MG won t start after warmed up.

[Buick35]

Hi,I've changed points,condensor,coil,fuel pump,cleaned gas tank,still after reaching operating temperature,my 74 MGB won t start until it cools off again.It's got a Weber carburetor(like that when I bought it).My son who borrowed it for a car show noticed that the new coil was hot to the touch.I let him use it because I thought I had it fixed.Could the coil be bad or bad ground somewhere? Any thoughts will be appreciated! Frustrating! Thanks. It always starts when cold.Greg

[Oldtech]

When it's hot and in a no start mood pull the coil wire out of the distributor and hold it 1/4 inch away from a grounded metal and have someone crank the engine. If there is a nice spark start looking at the carb. 

If no spark or very weak electrical troubleshooting is required. Check all connections and grounds, try a different condenser,  and yes, it could even be the coil. 

[Buick35]

Thanks

[Str8-8-Dave]

It certainly could be an overheated coil.  You could try getting the coil hot and confirming a no or weak spark condition then try cooling the coil with a gentle stream of cold water from a garden hose and see if it will start. Make sure the coil you use is designed for the electrics that support it, some systems have a resistor in the primary circuit to lower primary voltage when running and may have a bypass start circuit that bypasses the resistor when starting.  A mis-match here could result in overvoltage at the coil primary which could cause the coil to overheat.  If the coil you are running requires a resistor make sure to use one.   

 

These cars were pretty fussy about vapor lock as well so don't rule that out. Some of these cars had sheet metal heat shields between the carburetors and head to try to avoid this.

Edited October 10, 2021 by Str8-8-Dave (see edit history)

[Buick35]

Thanks,I installed a phenolic spacer under the carburetor and added a fan shroud and thermostat blanking sleeve so it runs pretty cool. Would a bad coil also cause the engine to quit while running?That happened to me the other day a few blocks from home.

[pmhowe]

A bad coil can definitely cause an engine to quit running. I have some bad memories of it happening to me. Not on an MG, but on a 1941 Dodge truck.

[TerryB]

Are the valves set properly?  Heat=expansion=loss of compression.

Might want to put the old coil back in and see how hot that one gets.  If you are handy with a electrical multimeter you could measure the resistance of the coil and also measure the current flow in the coil when the points are closed.  At 12 volts a typical current flow with the engine not running and points closed would be in the rage of 5amps +/-1 amp. 

 

 

Edited October 10, 2021 by TerryB (see edit history)

[Buick35]

Thanks,I looked for the old coil but couldn't find it.I think I have electrical issues because I noticed that the red dash light wasn't coming on after I turned on the key until I wiggled the harness a little.Now it dose.

[Oldtech]

Lucas, the prince of darkness strikes again??

[trini]

All right again. The prince of darkness. M G> does not not give a 1/4 inch of extra wire for the terminals. Those push type connectors lose connectivity and must be taken apart regularly to reboot. Exceedingly poor design. 

 

With the ignition turned off use a test light to make sure the "in" terminal is not getting feed . Another weak point is the insulator at the contact points pin.The plastic insulator breaks down.  

[Buick35]

Thanks,I'll do that.