Alternate Carby by PMCBURNEY. Not SU.

[wayne]

This from a board member.

Not all conversions need to be SU.

Pmcburney's A model with Mikuni BST40 carb transplanted from a DR650. I've had a good ride on this bike before he did the final sorting and even then it was flat spot and hesitation free.

[wiznorton]

But what do you do with the thirteen redundant cables??      


In all seriousness though , is it now on premix, or still using the pump?  Just wondered as the tach doesn't seem to be working so wondered if the pump was removed or modified?

[pmcburney]

I'll explain what I did, and that tach...


If you look carefully at close up shots of the carb, you'll see a 90 degree brass fuel line connector/junction behind/above it - you can see where I've drilled and tapped it and fitted the oil feed check valve on top.
That's as good a spot as any to get the oil feed into the fuel, and it meant I didn't have to drill and tap the carb fuel bowl (which would have been difficult to seal later if I removed it).

The tach does actually work, it's just very, very, very lazy!

I had my instruments refurbed by Allan (member here) and he did a superb job (they look brand new), but I think he used a little too much silicone damping fluid on the tach mechanism as it moves but too slowly to reflect fast changes in engine speed.
It's fine for cruising as it eventually settles, but is otherwise a bit useless when revving the engine.

As for adapting the carb to the OEM manifold, I tracked down a guy near me that does water cutting.
I got him to cut three 10mm thick (I think) aluminium plates that progressively transition the BST40's single venturi to the OEM triple venturi arrangement, bolted them together with some gaskets and sealant and then opened and smoothed out the completed passage using a die grinder.

The BST40 carb mounts on a stock DR650 rubber inlet manifold bolted to the adapter and the adapter then bolts directly to the OEM manifold using the original mounting points.
This arrangement was quite a bit of mucking about, but it means I can restore the bike back to stock seamlessly and I haven't sacrificed or otherwise modified any OEM intake parts.

The carb is jetted about the same as a DR650 that has been 'opened up' (Google DR650 carb mods), and uses a bit more fuel than the stock RE5 carb, but (seat of the pants) feels like it accelerates harder and has more torque.
This might be a product of the more instant throttle response this setup has - the stock RE5 carb is notorious for its flaccid feel while running only on the primary circuit.
There is a more pronounced 'lope' at idle than the stock carb which I think is related to being slightly lean on the pilot circuit, but it's not that problematic - I could likely fix it with a larger pilot jet but I can't be arsed at the moment.

The timing of the port valve seems to make little difference to performance - I have it operating approximately the same as stock to increase air speed at low throttle openings, but the bike runs well even with the port valve held open.

I removed all of the 'throttle' cables except two - one for the carb and one for the oil pump - this required modifying the OEM throttle cable destructively, but I figure they're not that hard to come by if I go back.
The DR650 carb is supposed to run a push-pull cable arrangement, but I haven't found it necessary.

I would not recommend anyone go down this path unless they have access to someone or the ability to make a new intake adapter similar to what I've done, plus BST40 carbs are not cheap, even second hand.
This is the result of many, many hours of trial and error and experimentation and a fair bit of cash over a long time, and while the result is very good, it's likely not that much better than a well-sorted OEM carb or an SU.

Ideally, I'd piff the BST altogether and fit a throttle body for EFI, but that's another story...

[wiznorton]

Nice work fella! Thanks for the write up. Your observations back up what I have found with my Norton and Hercules experience on peripheral port engines, namely that they are actually not over sensitive to fuelling issues, and the fact that the bike will run OK with the port valve open still makes me wonder if the stock setup is way more over complicated than it needs to be.

Do you have any drawings for the parts you got cut? I could probably get a CNC program written for that if people might be interested? But obviously I don't want to 'muscle in' on your project which has obviously taken a lot of time and effort to get to that stage.

It's fine to say that the stock carb works well if it is perfectly set up, but if there is a simpler solution that doesn't have the various fiddly add ons,(and with my racers' hat on weighs considerably less!) then why not???

Another slightly unrelated topic is the obvious problem the manufacturers had with tickover. Norton tried various solutions including what they called single rotor idle, which cut the ignition to one chamber so it acted as a drag to slow the other down. The Hercules had a vacuum chamber arrangement which bled air into the inlet at slow speeds to weaken the mixture. Certainly on the Hercules, you can remove the whole thing and block it up (as the vacuum chamber is a weak point) and the bike still runs and idles fine. Fair do, if your thing is making videos for you tube of the bike ticking over for 20 minutes at a time, it might not be perfect, but as an everyday ride it works fine.

Adding simplicity is my thing!!!!