Maybe working backwards???

[Rjpond]

I pulled the plugs from my donor motor in my 55 C-3-C. The donor motor is a 315 poly out of a 57 D-200. I was intrigued because the plug gap seemed wide. The plugs were Autolite 85 and were gapped at .040.

I researched part stores to see what they recommended for my engine and all suppliers recommended Autolite 295 and everybody said to gap them at .035. So I thought why not try the 295 plugs and gap them at .035. The truck starts as well or maybe even better with the new plugs (which makes since), however, now when shifting from 1st to 2nd, I need to feather the gas pedal because it sputters a bit.

I guess my question is what effect does spark plug gap has on engine performance? Maybe my carburetor now needs a little adjustment? Or maybe I should re-gap the plugs to .040?

Thanks

[Radarsonwheels]

Whenever I swapped to an MSD from points in an oldie I always increased the plug gaps to 35-40 thou but I think the slightly smoother idle and easier starting was more because of the multiple hotter sparks than the plug gap itself. Plus a new ignition usually replaced stuff that needed a tuneup anyway.

 

There is horsepower to be gained by playing with plug gap, heat range, even clocking, but I really doubt you’d notice it without scientific measuring equipment (like a dragstrip haha) and even then the sun setting making cooler denser air might have more effect than plug gap tuning.

 

I think too much gap can make the coil work too hard or not work and too small could tend to easily be bridged by fouling but if you are in that 25-35 thou range you won’t notice a difference. 

 

A plug with a cracked porcelain or a frayed plug wire leaking spark back to the block or a coil that gets too hot and starts throwing weak sparks or quitting- that stuff you will feel. 

[Radarsonwheels]

Check the performance of your accelerator pump and check for vacuum leaks. Also double check your plug wires, cap, rotor, and make sure the distributor isn’t wearing out and letting the rotor clip the plug tower connections or anything. Basic stuff first before you get happy tuning the carb.

 

exhaust leaks can lean out your mixture too- something to look for.

[Rjpond]

Thanks for all that info, I had to read it twice to let It all soak in. I even had to google MSD to learn what that is all about. This weekend I’ll take my truck for another drive and I’ll start checking the simple stuff