The 100 Forum

Vapour lock AH 100, 1955

Moss Motors
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor

Pablo Noodles Avatar
Pablo Noodles Paul North
Teddington, Middx, UK   GBR
Hello Folks

I recently bought a 1955, AH 100 which has been fully restored including aluminium radiator, multi blade fan, plus electric fan too. All the wiring is new etc.

I find after any trip when I get into traffic for a little while the car will begin to splutter and buck. I have read about the likely symptom being vapour lock. On a straight run the car drives perfectly and the temperature on the dial is fine. Also I do find the car will shudder a wee bit when I turn the engine off too!

I do not have a heat shield over the carbs not is the exhaust manifold wrapped and nor is the fuel pump insulated from the exhaust. I have read (on older links on this website and others) if I do add a heat shield and reposition the fuel pump and wrap the exhaust this should solve the problem. Some links even say it helps to remove the electric fan if an aluminium rad is fitted with multi blade fan!! It apparently helps improve the air flow to the engine bay.

Has anyone with the same problem solved the issue? If you have please, please could you tell be howsmiling smiley

Kind regards

. Hide banner ads & support this website by becoming a > Supporting Member <
francisthibaud Avatar
francisthibaud Francis Thibaud
Parmain, Ile de France, France   FRA
I had this same problem until I made a heat shield between the carbs and the exhaust.
Since then no overheating at all.
I made it from stainless steel sheet, angled at the edges, with heat insulation material backing. Carbs bolt through it.
Best regards
Francis Thibaud
100 since 1986.

Pablo Noodles Avatar
Pablo Noodles Paul North
Teddington, Middx, UK   GBR
Hello Francis

Thank you for your reply.

That gives me confidence to go ahead and get a heat shield fitted.

If anyone else reading this has any other interesting experiences successes or otherwise I would be very pleased to hear - pictures of any remedies also very welcome.

Thank you very much again Francis

. Hide banner ads & support this website by becoming a > Supporting Member <
dh100m Tony Chown
Brockville, ontario, Canada   CAN
curios the fuel pump is located behind driver seat on lhd cars where is yours the original bn2 had a very small heat shield bolted directly to manifolrd again made
larger one from ss
wish you well

Pablo Noodles Avatar
Pablo Noodles Paul North
Teddington, Middx, UK   GBR

I have been prowling the internet and have also spoken with one or two mechanics. My understanding of vapour lock is this.

Vapour lock occurs when the liquid fuel changes state from liquid to gas while still in the fuel delivery system. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. Restarting the engine from this state may be difficult.

The fuel can vaporise due to it being heated by the engine, by the local climate or due to a lower boiling point at high altitude.

Modern fuel has a much lower boiling point compared to older fuel that classic cars were originally designed and tuned to run off. One remedy is to add a fuel additive, for example - Milers VSPe. This additive helps to raise the boiling point and can solve vapour lock problems. I will try the additive first and see if it helps. The additive also helps to reduce wear and stresses on classic car fuel systems as modern fuel can be corrosive to certain hoses and joints. Also the removal of lead from fuel over the years has had a detrimental affect on the classic car engine wear. The additive helps with this problem too.

I shall report back in a few weeks when I next get a run out on the Healey.

Kind regards

Screenshot 2016-09-18 20.34.25.jpg    21.5 KB
Screenshot 2016-09-18 20.34.25.jpg

Pablo Noodles Avatar
Pablo Noodles Paul North
Teddington, Middx, UK   GBR
Hi Tony

My BN1 1955 AH 100 has recently been restored and there is no evidence of a heat shield around the fuel pump which is located above the exhaust. I drive a UK spec RHD car.


Penfold99 Avatar
Penfold99 Dave Buchner
North Hollywood, CA, USA   USA
I would also take a looks at your distributer, and mixture setup. These things can contribute to the problem by causing higher than normal combustion temps. Make sure your mechanical advance is not seized, and that you aren't running too lean...

jh24 Joe Herbert
Cambridge, Cambs, UK   GBR
Has similar problem. Added heat shield, and wrapped exhaust manifold in heat shield tape. Sorted! Joe

Pablo Noodles Avatar
Pablo Noodles Paul North
Teddington, Middx, UK   GBR
Thanks Dave, I will check that too.

