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#217496 - 10/07/08 4:14 pm monoblock vs concentric  
Joined: Aug 2007
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snowbeard Offline
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snowbeard  Offline
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US citizen in Lisbon, Portugal
so just how different are these two types of carbs? I've heard people speak of using a monoblock on a newer bike, and vice versa I think.

what do the numbers represent on a monoblock? say a 389 is what diameter?

I know 930 is 30mm, 928 is 28mm, etc.

my reason is I really want to see if my 71 thunderbolt will run at all, or if I should just pass it along to someone better able to restore or rebuild it. I don't want to dump a bunch into a great new carb if the rest of the bike is too far gone to even run.

I have a 389 that works great on my A10, so how much different would it be on an A65 single carb engine?


=================
/1957 BSA A10 Spitfire Scrambler (in a friend's shed)
/1960 BSA Super Rocket Basket Case (in the attic!)
/1987 BMW K100LT nekkid
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#217509 - 10/07/08 5:00 pm Re: monoblock vs concentric [Re: snowbeard]  
Joined: Jun 2007
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HawaiianTiger Online content
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HawaiianTiger  Online Content

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Maui Hawaii
SB,
The 389 came in several sizes. I've seen 1 1/8 and 1 3/16. As far as running on your A65 check mid 60's specs for jetting (Thunderbolt)and go from there. Typically the Monoblock being a less effecient carburettor uses larger main jets than the Concentric. ie. 250 vs 190 might be an equal setting from one to the other. Same problems with slides and rapid wear exist between the two. Unfortuantely there is no perfect carb for our bikes but a new Monoblock with chrome slide comes closest IMHO.
Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Flying Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
#217516 - 10/07/08 5:26 pm Re: monoblock vs concentric [Re: snowbeard]  
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Jim Hultman Offline
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Jim Hultman  Offline
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Minnesota, US
Originally Posted By: snowbeard
so just how different are these two types of carbs? I've heard people speak of using a monoblock on a newer bike, and vice versa I think.


Usually the "upgrade" is from the monoblock to a Concentric. Not sure why except that the monoblocks can be a pain if they warp. The air flow in a monoblock is better at open throttle because of the body design (more like a GP).

Anyway, I'm sure a lot of guys here like me have boxes of old concentrics (which is what came on your '71). Perhaps just borrow one to see if the bike will run? Anyone on the forum in your neighborhood? Give me a little time and I could mail you one.

#217545 - 10/07/08 8:06 pm Re: monoblock vs concentric [Re: Jim Hultman]  
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DMadigan Offline
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DMadigan  Offline
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ca, us
A defficiency of the monoblock is the offset float bowl. Leaning the bike sideways or leaning off in a turn causes the fuel to flow to one side or the other and effects the float level which can make the engine run rich/lean. That is one reason they went to the Concentric float (besides making it cheaper). The central support on the slide is far better than the narrow perifery support on the Concentric. The main reason the Concentric wears its slide at small throttle openings, the slide hammers back and forth from the vacuum pulses.

#217552 - 10/07/08 8:27 pm Re: monoblock vs concentric [Re: DMadigan]  
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glastrywhippet Offline
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co.down n.ireland
i have run monoblocs and Concentric on my big single, for a long time the Monobloc wears at a much slower rate and the only one that ticks over real slow! after all these years (30) im back to the original mono (resleeved) ... sayin all this i use concentrics on my racers... there more or less disposable but are ok the miles used


rickman trident racer
dresda t140 racer
1956 matchless g80cs
1960 matchless g2csr (wifies)
#217557 - 10/07/08 9:03 pm Re: monoblock vs concentric [Re: glastrywhippet]  
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snowbeard Offline
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interesting, so if I take a good battery and my 389 that is working well on the A10, I should have a good start to the test. I think that could be accomplished fairly easily. better check that frame for oil and water!!

