Forum

[mauri]

for road use a 32MM will suffice.

even with hot cams, ported, etc.

on the road were most of the time you'll be at half throttle max, there's only so much flow it can take.

in a maner of speaking if it can only take a pint class its no use feeding it a bucket.

a flat side is not really useful on the road also, and wasted on this type of engine.

as mentioned in an other thread:
no matter how far a donkey travels it'll never come back as a race horse

there more than one 612 bullet with the price tag of a modern race bike

[Presto]

Not sure what's being discussed here.
Is it a discussion about the ID of the inlet tract or the bore size of the carburettor?

There's a link between the two.

If the inlet tract has an ID of 32mm then a 32mm carb 'will suffice'.
If the inlet tract has a different ID then the carb - whatever make! - must have a bore size to match.
Non-negotiable.

Or is this too obvious to mention. From reading this thread I'm not so sure!

[Adrian]

If the inlet tract has a different ID then the carb - whatever make! - must have a bore size to match.
Non-negotiable.{/quote]

RE (India) didn't seem to think so with the Electra-X. There's an odd oval-ish inlet port on all the heads I've seen, roughly 32mm wide x36mm high, fitted with a 29mm CV carb...

A.

[Bullet Whisperer]

Regarding 32, 34, or 36 mm carbs, when I tuned a 500 Electra X for our hosts in 2008, I tried a 32 mm MK1, 34 mm MK2 and 36 mm MK2, one after the other, on the same day and on the same dyno at M&P, Swansea. Bearing in mind I was asked not to make any changes to the ports or valve sizes [they were left standard], I found with the 34 mm carb, power increased throughout the rev range over |the 32 mm. The same happened when I fitted the 36 mm carb - power increased from little more than tickover to flat out, so the 36 mm carb stayed on. That was on a 500 and here is the dyno sheet from the end of the day, when we were up some 2 bhp over what we had seen using the 32 mm carb.

[Presto]

I’m the last person (almost!) to question BW but if, say, a 36mm carb is fitted onto a 32mm inlet tract then the effective port size remains 32mm – regardless of the carb size.

However a disparity between port size and carb size will create a turbulence, which may be detrimental or beneficial – an outcome that can only be tested in use and not predicted beforehand.

I still say that the general procedure should be a match between port size and bore size of carburettor.

[Adrian]

Here's an original, un-modified and fairly typical Electra-X inlet port, parts of it are already 36mm! The tufnol carb spacer shown here has a 1.5" (38mm) hole, by the way.

A.

[windmill john]

However a disparity between port size and carb size will create a turbulence, which may be detrimental or beneficial – an outcome that can only be tested in use and not predicted beforehand.

I was going to say that. Seems pointless (not totally) polishing ports if you have a big step between the carb and manifold. Still, going to try the Mk1 900 carb on my 350 if I get a slide for it.

Added - Oops, haven't checked what bore the carb is yet! But judging by the zillion revs at tick over, it's bigger than my current manifold.

[Presto]

It does not follow that a bigger carb = bigger power. Often it’s the opposite. As the lower capacity engine fails to generate sufficient air-flow through the carb to draw the correct balance of fuel/air.

The hose with the thumb over – or the river going through a gorge - the narrower gap [smaller carb] increases flow.

[Adrian]

This still leaves us with the mystery of why RE (India) supplied a head with an inlet port that was, in your analogy, such a mis-match for their choice of carburetor. For what it's worth the carb-fitted 500 UCE Bullets for the Indian home market went up to a 33mm CV carb from the Electra-X's 29mm.

I notice that Allens' advise DBD34 Goldstar owners swapping to Mikuni VMs to use a VM36 instead of the VM38, which was the direct size match for the 1.5" Amal GP.

A.

[Bullet Whisperer]

I notice that Allens' advise DBD34 Goldstar owners swapping to Mikuni VMs to use a VM36 instead of the VM38, which was the direct size match for the 1.5" Amal GP.

A.

That could be as simple as availability [or lack of].