Carburettor Confusion - wrong carb for a Dommi !

A 389/12 carb would normally be fitted to a 500cc single ,so would be a bit too large for a 300 cc 99 cylinder. Yes it will run ,but not  well on a std tune engine.  

It will run on a 389 but if you want it to run at its best you need to fit a 376. The '59 99 has the smaller valve head and so really doesn't suit the oversize carb whereas it might be OK in a bike with a '60 onwards head with the bigger inlet valves. Attempting to squeeze more power from a 99 as the previous owner did isn't really such a good idea so just bite the bullet and get a 376. 

Whoever fitted the 389 may well have opened out the manifold to suit the larger size carburettor. This will need rectifying if you fit a 376.

Can you just change body, slide block and slide? Burlin will know.

Attachment shows relevant Carbs and & Settings.

I don't think that a 389 can be sleeved down to match a 376 body. However, as mentioned above the single carb manifold can be opened up enough to eliminate any steps and give a smoother gas flow. But...... the Model 99 engine does not get on too well with a 389 carb with regard to idling.

My recommendation would be to buy a new 376 carb. I have recently managed to get hold of a New Old Stock Wassell carb  for my Dominator and it it absolutely the Bs & Es. Easier starting and idling, smooth throughout the power range and up to 80mpg.

What is stupid, about ordering the correct carburettor for this model?   Possibly a new inlet manifold will be required, if the old one has been "Improved",

Looking back at the question to begin with...I think you are saying it had the wrong one (389), and you have ordered the correct one (376). Sounds like you've done the correct thing...

Perhaps Mike would like to clarify, but his original post seemed to me to be perfectly clear, as saying he ordered a 376 rebuild kit, only to find it was not suitable for the 389 carburettor, which he had not realised had been fitted by the PO. If he had ordered a carburettor, he would not have described it as a carburettor kit, would he?

Divided by a common language or what?!.

Ian

hello amal 376 and 389 some  parts are inter changeable So all is not losted yet  yours  anna j

Don't worry if the manifold has been opened out to match the 389. The resultant small step will energise the boundary layer and do no harm. It might even be beneficial - better intake charge mixing, better cylinder filling. One of the worst things you can have is a perfectly smooth mirror finish inlet tract. The boundary layer adheres beautifully restricting flow.  

Hi Mike,

I agree with Gordon, in the 60's you could buy an intermediate flange device which fitted between carburettor and manifold. it was intended to allow a spiralling leakage of air to enter the manifold in order to eliminate the 'boundary layer' of stationary gasses, as Gordon described. It was important that the fuel to air mixture was increased to compensate for the additional air introduced. The 'stepped bore that you will have is a much simpler solution as the mixture is not tampered with.

Regards

]Dick

Mike:

I suggest that you go ahead and fit the 376/67 unless the supplier will not take it back if it has been fitted.  Since you have all ready paid for it, what can you lose?

Mike

Would it be possible to fit a sleeve as was done with the 650SS when the factory found that it improved driveability.

Fit it. Having spent years working as an engine development engineer,  I can say that almost all high volume (more than 50000 made/year) engines have small to big 'steps' along the inlet tract to allow for casting and machining variations. 1/16" is nothing to worry about.

Regards,  George 

If the carb is a 1 1/16 inch 376 monobloc and the manifold bore is 1 3/16 inch as stated by Mike then there is a 1/8 inch step-up unless mathematics has changed a great deal since I was at school.   Regards, howard

The step will only be 1/16" all the way round, assuming the carb and manifold are concentric with each other. 

It still sounds excessive to me.  On my 650SS I still have the tin sleeves fitted in the inlet manifolds and protruding into the throats of the cylinder head ports as fitted at Bracebridge St.  I am using a paired set of 1 1/16 inch monoblocs (one float chamber chopped off).  When I did  some tuning work for production class racing I left out the sleeves and it did not perform well at all.  So I put them back in and in they remain.  On my Dunstall tuned race motor the ports are 1 3/16 inch, the manifolds are 1 3/16 inch and the carbs are 1 3/16 inch paired 389/689's.  If Dunstall didn't have steps in the port then my motto is don't have unnecessary steps.  Cheers, ride safe Howard   

Reference Catalogue .Saludos Gerardo

I find it surprising that, if it had been proven in the 1960's that a step from carburettor to manifold was beneficial, all vehicle manufacturers since then have the same bore of carburettor & manifold.

However, if I were Mike, I would try the new carb on his existing manifold and see how it goes.

I think Ian is right, after all will it run smoothly at 20mph, which is the new save the Planet speed limit in cities? I doubt that his worn old camshaft lobes will let it run that smoothly. Seriously, 0.062" is nothing unless you are running at over 6000 revs. Jetting the carburetor is the answer, I always use a Colourtune plug to set the mixture right, just a tad rich on tickover, a 31/2 slide to start.

My 1959 99 has the larger inlet valve head, which was an optional extra for 1959 - then standard from 1960.  Mine also had most of the other extras for that year including HC pistons, twin carbs, chrome mudguards, fully enclosed rear chainguard and polished head, but not rev counter kit or folding kickstart - not when I bought it anyway but by then it had eventually been run with a sidecar and single carb.  John Hudson called it a "99 Special" when I corresponded with him at the time. That's one with TWO Cadbury's Flakes!