Vapour Lock

[bob46320]

Some of you may have seen my post about a REV counter and temperature measurement.  Yesterday, we went out for a run with our local club, Renault Gourdinis, Porches, Fiats etc.  The RN ran well, however on climbing a moderate incline for 10 minutes or so in 2nd gear did raise the temperature some what.  Air temperature was 36 deg C and towards the top of the hill (about 400M climb) she started to miss and jump around.  Fortunately, we made the top of the hill without holding anyone up and continued without problems.  A long break for lunch, temperatures rising  and another run, but after about 40 miles,  another hill and some serious vapour lock problems. After wrapping some water soaked rags around the gravity fuel pipe, we continued for a few miles and we were then stuck !!.  It was my wife who came up with the OBVIOUS solution.  Why not remove the bonnet !! - Well that was so simple,  it fitted behind the front seats perfectly and we continued on our way home much to the merriment to our French drivers.
The temperatures I was able to measure on reaching home:-

Ambient Air temp    39 deg C
Water                    69 deg C  (With the bonnet removed, I guess these had dropped to avoid vapour lock)
Manifold                 74 deg C
Carburettor body     45 deg C
Fuel pipe                34 deg C

I think the culprit is the fuel pipe/ Bulkhead fuel tank temperature, because when cooled with a soaked rag, matters improve.  
I am looking at 1).  Placing insulation around the fuel tank
                      2).  Replacing the copper fuel tube with some sort of insulated tube  (Any ideas / suppliers for that ??)
                      3).  Making some sort of heat insulator for the exhaust pipe (Flitch plates not fitted because they don't fit).
Any thoughts on the above appreciated.  1932 RN Saloon, 4 blade fan, new radiator (Modern core), ignition timing about right, Heatwave.

[Reckless Rat]

Rather than taking off the whole bonnet, you could just open the nearside half, placing a cloth between the two halves of the bonnet to prevent chafing and a bungy twanger between the offside bonnet catch and the centre rail to stop the open half from bouncing about. It's what I used to have to do Bob, before I fitted the 4 blade fan. I think the secret is creating a path for the hot air to escape.

[Chris KC]

We took the bonnet off the Nippy over the Grossglockner many years ago, drove with it standing on my passenger's lap as the car was jammed with luggage!

I think Recks is right, you need the hot air to escape somehow. Failing that, the exhaust must surely be the biggest contributor. Have you tried binding it in exhaust wrap? (eBay) That might shave a few degrees off and looks quite 'period'.

[Rick F]

Not wishing to try to teach anyones ancient relative to suck eggs .... but , do you have a loop in the fuel pipe, and is it vertical or horizontal?

[Duncan Grimmond]

    'MIn both of my specials I have found that the bulkhead takes only a few minutes to become warm and the holes around the pedals allow a surprising amount of heat to enter the foot well. After a half-hour drive the feet are toasty warm despite the open cockpit.
Getting the hot air out from under the bonnet is critical to avoid overheating in hot weather. I thought I'd allowed for this with the large meshed vent I have fitted in Mathildeux but I think I'm going to have to fit a four-blade fan to help.

[AustinWood]

My 1929 Fabric Saloon has the exhaust downpipe wrapped and an aluminium heat shield between the updraught carburetor and exhaust.
There are no ventilation holes in the bonnet and I have had no vapourisation problems in use.
The only problem ever was on the way down to Malcolm Parker's Masham to Kelso run. We climbed to the top of the highest surface road in England in very hot weather (For England, high 20s). And stopped at the top to survey the view. When we got back it was a bit reluctant to start because the petrol was boiling in the float chamber. However it did and ran badly with not much power for a couple of 100 yards then resumed normal running.


   

[Dave Wortley]

Duncan,
Your special looks superb. Would you be able to post a few more photos?
Cheers,
Dave.

[bob46320]

lucky for Bruce, you have a rear tank! Rick - Which should it be ? Mine is 2" diameter loop in the vertical, but I see AW's is horizontal (and large).

[Rick F]

A vertical loop presents a really good opportunity for a vapour lock, a horizontal one is generally considered more satisfactory.

[bob46320]

OK, that's the first modification I will make.

Anyone got pictures for the "Flitch plates" on an RN or perhaps an RP ?