Figure this noise out

[Tiger Tim]

Hey all, I’ve been chasing a knock for a while and can’t really settle on what’s causing it. Something is banging in or around the engine, usually only under load Andy it sort of comes and goes. Here’s the weird part: today I forgot to switch to mag after start and there was no knock. When I switched several miles later from bat to mag the k o kill started up immediately. I switched back to bat and it stopped.

What would cause that, or is this just the weirdest possible coincidence?

[Scott_Conger]

These guessing games usually start out with simple described issue and a week later has evolved into a complex issue with multiple unresolved changes that took place and were unreported until well after the fact.

Under the assumption that this is not the case, and JUST the symptoms and parameters are exactly as you state, I would suppose the following: On 6VDC with a fixed time lag in the firing of the coil, and the low voltage available, I would expect that the spark would have to be considerably advanced to run well at any real speed. Switching over to magneto changes the voltage from 6VDC to 25+VAC. The time lag from energise to firing time will be considerably shorter and the spark considerably hotter with the magneto. At the same undesturbed spark setting as was set on 6VDC, you may be wildly over-advanced for running on mag, and are inducing a spark-knock which would have gone away if you had moved the spark lever back some (retarded it). Some engines are very susceptible to a noisy knock, particularly on a good fresh engine or an engine with a high compression head. Old tired engines will struggle under the same conditions but are not as likely to produce an audible knock.

[Chris Barker]

Following the thoughts above, Have your coils been properly and reasonably recently checked and adjusted?

You can easily 'set up' coils to give a spark and make the engine run, but the 'time-to-spark' may differ between the coils if the correct test gear isn't used (HCCT, Strobo-Spark or ECCT), and the cushion spring isn't set up correctly.

You may have one coil which sparks very quickly and so one cylinder is over-advanced.

[Tiger Tim]

I hadn’t even though about it being a coil adjustment thing. I will say that my engine is almost untouched in this knock chase (okay, it has a new adjustable crank pulley but that’s it). Three out of four coils were bought brand new about five years ago and came “adjusted and tested” when I bought them. The fourth of the set quit working pretty early on so I replaced it with either a grimy original or a plastic coil, I’d have to pull the coil box lid to see what the odd one is.

In any case, I guess I’ll have to try and make some local friends and see if someone has a coil tester. As luck would have it I saw the only other Model T I’ve ever seen on the road in this city just tonight. It was a very nice looking maroon 27 coupe on yellow wire wheels. Unfortunately we were going opposite ways on a busy street so I couldn’t stop to chat.

[Scott_Conger]

Chris may indeed be on to something, and from your description of the coils, I would not trust them any farther than I could throw them.

That said, nothing in my prior reply requires the adjustment or replacement of any part and will either solve your problem or rule out commutator timing as an issue. Methodically ruling OUT causes is useful in problem solving...unnecessarily or prematurely introducing variables is not. If you wish to introduce variables into the mixture prior to investigating anything else, that's obviously up to you.

Best of luck.

[Tiger Tim]

nothing in my prior reply requires the adjustment or replacement of any part

Sorry Scott, I must not have fully understood your first reply. I had taken it to mean that my coils weren’t particularly well matched and that just one or two was firing way too early causing the knock. Before the knock started my engine was running quite well on mag and I’ve been using about the same amount of advance all along.

The other thing is that ‘fresh’ and ‘tight’ don’t describe my engine at all. Old and tired is much more on the mark, and I was actually thinking it was a bearing until I found the ignition source makes the issue come and go.