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MG MGB Technical - Vapor Lock Problem Solved

I was having, what I think was, a vapor lock problem with my 3-main MGB engine fitted with HS-4 carbs. I solved the problem and thought I would share the solution with the group.

First the symptoms... On really hot (95 deg F) and humid S. Carolina days, when, after running at 50 MPG for 10 miles and the coming to a stop light the engine lost about 200 RPM from normal. The longer the engine idled the greater the loss in RPM to the point that it would stall if I did not give it some gas. Later, when arriving at my destination and parking for 15 or 20 minutes in the hot sun, I would have difficulty restarting the engine. It would start, but ran poorly. I think that the front carb was the problem and the engine was running on the rear car. After a while, I was able to drive the car, but just barely. After a mile or so the engine ran better but seemed to have less power, but still had the same problem at stoplights as mentioned above.

The solution... I suspected that I was having a vapor lock problem and looked for a solution. I concluded that the culprit was most likely the fuel pipes that carry gasoline from the bottom of the fuel bowl to the jets. I decided to insulate the pipes by wrapping them with two layers 1/8" thick by 1/2" wide foam tape and the covered the tape with a split 3/4" ID section of radiator hose. This would keep the tape from unwrapping and, at the same time, adding additional insulation. After 4 or 5 trips, under similar conditions, the problem seems to be solved. Hopefully!!!

PS - I would appreciate confirmation or a correction if the symptoms described are those associated with vapor lock

Frank Grimaldi
Frank Grimaldi

Frank, The HS4's rarely had vapor lock problems. Have you noticied any fuel coming out of the overflow lines beneath the engine? It sounds like you are running on only one carb when the problem occurs. This might be foriegn matter fouling the needle and seat or a stuck float. RAY
rjm RAY

Frank, Check the condition of the heat shield. If the insulation hasn't dropped off, this shield is usually up to the job of keeping excess heat from the carbs, but having said that modern fuels are more volatile. If it works and doesn't get in the way of proper choke probs. But I would be more inclined to beef up the insulation on the the heat shield and protect the under-bonnet (Hood) fuel line from engine and exhaust heat.
Allan Reeling

The HS4 jet pipe is about the only place vapour lock can occur on an MGB, but with everything as it should be i.e. normal engine temperatures, sound heat shield etc. it is still highly unlikely, these cars run in desert states without it occuring. Many think it can happen in the pipes from pump to carbs but it is impossible as any vapour will simply be pushed out of the carb vent as soon as the float valve starts to open, it would have to be vapourising at more than 2 Imperial pints per minute to cause a problem!

HS carbs do have an undocumented feature whereby the idle will gradually slow to a stall if left for a long period, it's why you have to run the engine at higher reves periodically when setting them up. HIFs have temperature compensated jets and bypass ports in the base of the throat which are pretty good at eliminating the problem.
Paul Hunt

Well I wouldn't be too sure of that, you've obviously never had an air lock in a water pipe defy the pressure in a system and stop water flowing altogether!!!
As for tha "undocumented feature", the rpm is raised to 1500 when tuning, to clear out the combustion chambers after running at less than optimal levels of air flow. All my B's have idled at 800rpm without the mythical stalling "feature" becoming apparant.
Allan Reeling


I had just about the same problem. Living in high temperture and humidity year round I thought it was vapor lock. As Ray and Paul stated, it is rare for this to happen.

It ended up being the ignition that caused the same symptons you have. Not the carb. You may want to check your rotor, plugs, etc.. If you have a pertronics ignitor ignition, check the round magnetics pick up inside the distributor.

Ray 1977mgb

This thread was discussed between 05/09/2009 and 07/09/2009

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