thanks! and thanks for the offer Jim, I haven't found anyone with enough extras to borrow one lately, maybe if the 389 does no good I'll drop you a line! beerchug


=================
/1957 BSA A10 Spitfire Scrambler (in a friend's shed)
/1960 BSA Super Rocket Basket Case (in the attic!)
/1987 BMW K100LT nekkid
#217584 - 10/07/08 10:38 pm Re: monoblock vs concentric [Re: snowbeard]  
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aprophet Offline
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aprophet  Offline
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Chesapeake Virginia
hey SB fwiw I use a Monobloc on my 1964 A65C I jetted it by the workshop manual after I got it running kina close I had to go a little richer but its been running good for about 2 years now the carb is a 389 1 1/8 I run a 320 main jet 25 pilot and a d needle i think the needle jet is a .106 hth pal beerchug


1964 A65C Rocket (Jeanie)
1971 T25SS
1971 A65T BSA (Lucile)
1965 A65D Lightning Rocket
#217613 - 10/08/08 2:35 am Re: monoblock vs concentric [Re: aprophet]  
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Mark Z Online content
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Mark Z  Online Content
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Owego, NY, USA
From the 1967 Factory Spares manual for A65 Thunderbolt:

Carb: 389-233 (Note that the carbs for the Lightning are 389-229 (right hand) and 689-229 (left hand)). I believe the second half of the model number denotes the bore size. The number for the Thunderbolt is larger, but I believe the bore size is 28mm vs. 30mm for the Lightning.)

main jet: 300
pilot jet: 25
throttle slide: 389/3 1/2
needle jet: .106
needle position: 3

I don't think there were any changes to the engine from 1967 to 1971 that would call for different jetting.

I like Monobloc carbs - these were the original carbs on my '67 Lightning and the bike ran very well with those (before I caused the bodies to get distorted from improper handling). They are also very attractive. Polish up that float bowl cover and you'll see what I mean...


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
#217627 - 10/08/08 5:15 am Re: monoblock vs concentric [Re: Mark Z]  
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BONZO R.I.P. Offline
In Remembrance
BONZO R.I.P.  Offline
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Michigan, USA
I have been livin/lovin/ridin/eatin/sleepin/breathin/ these machines for a long time , seen this question arisin since the first time I threw the sack across the saddle , and after a lotta miles , from what I can see there isnt a lotta difference on the road . The Concentric may be a bit more tuneable for the race machines , but the mono seems to be a bit more reliable for the roadrider. As far as I can tell , the Concentric carb (totally unfounded theory coming up) was a Mikuni idea . There were actual mono bloc carbs with AMAL AND Mikuni names on them , and the early Mikuni tends to resemble the Concentric , with some refinements ?? I wasnt there when the decision was made , but I would have to believe that the change was simply a cost cutting measure since the Concentric came about just as the industry was turning to twin carb machines . the stop gap measure was the half float bowl mono , then the left/right mono for a year or two , these used quite a bit more material , parts , and machining than a Concentric would , so a twin carb model with identical carbs machined from opposite ends made perfect sense rather than casting all those L/R specific parts in all those sizes ?? Built to a price ... nothing new under the sun ..

#217713 - 10/08/08 5:01 pm Re: monoblock vs concentric [Re: BONZO R.I.P.]  
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snowbeard Offline
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well then, I guess I'm much better off than I thought. I've put off messing with her all this time just for lack of a 928, I think I will shoot for a nice monoblock on eBay on the cheap!

thanks for the specs Mark and aprophet, I'll see what I can find to meet them along the way.

does anyone have a definite answer on how to tell the bore size? if it's marked in such a way that I can easily lead an unfamiliar ebayer to it? wink

Thanks again!!


=================
/1957 BSA A10 Spitfire Scrambler (in a friend's shed)
/1960 BSA Super Rocket Basket Case (in the attic!)
/1987 BMW K100LT nekkid
#217842 - 10/09/08 3:36 am Re: monoblock vs concentric [Re: snowbeard]  
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Mark Z Online content
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Mark Z  Online Content
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Wow, I'm about to do a 180 on yer - I thought you already HAD the Monobloc!

