Monoblocks vs Concentrics 650SS

elefantrider

elefantrider

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For those having experience with both, do you prefer Monoblocks or Concentrics?

I see most 650SS bikes were raced with Monoblocks.
 
There is no performance advantage to either in a single carb setup.
In dual configuration, the chopped Monobloc configuration has a severe fuel flow limitation as both carbs are fed thru one needle and seat. The maximum fuel flow that can be obtained in this manner may not satisfy the appetite of a hot 650, or a stock 750.

The mirror image 689 monobloc gets around this problem by having its own float chamber and needle and seat, but are hard to find.

Float chamber extensions were an attempt to get around fuel starvation at high rpm wide open throttle settings, but were not effective.

There was an aftermarket monster (0.200 orifice) needle and seat that was once offered by a now defunct company named Webco, or Webbco. Dual monoblocs with one of these would feed nearly 500 ml fuel per minute, which is adequate for a hot 650.

Slick
 
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No side stand option with chopped monobloc either - RH cylinder will starve.
Having said that, I have monoblocs on my '62 650ss and will be staying with them (as well as the K2FC) 👍 👍

I'm not certain but I think concentrics did not come out until maybe the last year of 650ss production, so nearly all of them will have had monoblocs fitted as OEM.
 
Theres Right & left and monoblocks . Concentrics were ' improved cheaper ' tho the magnesium ones might be worth a look .


Monoblocks vs Concentrics 650SS


As most chaps lean neutral to the ' G ' force , only sidecars'd benifit from asymmetric fuel concentrification .
And Morgans .
 
Matt Spencer said:
Theres Right & left and monoblocks . Concentrics were ' improved cheaper ' tho the magnesium ones might be worth a look

As most chaps lean neutral to the ' G ' force , only sidecars'd benifit from asymmetric fuel concentrification .
And Morgans .
Click to expand...

leaning a bike at speed has zero effect on fuel level in the float chambers, regardless of any asymmetry in the geometry.

Slick
 
Why did they fit a chopped monoblock instead of the mirror "689" one with the 2nd bowl? Fitment issues?

Agree they do look nicer than Concentrics, probably easier to service also. The bowl screws on the Concentrics are a PITA.
 
650SS were fitted with 376 monoblocs which were only available in the left & chopped version, 389/689 were only on the 750 Atlas, N15 etc.
 
Always looked weird to me that early t120 bonnies had two left hand 389s
Instead of a 389 and a 689 !
Were 689s a shilling dearer or something!!!?
 
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Did any 650ss owners upgrade their chopped carb for a 389/689 arrangement from an Atlas seeing that both bikes were produced concurently?

I would assume one could just swap in a 689 right side carby from an Atlas/N15?

I have also seen there is Amal remote fuel bowl assembly.

Seen previous posts on here saying Dunstall reckoned the Monos gave the most power.
 
You will see 389/689’s on the 1966-67 Atlas, 1968 were concentrics. I’m sure there are some anomalies found during transition years. Good luck finding good 689 slides.
 
baz said:
Always looked weird to me that early t120 bonnies had two left hand 389s
Instead of a 389 and a 689 !
Were 689s a shilling dearer or something!!!?
Click to expand...
Are you referring to Miss Schilling's orifice? ;)
 
650SS actually used inlet manifold sleeves 23874 with the 376 set up and from what I have read production racing teams that retained the sleeves were faster than teams that removed them so assuming this is correct why fit the larger 389 set?

www.rgmnorton.co.uk

INLET PORT SLEEVE

NM23874
www.rgmnorton.co.uk

The 389/689 was only introduced in 1966 prior to that the 750's used a "chopped" 39 for the right carb, the concentric carbs were introduced during 1967.
 
So good slides aren’t available either brass, chrome or anodised for the 699 it seems. I don’t need the bigger carbs, Just looking for a setup with dual float bowls. It sounds like a complete set is what I need.
 
elefantrider said:
So good slides aren’t available either brass, chrome or anodised for the 699 it seems. I don’t need the bigger carbs, Just looking for a setup with dual float bowls. It sounds like a complete set is what I need.
Click to expand...

EBAY has a 689 body and a slide.
www.ebay.com

amal 689 for sale | eBay

Get the best deals for amal 689 at eBay.com. We have a great online selection at the lowest prices with Fast & Free shipping on many items!
www.ebay.com

Slick
 

If yer put the bit with two on , theres one for each . not one for both .

Monoblocks vs Concentrics 650SS


Monoblocks vs Concentrics 650SS


Of course , its not just the needle , in the monoblock , thats superior .
Monoblocks vs Concentrics 650SS


They saved 1shilling per hundred leaving two grooves off on the concentric .
They were in two minds about the top , on the concentrics . But saved by only fitting two screws .

Monoblocks vs Concentrics 650SS


Theres the eccentric velocity stack , for the Concentric .
A little know fact is that above one fourty on the autostrada ,
the ' conventional ' stacks , with the inflo , can depress the floats .

Then , ' BANG ' , as its gone like L E A N . Which is not a good thing .
Should be fittable to the Mono's too . ?

Then youd have superior Top End as well as a superior idle and metering ,
as well as progression . 🌈
 
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possm said:
650SS actually used inlet manifold sleeves 23874 with the 376 set up and from what I have read production racing teams that retained the sleeves were faster than teams that removed them so assuming this is correct why fit the larger 389 set?

www.rgmnorton.co.uk

INLET PORT SLEEVE

NM23874
www.rgmnorton.co.uk

The 389/689 was only introduced in 1966 prior to that the 750's used a "chopped" 39 for the right carb, the concentric carbs were introduced during 1967.
Click to expand...
“During” is a better way to describe it. I have a set numbered for an Atlas with an early 1967 date stamp. The anomaly as stated above.
 
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It looks like the correct numbers for 650ss are 376 / 288 and 289, chopped though.

There's always a few GP carbs on Ebay but without an idle circuit, track-use only.
 
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Looks like 376 / 317 and 318 would also be correct for 650SS
389 / 88 and 89 would be the chopped Atlas.

389 / 236-241-242 and 689 / 237-241-242 would be the full size pair for Atlas.

If anyone has and wants to sell these, let me know.
 
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