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650SS single carb conversion

Submitted by thomas_newton on Mon, 26/10/2015
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Heavy Twins

I have a 1965 650SS which is fitted with twin 930 amal concentrics.

I am considering going down the single carb route using one of these carbs. Is it possible to get an inlet maniflod to suit ?

What would I have to jet the carb up to ? Both carbs are fitted with 270 main jets & size 20 pilot jets.

Wondered if anyone has experiance of converting to single carb.

Submitted by phil_hannam on Mon, 26/10/2015

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Both a version of the early 650 Manxman and later 650 Mercury used a single carb.

Checkout the attachment for the carb spec you need. It is a very smart move if you want easier starting, smoother running and better mpg. The downside of going for a single carb is your bike will probably now only do 110 instead of 120mph.

Attachments 1969-mecury-650-jpg

Submitted by thomas_newton on Mon, 26/10/2015

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Thanks Phil,

Looks like I will just have to jet up a size on the main & pilot valves.

Need to find a manifold for the doendraft head ?

Any ideas ?

Submitted by gordon_johnston on Mon, 26/10/2015

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When I converted to a single carb, I didn't alter the jetting at all, just used one of the carbs. I think Norvil may list a single carb manifold. I may have a spare one on the shelf if you can't find one.

Submitted by phil_hannam on Mon, 26/10/2015

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The single 930 carb I fitted to my 650 came with a 270 main jet. I added a K&N type Cone Filter which fitted snuggly between the oil tank and battery box. It works perfectly and the plugs are the correct colour over the speed range I use the bike.

Norvil were selling a SS head single carb manifold for around £54 plus the usual charges. P/N 069960

Submitted by thomas_newton on Wed, 28/10/2015

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Previously Gordon Johnston wrote:

Hi Thomas, £25 including postage. I'll nip out shortly and make sure it's where I think it is!

Gordon.

Hi Gordon,

Did you find it â?

Tom

Submitted by gordon_johnston on Wed, 28/10/2015

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After a search that a detective would be proud of, Ifound it still attached to a spare 650 head. Hiding in plain sight. So it's there if you want it. Gordon.

Submitted by thomas_newton on Thu, 29/10/2015

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Previously Gordon Johnston wrote:

After a search that a detective would be proud of, Ifound it still attached to a spare 650 head. Hiding in plain sight. So it's there if you want it. Gordon.

Gordon.

Thanks for taking the time to look for the manifold.

What else will I need to do the conversion. (gaskets, studs, nuts etc ?)

Tomâ

Submitted by thomas_newton on Thu, 29/10/2015

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Thanks for looking for your manifold Gordon.

What else will Ineed to do conversion ? (gaskets, studs, nuts etc)

tom

Submitted by david_cooper on Thu, 29/10/2015

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You should be able to re-use your heat resistant gaskets.You'll need a pair of studs for outside, but if it's like mine you'll need a pair of socket head screws to go inside the manifold. All 1/4" Whit. (I'm pretty sure. Certainly coarse thread into alloy head)Make sure the inside screws are snug. I'd suggest Loctite. You don't want them to work their way out unseen (please don't ask how I know that!).You'll need a simple throttle cable without splitter. (I suppose you could retain the one with splitter and remove one outlet - never thought of that before!)It's a lot more complicated going the other way of course.I still wonder why bother but the consensus seems to be on your side. I 'only' have an 88SS and I like to get what I can out of it. Carb balancing was much more difficult before I swapped to chromed slides. In my case the slides were much more worn than the bodies so limiting random air leaks was effective.

Submitted by fritz_wettstein on Wed, 04/11/2015

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Previously Phil Hannam wrote:

Norvil were selling a SS head single carb manifold for around £54 plus the usual charges. P/N 069960

Unfortunately this one has only the two holes on the outer edges, but it's easy to make the other two, by using the two originalheat resistantgaskets as templates.

Fritz

Submitted by phil_hannam on Wed, 04/11/2015

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Unfortunately this one has only the two holes on the outer edges, but it's easy to make the other two, by using the two originalheat resistantgaskets as templates.

Fritz

Check out the attachments...........the two studs have slots so that they can be turned a little more if there is any need. They actually only help to hold the heat insulators in position. I decided to put them between the head and manifold as this is done on the 88/99 models. However, on some SS head single carb set-ups, I have seen them between the carb and manifold.

Does it make any difference?

Attachments Single%20Carb%20Conversion%202.JPG single-carb-conversion-1-jpg

Submitted by Xanna_jeannett… on Wed, 04/11/2015

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Previously thomas_newton wrote:

I have a 1965 650SS which is fitted with twin 930 amal concentric.

I am considering going down the single carb route using one of these carbs. Is it possible to get an inlet manifold to suit ?

What would I have to jet the carb up to ? Both carbs are fitted with 270 main jets & size 20 pilot jets.

Wondered if anyone has experience of converting to single carb.

Hello we you should have for 1965 a pair of mono-bloc Amal 376 /317 and 376/318 , So then too go to single Carburetor you need the single manifold , and a single 389/71 1/1/8th carburetor with main jet 290 to 320 a 25 pilot jet a number 3 cut-way throttle side a 106. needle position second notch , this is the Set up for the 650 standard from 1961 to 1963 and you find that Amal mono-bloccarburetors work better than the later Amal type Mk 1 Concentric , has these tend too run too weak a mixture , witch can lead toprematureburning of the Exhaust valves seats so your better off with the mixture just a bit on the rich side but with No black smoke onrevvingup, you still can get the Gunsun colour tune kit see ebay.co.uk yours Anna J Dixon Bs I.mech Mme

Submitted by thomas_newton on Wed, 04/11/2015

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Thanks for the info. Anna, but it has been fitted with mark 1 concentrics in the passed & I think I will use one of these.

I have ordered a single carb. manifold from NORVILL today, & will post how things go later.

T

Submitted by thomas_newton on Fri, 13/11/2015

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I have just got round to fitting the single carb manifold.

I am so suprised at the outcome.

Bike started second kick with just a slight adjustment of the throttle stop screw to set a steady tickover using the jets already fitted. I took it out for a very short blast as it's so wet & cold here, & when I checked both plugs they are a light brown shade so the mixture can not be far out.

The bike also seemed to have evenmore pull when gunned upa steep hill near me.

First impressions "VERY HAPPY" !!

Submitted by thomas_newton on Fri, 13/11/2015

Permalink

I have just got round to fitting the single carb manifold.

I am so suprised at the outcome.

Bike started second kick with just a slight adjustment of the throttle stop screw to set a steady tickover using the jets already fitted. I took it out for a very short blast as it's so wet & cold here, & when I checked both plugs they are a light brown shade so the mixture can not be far out.

The bike also seemed to have evenmore pull when gunned upa steep hill near me.

First impressions "VERY HAPPY" !!

Submitted by gordon_johnston on Fri, 13/11/2015

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I am an advocate of single carbs on twins. How often do any of us ride with the throttle wide open for any length of time? Give me the simplicity and smoothness every time. Anyway, I found that just using one of the carbs complete with existing jets worked just fine, despite the manual suggesting differently. And of course I now have a ready-to fit spare carb.

 


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