Joined: Aug 2017 Posts: 606 Threads: 18
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Location: Sheffield South Yorks
Car type: 1932 RN saloon
Yesterday I did a 75 mile run in the chummy. It was a hot day.All was well for the first 30 miles but then the engine started running very lumpily and would not exceed 25mph in 2nd gear for a further 20 miles. On climbing out of lower ground onto the moors above Sheffield everything returned to normal. The same thing happened two years ago on the same run so I fitted a heat shield on the exhaust manifold and on the downpipe but these mods haven't done the trick. I checked and cleaned the distributor points,ap and all HT lead connections but all appear sound anyway. Any ideas re preventing this please? The chummy has a 4 blade fan and small SU carb and electric pump. Looked on the old forum but can't find anything.
Thanks,
Dave.
Joined: Aug 2017 Posts: 114 Threads: 8
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Location: NE England
Hi Dave,
Tuned up Mini's with SU's had the same problems. The fuel, as you rightly say, is boiling - in the float chamber - making the mixture too rich. You need a heat shield between the exhaust and the float chamber, a simple aluminium shield did the trick on my AH. Plus have an insulating gasket between the manifold and carb. Getting plenty of air round the carb is the last part, it comes in easily enough through the radiator but is there an easy way out with a flow in between? In these circumstances folding the side of the bonnet up can work.
Hope it works for you, Dave.
Joined: Aug 2017 Posts: 606 Threads: 18
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Location: Sheffield South Yorks
Car type: 1932 RN saloon
Thanks Dave,
I will have a go at extending my ex manifold heat shield to isolate the float chamber from the radiant heatt but keep airflow around it. Chummy bonnets have no louvres so will devise something to lft the nearside an inch or so. What material do I need for the carb to manifold insulator. Red fibre would be good but I have used up the 6mm thick stuff.
Joined: Aug 2017 Posts: 24 Threads: 5
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Yes, the volatile parts in petrol boil at a surprisingly low temperature as I found in a simple test as follows. I boiled a pan of water and took it outside. I then place a bean can containing petrol into the hot water with a thermometer inserted to measure the temperature of the petrol. It boiled at 40 degrees C !!
The boiling lasted for about 30 seconds before stopping although the temperature of the petrol continued to rise. Clear evidence of the very volatile component of today's petrol which is either lost or reduced when in storage and which boils just when you don't want it to when in your car!
Ron
Joined: Aug 2017 Posts: 606 Threads: 18
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Location: Sheffield South Yorks
Car type: 1932 RN saloon
Well! Thank you all. Never fail to be impressed by the almost instant helpful advice on this forum. I have managed to find my last piece of red fibre which is 8mm thick and just big enough to make a gasket. Nice quiet early morning job for we insomniacs.
Joined: Aug 2017 Posts: 691 Threads: 37
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Location: The Scottish Border
Ethanol/alcohol boil about80 degrees C.
Acetone about £60 degrees, so interested
to know what component of petrol boils at 40 degrees C?
Bill G
Based near the Scottish Border,
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Location: Ripon
DIN 51630 provides for petroleum spirit (also called spezialbenzine or petrolether) which is described as "a special boiling-point spirit commonly used in laboratory applications, having high volatility and low aromatics content." Its initial boiling point is above 25 °C, its final boiling point up to 80 °C.
Wikipedia
Joined: Oct 2017 Posts: 1,391 Threads: 44
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Yes one of RNs was prone to boiling the petrol in the float chamber on a long climb in bottom, on the same hill the other two RNs didn't suffer this problem. Then I realised that the one that boiled had the plate between the chassis and side under the exhaust pipe I removed the plate and continued the climb with no more problems.
