Question to Keihin flat slides

Thomas aus LAU

Thomas aus LAU

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Location
near Nürnberg/Germany where the best hops grows
I have a nearly new 4-way carburetor set Kehin Flat Slide FCR 39, which can easily be divided into 2 + 2.
Every two carburetors share an accelerator pump.
Since the float chambers are at an angle here, this is ideal for a 750 Laverda (old).

Questions:
- Is a 39 carburettor too big if the 750 cc engine with big head is slightly tuned, compression, air filter, exhaust, ...?

- If so, has anyone ever reduced the cross-section of a carburetor with a sleeve?
Is doable.

- Are there any tips which jets have proven themselves?

Thanks in advance for the reply from Thomas from LAU
 

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I'd describe 39mm as slightly too big for a street ridden stock 750. But a tuned one which may see track days, yes, why not.

I've inserted venturi shaped sleeves in carbs to take the bore size down to a certain size to satisfy rules for a class.
They are only short inserts and on the engine side of the slides. Don't have much if any effect on flow - but meet the rules for bore size.
Doing a full length insert would IMO be pretty difficult. And there's the drillings for idle fuel outlets too.....
 
The keihins are a sophisticated carburettor and likely offer enough tuning options to be usable. But no doubt will require some time and effort...

If you're going to that much trouble, why not take it out to 880cc?

cheers,

bazzee
 
Interesting looking FCR set there, Thomas. What are they off? I bought a bank of four off a Suzuki 1100 (H or 'horizontal' type - they look like they'll still work on my motor) and they had one accel pump for all four. The 39mm venturi goes through a tapered a manifold from 39mm down to around 37.5mm. They also had a hardchromed actuating rod right through connecting the bank of four with the cable pulley between 1 and 2. Splitting them was a lot of work; yours would be easier.

My brother Tippie will probably chime in soon. He has used 39mm FCRs on his stock street SF and hi racing SF. There is no problem regarding the size - they will function fine on a stock motor. I wouldn't bother sleeving, but you . Note that a stock big valve SF head has a 34.5mm inlet at the carb end (so Dells go from 36mm to 34.5mm in the inlet stubs and out again further down the port - I ported this out of my head. SFCs sem to have the 36mm ports as std as well. The SF system is called a double venturi and is suppossed to boost midrange. My motor had great mid and top end after the porting and fitting the 5/C cam. You can easily taper the intake side of the FCRs from 39 down to 36-37mm. We have base figures for jetting.

I'm in the process of setting up mine for the SFQ, 880cc, hi comp, SFC cams, big inlets valves, ported head and open exhaust. I will use the 2nd set on the more std motor for bike #2 (one day!!).
 

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piranha-bro2 said:
Interesting looking FCR set there, Thomas. What are they off?
Click to expand...
The carbs come from a Kawasaki ZXR-project from "Egli-Racing-Hamburg".

Yes, more cubic capacity would be a possibility, but the effort is great here and there are technical limits with regard to the diameter.
More stroke is not possible, because the piston speeds will get too high.

Never lose sight of your dreams!

One of my last big projects looks like this:

- remove the rear frame of an old 750 frame
- change the frame geometry similar to a Zane frame
- use the steel rear frame of a Zane (screwed on, two heights)
- use fork, wheels, brakes, mono-shock absorber and swingarm from a Zane (668 in this case)
- Remove the starter and replace the alternator with a Dynastarter that is available in the same diameter.
- swap the Dellortos for Keihin flat slides
- A roots blower (compressor) is installed in place of the starter.
- of course there is also a digital ignition and a simplified wiring harness.

Why compressor charging?
- You get a lot more torque and power without putting a lot more strain on the engine components.
- Compressors like the Eaton M45 are proven and affordable.
- In contrast to a turbocharger, the effort is significantly lower.
- The removed former starter gives you enough space to install a charger

You can get here good informations: compressor Guzzi

If global travel is not expected to be possible in 2023 due to Corona, the project will start next winter.
If you can (probably) travel to Russia, Mongolia and beyond in 2023, I'll be on the road with the Land Cruiser HZJ78 for the whole year 2023 and the project will have to wait until 2024.

We will see.......

Thomas aus LAU
 
Last edited:
Thomas aus LAU said:
The carbs come from a Kawasaki ZXR-project from "Egli-Racing-Hamburg".

Yes, more cubic capacity would be a possibility, but the effort is great here and there are technical limits with regard to the diameter.
More stroke is not possible, because the piston speeds will get too high.

Never lose sight of your dreams!

One of my last big projects looks like this:

- remove the rear frame of an old 750 frame
- change the frame geometry similar to a Zane frame
- use the steel rear frame of a Zane (screwed on, two heights)
- use fork, wheels, brakes, mono-shock absorber and swingarm from a Zane (668 in this case)
- Remove the starter and replace the alternator with a Dynastarter that is available in the same diameter.
- swap the Dellortos for Keihin flat slides
- A roots blower (compressor) is installed in place of the starter.
- of course there is also a digital ignition and a simplified wiring harness.

