Carbs intake tubes and heads

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chasbmw

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My new to me 71 750 is a bit of a bitza, I think.

On taking the carbs off I find that I have 932 carbs, 28mm manifolds and 28mm heads.

Is this normal?

The guy who built the bike used a silicone Delaney between the various carb intake flanges , I have never found that Silicone sealant and petrol work together very long, should a fuel resistant sealant be used on these joints?

Any tricks to reassembling the whole caboodle?

Thanks for your advice
 
Itsa bitsa ! I'd fit smaller 930 premiers and check the manifold sizes to match, Any internal steps or air dams are to be eliminated. Silicone not good here,heat insulating spacers go against the head itself and carb to manifold uses o-rings to seal in a groove. Perhaps the mounting flanges got warped by his overtightening carbs to manifold and he used silicone to compensate ?
 
I'd leave it as is with the air flow trip up lip entering small head ports for the vortex generation that eases the flow of the bend in ports and packs in mixture that's funneled down to trade pressure for velocity for some extra spunk. I had similar 28.5 head with both 34 Miki and then dual 32's Amals and lost significant response when I put on the 32 mm port head, that didn't help power till rpms into the red zone, so mostly useless drag down to ride anywhere but airports. Keeping it as it is now may definitely disappoint after 120 mph or so - so of course you will be thought slack owner not to configure to land speeder set up.
 
Thanks for all your advice, the 932s on the bike are unused, so I will keep them because as nice as a set of Premiers might be, there are beyond the budget. :roll: :wink:

No sealant on the joins, I will check carb flanges to make sure they have not been distorted
 
chasbmw said:
Thanks for all your advice, the 932s on the bike are unused, so I will keep them because as nice as a set of Premiers might be, there are beyond the budget. :roll: :wink:

No sealant on the joins, I will check carb flanges to make sure they have not been distorted
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Alway true up the faces whenever they are removed.

It is not hard to relieve the ID of the manifold that meets the carb. Go in 1/2" to 3/4" and bring it to equal the ID of the carb with a nice gradual taper using with a Dremal and a drum sanding bit or the like. Use a 32mm gasket as a guide. If you go over slightly, no big whoop.

It's hard to screw it up and manifolds are pretty easy get if you actually destroy them, which you won't.

I get the theory of turbulance to help with atomization but I think that these 32's maybe bre too big already, unless you have a hotter cam, added compression and some exhaust changes to compliment them.
 
I highly suggest before you follow the common wisdom of land speeders built for maxium flow with some definite sacrifice of lower rpm response that you try a rough cut a manifold gasket that sticks into the air path another ~1/16", especially on the the lower half of the port opening and get back to us what ya notice. You can still smooth the mismatch later and then try the intruding gasket to regain some the lost response back. Goes against the grain of most power and response scenario's but I've had two engines, an out board and a Combat that surprised the poop out of me & strangers after only doing the idiot expedient mods just to get running a while before doing it the right way...
 
If you do a search on some of the threads by Comnoz, you'll see his research is heavily in favour of small ports on Nortons. So I'd suggest that you should not open out the head.

I would suggest tapering the difference within the manifolds. I am fitting 34mm flat slides to my RH10 head that has 30mm ports and will taper the difference in the manifolds in exactly this way.

A bigger carb, feeding a small port, being sucked on by a long stroke, should (I hope) give good gas velocity to the intake charge.
 
I agree. I just to remeasure the manifolds to check whether not they are parallel or tapered.
 
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