Mk 2 Smoothbore Carb Set Up

Hi all, thanks to the watchful eye of Fast Eddie perusing ebay, I have now managed to purchase a couple of sets of 34mm Amal Mk2 Smoothbore carbs.
I had been talking to Amal and other vendors around the planet for close to three years and have finally landed them!!
Amal made noises about building more, although their Mk 2 Concentrics are now availble, not so the Smooth bores.
There are spare slides and manifolds, one set of carbs is near new, lucky to have had any fuel through them, the other set is a bit more used although in extremely good condition.
In the process of pulling them apart and checking everything.
Both sets come with reasonably heavy duty springs and the aluminium screw tops(no plastic)
The mains jets on the new sets are 260, the pilots 106 and the needles 2C3's

They are for a long term project race motor I am trying to build.

Has anyone on the forum had experience with setting them up ?
i have looked on other forums as well to look for set up experience.
Look forward to some healthy feedback
Regards Mike

PS Here is a .pdf document I found that I handn't seen before
http://www.tioc.org/amalbritbikeweb.pdf

That pdf looks useful !

Thanks John, has the bike of yours come out of retirement yet?
Yes I was searching every listing I could find for carb info specific to smoothbores, so that was a bonus as I hadn't seen it before.

There doesnt seem to be a lot on smooth bores other than most set them up the same as non smooth bore Mk 2's and work from there.

Hopefully i can get them to perform , although a long way off before I get the motor finished.
Regards Mike

Nope :-(

Its sitting in Wellington in a shed with a lot of Velocettes.

- and I'm sitting here looking out a window over a very warm Bucharest Romania ! Its a really nice day but Im at work !

My contract here finishes October next year and probably I will come home then. Turmoil in the oil industry these days with the oil price drop. I have seen over 200 contracts finish ahead of time here.

Mike,

They are essentially the same design as a mk2 in the way they work, which ain't that different from a mk1!

I believe the smooth bore encourages a faster gas flow though, which then draws harder on the main jet, I think that's a trap people fall I to and often over jet them.

In my experience, despite the similar design to more mundane carbs, smoothbores are more sensitive to adjustments of needle type, needle position, needle jet and slide cutaway, so patience and Dyno time will yield best results.

They also seem more sensitive to velocity stack shape and design. I found they tended to give higher (but peaky) power with the long Amal stacks but a better overall curve, albeit 2 BHP less, with some custom made stacks that were shorter but with much larger initial diameter.

But then again... None of my experience with them is on Nortons!

Nigel thanks for that and at least have a good base to work of.
I'll send them off for some flow testing with the head and the different length velocity stacks to see whats what!
I have access to 34 mm Mk 2 Concentrics (non smoothbore) as a comparison

John, yes in the same industry, at present on a long term (ha!!!) project in India, w e are contracted directly to RIL , although they are making redundant their shore based expats as well.

See you next year in NZ

Regards Mike

Hi Mike

What size are the carbs? I will look at the settings I have. Steve Maney runs 38mm I run 34mm.
Big difference for me was the alloy domed tops as they give the springs more room & ease the throttle, although the action ( I think its light everyone I have lent it to are shocked (Modern bike riders) is heavy.John (seeley 920) has a really need lifting linkage & his throttle really is light.

ps Dave Watson/Gary Thwaites bike ran 34Mk2 not smoothbore.

Chris

Just what is the difference between a MK2 & a MK2 Smoothbore? I am presuming the needle position isn't remote as the GP & RN types.

Martyn.

Matchless said:

Just what is the difference between a MK2 & a MK2 Smoothbore? I am presuming the needle position isn't remote as the GP & RN types.

Martyn.

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The slides are very thin walled, and the choke body is manufactured somewhat like monoblocks were, so the slide kinda slides through thin slots, so there is not a large slide-sized void when the throttle is opened. When coupled with the proper velocity stacks that omit the pilot intake holes and resultant step, they make for a nice smooth intake. In my experience they were worth a few BHP over the best fettled mk1s or 2s although some people claim they make a bigger difference than I ever found.

Perhaps Mike could indulge us with some pics?

In terms of flow, their main advantage is at wide open throttle when they are pulled completely out of the throat and then present a smooth bore to the air flow. Because the slides' shape is that of an upside down water glass, at all throttle openings less than full throttle, they present a void on the inside of the slide, as the air passes under the leading/forward edge of the slide. The air tumbles in that void on its way to the trailing/rear edge of the slide. Non smooth bore carbs all, more or less, fill that void. The smooth bore design, with that greater volume under the slide may pull a greater Bernoulli effect vacuum, explaining their superior performance. Flat slide designs . . . . .

Thanks guys, the size is on the start of the thread at 34mm
Will post pics when I gain access to photobucket, it is blocked by my server where I am at work
Regards Mike

Perhaps the reason that Amal are not making smooth-bores has something to do with not selling enough of them, because the advantages in using them are not great ?

The Evil Coils fed up the jetting attempts , between 220 & 270
theyed jet constant for one throttle posn. or one speed ,
but plug chops wernt entirely consistant .
A high speed run for some distance in the hills up there showed WHITE plugs , Zorsts & pistons .
Though the plug center posts wernt erroding
a gnats dick less fuel mayve been dis arse terous .

Therfore Decent Coils and SPARK would be a good start .

cool Weather and NO CHOKEs lead to tears too . though the exercise will warm you up .
Eventually when youve kicked some temperature into the engine , it MIGHT start .

The no ' Tickler ' to enrichen the mixture means that . By Golly .
even push starting dosnt work cold , chokeless .

Though the 34s are equivilant of 36 mm Mk 1s .
So we expect some pictures of wheel spinning tyre smoking froint lifting take offs .
If youve got your weight forward over the bars you shouldnt flip it .

Wots This Then ; ?

Matt - whether the plugs are white or not, you need to make sure that the black ring stays on the porcelain in the plug right down inside where it meets the metal. If you use hotter plugs for tuning purposes you will stay safe.

Matt Spencer said:

Wots This Then ; ?

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Hi Matt.
Search 'Cosworth' in the top-right corner search box.
Ta.

Matt Spencer said:

Wots This Then ; ?

Click to expand...

I would never attach the swing arm to the rear of the gearbox, without picking up the ends of the pivot shaft.