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650ss carb size

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HI all

 my bitza 650ss continues to make progress. Santa sent me new main bearings via Lapland to complete the bottom end. I reckon March 2020 I will have the engine running in the frame on this multi national assortment of Norton parts.

Bit of advice required - the motor is a 1965 Norton 650ss which originally would have had  a pair of  376 monoblocs   (not included on the motor I bought) 

 On measuring  the inlet ports its 28mm,  therefore I assume 28mm concentric would be the best fit. However I note that Norton went to 30MM Concentric when monoblocs ceased to be used. Was this a straight swap or were later 650ss inlets opened out to accommodate 30mm carbs?

 Thanks - and merry Christmas

 Mark

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My -68 650SS has 30mm bore in the head and a pair of 932 Concentrics. Manifolds are tapered from 30 to 32. Guess same PO who fitted a Ceriani  230mm 4LS front brake and some other goodies.

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Originally the 650SS had 1 & 1/2 376 monoblocs with metal sleeves fitted in the inlet tracks supposedly to increase torque, I have been able to duplicate the sleeves for my '67 650SS, I am currently rebuilding a late 1966 750 Atlas, my research suggests that sleeves were not fitted to the 750's,as my Atlas has 28mm inlet ports and a previous owner fitted 928 Concentric's as the 389/689 Monobloc's are as rare as I will probably stick with the concentric's. Al
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If I did my sums correctly, 1-1/2" will equate to 28.1mm

I wouldn't think that a .1mm discrepancy would make any difference.

Mike

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All the 650SS's with AMAL monobloc carbs had the same size bore i.e. 1 1/16 inch.  The port in the head was slightly bigger at 1 1/8 inch as was the inlet manifold stub.  For this reason a tin sleeve fitted through the stub and into the port to bring all the related parts to a nice smooth 1 1/16 inch entry.  When the AMAL concentric carbs were fitted in 1967 (at engine number 121307) to the 650SS (and Atlas) models the sleeve was done away with and the carbs were 28 mm or in some cases 30 mm.  Presumably the Atlas got the 30mm to replace the 1 1/8 inch monoblocs and the intention was for the 650SS to get the 28mm to replace the 1 1/16 inch monoblocs.  However money and time was tight in those" heady days" at Woolwich so some 650's got the bigger carbs.   None of the models had 1 1/2 inch carbs so that is an error Michael - you did not do your sums correctly. Please go in detention! signed Teacher (1.5 inch = 38.1mm)   Merry Christmas everyone, Howard

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All

 thanks for the replies. I wasn't aware of the sleeves in the inlet manifolds. I've just  checked mine and they are solid alloy at 28mm (possibly atlas?) but reading the above it appears 28 twin carbs shouldn't be a problem in performance terms. If I've misunderstood please let me know.

  I'm looking forward to spring and starting this old bike up. Its been 40 years since I restored a norton twin  (99ss)  as an impoverished 17 year old student. That bike ran fine for years, I'm hoping for even better success on this one .

 Happy new year to you all

 Mark

 

 

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Go for it Mark - You shouldn't notice that you don't have the sleeves fitted but be aware that the two inlet stubs are usually linked with a breather tube between them.  They make it easier to get the cylinders in-step.  Ride safe, Howard 

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Hello, Now for the first set of twin carbs Fitted to the First Norton Manxman 650s  where 376/256 and 376/257 then the home model 650ss  had the 376/288 and 376/289  and these were also fitted to the model 88SS and (99SS)  with splade inlet manifold, Now has for the Pre warming Sleeves these were fitted to the  Manxman 650 and later after April 1961 to the models 88SS and (99SS) and 650 and early Atlas  Now you can still buy these steel inlet per warming sleeves from RGM MOTORS  they were fitted to help stop the carburettor from icing up in cold weather and helped with the incoming fresh fuel and seemed to produce 3 more bhp why no one knows. but this was tested on the dyno  and theses were the results,  so have fun out there  yours  Anna J   

Sorry, I miss-typed the conversion

1.5" will equal 38.1mm

Sorry

Must have been caused by drinking up all the leftover Christmas cheer!

Mike

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Hi!

i am Looking for the correct types of Carburettors for my 1962 650 SS Export Version ( Austria). it shall be 376 / 288 and 289.

these are 1 1/16 Inch Monobloc

 

376 / 288 is witha  normal float bowl, is 376 / 289 with a chopped Bowl or also normal ?

 

Thank you in advance !

 

 

 

 

 

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The smaller ports producing more h.p as Anna mentions may be due to better flow or velocity as designed into the excellent Fullauto head for Commandos.

For a street 650SS this may be worth further investigation for the adventurous tuner. Like trying 28mm Concentrics with the 1 1/16" (27mm) sleeved ports but Mark shouldn,t have a problem with well sorted 28mm Premiers on a rebuilt engine 

 

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If you intend to ride the bike in temps as low as 5 deg or lower and in a temperate  damp climate then the sleeves will do what they were designed for. Carb icing is not funny and will cut the motor at the wrong moment in the warm up time. More so with E05 and E10 .  The 650's best feature is its eager ability to  rush up the scale from nothing  and this comes from the  good airflow velocity through the smallish carbs at low revs. I don't think I would like to trade this off with bigger carbs for a bit more HP at the top end which is rarely going to be used (by me anyway!).

In reply to by michael_sullivan

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My previous comment was supposed to mean a complete carb and a half carb[no float bowl]

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Carb icing was a complete pain on my 99, less so now that it has an Atlas engine. Low temperatures and high humidity bring it about. It can jam the throttle as ice builds up on the slide  and make the engine run very rich.

It shouldn't be so bad with E5 or E10 fuel. When I flew Hawker Hunters, they had a system to inject methanol to the fuel in the event of icing - it dissolved the built up ice. Similarly, the ethanol in petrol should absorb any ice building up in the carb. I have yet to try this, but again, I haven't suffered carb icing for a while so maybe it's working and I just haven't noticed.

928s should be fine on a 650ss.

   

Thanks for the information to all ! 

I intend to use the 650SS regularly and I also intend to have it as close to the original spec as possible.

 

so I might go for the 1 1/16 monoblocs - the right hand one with a chopped ( aka: no float bowl) and the inlet sleeves to prevent from carb icing.

 

Cheers 

 

Hermann

 

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