carb heat shield

[WIMPUS]

Hi,

Yesterday after fixing the OD on my spitfire we put it on a meter to look at the emissions..
It was around 7% !
Backed it off to 4,5%.
Car run great until it was warmed up, started to sputter & stuff 🤔

2 days ago i drove up to the netherlands with a friend.
After a while on the motorway i could hear the exhaust popping a bit.
When we drove off the motorway it was fine.

I've had a feel at the float chambers  & jets after a good drive and they are pretty hot ..
I do have a new regular heat shield on it, but seems it doesn't do the trick very well with the tubular manifold..

Any idea's ? 🙂

[WIMPUS]

I do have this one laying in the garage, but it's alu i think..
So would that work or better to have a stainless one? 🙂

[Rallyspit]

Hi Wim,

These heatshields are designed to prevent heatsoak which vaporises the fuel before it reaches the carb jets. They are ok, but you could improve them further by rivetting an additional sheet  of aluminium underneath with a small (5-10mm) gap between them. Heat will still radiate from a single sheet heatshield. The other thing to check is the SU carb to inlet manifold gasket - is it the thick one (about 6-7mm) which prevents heatsoak to the carbs from the inlet manifold?  If this is missing, the engine will die about every 15 minutes on a hot day with fuel vaporisation. The only cure is to pour cold water over the carbs or wait until everything has cooled down - ask me how I know! It's also worth installing exhaust wrap on a tubular manifold - but that only moves the heat further down the line, so the excess heat could be concentrated under your gearbox, which means a hot interior. Some say that exhaust wrap shortens the life of your exhaust manifold, but I've rallied cars fitted with 10 year old exhaust wrap without any ill effects - and they do get hot! Another alternative for keeping things cool on the manifold side is to duct air from in front of the radiator to that side of the engine. Also check your fuel line from the fuel pump to the carbs. Does it pass any "hot spots" on the way?

I don't think there is any appreciable difference between the aluminium and stainless steel heatshields unless you're after a bit of bling under the bonnet....

[markythesparky]

i made one out of stainless with an air gap between plates not had any problems

[Perfect 65]

I bought one off ebay just like that - seems to work ok. The shinier the better for retaining heat, just like old fashioned kettles used to be.
Shaggy dog story - Some cars (Vauxhall Astra with pump on the cam box was one) used to have a fuel return from the pump so it was always circulating and kept the pump cool in traffic. Down side was when the pump efficiency dropped with age there was not enough flow for feed and return so the engine died anyway. Cure when pump was weak was to shut off the return or get a new pump. The pipework had a tee piece with unequal legs so the carb got the major flow when needed.

[Chambers]

Quoted from markythesparky

i made one out of stainless with an air gap between plates not had any problems

What thickness did you make it from?

[markythesparky]

Its 1.5mm stainless with an 8mm air gap