Spark Plug gap?

[Copeland]

Hi, All - I'd appreciate it very much if someone could answer something for me. I just inherited my uncle's 1938 hand-crank-only Model B. It's been sitting and hasn't run for a good number of years, so I'm going to spend the winter getting it up and running again. There are Autolite 295 spark plugs in it now. I took one out and looked at it, and I'm going to put in new Autolite 295's - I just picked some up at the farm store here. I checked the gap on the old plug, and it was .028, but I don't know if that is correct/best for these plugs. So, here's my question: what SHOULD I make the gap for the new plugs? Thanks, in advance, for sharing your knowledge about this with me. Jeff

 

[wore out]

(quoted from post at 15:17:54 12/16/21) Hi, All - I'd appreciate it very much if someone could answer something for me. I just inherited my uncle's 1938 hand-crank-only Model B. It's been sitting and hasn't run for a good number of years, so I'm going to spend the winter getting it up and running again. There are Autolite 295 spark plugs in it now. I took one out and looked at it, and I'm going to put in new Autolite 295's - I just picked some up at the farm store here. I checked the gap on the old plug, and it was .028, but I don't know if that is correct/best for these plugs. So, here's my question: what SHOULD I make the gap for the new plugs? Thanks, in advance, for sharing your knowledge about this with me. Jeff

I would have said 0.025", but the AC tuneup page on this site says 0.035".

https://www.yesterdaystractors.com/atrp/specs/ac.htm

I will be interesting to see the responses of others to this!

 

[jCarroll]

Book says .035, but if you are bringing this B back from retirement, and if you don't know the condition of the mag, I would start with .025. Might make for a few hundred less turns of the hand crank.

 

[Copeland]

Hi, jCarroll - thanks for the reply. The magneto is original, but I don't know the condition it is in. This tractor is the classic it ran when it was parked scenario. If the mag is somewhat weaker because of its age, will going to .025 help make the mag work better or more efficiently by having the spark not have to travel so far at the plug tip? Is that what you were getting at? Thanks for letting me know. Another person replied and said the book says to use .035, but if the mag is weaker from age, is that too wide a gap for the spark that would be needed? If someone could clarify this, I'd be very grateful. Jeff

 

[jCarroll]

I am not an expert ........ The wider the plug gap, the higher the voltage required to jump the plug gap and create a spark. Your plug wire might have high resistance from rot, moisture, corrosion, etc. The mag might have moisture accumulation which soaks off some magnetic energy - or if the distributor portion has moisture, corrosion, etc - the mag will deliver less voltage than when it was new. You won't be able to know the exact nature of all these things, so give your mag a break - make the plug gap smaller - thus the voltage required to jump the gap is less than a wider gap.

Get her running - then regap one of the plugs to .035 - see if that plug misfires - if not regap another plug wider. See how it goes - even cylinder pressure affects the voltage to fire. One low cylinder might fire on .035- but not the other three.

And remember - this is not the SST.

 

[Copeland]

Thank you again for the advice and explanation. Everything you said makes sense to me. I'll start out with .025 and see how that works. And, once I have it running, I can play with the gap then to see what is best for this particular tractor. Thanks again -- your advice is greatly appreciated! Jeff

 

[Nordic]

I'll share this for what it may be worth. In the early 50's my grandpa had a '47 Allis B tractor. It had a habit of running uneven and misfiring somewhat. They consulted the best auto electric technician in the area. He said put the spark plug gap at 40 thou. Once they were reset to that it ran perfectly. I think trial and error in your case, as already suggested, will bring your answer. My D14 with electronic ignition seems to. run best at 40.