Inlet manifold doesnt sit square to the carb bore.

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Reggie

Reggie

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On my Commando, where the inlet stubs meet the carburettors, the transition from one to the other, there is a ridge of nearly 2mm at the 12 o'clock position where they are not meeting square on, and a slightly smaller amount on the other carb where it meets the manifold.

One manifold feels a s if it has been badly enlarged by hand, and the other has the part No. 06-2727 stamped on the inside, which as far as I can determine, doesn't exsist :!:

I was just wondering as to how this ridge is going to effect performance, should the transition be absolutely smooth or very nearly :?:

I see that Steve Maney retails 33mm CNC machined inlet manifolds. Can anybody explain why there is a 33mm size. Does the racing fraternity have their carbs bored out :?: If he did a 32mm I would have been interested in buying one. Does anybody have any experience of recent Andover Norton manifolds :?: Are they accurately machined :?: If they are this may be what I end up purchasing.
 
I think that you'd be best advised to look out for a second hand pair based on the size of your inlet tracts and carbs.

No step is the ideal. You certainly don't want a ridge.

33mm is indeed the maximum size that a 932 body can be bored out to and is quite a common mod.
 
Reggie said:
On my Commando, where the inlet stubs meet the carburettors, the transition from one to the other, there is a ridge of nearly 2mm at the 12 o'clock position where they are not meeting square on, and a slightly smaller amount on the other carb where it meets the manifold.
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Could your inlet manifolds be the 30mm type?
As there were three different types (30-30, 32-30 & 32-32). All three types look outwardly different.
 
I'd wonder that too. Port matching is normally performed when blueprinting an engine but it would be best to start with the correct parts.

If you are anywhere in the northern CA area (as I write this I can't look) I have a couple of types you can compare.

Oops, Yorkshire. I don't reckon it will be worth the trip to check out some manifolds.
 
The manifold which feels as if it has been enlarged from original, measures 31.8mm, 31.8mm, 31.8mm at three different points at the carb mounting end, the other manifold measures 32.1mm, 32.2mm and 32.4mm.

It would seem quite likely that the first manifold may have originally been a 30/30? The other one maybe a bit of exuberant home made "gas flowing?" Physically, they look identical except that one is slightly brighter in the metal.

It looks like I'll have to get some more manifolds as they are a poor fit. I just wondered if anybody had any inkling as to whether or not new manifolds were likely to be near perfect, or a bit hit and miss?

I was aware of the poor fit at the joint two years ago when I built the bike, but I'd been building it for about 5 months when I got to the stage of fitting the carbs, and just wanted to get it running with a view to coming back to the problem. Well now I'm back :P Ker-Ching £ $ £ $, Ker-Ching £ $ £ $

Thanks all.
 
I'll look at some of mine tonight to see if I can find a number. All I have off the bike would be a 32mm set from a combat.
 
Just out of interest, does anybody know the perceived advantages of this bit of kit found in RGMs catalogue? I think that I am correct in saying that this kit will move the carbs rearwards slightly, so no more standard air filter.

RUBBER MOUNT CONVERSION FOR STD CARB (1). THIS COMPRISES ONE CURVED MANIFOLD, ONE STRAIGHT MANIFOLD, ONE MANIFOLD RUBBER AND TWO
 
Conventional wisdom has it that in terms of maximum power, the Commando needs a longer inlet tract. The positioning of the carb isn't the crucial thing, rather the overall length to the inlet valve.

In actual fact, the ideal length isn't achievable in a standard frame (the gusset gets in the way) which is yet another reason why Seeley Commandos are faster !).

Rubber mounting helps prevent fuel frothing. I remember reading a piece by Mick Hemmings years ago where it was stated that the rubbers were worth another 1000 rpm at the top end (which is fine if you like to rev to eight thou. Personally, I'd worry a little about piston speed). :shock:
 
Don't swap 'em, use a sand roll to port match 'em.

The short side of the radius is where things stagnate, so you'll get the best results by matching the manifold top to the longer radius (twelve o'clock as you called it out.)

Smaller to bigger usually has an anti-reversion effect and isn't a killer. The opposite should be avoided if possible.
 
David wrote;
Smaller to bigger usually has an anti-reversion effect and isn't a killer. The opposite should be avoided if possible.
Click to expand...

Errrmmm :?: Could you explain please?
 
I seem to fuzzily remember that the heads are 30mm and the manifolds were 30mm in-30mm out on the older bikes with the 930 carbs and 32mm in-30mm out (tapered) on the later ones like my Combat with the 932 Amals. Sounds like you have a mismatched set there.

By sand roll I take it this means one of those cylindrical sanding disks that you would put in a drill? That could do it for sure to cure the step.
 
Hi there, just to make clear about manifolds sizes.
1- all 750 untill 1972 :30mm/30mm
2- except the last 750 after 200.001: 32mm/32mm
3- then the 850 with either RH 4 head :32mm/32mm, or Rh 10 head with the tapered :30mm/32mm.
But after all theese years many mixes could occure , and sometimes I like to put 32mm carb with the tapered inlet on a 28.5mm 750 head, it makes them happy and me too.......
 
I couldn't find any numbers on either set I have. I'd also probably just match it to the gaskets. You put the gasket on the manifold and scribe around the inside, machinist's blue or magic marker the area you want to remove.
Then you can use small hand grinders or sand rolls to match it.
 
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