Vapour lock

[jddevel]

Just had our weekend at the rally in Laon ruined by what I believe is vapour lock. That`s inspite of having a the exhaust manifold coated and a stainless heatshield. Symptoms were erratic power - stuttering (kangarooing !!) and carb/exhaust backfire. It was only really sort of eradicated when the engine speed was raised on the motorways thus increasing fuel consumption but slow at roundabouts etc and trying to accelerate away often hopeless. Managed to travel over 1000 miles but it was a nightmare and we nearly had to resort to the A.A. get us and the car home but limped along and made it with a good deal of nerves at such times as boarding and leaving the ferry. The point I`m coming to is what precautions/modifications have others made in an attempt to overcome this problem. It`s a TF 1500.

[Steve Simmons]

Is the engine running hot? If so then my first suggestion, if you haven't already, is to make sure the distributor advance is working. If you don't have at least 28 degrees BTDC at higher RPMs then it will run hot and lose power. Have you checked the exhaust manifold temperature? If it's too hot then it could suggest a lean mixture condition. I do understand that TFs are far more prone to running hot than TC or TD (or did you mean TF1500 engine in TC?), but it seems like you should be able to drive along without this issue.

Thinking further, I'm curious how you concluded that it's vapor lock. Stumbling at low speeds could be a condenser issue, or the points gap being too small. I also wonder if your carb float level(s) could be off? Just throwing out ideas here.

[Duncan M]

Sounds like a vacuum leak from the description. Of course the H series SU's known to (vacuum) leak from throttle spindle wear, and also can suffer the warped surface where they attach to the intake manifold. Similar symptoms as a bad condenser.

[jddevel]

Thanks for the thoughts Steve. Changed the condenser, checked the plug and distributor points gap and float levels whilst away- but no coil with me. Checking the advance at speed or even working and the exhaust temperature difficult whilst away. The fact that when struggling to get power to the wheels it was juddering and then would hare away which seemed to indicate the potential gas (vaporized fuel) blockage. Now back home will go through it in detail and perhaps get it on a rolling road. I did incidentally check the inline filters including the carbs but not able to check the in tank one for obvious reasons. We had had a little trouble during the nearly 900 mile trip a couple of weeks ago during the Founders Week on the I.O.M. but that was initially put down to running a bit rich which was addressed and because during the actual week apart from going to and returning home journeys were very short so engine and air temperature not that hot and whilst on the motorways we maintained 2500rpm plus ( I`ve a 5 speed box in yes a TF 1500) so I assume through put of fuel steady. Anyway I`ve a few weeks now before the next major run of an estimated 1500 to 2000 miles in France and Spain with higher air temperatures so I`ve got a bit of time to sort it - that`s assuming I can afford the fuel here in Europe!!!! Any other suggestions from all you out there would be appreciated.

[jddevel]

Good point Duncan. Could the heatshield, which was only fitted over winter, (between the carbs and the inlet manifold with gaskets) cause the vacuum problem?

[Rob Reilly]

If there is a vacuum leak at the gaskets on either side of the heat shield, that would cause erratic running.
My experience with vapor lock was that the carb bodies would get REALLY HOT!
The fuel was vaporizing too far down in the jets, not getting sucked up into the throats.
Putting 1/4" thick insulating spacers between the carbs and intake manifold cured that.

[Steve Simmons]

If it's juddering and then takes off, then it doesn't sounds like a timing advance issue. And if it were the filter screen in the tank, it would happen more at speed rather than the other way around. Stopping for a bit might even clear up the problem entirely... for a while.

It's easy to check for air leaks at the carburetor and heat shield by spraying a bit of starter fluid at them and see if the engine speed increases. Don't overdo it, as this is a fire hazard. Some people spray oil instead and see if the idle changes (might actually go down instead). But the thing about an air leak is that you would have to adjust the carbs very rich to compensate. A leak at the throttle shafts would be more variable than a leak at the gaskets, and can make it difficult to get a stable idle.

Also being a TF, your fuel pump is at the rear, which normally helps reduce vapor lock. You also have the better carb bodies that are not as prone to warping.

[Duncan M]

JD-
I think the "vapor lock" most people with the XPAG/XPEG are talking about is more accurately called "heat soak." True vapor lock where driving along there is enough heat somewhere near (usually) a fuel line to cause boiling of fuel. Causes the symptoms sort of like you describe or the complete stopping of the engine, with difficulty restarting it. The XPAG/XPEG is not known for true vapor lock that I know of. Rather, it is sometimes known for difficulty starting after the hot engine is shut off and has sat for a short while. Giving it some choke usually can get a heat soaked engine started.

[jddevel]

Thanks to all for the contributions. Gives me food for thought. I must admit while away I was performing a little like a headless chicken. My excuse -the circumstances and frustration. Not how I wanted to present the MG to the other 700 plus cars on show although the official photographer did insist on photos in the collecting point.