Carb size, advice please?

In the early nineties I bought a set of "33mm amal mk1 smoothbore carbs" as advertised and sold by Norvil, they do measure 33mm and are marked as 933 on the id. on the side of the carbs but to look through them I see no difference between the bores in these and any of the other amal mk1's I've got lying around, I've been told by amal that they never made a mk1 smoothbore only mk2, does anyone else have these, I have seen a reference to them in one other place on google.
However, my problem is that I also have 33mm inlet manifolds and ports opened out to 33mm and now I want new carbs, so I need advice on what my best option is, I could ask amal to ream out a pair of 32mm premiers to 33mm, then all remains the same, I could buy them and do it myself or could I bolt 32mms on a 32 manifold and taper it up to 33mm to join the head, or could I bolt 32mm carbs and manifolds straight to the 33mm inlet port etc,etc.
I understand that most normal people are trying to taper down due to velocity, I don't want to weld up and re-port the head or anything radical like that, I'm just after the most sensible option with off the shelf parts.
Thanks in advance Paul.

Indeed, Amal did not / do not make a mk1 smoothbores.

Some people refer to mk1 carbs as 'smoothbores' when they have been fitted with the 'off centre' velocity stacks that line up smoothly with the inlet, rather than encompassing the pilot orifices as well. Perhaps this is what Emery meant?

As to your conundrum of what to now buy... If you search 'head flow testing' in this forum you'll find a wealth of info on this subject, mostly posted by Comnoz.

Unfortunately you'll learn that your 33mm ports are probably not ideal! However you'll also read that Comnoz explains that tapering is not required, a small radius on the 'step' being all that is required. So 34 or 35mm carbs would be an option.

Also, so may not be aware that Keihin make fabulous carbs in flat slide (FCR) and round slide (CR). These are available in 33mm sizes.

Finally, you might do well to check out these bad boys sold by CNW http://coloradonortonworks.com/part-cat ... #post-1038

I fittec these to my 850 and have been very impressed. They are not the cheap end of the market, but are fabulous things and come jetted for Nortons. It would be very easy to open the manifold the small amount to match your ports.

In summary, lots of options open to you!

You could fit a pair of 34mm Mk2s with these... http://www.stevemaney.com/products/bill ... 0tract.jpg :shock:

But more seriously 32mm Mk1s s bored to 33 was a popular production race mod in the 70s, with matched bell mouths....and plenty of full race bikes used them too....the smoothbore bit has always been a bit of bullshit anyway....and I never heard of Mk1s referred to that way, or even Mk2s in the 70s/80s.

It is easy enough to get your local machine shop to bore the new carbs, if you strip and rebuild them, though if Amal are willing to do it before dispatch that sounds hassle free....and you pretty much know how they are going to work....like you have, only easier tuning and better slow running and transitions....

But, if you fitted standard Premier 32mms I don't think you would notice any significant difference in everyday use. If you really wanted you could blend the carbs to your existing manifolds, or use new 32mm manifolds, that 1/2 mm ridge is not going to upset much and may even help create a little turbulence, see Comnoz' thread on flowbench results with various diameter tubes and steps for an advantageous way to 'align' and 'blend' things for best flow figures.

Yes they do have off centre stacks so this could have been what emery was referring to. 34mm carbs tapered down to 33mm would be more normal but how do you know your not going to overcarb the bike, it's a 750 + 40' and a 4s cam.
I have had dreams of the cnw keihins but there really is a lot I could buy for the bike with that money. Jim's new cr carbs are 33mm which is appealing, anyone took the plunge with these yet, the only thing I can say is that the amals are dead simple and keep the original look, but I'm not too sure if that bothers me or not.
As they say "its my bike and I'll do what I want". It's only bolt on bolt off stuff anyhow!

Go with the 34mm duals and leave the flow turbulizing lip at the manifold head junction and open up exhaust some to handle it then try to hang on to get back to us. Otherwise $$$$ on 34mm FCR pumpers to relieve any bog off idle and get back us or better for us try both and get back to us.

Hobot, I could also try one of each at the same time, bet that hasn't been done before, or has it? :?

If you don't want to spend big bucks... And unless you want to commit yourself to loads of Dyno time and plug chops and experimentation... And if your bike ran fine previously... Then sticking to new Amals bored to match would seem the sensible choice methinks.

Huh, theres something sensable about motorcycles, especially obsolete ones? Make a circle, draw in pie sections, lable each with carb options, put a spinner dial in center, close eyes and have wife flick it then try that first, being her choise she'll be happy, then if still unreasonalble curious flick it again your self while looking for next go around. Quickest dam 850ish Norton I've seen/tried had a 36 mm single to out pull bigger cube Nortons and XR1000 by like a dozen bikes lenghts out last turn to a close finish line so don't think about that taboo apporach out loud. Still not over what a single did to drive me crazy on Peel > so only got unreasonable advice.

brxpb said:

In the early nineties I bought a set of "33mm amal mk1 smoothbore carbs" .

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What are you actually going to do with them ?
If this is for road use, unless you ride 10/10ths all the time, just bolting up 32mm Amals isn't going to seem much different ??

Are the 33's completely worn out.
Having them reseleeved can make them as good as new again ?

Rohan, no not completely worn out but struggles to idle, but idles good for a while if I smear grease all over the slides, if you add up the cost of sleeving or anodised slides then I'm still not sure slides alone would make a difference, but the premiers at £119 a piece is probably the cheapest option, but you just sort of know that Jim's cr,s would be more fun.

