Carburettor spacer length?

Somewhere within the text of the Dunstall and Robinson Norton engine tuning books, are mentions that reducing the thickness of the carb spacer has a detrimental effect on the flow and mixture into the engine. In the Dunstall book, page 21, it states' for road use, the spacers (inlet tracts) between the head and carb flanges should be 1.50" long.'

Phil. Is that the same for the 650SS?

The bible 'Tuning for Speed' has a chapter on inlet tract tuning. What you are trying to achieve is resonance so that a pulse of mixture arrives just as the inlet valve is closing at the desired rpm - max power rpm for racing machines. The principal considerations are rpm and inlet tract length - so capacity is immaterial so that thesame length would apply to an Atlas as a 650. Or anything else for that matter. For road use it is all a compromise and few road bikes have tuned inlets. For road bikes, a long inlettract can result in carb icing and poor starting - Douglas twins with single carbs suffer in this manner.

My 63 Atlas has 389 Amals. I have a K&N air filter, ostensibly correct for this model/carbs, but it will not clear the oil tank/battery box. What would be effect of shortening the carb spacer, perhaps by a centimetre?

thank you

Ernie

spacers are one and half inch ,thick, yours anna j

Not wishing to go of the subject, but my question is the opposite! on my model 50 (I know this is the wrong subject but I read all posts) I am thinking of fitting a thin cranked spacer to enable an air filter to be fitted between the oil tank and tool box via the hole, (so far I have been unable to get a filter thin enough), could this cause a problem? It seems a simpler solution than re-welding the inlet manifold to get the carb to line up with the hole.

Regards John O

Perfectly feasible. If you look at the twin carb manifolds for the pre-downdraught 88s and 99s, they are rather unsubtly cranked and still work OK.

Many thanks Gordon, I'll try it and If it works OK I'll post on 'Singles' !

Regards John O

Previously Gordon Johnston wrote:

The bible 'Tuning for Speed' has a chapter on inlet tract tuning. What you are trying to achieve is resonance so that a pulse of mixture arrives just as the inlet valve is closing at the desired rpm - max power rpm for racing machines. The principal considerations are rpm and inlet tract length - so capacity is immaterial so that thesame length would apply to an Atlas as a 650. Or anything else for that matter. For road use it is all a compromise and few road bikes have tuned inlets. For road bikes, a long inlettract can result in carb icing and poor starting - Douglas twins with single carbs suffer in this manner.

Gordon. Thank you for reply, but I was actually asking if the length of the inlet stubs are the same on the Atlas and 650ss. I think you were getting confused with inlet stub length and inlet tract length, two very different things. The latter is the length from the inlet valve face to the outer end of the carburettor air intake. If the distance from the valve face to the carburettor mounting flange is the same on the Atlas and the 650ss, then the stubs should be the same. However, that is dependent upon whether one uses Monobloc or Concentric carbs, as the Monoblocs are 1/4" longer than the Concentric, which in theory would require the latter to have inlet stubs 1/4" longer than the former; disregarding for the moment the variable of bellmouth lengths.

As I said, I was just asking about the length of the inlet stubs on the 650ss; nothing to do with engine capacity. Also, did the stub length change when they changed to Concentrics; anybody?

I am a little confused now. Ernie originally asked about carb spacers. Somehow we have progressed through inlet tracts to inlet stubs.

The inlet tract spacers used on the monobloc equiped 650SS and Atlas were the same dimensions. Both the early 650 and Atlas engines also had tubular inserts to narrow the diameter of the inlet bores and so speed up the gas flow through them. I am not too sure what happened when Concentrics arrived on the scene.

Phil. Thankfully, Ernie got his question answered early on, so he is hopefully sorted!

My impression is that inlet stubs and carb spacers are one and the same. Gordon brought inlet tract length into the conversation, which is of course something else. I hoped to clear up confusion, not add to it! Anyway, thanks for answering my question, where Monobloc-equipped 650ss & Atlas are concerned. When they changed to Concentrics, the 650ss carb bore sizes increased from 1 1/16" to 30mm, or 1 3/16", so maybe they did away with the tube inserts?

BTW, I forgot to say, I would like a pair of the carb spacers/inlet stubs, preferably without tube inserts, if anyone has an unwanted pair sitting on the shelf?

I am a little confused now. Ernie originally asked about carb spacers. Somehow we have progressed through inlet tracts to inlet stubs.

The inlet tract spacers used on the monobloc equiped 650SS and Atlas were the same dimensions. Both the early 650 and Atlas engines also had tubular inserts to narrow the diameter of the inlet bores and so speed up the gas flow through them. I am not too sure what happened when Concentrics arrived on the scene.

now dose this put some anwers to your Q yours anna J

Thanks for that Anna. Interesting that they shortened the spacers when they fitted shorter carbs, effectively reducing the overall inlet tract length by 3/4". To maintain the same inlet tract length one would have to increase the spacer length by 1/4", not shorten it by 1/2".

