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Hi,

I'm thinking about soldering the poppets in the butterfly valves close to get a better idle, but i'd like to hear about your experience first.
I found a thread similar to this one about the MGB, and most of them were quite satisfied with closed poppets.
As far as I could find, you get a quicker return to idle, and it may get you that raspy rumble when you quickly let go of the accelerator.
Besides, it may allow you to achieve lower idle speeds, especially if one of the springs has weakened a bit.

Are there also any negative side effects known? I guess some re-tuning may be needed but I don't see that as a problem.

I just swapped the carb throttle discs for some without the hole. The engine runs fine without them. I don't like the idea of anything like solder being there with the chance it could come loose.
I have experienced an air cleaner nut come undone and go into an engine on my rally car and it does make considerable mess. It scrapped most of the engine.

New ones could also be an option, yes. But can you define 'runs fine' please? Did you experience improvement in response or performance or something like that? Next to the effect of its operation, the poppet valve itself is also a restriction in the inlet, so it wouldn't be strange if higher end power would increase a bit.

I have to agree with Russ, put new discs in without the valves.

@Beans Great, the advice will be followed. But I was hoping for someone to tell me about the effects of the conversion to closed throttle plates, either with soldered ones or new solid ones. My bad, i should have aksed the question in another way. This question seems to have the emphasis the effect of soldering them, while I just meant 'a valve without a poppet'. Surely it must be beneficial, or no one would bother to replace them with closed ones.

edit: Edited main question to avoid misinterpretation.

Hi

I replaced my discs with the solid ones and it cured my issue I had with a inconsistent idle. When the engine warmed up it would idle between 1100 to 1800. The swap made a constant 900 rpm.

I have had 2 7s with SUs. One had solid butterflies, & my current 7 has silver soldered poppet valves. I had not driven this car before soldering the valves.

Both had a somewhat inconsistent idle, ranging from about 750 to 1300 RPM. The first I cured the problem by meticulously cleaning the throttle linkage, & replacing the oily/dusty accelerator cable with a new low friction lined Bowden cable.

I plan on a similar cable replacement of my current 7s cable, but in the meanwhile I overcame the problem by removing the old cable & washing it out thoroughly with petrol, & blowing dry. Then lubricating it with graphite powder, & making sure no oil or other contaminates that would gather dust gained access to it.

I believe it is dust gathering oil in throttle cables that cause a sticking problem at small openings. I have been "gunna" get a quality cable for my current 7 for over 10 years, but keep forgetting when ever I am near the only supplier I know of in our area, as the existing cable, now clean is working well.

Hasbeen

Just been refreshing my memory by reading up in the SU Carburetters Tuning book and it describes the valve as an "over run throttle valve". It is designed to allow air into the engine under closed throttle over run conditions when the mixture tends to get a bit rich, and so it reduces emmissions by weakening the mixture.

dursleyman wrote:Just been refreshing my memory by reading up in the SU Carburetters Tuning book and it describes the valve as an "over run throttle valve". It is designed to allow air into the engine under closed throttle over run conditions when the mixture tends to get a bit rich, and so it reduces emissions by weakening the mixture.

Nope, as the throttle butterfly is after the fuel metering. The valve is indeed used to reduce emissions. It does this by limiting the maximum vacuum in the inlet manifold under closed throttle. When the maximum vacuum is lowered, less oil is dragged down the inlet valve stems - giving less emissions.

As an example, my TR7 16V had new solid butterflies and one day, when I was driving energetically to work, I snapped the throttle closed at 6500 rpm. Deceleration was massive and so was the smokescreen behind - lots of emissions as it were - all from oil dragged down the inlet valve stems.

Valve indeed is to help combustion of unburned fuel when the throttle is closed.

jeffremj wrote: ... Deceleration was massive and so was the smokescreen behind ...

Badly worn engine springs to mind

Beans wrote:Valve indeed is to help combustion of unburned fuel when the throttle is closed.

I can't work out how, as it is still metered fuel, so mixture is appropriate to normal use - the valve isn't letting in more air, but an amount of air/fuel mixture.

jeffremj wrote: ... Deceleration was massive and so was the smokescreen behind ...

Badly worn engine springs to mind

Bottom end, no. Top end (valve guides) yes . It probably looked like a blown engine at the time .

Thanks for the replies guys!

So, notes taken: A solid butterfly valve can help inconsistent or high idling, replacing the accelerator cabel with one that is sliding freely may also help the problem. Both are very plausible causes of problems in my case.

Note2: Don't shut off the throttle completely at high rpm's if you have a solid butterfly valve, or your vacuum will get so high that all kinds of nasty stuff will happen that will bring tears to your eyes.

I'll just get a new set of solid ones, if it unexpectedly still gives problems, I can always revert back.

One question left; do solid butterfly valves give you that raspy rumble in the exhaust upon deceleration?
Because that sounds rather cool.

jeffremj wrote:I can't work out how, as it is still metered fuel, so mixture is appropriate to normal use - the valve isn't letting in more air, but an amount of air/fuel mixture

Yes and no, due to pressure differences when the throttle is shut there is the chance that fuel condensates and sticks to the manifold.
The extra air should help move and burn it.

Beans wrote:

jeffremj wrote:I can't work out how, as it is still metered fuel, so mixture is appropriate to normal use - the valve isn't letting in more air, but an amount of air/fuel mixture

Yes and no, due to pressure differences when the throttle is shut there is the chance that fuel condensates and sticks to the manifold.
The extra air should help move and burn it.

But it is not extra air. It is extra air/fuel mixture. I still think the valves are for when the car gets older and extra oil starts to flow down worn valve stems. Look at the backs of the inlet valves to see the burnt oil deposits.

jeffremj wrote:

Beans wrote:

jeffremj wrote:I can't work out how, as it is still metered fuel, so mixture is appropriate to normal use - the valve isn't letting in more air, but an amount of air/fuel mixture

Yes and no, due to pressure differences when the throttle is shut there is the chance that fuel condensates and sticks to the manifold.
The extra air should help move and burn it.

But it is not extra air. It is extra air/fuel mixture. I still think the valves are for when the car gets older and extra oil starts to flow down worn valve stems. Look at the backs of the inlet valves to see the burnt oil deposits.

One purpose of the valves is to keep the vacuum down when you shut the throttle while still in gear. Or even more extreme, downshifting while braking and using engine compression to help slow down. That can cause manifold vacuum to rise well above normal idle levels, and will draw fuel directly out of the jets, above what the air flow would support. That is why you sometimes hear popping out of the exhaust under those conditions, the mixture is so rich that it isn't all combusting in the cylinder, but does once it finds more oxygen in the exhaust system.

Depending on how stiff the spring in the valve is, it may or may not have any effect at idle. The overrun valve on the V8 FI engine doesn't come into play at idle. Aside from that, some people like to remove them because they do cause some restriction in the intake system at WOT, as the throttle plate is more bulky. I don't know how much that really affects things.