Why compressor charging?
- You get a lot more torque and power without putting a lot more strain on the engine components.
- Compressors like the Eaton M45 are proven and affordable.
- In contrast to a turbocharger, the effort is significantly lower.
- The removed former starter gives you enough space to install a charger

You can get here good informations: compressor Guzzi

If global travel is not expected to be possible in 2023 due to Corona, the project will start next winter.
If you can (probably) travel to Russia, Mongolia and beyond in 2023, I'll be on the road with the Land Cruiser HZJ78 for the whole year 2023 and the project will have to wait until 2024.

We will see.......

Thomas aus LAU
Click to expand...
Wow, that is quite a project Thomas. With a blower are there other issues with carburetion regarding pressure differences?
On banks of FCR there is often just one needle roller supporting the operating shaft instead of one each side of individual intended carbs, it depends on your setup if that is an issue.
I bought my FCR39 from a bike that went down to FCR35 because a tuning guru couldnt get them right, told the owner they were to big. I got them perfect on my 750. If you can fit longer intakes between cab and head they are even better, but with a blower you are in different territory so not sure if this is relevant.
keep us up to date.
 
Some thought points I used when I first installed FCR Carbs on a triple...:

-a 750 engine is - per cylinder - very similar in terms of bore and stroke and capacity to a 1200ccm engine (750 ./. 2 = 375ccm; 1116ccm ./. 3= 372ccm; both have 80,xmm bore and similar stroke).
-Guzzis very often use 40mm, even on the 750 and 850's, at least though 38mm
-Cico's racers where equiped with 40mm PHM carbs
-I might be wrong, but I think also the Sulzbacher racers used 40mm at some point in time
-the FCR are WAY better and more odern carbs than the Dell'Ortos

On the contrary side, there is the inlet channel with 34,5mm as stated above. It would require some machining to get them to 36mm at least I would say. then you could work with tappered inlet manifolds (I did this on all of my machines using FCR's). Also, I produced my own version of the "Power blade" on the intake side of the carbs and installed them in the "trumpets". the Jota is a real joy to ride with them on. On the 1000 SFC, I have to adjust settings again after the engine rebuild. For that though, the engine needs to be fully dialed in, which will take another 1-2.000km

HTH
 
That Saxon Mk 5 that Cathcart raced used 40mm Dels with some pretty intense porting, that raised some eyebrows back then. Those Power Now gadgets work very well.
 
I was always intrigued with the Kehin Flat Slide Carb. Never got around to using them mainly because I ran out of time and got too old.

Would loved to have witnessed a Breganzie Triple EFI Model ;o(
 
Vince said:
That Saxon Mk 5 that Cathcart raced used 40mm Dels with some pretty intense porting, that raised some eyebrows back then. Those Power Now gadgets work very well.
Click to expand...
True. But the bottom line was rather sobering. The engine produced around 104hp, from 1200cc, at the wheel on the Moto Witt dyno. Discussing the bike with Paul Taylor, we came to the conclusion that the basic port shape was pretty crap and difficult to improve upon, despite the many hours that had gone into this engine. Coincidentally, we had the Walter Prünner/Haider framed bike there a week later, both these bikes had clashed at the Zeltweg Ducati Speedweek a few weeks before with Cathcart and Prünner quite evenly matched, surpassed only by a very, very trick Guzzi. The Prünner bike made almost identical power and had 40mm carbs as well.

Paul Taylor took a Saxon Triumph triple to Daytona in 1995 for Scott "Z-Man" Zampach to ride. Paul was ecstatic, the Triumph engine finally fully exploited the merits of the chassis, which basically followed the lines of the Motodd MkV. IIrc, it was making 140+hp from 980cc at the time.

Scott was the current H-D 883 Twin Sports champ and helped me enormously to set up the Moto Witt bikes for the Daytona 200 supporting event. We didn't stand a chance actually, the field was full of future AMA and WSB stars, but we managed to put up a good show without disgracing ourselves.

piet
 
Some adventurous ideas for a humble old SF there Thomas. I like it! Other than the supercharger not too different from what I've nearly finished building, though I opted to retain the classic overall vibe by retaining the twin shock, albeit with a biggish alloy swingarm. Rejigged steering geometry, more fwd weight bias, 17 inch modern rubber, fully adjustable USD Kayaba forks from an OW01 Yamaha, R1 monobloc calipers 320mm Brembo rotors. And on and on it goes!

When i first started on this project about two decades ago I had planned to fit a subframe and an FZR1000 Deltabox swingarm and monoshock (still got them!) but decided they were overkill and out of place on the bike.
 
If you're going to supercharge it, don't use flatslides in a blow-through setup. You must use a butterfly throttle type so avoid throttle sticking.
From experience - I've built a couple of supercharged bike engined setups - I wouldn't use a slide type carb at all. even on a draw-through layout.
Because the flow is practically constant - unlike unblown, - a car type carb is better suited.
 
bazzee said:
Why is this head with Weber carby mounted on a wooden plinth?

confused,

bazzee
Click to expand...
Had to sell the 750 he raced but not the part he developed
Laverda4.jpg
 
Interesting approach, I know Webers were popular as a performance modification for cars but not usually installed on bikes.

bazzee
 
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