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MG MGB Technical - Hot Weather Fuel Problems?

I began driving my '67 BGT in November, 2005, and it ran beautifully except in hot weather. By June of this year, when I retired it to storage for the summer, the symptoms became predictable. Whenever the outside temperature was above 90°F, and the car had been driven until fully warmed up, then parked for 30 minutes, gas would leak from the carburetors and the engine would barely run when it was started. The car would limp along for five minutes, then it would be o.k. again.

It was as if there were too much fuel pressure coming from the Facet electronic pump, so I installed a pressure regulator and dialed it down to 1.5 psi. This seemed to help, but did not completely cure the problem.

Another problem appeared on days when the outside temperature was 95°F or higher. When the car was fully warmed up and driven at freeway speed for twenty or thirty minutes, it would act as if it were running out of gas. The engine would sputter and lose power. If I pulled to the side of the road and idled the engine for a couple of minutes, everything was back to normal.

I am driving the car daily again in anticipation of cooler weather, so I'd appreciate any ideas for fixing these problems. I am planning to rebuild the carbs and replace the fuel pump, but I'm also wondering if the symptoms might be caused by overheated fuel pipes or something less obvious. Thanks. -G.
Glenn G

Glenn. Might I suggest that you visit my website, www.custompistols.com/ and view the tech articles on the MG section? David DuBois was kind enough to write several tech articles dealing with the fuel pump and the fuel system, in general. His e-mail address is hot-linked in the article on the fuel pumps.

I live in the Phoenix, Arizona metropoletan area. Thus, when the temperatures drop to 95 deg F, we know we have survived another summer of 115+ deg F temperatures and winter, our primary driving season for many of us, myself excepted, has arrived. The only problems I have, in summer driving (beyond basic survival in the heat) are some minor engine roughness in stop and go rush hour traffic.

Thus, a good fuel pump, adequate insulation on the heat shield, and the factory routing of the fuel lines seems sufficient at temperatures far above what you are experiencing.

Make sure the heat shield is in good condition and replace the insulation as required. Read Dave's tech article(s) and check out the performance of your fuel pump. Contact Dave if you have any questions.

Les
Les Bengtson

Glen - It sounds like the float bowls are becoming pressurized when the temperature go up. The bowls should vent through the overflow tubes and if they are clogged up when they soak up the heat while the car is sitting still, the fuel is pushed out through the carburetors. Cheers - Dave
David DuBois

This is a copy of a response I sent to someone with similar problems. Regulators will not solve it, though they may help in minor situations.
There is another possibility, given the weather lately. "Vapor lock" is caused by fuel boiling in the lines and filling the pump chambers with vapor, which it can't pump. Cars like the B with electric pumps at the rear don't usually suffer from this, since they are sucking cool fuel, and then sending it under pressure to the engine, but there is a related problem, which happens under high temp conditions. It usually happens in the spring, when you get a hot day; the winter blends have higher vapor pressures and therefore boil at lower temperatures (so the car will start when cold). But this could also happen anytime fuel temps get high. We once had a period of several 90deg days in early April, when winter fuel blends are still in use; I had 2 MGBs, a TR6, and a midget, all flooding when you came to a stop. After checking everything like levels, I put a tee in the fuel line to the carbs, ran a line to a fuel pressure gauge in the car, and drove the cars. Got them good and hot, and, on stopping, I watched the fuel pressure climb from the correct 2psi to way more, 20+ psi in one case. One of these cars had a fuel pressure regulator, which cannot control this problem. After much thought, and a call to Walbro fuel pump engineers - who said it was impossible! - I figured out what was happening. Under open throttle conditions, fuel is flowing through the system, being replenished by cool fuel from the tank; When fuel demand, and therefore flow, diminish as you slow, the fuel starts to get hot, aggravated by low airflow. When the fuel gets hot, it boils, and since it can't go back through the pump check valves, it pressurizes the downstream side of the system. You can easily test for this as I did - a common vacuum/fuel pressure test gauge costs about $10 at the autoparts store, and is a v useful thing to have. That and a tee and 6ft of hose is all you need.
Trouble is, if this is the problem, while it's good to know and you can quit worrying, it's way hard to fix. The only way is to provide a system which bleeds off a bit of fuel and the vapors, and returns it to the tank. I have built such, and it does work, but is quite a production on most older cars (easier on yours). I had a Chevy pickup which suffered from both vapor lock AND this pressurization trouble at the same time - drove me nuts figuring that out, since it was obviously flooding/rich and starving/lean over a less than 30 second timespan! I suspect this problem is responsible for a lot of borderline bad running under hot conditions. If you do the test and the problem shows up and you can't live with it, I'll tell you how. Sometimes a different brand of gas will fix a borderline problem, I once had a lot of trouble with a blend with a lot of alcohol in it.

Back to the beginning, I have posted this several times to the board in response to flooding problems and as far as I know, not one person has got off his ass to do the test, or report back - but the flooding complaints continue, and I believe some people have actually replaced pumps and carbs in an attempt to deal with it!

FRM
FR Millmore

I figure if others have stock cars that work o.k. in the heat, then I need to make sure the relevant parts are set up like stock. If I do this and still have problems, then I'll do some testing and possibly some modifications. The archives have been helpful, too. Thanks for the responses. -G.
Glenn G

This is surely a problem on the car which can be fixed, not a generic problem which needs mods. It depends when the fuel leaked from the overflow - while it was parked or when you turned the ignition back on. If the former then maybe the floats are set to high, or partially sunk, this used to happen on my V8 until the carb started flooding with the ignition on and I realised the float was partially full of fuel. If the leakage only starts when the ignition is turned on then the fuel pressure is overwhelming the float valves. It is quite comon for after-market pumps to cause this, but can equally be caused by a defective valve. The resultant rise in fuel level from either cause will result in a rich mixture which *will* make hot starting more troublesome. Once pressurised the SU pump shouldn't click more than once per 30 secs, although if it does click more frequently it could be its non-return valves leaking back rather than float valves leaking, and cars fitted with the charcoal canister will 'hide' float valve leakage for quite a log time so you need to remove the pipes from the float chamber vent ports to make sure that isn't the cause. This could take quite a few minutes to start happening, so disconnect the coil as well to stop it overheating.
Paul Hunt 2

This thread was discussed between 11/10/2006 and 13/10/2006

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