twin carb set up

[Dave]

G'day,

Could anyone tell me if they have run a twin carb set up on a grey motor - how did it go, what sort of set up?

If strommies are used, what type would you use?  Do you simply set up two grey motor carbies on a twin manifold - and are there problems with the linkages?

Is it all worthwhile...?

thanks
Dave

[Effie C]

Dave
I too have twin strommies, used the twin carby linkage set up from American Auto Parts Sydney(Rare Spares in Sydney). Works like a charm, more top end and looks 100% over the old oil strainer air cleaner.
John M

[robbzfc58]

dave i have twin strommies on a grey
runs real sweet and looks cool...no problem getting linkages for it

[twitch]

dave

i put twins on my grey, the motor was worked so i couldn't run twins as the cam in it was to big and was sucking too much air and also it wouldn't idle under 1000rpm.

twitch

[fc_1958]

Cool - I have acquired a twin manifold and I would like fit it  to my FC - standard 138 so should not pose any problem??

However what havens to the vacuum advance - is it split between both carbies?

Also I read on another posting "it is importatnt to make sure that both throttle openings are matched and the easiest way is with a mechanical balance shaft" what is this? Has anyone got a picy?

Thanks

[nicko]

the vacuum advance is blocked on one carb and connected to the other

[TorqueFC]

are you sure. when we got our twin carb setup it had a t-piece so the vaccum line could run to both carbs. lucky he brought this topic up otherwise we wouldve just set it up that way

[craiga]

Darcy,

You'll find that your particular engine isn't going to produce a lot of vacuum anyway, especially at idle, because of the cam overlap.

To my way of thinking the vacuum can be tapped from one or both carbies, its really just going to be the same no matter if you connect one or both. The vacuum is produced by the air flowing through the carbies acting as a venturi and is used to operate the vacuum advance in the distributor and the wipers if original. Without sufficient vacuum the timing will be less advanced under load, specifically during full acceleration. With a larger cam you should look at getting the dizzy regraphed or even made full mechanical advance.

Spark timing with an altered cam profile is critical to making the best power, as is carburation and exhaust.

Cheers,

Craig.

P.S: Another one of Trev's old posts pop's up. Vale Trev.

[earlyholdenfan]

What about jetting on the twin Strombergs or model numbers?? do you just use two std carby? this is for a fairly standard motor
Matt

[craiga]

Matt,

Use two standard carbies, but fit smaller main jets. I think '46' (or 52??) is the right size, but someone else will confirm.

Cheers,

Craig.

[fc_1958]

Thanks guys now I am more confused.

On my set up the vacuum advance goes directly to the dizzy.
There is a separate pipe that runs from under the carby via  the fuel pump to the wiper and tees off to the washer control (borrowed from a FB). This vacuum point is replicated on the twin manifold.

However to my way of thinking there is approximately 138 cubic inches of air going through the carbie per revolution, so if two carbies are used this will be around half per carbie, this should cause a decrease in the venturi effect? Yes? Which will create less advance in the dizzy at crusing revs.

BUT joining the two together will not change this - for example if we have two compressors charged to say 90lb and  join them together using a tee and some hose, the outlet pressure at the tee will not be 180, it will still be 90. I assume the same is true for a vacuum.

Maybe the timing needs to be changed?

What do other members do - use the standard timing specs or are they modified?

And yes I realise that both carbies will need identical jets.

Thanks

Andrew  

[Burnsy]

Thanks guys now I am more confused.

On my set up the vacuum advance goes directly to the dizzy.
There is a separate pipe that runs from under the carby via  the fuel pump to the wiper and tees off to the washer control (borrowed from a FB). This vacuum point is replicated on the twin manifold.

However to my way of thinking there is approximately 138 cubic inches of air going through the carbie per revolution, so if two carbies are used this will be around half per carbie, this should cause a decrease in the venturi effect? Yes? Which will create less advance in the dizzy at crusing revs.

BUT joining the two together will not change this - for example if we have two compressors charged to say 90lb and  join them together using a tee and some hose, the outlet pressure at the tee will not be 180, it will still be 90. I assume the same is true for a vacuum.

Maybe the timing needs to be changed?

What do other members do - use the standard timing specs or are they modified?

And yes I realise that both carbies will need identical jets.

Thanks

Andrew  

I believe being four stroke it would be 138 CI per four revoluions but I could be wrong - I can remember this being discussed in year 12 industrial arts but for some reason I was not paying attention - must have been that I could hear the grinders and welders calling me from the other room

Also you have answered your own question in that if you take off from both carbies and tee the two pipes together the pressure will not double back to the original (that you had with just one carby), it will stay diluted.  Just the same as your two compressers joined together..........I think

[fc_1958]

Thanks Burnsy, Year 12 was not invented when I went to skool

However I don't wana be a smart ass   but now I do recall that in Year 9 motor workshop we were told that inlet and compression strokes = 1 rev, power and exhaust = 1 rev. So lets split the difference and call it 2 revs per 138 cubic inches!

Now that we have the basics sorted -

Are the tuning specs the same

And

What is a mechanical balance shaft - the rod between the two throttles or something more advanced

Thanks
Andrew

[bob_hawke]

can i ask is this talk much the same for a twin strommie on a 179 red motor with all of the linkages and stuff?

cheers

[craiga]

Mr Hawke,

On a red you will be best off using two 149 carbies, the manifold is obviously different but same principles apply.

Cheers,

Craig.

[fc_1958]

Hey Bob

It can't be much different - I want to fit two strommies on a standard 138. The vacuum to the wiper happens from the manifold - after the cabie operation.

Andrew

[4hammers]

FC-1958,
I have seen on ebay a couple of times, aftermarket "Vacuum Storage" taanks for car that have been slightly modified & for cars in hilly areas. As you probably know, the vacuum is gone once a load is put onto the motor, so this tank storess it for just that eventuality. I actually think there is one on ebay at the moment.



Rob J

[colt]

The shaft between the 2 throttle butterflies is used to open the throttles if your linkage goes onto the rear carby. There are different types of linkages available. Craig mentioned you will have to change your main jets to a smaller size. All other jets must be identical. I also went for smaller idle tubes. I was getting too much petrol on idle. Have fun.
Colin

[fc_1958]

Thanks for all the technical advice guys - I think I now have the basics to start the project - so we will let this topic rest.

[craiga]

Topic has been moderated - cleaned up AND locked.