Which carb to go with?

Uh, Mr. Beefco bought a VM 34 back on page 2, so we're all just flapping our gums.

mschmitz57 said:

Uh, Mr. Beefco bought a VM 34 back on page 2, so we're all just flapping our gums.

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Yes, sometime it take weeks for the fiery momentum ot settle down, usually with no intervention from the author.
We're a crazy bunch!

pete.v said:

mschmitz57 said:

Uh, Mr. Beefco bought a VM 34 back on page 2, so we're all just flapping our gums.

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Yes, sometime it take weeks for the fiery momentum ot settle down, usually with no intervention from the author.
We're a crazy bunch!

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Well yes, but the original post was nearly 2 years ago!

I want to hear more about the Amal MkII Smoothbore option ;-)

Blame canampro. He stated this thing up again. I just got cought up in it (again).

My post about the MKII smoothbore was, as mentioned, 2 years ago. At the time i was just rescearching. I have no experience with this carb, only a single regular MKII 34mm for a very short time.

mschmitz57 said:

If it's a single modern carb you want, I'd check out Jim's single flat slide PWK.
Jim's carb and 2-into-1 manifold is much more compact than the Mikuni setup, and I'd wager the more modern flat slide design will perform better. I swapped a single Mikuni from my 850 for dual PWK's largely for aesthetics (sorry, it just looks wrong) but was very impressed with the performance gains of the dual flat slides, and I can putter around town just fine thank-you. Yeah, the Amal Premiers look correct if you want a museum piece, but they're still a 50 year old design with a goofy + complicated choke setup (slides within slides? Really?).

And If you don't care about aesthetics, then why ride a 45 year old Brit bike anyway?
Does anyone really think this looks good? C'mon. It sticks out like a sore thumb on an otherwise beautiful engine.

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Heres the JS version of the single (34mm flatlside) carb kit for Norton. The 34mm round slide mikuni will starve for fuel at sustained WOT. The JS flatlside has reworked fuel passages and modified needle jet to give it enough fuel flow at sustained WOT. Putting a single carb on a twin doubles the fuel air demand.

Looks like half as much trouble.

[/quote]

...and half as much FUN!

I have been VERY impressed with the "plug and play" performance of the FCR kit from Matt at CNW. I have yet to properly prove the settings on a Dyno, but general performance and plug colour etc is really very good.

For 30 years I battled Amal carbs and Lucas electronic ignition! Always idled great after a run through the hills, but around town it was always a guessing game if it would idle without dying at a stoplight! So a couple years ago I put on a Mikuni 36 mm, but still experienced some idle issues at stoplights around town. Seems old Lucas electronic ignition need more juice to be happy. So with a trispark electronic ignition and with Mikuni 36 mm my bike will idle all day at 800 RPM

Thinking about putting the Amals back on now that the electronics are updated. But why mess with what works!

jseng1 said:

mschmitz57 said:

If it's a single modern carb you want, I'd check out Jim's single flat slide PWK.
Jim's carb and 2-into-1 manifold is much more compact than the Mikuni setup, and I'd wager the more modern flat slide design will perform better. I swapped a single Mikuni from my 850 for dual PWK's largely for aesthetics (sorry, it just looks wrong) but was very impressed with the performance gains of the dual flat slides, and I can putter around town just fine thank-you. Yeah, the Amal Premiers look correct if you want a museum piece, but they're still a 50 year old design with a goofy + complicated choke setup (slides within slides? Really?).

And If you don't care about aesthetics, then why ride a 45 year old Brit bike anyway?
Does anyone really think this looks good? C'mon. It sticks out like a sore thumb on an otherwise beautiful engine.

Click to expand...

Heres the JS version of the single (34mm flatlside) carb kit for Norton. The 34mm round slide mikuni will starve for fuel at sustained WOT. The JS flatlside has reworked fuel passages and modified needle jet to give it enough fuel flow at sustained WOT. Putting a single carb on a twin doubles the fuel air demand.

Click to expand...