Monoblocs AND Concentrics are made out of zinc, and the upper tube is so close to the mounting flange that anything but the gentlest treatment will cause the carb body to get out-of-round. If you buy a used carb, you will have no guarantee that the body is true, and you may have to send it off (to Lund's, for instance) for a rebuild.

On the other hand, brand new AMAL Concentrics are available for about $150.

I still like Monoblocs; all I'm saying is that, if cost is an issue, you may get off easier with a new Concentric.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
#217850 - 10/09/08 4:50 am Re: monoblock vs concentric [Re: Mark Z]  
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nert Offline
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just a guy from Jersey [Hopewe...
Originally Posted By: Mark Z
. They are also very attractive. Polish up that float bowl cover and you'll see what I mean...


I agree.... very attractive, I like/prefer the Monobloc, however, i strongly believe that the correct installation of twin mono carbs (read lightning) place the fuel bowl covers inside, facing each other. Not very visible.


keep your "oddies" lubricated, and carry a dime
#217875 - 10/09/08 11:54 am Re: monoblock vs concentric [Re: nert]  
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aprophet Offline
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Chesapeake Virginia
"I agree.... very attractive, I like/prefer the Monobloc, however, i strongly believe that the correct installation of twin mono carbs (read lightning) place the fuel bowl covers inside, facing each other. Not very visible."

I never realized this checking the workshop manual for 66,67,68 and the parts list for 66 it shows the 389 as the right hand carb and the 689 as the left hand carb 66 parts list shows the same that would put the ticklers to the inside? beerchug


1964 A65C Rocket (Jeanie)
1971 T25SS
1971 A65T BSA (Lucile)
1965 A65D Lightning Rocket
#217917 - 10/09/08 4:55 pm Re: monoblock vs concentric [Re: aprophet]  
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snowbeard Offline
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snowbeard  Offline
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US citizen in Lisbon, Portugal
mark, you had it right, I do have a 389 on my A10, so that was what I was considering to "test" out the old rust pile, but I won't leave it on there. so I do need to find a permanent one, and I see a lot more 389's go for 9.95 on eBay than a 928 Concentric. I'd rather start there and once she's a good runner I'll step up for a new whatever works best.

or just demonstrate that it runs and sell it, either way. but I'm not quite ready to plunk down 150 just to find out.

thanks again!


=================
/1957 BSA A10 Spitfire Scrambler (in a friend's shed)
/1960 BSA Super Rocket Basket Case (in the attic!)
/1987 BMW K100LT nekkid
#218004 - 10/10/08 1:57 am Re: monoblock vs concentric [Re: snowbeard]  
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Mark Z Online content
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Mark Z  Online Content
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Owego, NY, USA
Oh, if you can get a Monobloc for 9.95 (that's dollars, right?), go for it. I had no idea (I may be the only person left who doesn't shop on eBay).

Pictures in sales brochures and magazines reveal that the Monoblocs on twin carb models were indeed mounted float bowls-in at the factory. I guess this was to provide easy access to the adjustments. I always had mine mounted float bowls OUT.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
#218100 - 10/10/08 6:32 pm Re: monoblock vs concentric [Re: Mark Z]  
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snowbeard Offline
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snowbeard  Offline
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occasionally, not as the rule, but it happens.


=================
/1957 BSA A10 Spitfire Scrambler (in a friend's shed)
/1960 BSA Super Rocket Basket Case (in the attic!)
/1987 BMW K100LT nekkid
#218154 - 10/11/08 6:14 am Re: monoblock vs concentric [Re: snowbeard]  
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trevinoz Online content
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trevinoz  Online Content
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newcastle australia
Norton mounted them float bowl out on the '66 Atlas. It was the only year the 389/689 combination was used.


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