28 replies to this topic

[Rob Jones]

Over the last four or five years I have searching for a good Dellorto 45 carb at a reasonable price to fit to my 1380 rally spec Mini, but with no joy. Therefore, after a mate of mine bought a rally spec Nova equipped with R1 carbs it got me thinking of putting some on the Mini.

After searching the web for info on what carbs were fitted to what bikes, I came across the 2 cylinder Yamaha 850TDM, which obviously only uses two carbs, rather than the usual bank of four which are often sold on E-bay. These carbs are 38mm choke size, therefore equivalent to the 1 1/2” SU and being a modern bike, has a TPS fitted as standard, so ideal for the Megajolt system which I installed in 2006 (although at present runs on the MAP system).

So the search for suitable carbs began … with E-bay coming up trumps with a set of 1999/2000 carbs at a price of just £30 including p&p, ideal! I already had a twin SU manifold, which I was going to use to mount the carbs onto. E-bay again, and SDRAM 600 mounting rubbers £15 for a set of four were obtained.

After receiving the carbs, it became apparent that the angle of the carbs wasn’t right for the carb fuel bowls, so I acquired a lump of aluminum (£20) to mill a 10 degree wedge to bring the bowls level.

Then began the fun bit, respacing the carbs to match the SU manifold. Fortunately my father is a model engineer and was able to do the milling and lathe work.

After a few months of work (part timer!), it came time to mount the carbs onto the engine. Guess what, they fired first time, much to my surprise!! Although they worked, it was apparent that they weren’t set-up, with a big flat spot when you banged open the throttle. Talking of the throttle, the pedal needed modifying, as it ran out of travel only at half pressed.

The rolling road session went well, after a few holes in the carb were blocked up and the main jets changed, we had gained 6 brake and a flatter torque curve.

After a few miles of driving, I am quite pleased with the results, with an engine that feels more responsive (quicker?) but I have a few issues. Firstly the throttle pedal is very light and unfortunately the loud Weber bark has gone!!!

TDM Carbs


Carbs respaced apart


Clearer view


Manifold modified, studs relocated, inlet bores opened up


10 degree wedged spacer fitted


Carbs fitted to manifold


Side view shows wedge




Fitted to engine






Rolling road graph

[Udo]

Good work in making the effort to space them out, I couldnt but bothered at the time when i did R1 carbs, see link in signature.

Additional spring on the throttle arm of the carbs helped loads in returning the pedal etc, standard spring isnt strong enough as all is used to do was return the grip when on the bike if that makes sense

[george91]

Good job, so whats that 105bhp at the wheels or crank?

[mike.]

Nice work! I love the idea of bike carbs, how do they sound in comparison to an SU - If you've ever driven your engine with an SU fitted.

Also what sort of filters are available?

[Rob Jones]

Udo - The spindle already has an extra return spring fitted (to satisfy the MSA scrutineers), but is still light. I don't want to put more springs on the spindle (although it is supported in the middle) as it would put extra strain on the shaft itself. I am thinking of putting a torsion spring on the throttle pedal itself which hopefully will help.

George 91 - The bhp is the lower lines on the graph, so 82 at the wheels, which shows a 7 bhp improvement over the Dellorto. The upper lines are the torque curve. As you can see, it is a much flatter line and doesn't drop off when approaching max bhp.

Mike - They sound very similiar to SU's. Although I haven't had twin SU's on the Mini for at least 20 odd years!! They still have a roarty sound though, but not the bark as the Dellorto. You can get "sock" filters made from foam, which I have ordered from Demon Tweeks for around £25 a pair.

[Rob Jones]

To alter the super light pedal, you can shorten the pedals leverage ratio.
You need to cut down your pedal and re-dill it for the cable, hacksaw a slot down to the new hole. Then drill a new hole in the bulkhead.

I had already made a new pedal, with the correct leverage, and with the correct full travel, using the original mounting points and hole in the bulkhead. But as Udo has said, the bike carbs are intended for hand throttle. At present I am looking for a correct sized and stronger enough torsion spring to put on the pedal, to stiffen it up.

[Udo]

I used a return spring off a set of twin SU carbs as an additional on the carbs if its of any help

[camp freddy]

Hi Rob I was looking at some Ducatti monster carbs , they are 38mm, but they fetch a
lot more than £30 ( , I keep an eye out for the Yam ones. looks the dogs by the way..

[Rob Jones]

Hi Rob I was looking at some Ducatti monster carbs , they are 38mm, but they fetch a
lot more than £30 ( , I keep an eye out for the Yam ones. looks the dogs by the way..

I looked at the Ducatti 990 S carbs, which look exactly the same as the Yamaha ones, but as you say, they are more expensive!!!

[miniman92]

Does anybody know of any bike carbs that are 1.25 inches?

[greenlaner1]

Does anybody know of any bike carbs that are 1.25 inches?

how about the mikuni BSR32 twin carbs from a CCM R30/644... they also run upright so there would be no messing about getting the angle right

GL1

[miniman92]

Would a pair of gsx750 carbs work ? They are also 32mm and seem to be easier to get hold of. Do they run upright ?

[craig 1010cc]

What about looking at a MPI pedal set up as that has a return spring on the pedal?

[crock]

Don't you find that the carburettors starve for fuel in long turns? Motorcycle carbs are not designed to handle the G forces from lateral acceleration.

[Rob Jones]

Don't you find that the carburettors starve for fuel in long turns? Motorcycle carbs are not designed to handle the G forces from lateral acceleration.

No problems at all. Competed on the Cambrian Rally a couple of weeks ago in the forests of North Wales and the tarmac road around the Great Orme.





The bike carbs performed great ... pity about the rocker shaft breaking on stage 3, but after borrowing a shaft assembly from a local garage, we finished, despite the headgasket failing.

Edited by Rob Jones, 31 October 2011 - 11:54 AM.