Jim, if the carb gives enough fuel at sustained WOT, then does this mean there is no loss of top end like other single carb setups, which I persume lose the top end by not giving enough fuel at WOT, or is it the "charging/recharging of the 2 into 1 manifold between induction strokes that usually causes the loss of top end speed?

brxpb said:

jseng1 said:

mschmitz57 said:

If it's a single modern carb you want, I'd check out Jim's single flat slide PWK.
Jim's carb and 2-into-1 manifold is much more compact than the Mikuni setup, and I'd wager the more modern flat slide design will perform better. I swapped a single Mikuni from my 850 for dual PWK's largely for aesthetics (sorry, it just looks wrong) but was very impressed with the performance gains of the dual flat slides, and I can putter around town just fine thank-you. Yeah, the Amal Premiers look correct if you want a museum piece, but they're still a 50 year old design with a goofy + complicated choke setup (slides within slides? Really?).

And If you don't care about aesthetics, then why ride a 45 year old Brit bike anyway?
Does anyone really think this looks good? C'mon. It sticks out like a sore thumb on an otherwise beautiful engine.

Click to expand...

Heres the JS version of the single (34mm flatlside) carb kit for Norton. The 34mm round slide mikuni will starve for fuel at sustained WOT. The JS flatlside has reworked fuel passages and modified needle jet to give it enough fuel flow at sustained WOT. Putting a single carb on a twin doubles the fuel air demand.

Click to expand...

Jim, if the carb gives enough fuel at sustained WOT, then does this mean there is no loss of top end like other single carb setups, which I persume lose the top end by not giving enough fuel at WOT, or is it the "charging/recharging of the 2 into 1 manifold between induction strokes that usually causes the loss of top end speed?

Click to expand...

I think Jims comments are regarding some carbs that do not flow enough fuel and cause fuel starvation at WOT.
The motor needs air and fuel. My own experience with a single 36mm Mikuni is that it cannot provide enough air/fuel flow to keep up with the motor at much more than 5k.
Personally, I have always thought that the single carb manifold is part of the issue on Norton's as the bends in them are so tight.

If a single carb is delivering enough fuel at WOT this is obviously the same amount of fuel req for the cylinder whether single or dual carbs are used, so where is the loss of speed coming from if your getting enough fuel at max revs, is it only with the single carbs that starve for fuel at higher revs? Does Jims carb provide the same flow per cylinder as twin carbs, or does it just not starve for fuel at WOT? If it wasn't the correct amount or it was starving does this give a "lean" condition.
What do you perceive to be the best 2 into 1 manifolds, I'm always tempted by Jims single set up but haven't had the courage to jump in with both feet if you like!

The motor needs air and fuel. My own experience with a single 36mm Mikuni is that it cannot provide enough air/fuel flow to keep up with the motor at much more than 5k.

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true

but the real issue is that a single carb setup will not flow enough AIR at WOT

even single 34 flow plenty of fuel, just not enough air

and what limits this is the fact that two straight manifolds simply flow more than a single that splits and actually somewhat disturbs the air flow, which is more helpful at midrange but is limiting above say 4500rpm or so

I believe Jim Comnuz has confirmed with a post the horsepower differences on his dyno

1up3down said:

The motor needs air and fuel. My own experience with a single 36mm Mikuni is that it cannot provide enough air/fuel flow to keep up with the motor at much more than 5k.

Click to expand...

true

but the real issue is that a single carb setup will not flow enough AIR at WOT

even single 34 flow plenty of fuel, just not enough air

and what limits this is the fact that two straight manifolds simply flow more than a single that splits and actually somewhat disturbs the air flow, which is more helpful at midrange but is limiting above say 4500rpm or so

I believe Jim Comnuz has confirmed with a post the horsepower differences on his dyno

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Exactly!
I think we may be mixing up topics here though. It may be best to email Jim directly for a fuller explanation, but my understanding is that these carbs were originally designed for dirt bikes, and of a smaller capacity than a Commando. Some of them did not supply enough fuel at WOT to satisfy what a Commando needs INTO the float bowl, so after a little period of wide ish or WOT, they would empty the float bowls, and then run weak.
Tis was their fueling issue, Jim has solved this.
But, as 1up says, the concensus is that a 34mm carb on a 2:1 manifold cannot flow enough air. Apart from a few people who have put 40mm pumper carbs on, the general consensus is that a single carb (and certainly a 34mm one) will cost you power in the higher rev ranges on a Commando.
My own bike, with a 36mm Mikuni, revved nicely up to 5k, it then struggled to get much above that, it would rev to 6k, but only if I 'forced it' and power certainly peaked at just over 5k.
Jim's carbs are a cheap way of fitting flat slides to a Commando for sure.
The Mikuni is a nice carb IMHO, its fairly traditional technology (and not flat slide) but they are very well made.
A friend has just put 2 new 32mm concentrics on his stock 850, replacing a 34mm mkII, and he says it has transformed the bike, much more eager to rev, with no noticeable loss of power in the bottom end either.
I see you're in the UK, these guys are very clued up on Mikuni's and Mikuni kits, cheaper than Allens too: http://www.motocarb.com

