Engine fires but won't sustain idle

My 78 MGB was running fine , idled fine and started first or second tuen of the key. I replaced the water pump today after noticing a rattle in the engine. The replacement went without incident. I replaced the fluids , checked for leaks and tried to start the engine. It fires and will rev high as long as the ignition is in the start position. As soon as the ignition is released it dies. I checked fuel to carb,ok . i'm obviously getting a spark because it fires. Checked the carb, the float appears to be functioning properly... I checked the enine compartment for any loose or disconnected wires our vacuum hoses. nothing is apparent. Any suggestions on what to try next is appreciated.

Could you have knocked a wire loose around the alternator? That's close to the water pump. Not sure that it would cause the problem you describe, but it sounds like you're loosing power once you go from starting to running.

Check the wires to the + post of the coil. And check for power there when the ignition switch is turned on. It appears that the only power the coil is getting is from the wire connecting the coil + to the starter solenoid.

You probably do not have spark when the ignition switch returns to the run position. Later
cars used a direct connection from the starter solenoid to the positive coil wire to bypass
the ballast wiring in the ignition circuit. After the switch is returned the direct circuit is disabled
and the firing circuit comes from the ignition relay by a white/brown wire to junction of the
ballast wiring.

The above infromation assumes the wiring to the distributor hasn't been changed. Some
cars have been changed because a different distributor or the use of a Pertrinix or other
electric ignition module. Look for 12 volts with the ignition in the run position on the
white/brown wire on the fuse box at the second fuse up from the bottom. If no voltage
there chase it back toward the ignition relay. Ifyou have12 volts at the fuse box trave it
toward the distributor.

Clifton

I type slower than John.

Clifton

Don't feel bad, Clifton, most of us do.

I'm just able to correct my typing mistakes quicker than you guys.

My vote is the wire from the solonoid as well. It has all of the usual symptoms. RAY

Thank you all for your suggestions, Ill let you know how I make out.

Good news and - not so good news. It starts and idles just fine. (good news)
I have know idea why. (not so good news) - I checked all electrical connections in and around the coil , distributor and alternater. All appeared fine , but I disconnected and cleaned each contact as precaution. It took a few turns of the key - but now it starts just fine. I'm happy but leary.

You can replace all that old coil wiring with a new jumper that runs directly from the "white" conections at the left side of the 3rd fuse down to the + terminal on the coil. Just tape up and get rid of the two green/white wires that you're using now. That eliminates the strater wire and the original wire that comes from the relay. This is exactly how the early cars are wired, and we have none of your issues! :D

After a few weeks of frustrating irratic recurrences of the problem- I isolated the problem to the ignition switch on the steering column. I ordered a replacement switch from Englishparts.com and all is well again.