Does anyone have any information, anecdotal or otherwise, as to how the performance of the early, Monobloc-equipped 650ss' compared with the later, larger Concentric carb'd version?

A bit of mixing and matching to add to this thread.

The Dunstall Tuning book recommends using Concentric carbs. The claim being that these are less prone to fuel frothing. The Robinson book agrees to some extent but equally says if you have 389 Monoblocs these are also fine.

On the early machines carb spacers are all 1.50" long but once Concentrics become available this changes to 1.00" . The Dunstall 'go-faster' items now being finnedor rubber mountings.

According to the spares books inlet tract inserts were used on 650 and Atlas engines up to the change to Concentrics. As Anna suggests these are like finding 'hen's teeth' but they are about. Often in A/J junk boxes because most selllers do not have a clue as to what they are. I saw a set at Kempton last year but they were too badly corroded to be of use.

Part numbers:- Inlet Port Sleeves - 23874

Carb Spacers Thick - 23872

Carb Spacers Thin - 23358

To finish......Robinson recommends a total inlet tract of 14". That is from valve seat to bellmouth.

Interesting stuff Phil. I have a pair of 389's; one with chopped floatbowl; which I was thinking of fitting to my 1964 ss engine, which at present has a single 389. I have a pair of 1" tufnell spacers I could use, but am wondering if rubber mounts are the way to go, to help prevent frothing. Inlet tract length is interesting too. Phil Irving reckoned 8" - 10" was about right, but things have moved on since the '50's.

Interesting stuff Phil. I have a pair of 389's; one with chopped floatbowl; which I was thinking of fitting to my 1964 ss engine, which at present has a single 389. I have a pair of 1" tufnell spacers I could use, but am wondering if rubber mounts are the way to go, to help prevent frothing. Inlet tract length is interesting too. Phil Irving reckoned 8" - 10" was about right, but things have moved on since the '50's.

hello if you fit 389s pair of carbs you will need small er jet sizes like 240 main jet, has you be running on ethanol, you need to down the jey by one size 250 to 240, 25 pilot jet .106 needle , pos 3 yours Zranna3

Hello here,

I apologise in advance for passing comment on the heavyweight twins but in the classic car racing world (well MGBs - not F1 ) accepted wisdom is that short inlet tracts give max power at the top end of the rev range, plus a revvy engine and longer tracts give best mid range power but at the expenses of top end power. So carb sizes and inlet manifolds were changed to suit particular circuits. For road use good mid range is better than all out max BHP at the top end unless you want to ride with the throttle pinned open all the time.

Patrick.

Thanks Zranna3! I don't run on ethanol fortunately, or not yet at least, so I will probably start on 250 main jets to be on the safe side.

Previously patrick_mullen wrote:

Hello here,

I apologise in advance for passing comment on the heavyweight twins but in the classic car racing world (well MGBs - not F1 ) accepted wisdom is that short inlet tracts give max power at the top end of the rev range, plus a revvy engine and longer tracts give best mid range power but at the expenses of top end power. So carb sizes and inlet manifolds were changed to suit particular circuits. For road use good mid range is better than all out max BHP at the top end unless you want to ride with the throttle pinned open all the time.

Patrick.

Patrick. I wasn't sure if that was the case with 4-strokes, but it certainly is with 2-strokes. My other passion is for Greeves, and the scramblers have the carburettor bolted straight to the cylinder barrel inlet flange, whereas the trials bikes, needing much softer power delivery low down, have a 3 or 4" long carb spacer in between. 2-strokes, incidentally, unlike 4-strokes, don't like a long bell-mouth.

Previously Phil Hannam wrote:

A bit of mixing and matching to add to this thread.

The Dunstall Tuning book recommends using Concentric carbs. The claim being that these are less prone to fuel frothing. The Robinson book agrees to some extent but equally says if you have 389 Monoblocs these are also fine.

On the early machines carb spacers are all 1.50" long but once Concentrics become available this changes to 1.00" . The Dunstall 'go-faster' items now being finnedor rubber mountings.

According to the spares books inlet tract inserts were used on 650 and Atlas engines up to the change to Concentrics. As Anna suggests these are like finding 'hen's teeth' but they are about. Often in A/J junk boxes because most selllers do not have a clue as to what they are. I saw a set at Kempton last year but they were too badly corroded to be of use.

Part numbers:- Inlet Port Sleeves - 23874

Carb Spacers Thick - 23872

Carb Spacers Thin - 23358

To finish......Robinson recommends a total inlet tract of 14". That is from valve seat to bellmouth.

Hello Phil Then Pr-heater Sleeves may past there best but they can be used as Patenes too make new ones , out of Stainless steel Has I can now Weld Stainless steel Yours Anna J