Stromberg Vacuum Connection

[terryj]

There was an interesting article in the  August 2020 Courier No. 482 p.62 on SU Carburettor Balancing.  However, my Gentry has twin Stromberg CD150’s on a Vitesse Mk2.  The carbs were originally from an Avenger GT I believe so linkages and chokes are different from the usual Vitesse setup. 

The SU balancing requires a vacuum connection from both carbs but, although my carbs both have a tube for attaching  a vacuum pipe to, only one of them is drilled through and in use for the vacuum advance connection.

Is it feasible to drill through the tube on the unconnected carb into the air intake to create a vacuum connection to provide the two vacuum connections required for carb balancing in the Courier article?  I would, of course, have to devise a method of blanking this second vacuum tube for normal running purposes.

I’m not really au fait with all the drillings and workings within the carbs so any expert advice will be much appreciated.

[PeteH]

Never done/seen it on Stromberg`s, but the Bing Carbs fitted to my Beamer (bike), both had drilling s which where blanked off with small machine screws and fibre washers. I only remember balancing the Vitesse carbs (1980`s) by listening to the "hiss" in the Carb mouth with flex hose? a sort of stethoscope . As shown in the Haynes manual?. The vacuum connection was for the vac advance on the distributor, hence why only on one carb;.

Years ago I had one of these, which could be used on twin carb setups. https://www.halfords.com/tools/hand-tools/automotive-tools/gunson-carbalancer-495254.html?cm_mmc=Google+PLA-_-Tools>Hand+Tools>Automotive+Tools-_-Tools>Hand+Tools>Automotive+Tools-_-495254&istCompanyId=b8708c57-7a02-4cf6-b2c0-dc36b54a327e&istFeedId=62b447cf-331e-4fec-a47a-9985ff72d404&istItemId=pplxwaqim&istBid=tzwt&_$ja=tsid:|cid:11902546034|agid:113456751337|tid:pla-331014970735|crid:487900199209|nw:u|rnd:18286968906922132218|dvc:c|adp:|mt:|loc:1006816&gclid=Cj0KCQjwt-6LBhDlARIsAIPRQcJeOhglR_teN89wR-N3Nq6FAmrvWfkGNpdtdZWmyOnIXgjLyjvrOigaAj9dEALw_wcB

Pete

 

[Nigel Clark]

Like Pete, I've always balanced twin SU or Stromberg carbs by listening to the hiss with a tube held in the inlet,  or by holding an airflow meter in the inlet. This type of meter is widely available and works well enough:

https://www.ebay.co.uk/itm/STE-Carburettor-Synchrometer-Air-Flow-Meter-SK-40-42-45-48-SU-Weber-Dellorto-ADV-/352992790086?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l49286&mkrid=710-127635-2958-0

Nigel

 

[terryj]

Sorry Pete and Nigel but I forgot to mention that in the Gentry there is very little room for air cleaners.  Many owners have been unable to fit them at all so using a tube to match the hiss of intakes is possible for them.  However, I do have pancake air cleaners fitted and they are a pain to remove and replace so I would like to be able to balance the carbs without access to the air intakes, hence my interest in the vacuum pipe method.

Terry

[PeteH]

My thoughts on that are the Vacuum connection is on the wrong side of the carburetor? you will be effectively measuring the manifold pressure?. Any take-off needs to be on the carb; inlet side?.

Pete

[Nigel Clark]

I've only ever seen vacuum take offs for carb balancing downstream of the throttle butterflies on multi-cylinder bikes, where each inlet tract is separate and not connected to the others while testing. I've tuned  three and four cylinder bike engines this way with a bank of vacuum gauges but never a twin carb car engine. With a Triumph four or six cylinder set up, there is a large balancing pipe on the manifold which will tend to equalise vacuum downstream of each butterfly.

For balancing these carbs, I believe it's the airflow through each carb is what needs to be matched or balanced. And the airflow is detected either by listening to the hiss with a tube in the mouth of the carb, or with an airflow meter. I'm happy to be corrected if wrong.

Nigel

[Pete Lewis]

do it mechanically    disconnect the link rod clamp   

back off the idle screw turn in to just nip a 005" feeler between screw and casting stop

turn screw in one full turn   do both carbs    run engine to operating temperature  adjust each idle screw the same amount  then when happy

nip up  the link  rod clamp

idle mixture is done the same  raise jet to be flush with bridge and back off 2 full turns  

again  when hot  adjust to best running  in equal amounts on each  

remember in the factory carbs were supplied to simple pre set idle and jet heights 

flow tested carbs were not dreamt of till the 80s  to aid emissions 

so in the day they were just preset and thrown on at engine dress and no faffing around with any   gadgets

Pete

 

 

[PeteH]

I think, (it a bloody long time ago) that I set mine, and then double checked with a couple  of colourtune plugs in 2 and 5?.

Pete

[Pete Lewis]

theres a lot of myths and fafs about balancing at idle   its not rocket science   its all  basic stuff

once you open the taps its pretty well all lost apart from basic mixture settings 

 

Pete

[terryj]

There is plenty of food for thought in everyone's answers so I had a look at a pair of old CD150's today and see that the drilling from the advance/retard vacuum pipe emerges in the airway in front of the butterfly on the intake side, not the manifold side, which seems to be not what PeteH thought.

I will probably not pursue this and use Pete Lewis's manual method which is pretty much what I have done in the past.  I just wanted to be able to check the balancing by an alternative method.

Incidentally, I wanted to check what my old carbs are from.  No brass tags but number C1854F & R stamped on the flange.  I remember having seen a website which listed what ref. No. carb was used in a whole list of cars but have been unable to find that website again.  Does anybody know what that website is please?

[NonMember]

1 hour ago, terryj said:

the drilling from the advance/retard vacuum pipe emerges in the airway in front of the butterfly on the intake side, not the manifold side,

Beware such descriptions!

The space in front of the throttle butterfly on a Stromberg or SU is decidedly NOT the "intake". It's the "constant depression chamber". Also, the vacuum advance drilling is only just ahead of the throttle plate. It's more of a venturi sense tapping than a pressure tapping - with the throttle anywhere near closed it's measuring the speed of the air.

[Pete Lewis]

the old website for the carburettor exchange in leighton buzzard had a listing of models abd spec numbers but its long gone 

was carbex.demon  but not there now   but a call to their site should give you what they are from

now its    https://www.carburetterexchange.co.uk/ 

have a browse at 

http://zenithcarb.co.uk/cf/vehicle/list/?manufacturer=Chrysler&vehicle=Avenger

never  ever  use the vacuum dizzy port for any manifold vacuum measures  its as Rob says very different and the vac is generated by air flow over a small drilling next to the throttle plates  its output has nothing  to do with manifold depression 

there is no reason the avenger spec carbs wont be ok  the main differences are may have  temperature compensators ( these need to be closed at normal running)

 could be rod or cable /cam operated throttles   being Rootes   it will have the two stage choke stop  

can be cds or cdse  

Pete

 

 

[PeteH]

18 hours ago, Pete Lewis said:

its output has nothing  to do with manifold depression 

That make more sense actually. The movement of the Vac; advance needs to be proportional to the "Throttle opening". Whereas the manifold pressure would not be as accurate. The location of the take off, is confusing, until you point it out. And the supply needs only to be from one carburetor.

Pete