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Austinsevenfriends
removal of obstructions/hot spot Ruby manifold - Printable Version

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removal of obstructions/hot spot Ruby manifold - JonE - 29-09-2018

Now the manifold is off, I've removed the hotspot for modern fuel, and I thought I'd post here as I couldn't find a pic of what I need to do prior.
I've also widened the inlet manifold obstruction (both pics have another standard manifold next) ready for the better flow from 1" SU, and am just going to keep on with this til it feels like it widens/tapers nicely into the main tube beyond.

Anything else I need to do, profitably. Exhaust bits? Someone mentioned making sure the holes line up, but does that mean taking it out to the size of the gasket rim.. can't see anything if I offer it up to the block...


RE: removal of obstructions/hot spot Ruby manifold - Chris KC - 29-09-2018

I find the simplest way to match the ports is to make a stiff paper gasket. Punch at least two holes to neatly locate it in position on studs then rub the outline of the ports. These can then be carefully cut out and the gasket transferred to the other side for comparison, keeping the right orientation of course. Mark with a felt tip where it needs work. I've tried other ways (blueing, spray paint through the port etc) but found this works most reliably. You can be as fussy as you like but the returns diminish.


RE: removal of obstructions/hot spot Ruby manifold - JonE - 29-09-2018

thankyou. will report back!
Does the gasket ever need 'work'?
And will I be working on the manifold, or the engine side? Or the possibilty of both?


RE: removal of obstructions/hot spot Ruby manifold - Bob Culver - 29-09-2018

Like everybody else I slavishly followed the Willaims book years ago and removed the "obstruction." But is it? Or a transition to the small diameter of the manifold? has anyone done a before and after with no other changes? Some tuning books favour an inlet manifold with apertures smaller than the in the block. Curiously a homemade manifold just over  1 inch id seemed no better on my car but perhaps needle was not optimum.


RE: removal of obstructions/hot spot Ruby manifold - Peter Naulls - 29-09-2018

Jon,

I'm interested to know the theory behind removing the hotspot. I always understood it was there to help vapourise the fuel after atomisation. I realise that modern petrol has a lower vapourisation point but doesn't this create problems in the body of the carb rather than in the choke area? Are you trying to reduce heat transfer back to the carb body?

The reason I'm asking is that I have a special with a tubular exhaust manifold and SU carb which runs well in warm weather but badly when it's cold. Living in Inverness this reduces the Austin Seven season down to a couple of months! I suspect part of the problem (just part) is lack of vapourisation.

Peter.


RE: removal of obstructions/hot spot Ruby manifold - jansens - 29-09-2018

(29-09-2018, 09:54 PM)Peter Naulls Wrote: The reason I'm asking is that I have a special with a tubular exhaust manifold and SU carb which runs well in warm weather but badly when it's cold. Living in Inverness this reduces the Austin Seven season down to a couple of months! I suspect part of the problem (just part) is lack of vapourisation.

Peter.
 My old MG mechanic used to tell me he'd re-tune his SUs between summer and winter in his B.


RE: removal of obstructions/hot spot Ruby manifold - JonE - 29-09-2018

I'm just a sheep... sorry. I was thinking it was about heat transfer, but it just seemed a good idea as saw it written down. Hopefully others will answer!
(If it doesn't work then I'm happy that I've removed a few grammes, which is sure to help acceleration)


RE: removal of obstructions/hot spot Ruby manifold - Steve Bryant - 29-09-2018

(29-09-2018, 10:07 PM)jansens Wrote:
(29-09-2018, 09:54 PM)Peter Naulls Wrote: The reason I'm asking is that I have a special with a tubular exhaust manifold and SU carb which runs well in warm weather but badly when it's cold. Living in Inverness this reduces the Austin Seven season down to a couple of months! I suspect part of the problem (just part) is lack of vapourisation.

Peter.
 My old MG mechanic used to tell me he'd re-tune his SUs between summer and winter in his B.

Re your MG mechanic : This is because of the difference in air density between summer and winter.


RE: removal of obstructions/hot spot Ruby manifold - Chris KC - 29-09-2018

I think it's a good idea to blend out obvious steps in the ports and this implies making sure the outlines line up more or less. I would always by preference remove metal from the component which is cheaper and easier to replace!

I've come across a few slavishly polished manifolds in my day but am not convinced that they bring much real benefit and they must take many, many hours of work to produce.

As far as I can fathom A7's seem to run perfectly well on absurdly narrow inlets and exhausts, so I wouldn't expend enormous effort making them bigger either. What you've done there Jon is as much as I would.

Just my opinion!


RE: removal of obstructions/hot spot Ruby manifold - JonE - 30-09-2018

Thanks. Having - after removed the head and the manifold - relaxed into observing the engine ports, gasket and Ruby manifold respectively, it appears the the block pretty well represents the shape of the gasket, and the gasket thus makes a good prompt for where I can get a few more grammes off of the manifold which definitely sticks out a few mm further...