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Hi chaps

I took my 745 out for its first run of the season yesterday but quickly found there was a misfire when on the road. At tickover and pootling around the driveway all seemed fine but, once under load (strangely only in top gear?!) a significant misfire occurs.

Now I did have the distributor off over the winter and the engine is harder to start from cold, requiring some retardation of the spark which I never had to do.

Any advice on how to go about solving this?
Thanks in advance

I should also throw in the fact that I've adjusted the carb as the tickover seemed low (kept stalling in fact).

Following the owners manual instructions the adjuster on the detroit lubricator carb was turned fully to the right and then slowly to the left until running is smooth and tickover adequate.

Would unsufficient fuel/air resulting from my meddling cause some fuel starvation on the road giving a similar symptom?

Some of our good troubleshooters may have something specific to recommend, but to me since it only occurs under load on the engine (top gear) would make me suspect ignition first over carburetion.

if the fuel is borderline old it will do this.

I'm suffering these symptoms. I took it to a shop and they scoped it to be the carburetor pump sticking and not delivering enough fuel under load, such as up a steep hill when you keep the pedal down. Pumping the pedal corrects the situation temporarily (so you at least get home). Mine's a 40, so I would rely on the advice of someone familiar with your year and model.
I added some fuel stabilizer and aviation fuel. Both improved performance, but just incrementally. I have to rebuild the carburetor.

All good advice - thanks

I did remove the plug leads from the distributor and the way in which they connect is quite fragile ( a small pin which perforates the side of the lead) so I might double check those connections.

The fuel theory is an interesting point - I have added some Redex lead additive (only because I put some in the tank of my 67 Mustang and thought it might be a nice "treat" for the Packard to have some too). That might actually be the problem as in fact the misfire could have started at about the time that the fuel mixture would have been drawn through.

I'll experiment and report back

I'm not an expert, but have run into this problem on several older engines, that a weak coil, or bad points, will often cause the situation you are speaking of. They idle fine, but when placed under a heavy load the weakness in these parts then shows up.Make sure the gap in your points is correct. Just a suggestion to consider.

Dear KTS:

As noted above, misfire under load is typical for weak ignition. You can test by using a KD spark tester. You should have at least 20kv cranking and it should be a nice strong regular spark.

However, there is also a service bulletin from Packard circa 1932 that advises to install a pigtail between the distributor and the cylinder head tower bolts due to poor ground.

Check that out first, then if you are still using the vacuum tank, make sure the needle valves are closing.

Interesting suggestion Tim

I did repaint the whole of the distributor while it was off. I'll make sure that the earth connection isn't hampered by my repaint.

I've ordered some new ignition leads as the current ones must be at least 10 years old, maybe much more.

Well I installed new ignition leads and made sure the connection to the coil was good and the result is that the car starts very much easier than before and runs smooth at tickover. It seems lumpy at higher revs and theres now a bit of black smoke.

Would this be the ignition too far advanced or the mixture?