Inliners International Forums General Discussion Engines Vac. Advance

I am runnning three Stromberg BX-35's on my 235. Does it matter which carb I run my Vac line to for my advance? Thanks in advance.

That might depend on what kind of linkage you have on your carbs. If you are running progressive then you probably want to come off the primary carb. There is also a question of whether you want the vacuum before or after the throttle plate, I'll let some one else chime in on that.

Larry

Take it of the manifold and you will get better cruising mileage.

Frank

Frank; I was told previously on this forum, I believe, not to take it off the manifold. That is where I should feed my wipers and vac. shift from. It would be alot nicer install though then on the carb. Thanks for your reply.

I did both with some T-fittings
wipers, vacuum shift can and advance.

If you don`t have a super wild cam there is enough vacuum for everything plus a vacuum fan.

Frank

Dear Mr. Morgan;

Your vacuum advance line goes to the center carb ONLY as; It's "metered vacuum". Other acessories go to the manifold, as stated.

Good luck.

Although i do not need the vacuum supply for the wipers or shift as that will supplied from the pump, answer me this if you will. If the center carb is metered why would the two outside carbs not have the same venturi vacuum. I am not running a progressive linkage so should not the vacuum be the same on all three carbs? Also ,if I'm taking the vac post the throttle plate what is the diff if i take it from the carb or the intake. The cam is a very,very, mild grind just a bit better then stock. Bored 30 over with 848 head and offy triple intake with corvette split exhaust. The stombergs are single barrels BXVD-35's part #380419. 2" exhaust through steel packs. Any suggestions and help to wrap the ole gray matter around the theory would be appreciated. Thanks.

It can go to any one of them.

I said "only" because; some have wanted to connect all 3 to the distributor etc.

Often the linkage is (temp) changed, to only use the center one to save on gas too. If you use an end one, you lose that option is all.

Thanks for taking the time to reply John. So what would be the diff. if I take it off the manifold or the carb? I know this was explained before but I simply just don't remember.Good memory just not too long.

The vacuum needed for proper distributor advance, is 'metered' via the trottle position in the carburetor.

Manifold vacuum varies depending upon the engine's load.

Happy trails.

I'm making a nice modification. I'm getting vaccum from the intake manifold and from there I use it in three different things, so there is a big "T" pipe junction. One way out is for the vaccum advance of the distributor, another is for a vaccum gauge in dashboard and the other one is for water injection.
I think you can install needle valves - the ones that allow you to regulate the obstruction of the tubing - so you can adjust how much vaccum you want.
For distributor I adjusted the vaccum advance curve (very similar to a straight ramp curve) with the use of needle valve, tuning it with an eye in the timing pistol reading.
For the gauge, it's direct vaccum from the intake.
For the water injection I use a needle valve too, to control the amount of water/alcohool/oxygen injected in the base of the carb. I think it will work, but not sure about it. I'll run it when I got the extra tubing that is missing - I'll make it all using plastic and next I'll go to cooper tubing.
So, one idea is to use needle valves to regulate the amount of vaccum. I don't know, but there is a possibility if using big cams getting good vaccum at iddle but very high vaccum at high revs.
Am I crazy at all??? Someone just say if I'm thinking right or not, please.
Thanks a lot!

Saunds like an intresting project, similar to my 261 (water injection/dash vacuum gauge/needle valve to controll dizzy)

funny how differt guy on the other side of the globe come to same solutions.

You have some pics?

Frank

I would do a little experimentation. Try it both ways, and see which gives the best combination of idle and part-power performance and economy. With three carburetors all linked together, each carburetor will only be opening about one-third as much as a single carburetor would be to supply the same amount of power. Thus, the "metered vacuum" from any one of the carburetors in a triple set-up will be different from what would be produced from a single carburetor manifold, as the metered-vacuum port is usually just slightly above the fully-closed throttle plate. Where is it on a Stromberg BX-35?

Get a couple of vacuum gauges, connect one to the manifold and the other one to one of the "metered" ports on the carburetors, and see just how the vacuum signals differ during the different modes of operation - idle, off-idle, low speed cruise, high speed cruise, wide-open, etc. The primary reason for running for running progressive throttle linkage is to prevent the linked-together carburetors from operating in the off-idle mode (kind of a no-man’s land for the carburetor) during low speed cruise. If you have a wideband air-fuel meter, you have the makings of a real science fair project. I think that Frank may have done some of this.

Back in the late fifties-early sixties, I ran a 235 with two Stromberg BXOV-2 carbs, and a couple of 261s with both the two Strombergs single barrels and with three 97s. With the 97s I had no choice but to use manifold vacuum, and I found that manifold vacuum seemed to provide better overall satisfaction than carburetor vacuum to the distributor with the dual BXOV-2s. Perhaps the results would have been different if I had modified the centrifugal advance.

If I recall correctly, some GM cars from fifty years ago also sent manifold vacuum to to the distributor at idle. Some cars also have dual diphragm arrangements, with an additional vacuum retard function.

I once tried to hook up a vacuum gauge to the rear intake port of my GMC. The gauge needle vibrated wildly every time #5 or 6 cylinder drew vacuum. From that experience I learned it is better to pull vacuum from as close to the center port as possible so the vacuum effects of all 6 cylinders are felt evenly. I run 3 Weber 32/36 carbs on my GMC and use a small tube manifold to combine the metered vacuum off all 3 carbs into one for my vacuum advance signal. It seems to work fine.

I screwed up on the info. The vacuum would be coming out pre throttle plate if I use the outlet provided. There is no outlet for vacuum post throttle plate. The only place would be on the manifold. Guess we'll try it there and see what I come up with. Any other suggestions I'm all ears. I do appreciate all the info being posted. Thanks a bunch all.