Pablo Noodles Avatar
Pablo Noodles Paul North
Teddington, Middx, UK   GBR
Hi Joe

Sounds like that's the way to go. Many thanks

cpcooper Craig Cooper
Chico, California, USA   USA
Consensus around here it the problem is due to current fuel formulation. Whether that's true, I don't know but I didn't have the problem 10 years ago and now I do. On suggestion of another Healey owner, I put together an inexpensive bilge blower and a piece of 3 inch aluminum pipe, mounted it on the steering box with the outlet blowing directly on the front carburetor. Problem solved, but when I proudly sent a picture to my vintage race mechanic friend Andy, he promptly laughed his head off. Well, heck... it works. He says the problem is the "daisy chain" fuel line arrangement which delivers fuel to the front carb first, then goes to the rear carb, but the joint is right at the point with no heat shield on the front carb. He said their race cars had two fuel lines all the way back to the fuel pump, but suggested splitting the fuel line at the frame, beneath the carburetors, should be adequate for a street car. I'm scrounging parts to try this out without ruining my stock fuel lines, but in the meantime my bilge blower carb cooler works just fine, thank you very much.

Pablo Noodles Avatar
Pablo Noodles Paul North
Teddington, Middx, UK   GBR
Hi Craig

Thanks for that explanation. Glad you have found a short term fixsmiling smiley. I may very well try the splitting of the fuel line as your mechanic suggested at the carbs before paying for a heat shield to be fitted.

I drove with my inspection cover, behind the seats, open today to see if this would help with heat reduction and improve things. I can't say it made any difference really - still started to buck in traffic. When I felt my fuel pump it didn't really feel hot at all, so I guess all the problem must be in the lines near the exhaust manifolds and the carbs themselves getting hot and vaporising the fuel.

My fuel additive (Millers VSPe) has not stopped my misfiring problems so I will have to get a heat shield fitted in the next few weeks. I will keep any interested parties up to date.

Thanks again Craig and all the others who have kindly given your time and advice.

Fredaston Avatar
Fredaston Fred ASTON
DUNKERQUE, Nord, France   FRA
I completely agree with Penfold99,
I had the same trouble whith my 3000,after checking advance, balance and mixture, never again problem of vapour lock for since 5 years !
Nothing else done.
Good luck


Pablo Noodles Avatar
Pablo Noodles Paul North
Teddington, Middx, UK   GBR
Thanks Fred

I have now had my car fitted with a bespoke heat shield and wrapped the exhaust too - great job BUT I still have the same problem - agh!!! The temperature gauge shows about 60-70 degrees only when the rough idle starts in traffic of when the car is stationary. Drives beautifully otherwise. I can't see it can be vapour lock now!!!!!

Curiously my battery is not holding its charge and has gone flat several times and has now died. I wonder if that is part of the problem? I will replace my battery in a few days (Bosch S4021).

My next step is to get the carbs and mixture and timings looked at and to see if the dynamo (alternator) is working properly - cause of flat battery perhaps and lack of spark!!!. I am determined to sort out the rough idle problem if I can.

Any other thoughts or experiences on this subject most welcome. Hopefully others reading this will benefit going forward too.

I will keep you all posted.

Regards to all.

100M Avatar
100M mike lewis
harbor island, sc, USA   USA
Hey Pablo,
One more thing that could possibly be giving you this problem - a bad coil. Once it heats up starts to cut out and gives you same symptoms as vapor lock. Had this problem years ago on an MG TC and drove me nuts until figured out it was the coil and has happened on my 100M as well. Worth checking out. PLUS, check out the wiring on the kill switch in the boot. Actually wiring would be underneath but if you have a loose wire that either grounds out or comes loose from the switch, you will get the same problems of cutting in and out. ONE final thing to also check - the white wire going from your coil - I think, could be from the dizzy - goes to the fuse block on the firewall. If it comes loose can cut out all electrics but still look like it is attached. Typically connected at the fuse block with 2 or 3 other white wires so easy to miss if just barely loose.

Good luck,

. Hide banner ads & support this website by becoming a > Supporting Member <

To add your reply, or post your own questions

Members Sign In   or   Create an Account

Registration is FREE and takes less than a minute!

Having trouble posting or changing forum settings?
Read the Forum Help (FAQ) or contact the webmaster

Join The Club

Sign in to ask questions, share photos, and access all website features

Your Cars

1960 Austin Healey Bugeye Sprite

Text Size

Larger Smaller
Reset Save

Sponsor Links