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#657695 - 06/20/16 6:07 pm What Size Carb?  
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Stephen Hill Offline
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Victoria, BC,
My 1956 DB had a 30mm Monobloc when I recently bought it. The bike was a bitch to start, so a friend lent me a 30mm Concentric, jetted for his Gold Star. Now the bike starts and runs nicely, which tells me that the Monobloc had issues.

I dug through my collection of carbs, and am wondering what I should put on the bike. The intake measures 30mm at the carb end. I know that a step down in size between carb and the inlet can be beneficial.

So what carb should I use?
-sorted out 30 mm Monobloc
-30 mm Concentric
-32 mm Concentric
-34 Mikuni VM
-36 Mikuni VM

BTW, I am really asking about function, rather than the aesthetics, originality, Jap vs Brit, etc.

Stephen Hill

BSA Gold Star eBay items

BSA Gold Star forum This board is dedicated to BSA Gold Star motorcycles.

#657736 - 06/21/16 2:11 am Re: What Size Carb? [Re: Stephen Hill]  
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Kerry W Offline
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Well! It depends on what you have (cheapest is usually the one you already own), and preference for period items.

For an 'in-period' go with the Monobloc (should work just fine)

For simplicity and ready availability, the Concentric would be OK. I'd go for 30mm. Ran both 28 and 32mm Concentric MkI's on DB's with good results.

For function, ready availability of parts, ability to fine-tune (to a degree not possible with the Amals), build quality and negligible wear rate (not an AMAL strong suit), go with the Mikuni. Use a round slide VM30.

The Mikuni will flow as much or more (most likely) than the the same size AMAL. Adapters for the Mikuni are readily available (Surry Cycles is a good starting point, in the UK).

I've said it here before, but once I stepped from AMAL to Mikuni, ignoring 'correctness' it would be hard to go back.


No generalisation is wholly true, not even this one.
Oliver Wendell Holmes
#657740 - 06/21/16 3:07 am Re: What Size Carb? [Re: Kerry W]  
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David Cass1 Online content
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I would go with the Concentric.
The seller of my rolling chassis had supplied a Concentric in it's box from 1990 or so. New and selected for my machine, but in it's box, never fitted for 25 years.

Without exaggeration- the bike started and ran perfectly after 3 kicks- I did not even adjust the tick-over ( which it does) and have used it for the past 3 years without fault (touch wood)

Sort of looks the part as it has a stack on the back

Can't fault it

David C [img][IMG]http://i1267.photobucket.com/albums...99oen.jpg[/img][/img]


#657915 - 06/22/16 1:47 pm Re: What Size Carb? [Re: Stephen Hill]  
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Stephen Hill Offline
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The bike is a DB34 with a 30 mm inlet. Would a 32 mm Concentric be too big? How about a 34 mm Mikuni VM?

Stephen Hill

#657917 - 06/22/16 3:06 pm Re: What Size Carb? [Re: Stephen Hill]  
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Ralph433 Offline
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I used a Monobloc carb on my first '56 Goldie for many miles (In the 1960s). It would usually start on the first kick. Since you already have the Monobloc it might be worth your time (and money) to check out some different jets and/or slides.

Ralph

#657926 - 06/22/16 4:40 pm Re: What Size Carb? [Re: Stephen Hill]  
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Magnetoman Online content
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Originally Posted By Stephen Hill
The intake measures 30mm at the carb end. I know that a step down in size between carb and the inlet can be beneficial.
While this is not exactly an Old Wives' Tale, it might as well be. The BSA factory found with a 1-1/2" GP carburetor on a new DBD head that the engine had a slight increase in peak h.p. when the small step was left between the carburetor and inlet. For racers every fraction of a h.p. matters so that information was important 50 years ago. But you have a DB head, there's no telling what has been done to the inlet track over the years, or how much the valves have receded into the seats, so even had the information applied when your head was new it likely doesn't now anyway.

Originally Posted By Stephen Hill
Would a 32 mm concentric be too big? How about a 34 mm Mikuni VM?
While a bigger carburetor might give you another 1 mph at the top end it will come at the expense of worse performance at normal speeds. The problem you are having starting your bike with the Monobloc is very likely due to a plugged pilot jet. Clear it with a #78 drill bit and all should be well.

If you decide to put a 30 mm Concentric on it the following should get you very close to the correct jetting:

106 needle jet
needle on top groove
260 main
slide 3-1/2
Air screw 3/4 turn out

#657964 - 06/23/16 2:40 am Re: What Size Carb? [Re: Stephen Hill]  
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Kerry W Offline
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Middle East,
I wouldn't go bigger than the diameter of the inlet port, regardless of the carb you choose. The folklore has it that the 1.5" GP was usable, with the 1 3/8 port, as the resulting 1/16" step improved mixing or flow or both, and that removing/blending the step reduced power.

