Thinking of removing secondary throttle assembly

[JGeezer]

I am thinking of discarding the whole secondary throttle assembly on my '69 early Federal project. It only serves to support the crossover for EGR and the idle retard, both of which I am planning to remove anyway. The Strombergs mount beautifully directly on the intake manifold (well, they will once I put in shorter studs).

Anybody have a [good] reason why not?

[CBUEB1771]

Ideally you would replace the mixture pre-heat assembly with the throttle blocks and balance tube fitted to non-federal engines with Stromberg carburetors. You really want the balance tube to prevent starving cylinders with the Stromberg arrangement. These are shown in the Service Parts List.

[rdssdi]

I have attached a picture of the secondary throttle assembly removed on my TC engine. Also a 1969 federal model. This was done by Marcovicci and Wenz when they rebuilt the engine. I have since returned to the original air box as the front filter assembly contacted the hood (bonnet) when closed.

Bob

[collins_dan]

You can remove the crossover tubes, but need the secondary throttle bodies as shown in Bob's picture. Most put simple covers over the holes like in Bob's picture. The secondary throttle bodies are needed to balance the carbs. Dan

[RogerFrench]

Indeed, keep the manifold, O-rings, Thackeray washers etc. You can keep or discard the secondary throttle plates and spindles. If you keep them, lock them in the open position; if you remove them (as I have done on my TC Europa) then block the holes. I tapped them (3/8" coarse thread if I recall correctly) and put aluminium studs in with some epoxy to keep them in place.

[JGeezer]

So the reason you would all keep the secondary is just to balance the carbs - OK, but why don't the Webers need that? They were mounted directly on the intake manifolds, with no interconnection. If I get Webers I can remove it, but the Strombergs need the balancing?

[garyeanderson]

So the reason you would all keep the secondary is just to balance the carbs - OK, but why don't the Webers need that? They were mounted directly on the intake manifolds, with no interconnection. If I get Webers I can remove it, but the Strombergs need the balancing?

Webers have an individual runner per cylinder where as the Stomberg head has 2 runners for 4 cylinders and have a Siamese intake tracts so what happens in practice is that they do not balance well. Cylinder 3 steals some of the intake charge from Cylinder 4 (likewise cylinder 1 steals from Cylinder 2, 2 from 1, and 4 from 3) when there is no balance tube. Why a balance tube works I can't really say but without it it won't run (or drive) well.

Gary

[oldelanman]

So the reason you would all keep the secondary is just to balance the carbs - OK, but why don't the Webers need that? They were mounted directly on the intake manifolds, with no interconnection. If I get Webers I can remove it, but the Strombergs need the balancing?

Webers have an individual runner per cylinder where as the Stomberg head has 2 runners for 4 cylinders and have a Siamese intake tracts so what happens in practice is that they do not balance well. Cylinder 3 steals some of the intake charge from Cylinder 4 (likewise cylinder 1 steals from Cylinder 2, 2 from 1, and 4 from 3) when there is no balance tube. Why a balance tube works I can't really say but without it it won't run (or drive) well.

Gary

Beat me to it as usual Gary Here's my understanding.....

The firing sequence of the engine - 13421342 - means that the front two cylinders fire followed by the rear two so with the paired runners of the Stromberg set-up the manifold depression "seen" by each carb tends to fluctuate. The air valve in each carb tries to follow these fluctuations leading to unstable air/fuel ratio and rough running, particularly at idle. The balance pipe is necessary to overcome this problem.

The twin choke Webers and Dellortos are effectively 4 individual carbs each feeding one cylinder through it's own intake runner so the above problem does not arise and a balance pipe is unnecessary.

[JGeezer]

Thank you so much, Dan, Gary, and Rodger! You guys do know your stuff, and unfortunately, that now makes a lot of sense to me; having that whole (in my judgement, rather ugly) extra manifold just for that is now a burden I accept.

I started looking into the dual choke conversion, but now see that's whole different head. My only option is Dan's, and I will try to source the non-federal parts:

Thanks again! You guys are really good.

John G
New Hartford, CT

[RogerFrench]

Gary and the other Roger have explained 99% of the issue - there is one more bit to it. Consider one pair of cylinders - say 1 and 2. Since there's a "suck-suck-pause (2, 1, pause)" flow through the carb, you can consider that the first inlet to open is No2. Before it's completed its fill, however, along comes greedy No1 inlet valve, opens up and steals charge from No2. Having a connection between the 2 inlet runners (1&2, 3&4) helps this issue too, though doesn't eliminate it.

You don't absolutely need the non-Federal solution, though I must admit to it being slightly better and am in the process of converting my Europa, having found the bits and pieces through this forum. Good luck with your search, but you can go ahead with the Federal manifold anyway. It does have one small advantage over the non-Federal, in that it provides you with somewhere to hold the throttle cable outer sheath, which you otherwise have to work out yourself.

As to whether it's ugly or not - I don't think Bob's or my setup look ugly.

[JGeezer]

That is about as nice as I would think it can be done. Thanks for showing me!

[rgh0]

The reason that Strombergs have a balance pipe is to keep the manifold vacuum steadier at idle speed. Without the balance pipe the period of non flow is signficant at idle spped and this causes the piston to drop and then lift with each suction cycle which gives a poor idle and throttle response of idle. The balance pipe enables a more constant vacuum signal to the piston and thus constant piston postion in both carbs by ensuring they both see the same vacuum vacuum signal during the flow and non flow parts of the cycle..

At higher engine speed the balance pipe is not really needed as the piston does not have time to respond the shorter time period loss of flow and its mass and dmaping achieves the required averaging of the flow rate required.

cheers
Rohan

[oldelanman]

The reason that Strombergs have a balance pipe is to keep the manifold vacuum steadier at idle speed. Without the balance pipe the period of non flow is signficant at idle spped and this causes the piston to drop and then lift with each suction cycle which gives a poor idle and throttle response of idle. The balance pipe enables a more constant vacuum signal to the piston and thus constant piston postion in both carbs by ensuring they both see the same vacuum vacuum signal during the flow and non flow parts of the cycle..

At higher engine speed the balance pipe is not really needed as the piston does not have time to respond the shorter time period loss of flow and its mass and dmaping achieves the required averaging of the flow rate required.

cheers
Rohan

That's what I tried to say in my post above..........but I bow to your superior explanation

[rgh0]

Hi Roger
I was trying to reinforce what you had said as some of the subsequent posts appeared to be missing the point.

cheers
Rohan

[collins_dan]

The federal assembly is fine. The o-ring and thackery washer set-up is critical to good performance. Notice the gap between the carbs and the assembly, that is the o-ring and thackery washers at work. Good Luck, Dan