[chr646566]

I was trying out different mufflers on a '65 289 today. Would put one on, fire it up, listen for about 10 seconds and try another one. After a little while the temp started to rise but not enough to expend any fluid when the car was turned off. It started and ran well every time and has started fine in the past few weeks, even when very hot. Went out about an hour later and started it and there was a terrible miss. If it's revved up a bit it smooths out slightly. Darn thing sounds like an old six-cylinder truck. Not good.

Why the sudden change after running so well the rest of the day? It has new points, cap, wires and plugs. Wires are nice and firm on the cap and plugs, cap is clipped down tightly. What are the next things to check here?

 

[sharkmouth]

did you change the condensor too?

Look at the points under good light.....

 

[camachinist]

Carefully watching a timing light while the engine is running might help spot an ignition miss.

Sequentially removing plug wires might as well...

It's possible, with all the stop/start and idling, that you fouled a plug...long shot, but there ya go..*G*

Could also be pure coincidence that you got a chunkie in the idle circuit of the carb...if it smoothes out when the main wells take over, that could be it. What kind of carb?

Try a few things and get back to us.