Atlas and 650SS inlet sleeves

There were no sleeves in 2 650SS heads of mine nor are there any in my Atlas head. Would they make any noticable difference in normal riding? Probably not I suspect. For both my SS and Atlas I fitted a single carb - in my viewone of the better modifications if you actually do any serious riding.

My 650 had these sleeves fitted ......the idea being to give the inlet mix a constant 26mm flow through to the ports from the 376 Monobloc carbsthenfitted. Many owners, like myself, promptly removed them and then fitted 389 carbs. These gave better top end performance but drank fuel and resulted in a lumpy tickover.

I don't believe the Atlas ever had them as it was either a 376 single carb or twin 389s or later 928/930 Concentrics.

Question for David..........What size are your inlet ports? The first 88SS models with the proper 650SS type of head were supposed to have just 26mm size ports whereas on the 650 head, these standard at around 28mm. (1 &1/8") These early 88SS heads would need the sleeves in the spacers.

Nobody has dived in so I'll start the ball rolling - maybe.My 88SS had no sleeves when I acquired it but they are present on the parts list. Descriptions at the time said that the inlets were too wide - the optimum carburetor size for road use was the 1+1/16 Monoblock so presumably there's no point in having tracts any wider. The gas would have to suddenly decelerate after leaving the carbs. But for racing at high revs then a bigger carb would be more suitable so racing owners could remove the sleeves and not have to do any machining.I put home made alloy sleeves in my 88SS.It's a bit of a surprise that they were also fitted to the bigger motors - although the Atlas wasn't intended to rev as high anyway.

Well No it dose not decelerate if you have a tube the gases flow though the tube and the you put another tube with a narrow middle the gases speeds up not down , this it what these were for, it gave you 3 more bhp, it just like having fuel injection , and then they had the air box on too, my great uncle Freddie Dixon was the first to use a air box system just like they do too day ,on motorcycles

You have it back to front again Anna. You are quite right, that gasses accelarate when passing through a restriction, it is call the venturi effect.

However, without the sleeves fitted, the bore of the carbuettor is narrower than the inlet tract, so, the mixture does lose speed as it enters the head.

The inlet sleeves issue is a bit of a red herring. On the downstroke there is a negaitive pressure gradient from carb to cylinder and, crudely, the inlet charge isn't that much affected by moderate changes in the inlet tract. Where ideal inlet tract profile can help is as the piston is starting to rise and the inlet valve is shutting when the inlet charge ismeeting rising pressure. With an ideal profile, the momentum of the charge will briefly better overcome the adverse pressure gradient and stuff a little bit more mixture into the combustion chamber before the valve closes. All very simplistic. The effect of sleeves is minimal. What would make a significant difference is inlet tract length, but that's another story.

Gordon...agree totally with you. Adding a touch more to this thread. Before the age of computers turned testing onto its head, quite a lot of carburretion work was basically an ongoing 'suck and see' experiment. Several factors had to be taken into consideration. This included the exhaust system, camshaft & valve timing, carb sizes and length of inlet manifolds. People like Paul Dunstall discovered that putting the carb right next to the head actually made the carburretion worse. Especially at certain rev ranges. Trial and error, eventially determined values for certain engines both for production bikes and the race boys.

Please don't ask me what works best.

And hence the development of the variable length inlet manifold. this is basically longer at low revs and shorter at high revs. A lot of cars have this and a few high end motorcycles.

Previously David Cooper wrote:

Nobody has dived in so I'll start the ball rolling - maybe. My 88SS had no sleeves when I acquired it but they are present on the parts list. Descriptions at the time said that the inlets were too wide - the optimum carburetor size for road use was the 1+1/16 Monoblock so presumably there's no point in having tracts any wider. The gas would have to suddenly decelerate after leaving the carbs. But for racing at high revs then a bigger carb would be more suitable so racing owners could remove the sleeves and not have to do any machining. I put home made alloy sleeves in my 88SS. It's a bit of a surprise that they were also fitted to the bigger motors - although the Atlas wasn't intended to rev as high anyway.

Previously phil_hannam wrote:

My 650 had these sleeves fitted ......the idea being to give the inlet mix a constant 26mm flow through to the ports from the 376 Monobloc carbsthenfitted. Many owners, like myself, promptly removed them and then fitted 389 carbs. These gave better top end performance but drank fuel and resulted in a lumpy tickover.

I don't believe the Atlas ever had them as it was either a 376 single carb or twin 389s or later 928/930 Concentrics.

Question for David..........What size are your inlet ports? The first 88SS models with the proper 650SS type of head were supposed to have just 26mm size ports whereas on the 650 head, these standard at around 28mm. (1 &1/8") These early 88SS heads would need the sleeves in the spacers.

Phil/David - I have measured my '63 88SS downdraught head (with tin sleeves removed) as 1.1055/1.1030 inches. '62 650SS ( with tin sleeves removed) 1.1105/1.1115 inches. Also pre-downdraught 99SS head (No tin sleeves) 1.0105/1.0045 inches. The manifold for the 99SS seems to be exactly 1.0625 (1 1/16 inches). The two measurements are for right and left ports. I had a '66 650SS without tin sleeves but it had concentric carbs (30mm). So either there was a step-down at the induction spacer /head junction or the later head had a Iarger port diameter. Also had a '65 Atlas without tin sleeves and that had a pair of 1 1/8 inch monoblocs. I have also measured a Ken Curley downdraught racing head for interest and that's 1.1845/1.1867. Near enough 1 3/16 inch. Hope this helps your stats !!! Howard

Howard........Mega thanks for the information/data. You must have a good collection of bikes and spares. People are often asking me for details about the cylinder heads. Especially for 1959 to 62 bikes that are being restored.

That era must have been a bit bonkers at the factory as you had standard single carb heads, 99SS heads, 650 and Atlas cylinder heads in production. Plus some specials for the Nomad and racing bikes.

Please let me know if you ever come across a very early 88SS head with the smaller inlet ports.

Previously phil_hannam wrote:

Howard........Mega thanks for the information/data. You must have a good collection of bikes and spares. People are often asking me for details about the cylinder heads. Especially for 1959 to 62 bikes that are being restored.

That era must have been a bit bonkers at the factory as you had standard single carb heads, 99SS heads, 650 and Atlas cylinder heads in production. Plus some specials for the Nomad and racing bikes.

Please let me know if you ever come across a very early 88SS head with the smaller inlet ports.

Thanks for that Phil - I did once own a Dommie 500 ('57 I think) that had the big-fin head with both inlet and exhaust valves all at 1 5/16 inches. It also had a twin carb manifold (genuine Norton) that had the 1 1/2 inch mounting stud centres and 1 inch monoblocs. Can't say if it was a "Special" as delivered but it didn't say so in the logbook. It was quite quick but difficult to keep an even tick-over. I eventually converted it to single carb to sell it. Shame really - impetuosity of youth ! I sold the manifold only a few years ago following a wanted advert. I shall certainly keep my ears and eyes open for another . Regards, Howard