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MG TD TF 1500 - TF 1500 Hot Misfire

Help..... I just finished placing a 1500 engine back into my 1955 TF. It has always had an 1250. It now has a very bad missfire when hot. The car runs absolutely beautifully, but after driving for some time, if I get stuck in traffic the engine missfires when accelerating, but goes away when the air flow cools down the engine. Also if I park the car hot and then return within fifteen minutes to drive away it missfires, again "clearing out" once driven for a few minutes to cool down. It seems to only do it when the carb's have had time to heat soak.
I have never had a problem like this after driving it for fifteen years in the heat of Southen California. And upon the engine instillation I added an oil cooler.
I checked/replaced the coil, points, condenser, cap, rotor, and changed heat range for the plugs. I have pulled apaft the carb's, making sure they are clean, adjusted etc..., put in a fuel filter, moved the carb's 3/8 inch further away from the manifold, moved the floats up and down, moved the jets up and down and banged my head against a wall.
I drove 400 miles yesterday through beautiful winding hills, and had no problem, other then stopping to fill up with gas and missfiring while driving away.
My only thoughts at the moment is the California "oxiginated" gas and if that has anything to do with it.
Any help to save me from making this car into an anchor.....
Ben.
Ben Simpson

It sounds to me like vapor lock. Are the carb float vents clear? If there's too much vapor pressure in the carbs, your car will run lean and miss when hot, especially after being parked.

The only time my car has missed like this is when it is running too lean. Another thing to check is to look for vacuum leaks around the intake manifold.

I would set the mixture while the car is warm, and I always set my car to run a little rich in the winter, but not too rich.

Fuel manufacturers use different formulations for different seasons, so try some different brands of fuel.

If the scattershot method of troubleshooting fails, then go back to first principles. Often what appears to be a carbeuration problem can be an ignition problem. The coil could be weak when hot. Test for good timing and strong spark.

Mark B.

A bit of vapor lock after a short hot soak is sort of normal- only for a minute or so, but not poor running in traffic. Do/check what Mark says- esp. change the coil. Do you have the correct damper caps-vented or not- whatever TF's have? Oil in the dampers? Does choking help? Idea- after a hot soak restart with bad running, shut it down, carefully remove the float chamber tops and see if gas is in there, and its level. Is the fuel line close to the exhaust header pipe?
George Butz

Ben, Definitely sounds like a carb problem to me. You don't say if the 1500 engine is a bored out XPAG or an XPEG. The XPEG has siamesed cylinder bores and thus fewer water passages - so is likely to run a bit hotter than the XPAG. Have you noticed a higher temp reading on the gauge? Even with a 1500 XPAG you will be burning more fuel and producing more heat. The TF, because of its narrower bonnet (hood) has a tendency to run hot, which is why the carb flange spacers were used to move the carbs further from the heat source. It could be worth fitting a heatshield between the carbs and the manifold - Brown and Gammons do a good-looking one - I have one fitted to my XPAG, which has stopped a tendency for difficult starting when hot. AB
Adrian Bennett

Ben,

This was my scenario: Start up and drive a bit. Stop, and there's a mis-fire, until the revs get up and we're underway. Park in the garage and try to adjust the carbs. When it warmed up, there was nothing I could do because it ran so poorly. I thought it was the throttle shafts. Even after the carbs were rebuilt by Joe Curto, it still happened. I was going crazy. I checked everything, even going over the entire ignition system with a voltmeter. Changed the condenser just in case I missed something. The very last thing I checked was the points gap. The gap was way too small. Seemed the tiny gap was good enough when first starting, when cool, and when the revs were up. It was so simple. I set the points and it started up and ran perfectly, and has ever since.

I would say check the points again. The screw that secures the points to the base has to be REALLY tight, or they will just slip closed again.

I hope this helps.

Greg
TF 6846 XPEG 0797
Greg Van Hook

This thread was discussed between 05/02/2003 and 10/02/2003

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.