I suspect this was something of an aberration. recent experience shows that a 36mm (approx 1 3/8") Mikuni makes more power than a 38 (practically 1.5")

It ain't all about size...


No generalisation is wholly true, not even this one.
Oliver Wendell Holmes
#658165 - 06/24/16 8:35 pm Re: What Size Carb? [Re: Stephen Hill]  
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We must bear in mind that the throttle opening at full throttle is the determining factor choosing carb size. At starting or slow speed operation the engine has no idea how large/small the carb bore is with a small throttle/slide opening. Only at WFO is the carb size an issue. Make sense?


dave - NV
#658175 - 06/24/16 10:58 pm Re: What Size Carb? [Re: dave - NV]  
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Magnetoman Online content
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Originally Posted By dave - NV
At starting or slow speed operation the engine has no idea how large/small the carb bore is with a small throttle/slide opening. Only at WFO is the carb size an issue. Make sense?
Not exactly. Imagine two carburetors of different bore at 1/4 throttle but at the same rpm. Since the rpm is the same identical amounts of air are sucked into the engine on each stroke. However, even though the slides are only 1/4 open the carburetor with the larger bore will have somewhat more cross sectional area open (because its diameter is larger). This means the air speed will be proportionally lower to supply that amount of air. Hence, the pressure drop drawing fuel from the main jet will be lower than for the smaller carburetor, less fuel will be drawn in, and the h.p. will be lower. I don't have it handy but I'm pretty sure the Mikuni manual (and other books) discusses this.

A larger bore gives a bit more h.p. at the peak, but at the expense of a bit less h.p. at low rpm. Rather than h.p. another way of looking at this is the reduced air speed of a larger bore reduces the pressure "signal" on the jets thus making it less responsive at low rpm than one with a smaller bore.


#658182 - 06/25/16 2:53 am Re: What Size Carb? [Re: Stephen Hill]  
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...and to add to what MM posted above, the resultant fuel droplet size, from the larger bore carb is less well regulated and the drops are larger and take for effort to burn, resulting in less accurate fuel control and overall efficiency.

Also, while accelerating, at larger throttle openings than would be required for content rpm, the smaller bore carb will produce (in most cases) better acceleration.


No generalisation is wholly true, not even this one.
Oliver Wendell Holmes
#658187 - 06/25/16 3:50 am Re: What Size Carb? [Re: Magnetoman]  
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Originally Posted By Magnetoman
Not exactly. Imagine two carburetors of different bore at 1/4 throttle but at the same rpm. Since the rpm is the same identical amounts of air are sucked into the engine on each stroke. However, even though the slides are only 1/4 open the carburetor with the larger bore will have somewhat more cross sectional area open (because its diameter is larger). This means the air speed will be proportionally lower to supply that amount of air.


I can't agree with some of that. 1/4 throttle on a big carb is more throttle (more open area) than 1/4 throttle on a little carb, so they are two different throttle conditions. The big carb is delivering more mixture at 1/4 throttle, so vacuum, rpm and/or torque are not the same.

What I have found, when I've over-carbed a bike is that The "low speed" adjustments (pilot screw and slide cutaway) are metering the mixture up to considerable road speed, when it should really be the needle jet and needle position which are controlling things.

Last edited by triton thrasher; 06/25/16 3:51 am.

Amateur Loctite enthusiast.
#658196 - 06/25/16 6:50 am Re: What Size Carb? [Re: Stephen Hill]  
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John Alexander Online content
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Hi stephen, are you sure your G Star is 1956 or just registered in 1956, for a DB34 is a 1955 model and that does have a one and three eights bore carb [30mm] which is the correct size, the step in the bore you talk about only applies to the 1956 DBD 34 which has a one and a half inch bore [40mm].Why not stick with the Concentric, sounds to be running nicely.
Goldie John.

#658199 - 06/25/16 7:09 am Re: What Size Carb? [Re: Stephen Hill]  
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One and three eighths is nearly 35 mm.

Inch and a half is 38 mm.

Last edited by triton thrasher; 06/25/16 7:10 am.

Amateur Loctite enthusiast.
#658202 - 06/25/16 7:20 am Re: What Size Carb? [Re: John Alexander]  
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Triless Online content
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OZ
Yes to what MM and Kerry W say. In another life, the dope carb on my BSA ZB33 engine was a 30mm Concentric. This gave the punch out of corners, and the requisite go for the relativly short straights. Relative, really, as antiquity was being swamped by modernity then.
The point is, if the discipline ( was a scrambles outfit ), was of long straights and fast corners, the carb I would've used would have been a 32 mm ( suitably rejetted, of course ).
Horses for courses, really !

