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MG MGA - First Vapor Lock Experience

Just wanted to share with you guys my first Vapor Lock experience which was embarrassing as well as a PITA.

So some reason I got into a traffic jam yesterday, when the temperature was VERY hot in Miami, and +90% humidity.

The car had been running cool as it usually does (around 170-190). At some point I see the gauge an it is creeping up to 230! and I felt the engine starting to stumble. Immediately I tried to get out of the traffic and drive on an open road to cool it down. It kind of worked but eventually it turned off at a traffic light.
I had to push the car out of the middle of the road as it was impossible to restart.
I was really worried about the temperature earlier, as the first thing that came to my mind was the head gasket blowing...

I guess it was just the carbs who were cooking!

Got the car running again after a while, an drove off back home, but the same thing happened several times, and had to pull to a side an wait for it to restart.
On another traffic light the engine went out again, pulled the starter, and the cable broke!
I had to open the bonnet and start it with the engine compartment button... I had to do this several times!

My wife who was in the passenger seat was not too impressed.

After reading Barney's section on vapor lock, I understand I should have pulled the choke...nevermind.
At least it is nice anecdote I can laugh about now.

I will be however looking at improving the heat shield on the carbs.

Just wondering how some of you deal with this, particularly in the very hot summer months.
Gonzalo Ramos

I've had problems the past couple of years while burning gas with ethanol.I started using ethanol free gas and have not had any problems since!

Hi Gonzalo,

I'm experiencing a similar problem here in New Orleans. Only during the hot summer 80-90% humidity and 85+ degrees - while in slow traffic, city driving and having to restart after shutting off the motor (temperature spike.) My MGA runs at the same temperature as yours. I have the problem of the cylinders running progressively richer from front to back although the carbs are set exactly the same. Currently I suspect that my carb tuning to adjust out this progressive richness may be the culprit for the vapor lock - but maybe not. Right now I'm setting both the carbs back to .060 and see what happens. Its a problem I don't like troubleshooting for solutions as I have to drive until the MGA stalls out , never knowing where its going to happen, then push it off the road and wait, sometimes an hour for it to start again. Wrong neighborhood and the car's history "what dat be, how much it do you?" I made a heat shield to deflect the temperature from the exhaust manifold but this didn't help the problem. Should I find a solution I'll let you know. -Martin Straka
Martin Straka

It seems that you guys down south suffer from this more than I thought. I wonder if this idea might work. Install an in-line bilge blower (such as in the tube that feeds fresh air to the carbs from in front of the radiator. I found a 230CFM 4"dia units that should be able to bring in enought fresh air to keep the heat circulating around the carbs. You could just put it on a switch and turn it on when you get caught in traffic. Cheap enough to give it a try. Just thinkin'.
Chuck Schaefer


Do you have the short hose on the carb side in front of the radiator installed? How about the other side?

Vapor lock is caused by the engine getting too hot.

Remedy is to insulate/isolate the fuel lines and float bowls - but I have been told that it is really a fuel line phenomenon vs. carbs themselves...dunno if that applies to MGA's though.

I think the best solution would be to install an electric fan - runs all the time even at low RPM/traffic congestion - keep the engine cooler and you won't have the problem.

Make sure that your radiator is topped up, and that the heat shields and ducting are all in place first.

I like the comment about non-alcohol gas. Up here, it's ALL got alcohol in it...nowhere to get anything different. I don't know if FL is the same.

Too hot in NH lately - wouldn't want to be in FL for a whole summer!


AJ Mail

Just recently, I've had 2 vapor lock episodes despite normal, hot summer temperatures here in New Mexico (I've driven in hotter weather on longer, slower & faster drives before with no problem, for several years). This problem is new. The symptom is lean fuel valve chatter, my see-thru fuel filter is almost empty, and my fuel pressure gauge is zero. What has changed? Could my SU fuel pump (several years old) be going bad?
thanks, Tyler
C.T. Irwin

It is ironic that I put the ducting in just before this last drive. I never had this happen before; but then again I never got stuck in traffic on such a hot day.
As a side comment; The ducting I got from SF, I found was poor quality. Made out of paper and thin aluminum, as soon as you start forming it, it will bend and break everywhere. I was not very impressed... Is this recreating the original standard? If not I would suggest to go with some cheaper alternatives that are readily available for a 10th of the price. Any experience there?