There are a few on this forum that giggle along with Kelly George poking me in ribs on forum wisdoms about what the big single carb engines are missing out on. Flat trackers go straight to top end snapping throttle on dirt and this '73 850 single intake flow restricted example way out pulled National level XR1000 and 2 other dual intake Nortons by over a dozen lengths from last turn to finish line, one with maxed out 920 Maney/Fullauto engine from Canada. Ms Peel lost 15 mph when putting on brand new dialed in 32 Amals but did spin tire easier on THE Grit up to 30's mph. There are exceptions to every rule w/o exception...

hobot said:

There are a few on this forum that giggle along with Kelly George poking me in ribs on forum wisdoms about what the big single carb engines are missing out on. Flat trackers go straight to top end snapping throttle on dirt and this '73 850 single intake flow restricted example way out pulled National level XR1000 and 2 other dual intake Nortons by over a dozen lengths from last turn to finish line, one with maxed out 920 Maney/Fullauto engine from Canada. Ms Peel lost 15 mph when putting on brand new dialed in 32 Amals but did spin tire easier on THE Grit up to 30's mph. There are exceptions to every rule w/o exception...

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Interesting!

That manifold looks quite different to the ones in the off the shelf kits though, that might be something to do with it.

Most kit manifolds are designed to fit in with standard air box / filters, hence the bends are very tight.

So many factoids to account for power results but Kelly laid out all his 'secrets' to me and one I can share is his 850 liked 38 mm best with megacycle cam, next size down form Norris D which Kelly said was next experiment to try for more. He lost power on 36 & 40 mm singles. I once believed firmly in dual cabs and all smooth air paths - until setting up Peel for mere break in use in depression waiting on dual 'proper' induction and head installed to kick up heels but got out pulled by sport bikes and ordinary good Commandos when I finally did, ugh. More than one way to skin induction cat and comnoz obviously got more with his duals injectors and most racers so who can say till experimenting/spending enough. I want to experiment with dual carbs on Peel, one rather small one, one rather large one, sort of like a GM QuadraJet spread bore 4 brl to see what happens w/o clutter of Drouin single 40 mm.

So if we're saying that single carbs struggle not so much with fuel but with delivering enough air, what would be stopping you just putting on the next size carb, would larger carb bore result in a weaker signal at the main jet?
Sorry, it's a long winded way of me chasing the o.p's question " Which carb to go with?" even though I'm quite happy with my dual amals at present.

Here's a pic of the single Mikuni that was installed on my on my 850 before I tore it down (this is the P.O.'s work, not mine).
The length of the manifold really limits the depth of the air filter, especially if you retain the rear air filter plate.
The whole assembly is like a foot long! There's no way the skinny air filter can supply enough air flow at WOT.
There's barely 1/2" of useful filter area after you subtract the end cap and the excess glue holding it together.

what would be stopping you just putting on the next size carb

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what is stopping you is the undeniable fact that, regardless and everything else being equal, a two into one manifold cannot flow the same amount of AIR into the combustion chambers as a two carb/two manifold setup can and the primary reason is because SOME of the mixture on a single manifold is sucked into the restriction inside where it splits into two paths to the chambers. For this reason two straight manifolds flow more air/mixture.

I have the Cycle magazine road test here where the testers bolt on a 34 single Mikuni and test it against the stock twin Amals.

Their conclusion was that single Mikuni was very slightly "faster" in midrange but the twin carbs were significantly stronger thereafter, with the difference being roughly a full second over the course of a quarter mile drag strip and also in top speed testing. The test also said that the Mikuni being simpler and not subject to being needed to be balanced and fiddled with as the twin carb requires, that for the majority of street riding therefore the Mikuni may be the better choice and particularly if one's Amals had over 5000 miles on them and were wearing between the slides and carb bores, mostly due to the dissimilar and softer metal material.

I ran with a single Mikuni 34 for 15 years. I recently installed a new set of Amal Premiers and the difference in felt acceleration above say 4500rpm is very noticeable.