#658205 - 06/25/16 7:33 am Re: What Size Carb? [Re: Stephen Hill]  
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Bodger Offline
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sounds like a small port head..a scrambler or trials version?..they came with a 1-3/16 Monobloc IIRC right. They were tractable and easy starting.

These bikes are 50-70 years old, as are a lot of us, haha, getting the absolute top rpm power band seems plenty odd as they go well with more useful modest carburation...but, yeah, I get it, wfo baby! But are you really taking into the rpm band where the 1.5" GP would give any more meaningful performance?...(get a Pearson 600cc then, more grunt, won't blow up. yeah, and talk is cheap, lol)

And just fer giggles, does Pearson and the others make their new intake ports 1.5" now?

Maybe for looks a smaller GP or even a period interesting Gardner carb as they are said to give better power per size, and still available, flat slide, intake cone whatsit looks right too. del'Orto was another period mod, giant intake thing and all shiny..

IMO the Concentric looks odd, as does any of the later AMAL carbs, function aside.
they were a late period mod but the Monobloc and GP are the carbs they came with, after that yer just playing with it, lol.

Another period mod that worked well with the big port heads was a reducing inlet manifold, weber maybe, that took the 1-3/16 AMAL and adapted to the sorta 1.5" port.

Me, I liked the 1.5 GP, it was fine, no idle?...right on the cabl;e was good enough, plus it looks right and makes better noises too. ( and adapting that size to a small port head would be silly, imo.)

#658219 - 06/25/16 8:48 am Re: What Size Carb? [Re: Bodger]  
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Bodger, my ZB 33 was a hotted up cast iron road engine originally. The only things Goldie were the Alpha bigend ( big nuts ), and possibly the big overlap, etc, cams ground to Golfie alcohol spec.
I originally ran a TT9 carb ( dope ), but, honestly, although it didn't look as trick, the Concentric was less hassle, and less messy!
I ran 11:1 comp. We weren't world beaters, or any sort of beater really.Only ever won one race but got an occasional second, and a few thirds which enabled the purchase of more metho to have another go at achieving not much!
A long time ago, early "70's. But I wouldn't have missed it for quids !

Last edited by Triless; 06/25/16 8:49 am. Reason: spelling
#658266 - 06/25/16 4:19 pm Re: What Size Carb? [Re: Stephen Hill]  
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A smaller carb that flows air well will not have a dramatic effect if it is slightly undersize. To get a bigger carb to work well the carb needs to meter well (concentrics and monoblocs flow quite well but are piss poor on metering) Really big carbs need an accelerator pump to avoid bogging down when grabbing a handful


1955 BSA B31 400cc "Stargazer"
1962 Greeves 200cc "Blue Meanie"
1962/67 Greeves 350cc
1967 Greeves 360cc Challenger
1984 Suzuki GSX1100 EFE "Sorcerers Apprentice"
Modified Nu-Trak GM500cc sprint/LSR bike "Deofol"
2003 Aprilia RSV Mille "Lo Stregone"
#658269 - 06/25/16 4:39 pm Re: What Size Carb? [Re: Stephen Hill]  
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MM ... The gents initial question was regards carb size to a difficulty starting right?
You're telling me that a small bore carb at idle setting for starting, has a noticeably smaller opening under the slide than a larger bore carb at idle?

There's some confusion out there I can tell. Oh well....

Last edited by dave - NV; 06/25/16 4:44 pm.

dave - NV
#658272 - 06/25/16 4:54 pm Re: What Size Carb? [Re: dave - NV]  
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Originally Posted By dave - NV
The gents initial question was regards carb size to a difficulty starting right.
Yes, that was his initial question. For that I suggested he clean his pilot jet with a #78 drill bit.
Originally Posted By dave - NV
You're telling me that a small bore carb at idle setting for starting, has a noticeably smaller opening under the slide than a larger bore carb at idle?
No, I never said that. If idle is all that mattered a 96 mm carburetor probably would work as well as a 26 mm. However, my response about carburetor size was to the question in his subsequent post where he wrote:
Originally Posted By Stephen Hill
The bike is a DB34 with a 30 mm inlet. Would a 32 mm concentric be too big? How about a 34 mm Mikuni VM?
If a bike is to be used only at idle bore size isn't important, and if used only at WFO bigger is a little better. Everywhere else, moderation is best.