It is difficult for me to check the coolant level, but when I take the cap off I see coolant at the bottom of the filler neck. Maybe it is worth adding some distilled water?

Chuck, you idea is good regarding the blower, I guess that is properly installed it could fit snugly into the metal bracket where the inlet hose goes into.
Have you done it already? any pictures?

BTW I have never seen Ethanol free gas here. In fact there is always a sticker stating 'This Gas may contain up to 10% ethanol', so you never know how much it contains!

Jim, you are suggesting to insulate the float bowls. How is this done?

I am definitely thinking of improving the heat shield but I need to find the right material. I saw on Barney's site that 'Australian Ceramic Aluminum Sheet', but I think something like that is not readily unobtainable here.
I was thinking about going to HomeDepot or Ace hardware to look for something suitable that can handle the temperature. Anyone gone down a similar route?
Gonzalo Ramos

Nope. I haven't done it. I was just thinking of something that might help those of you with the hot carb problem. And something that could be hiden inconspiculously. I saw these on another forum I visit where they were being used for heater blower motors. The metal bracket is where I thought it could fit. Adding some 4" dia tube extending up to the carb bowls would direct the fresh air to the needed location. At 230 CFM it is more that the engine breathes @6000 RPM so it shouldn't be a detriment to performance. At ~4.3 Amps draw, the electrical system should handle it OK too, as long as the factory heater blower was not on at the same time.

FYI, the Moss ducting tube is VERY stiff and durable. It is a paperish-based (is that a word?) product. I don't know the difference between SF and Moss though.
Chuck Schaefer

Gonzalo, check out Brown and Gammons. They carry a highly recommended heat shield for the MG's.
Richard Taylor

It seem like B&G's heat shield looks good. I will put i on my xmas list! ; )
And definitely cheaper than the moss one.
Gonzalo Ramos

I insulated my float bowls and fuel lines. I used insulating tape that is made for wraping headers. I put several layers under and around the float bowls and covered the fuel lines with it. It is "peel and stick" but I secured it with zip ties just in case. It isn't an absolute fix, but it has allowed me to sit in traffic much longer before it starts running rough. This fuel with alchol doesn't do well in hot weather and traffic. There is one filling station near here that has a sign advertising non-alchol gas. I have not stopped to check prices so don't know if it's practical to run it.
Ed Bell

Ed, do you have any pictures of how you put the tape on?
I guess you put it over the Input fuel line and also the fuel line across the carbs?
I am going to look into it...

Gonzalo Ramos

Here is a shot of what I did with the header wrap tape. It's not pretty but it beats sitting on the side of the road waiting for things to cool off. I wrapped several strips under the float bowls and up the sides. Then I wrapped it around them several times. The zip ties are in case the adhesive on the tape didn't like to be around fuel. So far it is holding, and seems to help delay the onset of vapor lock.

Ed Bell

I use to have a 77 midget that would start vapor locking as soon as it got warm outside. I finally cured the problem by installing an electric pump back by the gas tank in addition to the mechanical pump on the engine.
I can understand a car vapor locking when you get it hot and then shut it down for a few minutes and then start it back up but as your cars and that Midget would vapor lock anytime, anywhere; even going down I-75 .
I use to think it might just cure the problem to increase the diamter of the fuel line because it was very small.
Richard Taylor

This Vapor Lock thread has grown since I submitted. I'm including some pictures of the heat shield I made from flexible reflective insulation purchased at Lowes. $15 for a large roll, much more than what I needed o make the shield. After trial and error making a cardboard template that closely followed and then closed the open space below the carbs and with a cut out for the frame brace and carb tubes running down the side of the motor. I attached it with reflective foil tape to the MGA original heat shield, fender, and frame brace for a clean installation. This also aids the fresh air flow to the carbs from the left air hose. (step #1) Still experienced vapor lock though less time was required for the motor to re-establish running conditions. (step #2) I readjusted the carburetors so both are at .060 (see original post). After returning home and shutting of the motor and waiting for the maximum temperature spike, the motor then restarted without any problem. I'm not overly optimistic because it will need more tests under different conditions that caused the vapor lock before I feel the problem is solved. -Martin Straka

Martin Straka

Vapor lock can also be an indication (I have read - no first hand experience yet) that your carbs are running too lean. You might try enrichening them a bit during the hotter weather.

AJ Mail

This thread was discussed between 19/07/2010 and 22/07/2010

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