Fair enough.
New slides can make a big difference though, especially if all the jets are new and unworn.

brxpb said:

if I smear grease all over the slides,

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Be careful of that, if the slides stick and you get lots of throttle when you least expect it,
it may not be good. Quick and dirty diagnostic test though.

Yep greasing Amal slides may be hobot's stupidest most dangerous suggestion, but if surviving it to find it distinctly helps for a while then anodized slides with less cut out to compensate for the bore wear leaks will give quite adequate operation, dead cold start, idle to full heated 'lack luster' Amal spunk while spinning the bottle to non-factory erotica exotica.

The f750 bikes ran 33 Concentrics .

32 Mk IIs are equivilant to 34 Mk1 Concentrics , so they say . 34 Mk IIs might be excessive for a 750 .
Robbie Dean ran 32s on his 850s , though eventually went to 34 Mk IIs to counter the whizz bang threat .

Matt Spencer said:

Robbie Dean ran 32s on his 850s ,

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32 mm Amal Mk1 are stock on any 850.
Who was Robbie Dean when he was at home ?

Rohan said:

Who was Robbie Dean when he was at home ?

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A long time and successful racer at domestic level in NZ, it's not too many years since he last raced although i don't know when he last raced a Norton.

As has already been stated, I think that if you put some new premier 32mm carbs on you wont notice any difference from having 33mm bore carbs on. If you were to check the alignment of carbs to inlet tracts and the inlet tracts to heads on un-molested Nortons, most of the time they don't line up perfectly unless they have been worked on by the owner, and so I doubt if a 1mm difference will have any noticeable effect.

A few years ago when I was having issues with my 34mm Amal Mk2s which were matched to 34mm inlet manifolds and a ported cylinder head with 34mm inlet tracts, I re-fitted my standard 32mm Mk1's and (32mm) inlet tracts on to the (34mm) head and the bike ran very well with no noticeable difference.

I would say if it really bothers you to get the new carbs bored out to 33mm, but try it first with the carbs unmolested at 32mm.

It is relatively easy to bore out carbs, But and a big but, you can’t really bore out if the bore is bigger than the slide, as the slide will have nothing to support it, also big bore carbs WILL slow the gas speed down if you bore out the cylinder head to the valve guide, so in effect you WILL make the bike slower. I did all trhis in the 1970s.

Bernhard said:

It is relatively easy to bore out carbs, But and a big but, you can’t really bore out if the bore is bigger than the slide, as the slide will have nothing to support it, also big bore carbs WILL slow the gas speed down if you bore out the cylinder head to the valve guide, so in effect you WILL make the bike slower. I did all trhis in the 1970s.

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Correct, and that is why 32mm Mk1s were bored to 33mm and not 34mm......it was about as much as you could do without buying more carbs...

I ran 36mm Mk1s in my 850 race bike....they worked fine but I wanted to upgrade to Mk2s, we had a long debate about the 36mm being too big and that a Mk2 should flow better....well...in the end I bored the 34mm Mk2s to 36mm and that was the best set up of the lot.

When the bike was first built and I was having trouble getting it started I did put a pair of 32mms on the manifolds....the head had 34mm ports....that ran fine except flat at the top...no surprise there...and the problem that I had had getting it running turned out to be bad brand new 6 volt coils and nothing to do with carburation...

As for gas speed.....well I didn't have a flow bench in the shed in the '70s, but this bike pulled harder out of bends than any 750 or 850 Norton I cam up against.....te head porting was done by John Baker of the Baker Rawlins drag team using a works 750 shorts stroke as a base.....it worked with big carbs better than small ones....but it was fully sphered and had big valves and works Omegas runnng at about 10.25:1....and has a Peter Williams cam of about '74 vintage... people will tell you .....that wont work.....but it did....as a race bike. I wouldn't build that engine today....because I agree with keeping gas speeds up....but if it didn't work going large for you, something else was not supporting the package....maybe valve head size or bowl restictions, or even just port shape and too low a port floor....

Having said that, the only Nortons I ever ran in the same race as that I knew to be faster than mine were Thruxton Motorcycles ridden and prepared by Tony Smith, with 36mm Mk2 carbs, Cosworth style bellmouths and.....30mm ports at the head....

Anyway, for the OP.....I and others have said it already....you probably won't notice the 32mms compared to your 33mms

But....... if Amal do a good deal boring them to 33mm and deliver ready to install....easy, no risk, they will work nicely....and present a pretty economic long term solution that looks totally original. That actually sounds like more fun to me than flying carbs across the Atlantic and working a new installation. But hey, different strokes for different folks.....your choice....time....and money

An alternative idea for yer...

A 34 or 36mm Mikuni single carb conversion, and open up the mating orifices of the manifold to fit your head.

Should work well really.

Trouble is, a single carb is a bit gay...

Fast Eddie said:

An alternative idea for yer...

A 34 or 36mm Mikuni single carb conversion, and open up the mating orifices of the manifold to fit your head.

Should work well really.

Trouble is, a single carb is a bit gay...

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Haha
To be honest, I've been frigging around with a pair of 34's for days. Stripped and cleaned thrice including float needles etcetera. RHS keeps failing me. Flooding and the like. I've checked on the bench whether the float needle is working and it is so no problem there. They are as synchronized as I can get them by the old coat hanger trick.
They get one more chance today. One more I say!!
Any drama, I'm goin back to the single, problem free, rock steady and easily maintainable bulletproof single 36mm mikuni.