controls
the extent to which the venturi is opened up, and thus (when correctly set up) even with butterflies fully open, it (roughly speaking) won't
allow the effective diameter of the venturi to exceed the optimal value (and hence reduce the velocity of the mixture, hence "Constant
Velocity" / CV carb). So if you whack open the throttle at low revs in a high-load situation, the slide will limit the effective opening to a
usable amount - excess throttle action won't affect fuelling.
This was an advance on your traditional round-slide bike carb, where there's a bloody great slide in the middle of the venturi and one cable
goes down the carb top and raises and lowers the slide directly. If you whack open the throttle in above situation, you can enlarge the venturi so
much that airflow slows, causing rough running as fuel is not picked up effectively or is actually dropped out of the mixture to drip down the venturi
into the intake valve / reed block (if it's a reed valve 2-stroke). Open just enough, you get optimal response; open too much and the engine
"chokes".
NOW, a flatslide carb (Keihin's racing flatslides for bikes are usually "FCR" series, whereas Mikuni make "TDMR"s) is
essentially like a roundslide, but the slide itself is blade-like, which basically gives much better flow characteristics than a round slide, and
eliminates the butterfly valve as an obstruction from the middle of the venturi. In the right engine speed and load range and at WOT, an
appropriately designed and jetted flatslide gives you maximum flow and a very sharp, precise throttle response.
It can be hard to get there, however. Some setups work well over most of the rev range, but with others the engine will basically stutter and shudder
its way to 50-60% rpm and then go like ****. My favourite FCR installation was the old YZF750SP, which did exactly that. When it got to 6-7,000 rpm,
the induction noise was incredible and you had to just hang on as it tankslapped down your local A-road with the front wheel skimming tarmac.
Brilliant! You have to remember that it also had a 1st gear ratio topping out at 90+mph, so by the time it picked up you were about to break the
national speed limit
Now that was a factory setup. Imagine what happens when you fit them to your kit car and bugger up the jetting.
Personally I would stick with a good set of CVs on non FI bike engines over flatslides. And if you have FI, although it is not ideal for road bikes
(complex, bulky, poor throttle response compared to silky CV carb feel), I certainly wouldn't remove it in favour of flatslides whose fuelling
will be all to **** unless an expert sets them up.
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locosaki
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| posted on 3/2/09 at 02:58 PM |
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Thanks for the replys guys
So basically with what I have just read,I am right in saying I woul be better leaving my carbs well alone !!
I was offered a set of 41mm fcr carbs that came of a racing side car,They aslo had this flow-commander(unsure of what this is)
I was getting the complete kit plus filters etc for £275.
Theres nothing wrong with my own carbs,there just the standard 36mm cv carbs that came with the old zx10 although they have been dyno-jetted.Just
thought I could gain a few bhp !!!!
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Memphis Twin
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| posted on 3/2/09 at 03:04 PM |
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It all depends on your application. I had a set of FCR39s on my modified GSXR1100WP engined Mallock hillclimb car. Throttle response and power were
amazing (180+bhp), but I never tried trundling through town with them! In fact the operating range of the motor was between 6500 and 11000 rpm.
If yours is primarily a road car I'd stick with the CVs. Also 41mm is overkill for a non race-modified engine.
Chris.
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locosaki
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| posted on 3/2/09 at 03:17 PM |
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So the 41's are too large,The car is mainly for track use,Don't think I'll bother putting it on the road this year
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Memphis Twin
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| posted on 3/2/09 at 03:36 PM |
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Do you know your valve and port sizes?
As a rule of thumb, with flatslides you dont need a venturi size any bigger than your port size (or twice each port runner size for a 16v). If you
had 2 x 28mm inlet valves, you'd be looking at a 36mm venturi - approximately!
[Edited on 3/2/09 by Memphis Twin]
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Norledge
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| posted on 4/2/09 at 12:42 PM |
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Am i right in thinking (well been told) that standard bike carbs don't take too kindly to the horizontal g's developed in a car?
Confuses the float chamber?
Is this right or pure rubbish?
www.built4track.co.uk - cheap track days and the like!
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idl1975
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| posted on 4/2/09 at 05:26 PM |
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quote:
Originally posted by locosaki
So the 41's are too large,The car is mainly for track use,Don't think I'll bother putting it on the road this year
41s are actually not necessarily too large. If you look at the Sudco (a major reseller/distributor of bike carbs) catalogue, 39s or 41s are listed
for most large Kwakka fours, including the ZX-7R. As the 7R is basically a ZXR750 (vintage 1989) with a body kit, I doubt the flow characteristics of
a ZX10 lump are greatly inferior.
Obviously with the larger bore, the 41s will tend to give better results at higher engine speeds and 39s will tend to meter better. My limited
experience with FCRs suggests that which is better is very much a machine-by-machine thing. Sometimes you end up with similar power figures.
£275 is a good price, so IF you use the car just on the track and don't mind spending a bit of time fine-tuning the jetting (it probably
won't work out of the box, unless the donor engine was also a ZX10), it's a very cost effective upgrade.
Just be aware they are unlikely to just be a bolt-on mod. You will probably need at least a day of swapping jets and cursing to get them working
acceptably, even with a general baseline setting for a ZX10.
One practical issue is that if the car ain't road legal, you will have to do this on a track day, a very big private road or someone's
dyno. You won't be able to check fuelling without load - it may fuel fine in neutral in your garage, but at 70 mph huge holes appear in the
jetting.
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locosaki
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| posted on 4/2/09 at 05:41 PM |
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The carbs are off a powertec zzr1100 lump so your right enough I would likely have to alter the fuelling.Get it on a dyno and make sure all is
well.
Cheers
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mad-butcher
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| posted on 4/2/09 at 06:53 PM |
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Thanks for a very interesting topic, I was brought up on monoblocs and concentrics, these new fangled carbs are to complicated for me, you've
explained to me why when I said to one of the local sidecar racing teams I was thinking of changing the original honda airbox for a foam filter thay
said the easiest way was to fit flatslides as they would be easier to set up than blanking jets off etc, had at one stage thought of fitting 4 amal mk
2's
Tony
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