[Healeys] Vapor lock gas

Bob Spidell bspidell at comcast.net
Thu May 6 09:03:15 MDT 2021

re: "Methanol[sic] lowers the boiling point ..."


On 5/6/2021 6:51 AM, Michael Salter wrote:
> Hi Ken, yes I think everyone with a 100 has encountered fuel delivery 
> issues with modern fuels. Methanol lowers the boiling point of petrol 
> to a point where it becomes a problem.
> With the 100 there are 2 distinct issues.
> Firstly, because the fuel pump and its associated lines are directly 
> above the exhaust pipe, hot air heats them. The fuel in the suction 
> side of the pump has, effectively, a lower boiling point than it would 
> at atmospheric pressure. This fuel tends to boil if there isn't 
> sufficient air circulation to keep the pump and lines cooled, as 
> occurs while stopped in traffic on a hot day. True vapor lock occurs 
> because the pump will not pump  sufficient volume of vapour to exceed 
> the rate of its production by the boiling process.
> Secondly the carburetor float chambers and the small fuel passage to 
> the jet are very close to the exhaust manifold.
> The manifold radiates lots of heat which heats them. Again when the 
> car is moving at a reasonable pace the incoming air through the grille 
> and around the radiator is sufficient to maintain the carburetor at a 
> low enough temperature to prohibit boiling but, again when stopped in 
> traffic this doesn't happen.
> I have managed to eliminate the first problem by installing a 
> insulating sleeve from a modern car's AC hose over the fuel line from 
> the tank to the pump.
> For the second I have made small heatshields from 24 gauge galvanized 
> steel to shield as much as possible of the carburetors from the 
> radiated heat. This has helped considerably but not completely 
> eliminated the problem.
> For the most part neither of these modifications are visible and I 
> have found that fuel delivery issues, although not completely 
> eliminated, are manageable.
> I am considering installing a small blower and duct sourcing air from 
> behind the grille to see how much difference that makes but if course 
> that will be visible.
> Hope that helps, others may have found better solutions.
> M
> On Mon., May 3, 2021, 2:58 p.m. Ken Fleming, <ahmg at aol.com 
> <mailto:ahmg at aol.com>> wrote:
>     Michael , I thought you maybe have experience this ans have a
>     solution . On my Bn1 which is M spec with carbs, advance
>     distributed, LeMans Cam and Pistons has always suffered from what
>     I call vapor lock of gas. Especially in hot weather .
>     I am assuming the heat is from
>     Manifold and it placement near carbs or fuel pump/ battery near
>     exhaust play a role but I could be wrong . I have asked other
>     owners and they seem most of them not to have same problem. I do
>     have a heat sheild added with little result.
>     I am at a loss as to
>     What to do
>     Next , but have you experienced this and have any suggestions. Are
>     my assumptions
>     Correct on heat causing the issue.  Could today’s quality if gas
>     be a issue?
>     I recall reading this was a common issue for 100’s to have vapor
>     issues.
>     Ken

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