#658285 - 06/25/16 6:53 pm Re: What Size Carb? [Re: Magnetoman]  
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I think I get what Mman was trying to get at
I think..lol.

I think he is saying for the same load, the carb is flowing faster in a smaller bore and therefore with higher vacuum, better mixing is occurring so better flame propagation, thus efficiency and smoothness...or something..and it's pretty intuitive really.
At idle with a larger carb there would be still more air sneaking around the jet so it still would be less efficient, therefore more burbly facepalm altho by the time the intake air/fuel gets squished the cam and timing probably has more meaning burblewise confused than the WFO carb size.

To me the issue is looks, you are bike staring more than riding in traffic so for traffic, get a Honda. I hear they can be pretty smooth... laughing
You can send me your 'hate to use it in traffic' 1.5"GP. thanks.
.
.
I am only kidding a little bit.

I bet if Stephen wants a big bore/WFO/DOtheTON head someone would be happy to swap it for the rarer small bore head..eventually.
IMO the small bore was not designed to be a WFO type DOtheTON machine, that the trade off is low speed tractability, emphasis on tractor. IMO it's a rare thing and should not be hogged out at all. But that's just me, lol.


Last edited by Bodger; 06/25/16 6:58 pm.
#658287 - 06/25/16 7:05 pm Re: What Size Carb? [Re: Stephen Hill]  
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A slight side note on carbs.
Speedway bikes are limited to a maximum 34mm carb bore, the latest speedway carbs (Blixt, BZ, Finn Jensen) have quite short bodies, flat guilotine type slides about 2mm thick and smooth bore. good enough for over 80BHP @ 12000RPM on a 500 single


1955 BSA B31 400cc "Stargazer"
1962 Greeves 200cc "Blue Meanie"
1962/67 Greeves 350cc
1967 Greeves 360cc Challenger
1984 Suzuki GSX1100 EFE "Sorcerers Apprentice"
Modified Nu-Trak GM500cc sprint/LSR bike "Deofol"
2003 Aprilia RSV Mille "Lo Stregone"
#658291 - 06/25/16 7:26 pm Re: What Size Carb? [Re: Stephen Hill]  
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Stephen Hill Offline
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Thanks for all the helpful responses. I originally mentioned I had starting problems with the 30 mm Monobloc.
But my question had more to do with which was the better size carb, given the selection of carbs I have to work with.

John Alexander asked if my bike was a 55 DB34, due to it having a 30 mm inlet. I am pretty sure it is a 1956, and think a 30 mm inlet was still available on a 1956 DB34. Happy to hear if I have this wrong.

Sounds like the smart money is to stick with a 30 mm carb to suit the 30 mm inlet. Ideally, I will sort out the Monobloc, because it looks more original.

Stephen Hill

#658292 - 06/25/16 7:31 pm Re: What Size Carb? [Re: Stephen Hill]  
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Bodger Offline
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wowie, that would be fun in traffic!
Crikey, thanks Andy, very interesting..4 valve heads too.

Now I think they might be keeping the oil in, it used to be that they 'just' dribbled it out on the track..and to use one on the street a sump and pump was added. And the 80hp is on methanol and very high compression right?..so even 50-60 hp on gas would be hot stuff indeed.

For all the trouble to make specials anyway, you'd think there would be more than the handful of road conversions seen over the years. And they do look the bee's knees too. And then there was the speedway engined one done here by...gimme a minute, I'll remember him, it was great..err...ummm... facepalm.....

#658293 - 06/25/16 7:41 pm Re: What Size Carb? [Re: Stephen Hill]  
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good choice Stephen, remember bike staring usually uses up more hours than riding wfo doing the ton, lol..that will work well.

As I recall one of the 'selling points' of the Concentric was that it was 'an improvemnet' over the Monobloc, and could be bigger I think,(the WFO theme again) and parts were available for them etc..in practice they were ok, not much better, and now with new Monoblocs available...and they were easier to get and less expensive than the GPas well...so they made more sense at the time when trying to tune and failing, lol, the Concentric was 'available'.

#658295 - 06/25/16 7:59 pm Re: What Size Carb? [Re: Bodger]  
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Yes the oil is contained in the timing chest, about 0.8L. It is pumped to the engine, and then returned to the timing chest via a reed valve in the sump using crankcase prsssure


1955 BSA B31 400cc "Stargazer"
1962 Greeves 200cc "Blue Meanie"
1962/67 Greeves 350cc
1967 Greeves 360cc Challenger
1984 Suzuki GSX1100 EFE "Sorcerers Apprentice"
Modified Nu-Trak GM500cc sprint/LSR bike "Deofol"
2003 Aprilia RSV Mille "Lo Stregone"

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