Engine Vaccum readings
 

What vacuum should a good Model A engine pull at say 40 miles per hour.
At Idle my engine pulls a steady 18. At 35 - 40 it drops to above 10.

Re: Engine Vaccum readings
 

Man pulls 20 in at Idle and the same at 40 miles per hour steady speed

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Re: Engine Vaccum readings
 

Could this indicate a muffler restriction?

Re: Engine Vaccum readings
 

were is the spark advance ---too retarded and vacuum will be low

Re: Engine Vaccum readings
 

spark is fully advanced.

Re: Engine Vaccum readings
 

Ignition advanced fully usually not good either. All things being correct, the lever can provide a timing that is more than what is safe. Try retarding the timing slowly at a constant speed and see if the vacuum increases, if it does, that's good. Find the best and use that or 1"Hg below the best when backing off (retarding) lever. this is for any speed. Loosen the clamp that holds the muffler down pipe so it has an intentional exhaust leak. This will relieve back pressure if the flow is restricted as a test. The vacuum test is good and might indicate restriction. Dropping the pipe is proof.

Re: Engine Vaccum readings
 

Look at these

Re: Engine Vaccum readings
 

Depends on several items, but mostly it is power output vs wind resistance. At around 40 mph, out are getting close to using most of your power just to push the car through the air. If you have some modifications that allow more power to be generated than the stock 40hp, then you would expect less vacuum loss at that speed. However, on Elvira, my mostly stock, but high compression head, model A, the vacuum drops off pretty quickly between 40mph and 50mph (and above). I made several measurements when installing the Honda vacuum advance distributor, and I was at 10 inches of vacuum at about 43-45mph. I was down to 6 inches, where the vacuum advance drops out on the Honda Distributor at around 50mph.

It all depends on how far your foot is into the carburator, which is based upon how much power your engine puts out, vs what the resistance to vehicle motion is. For a totally stock engine in no wind conditions, 10 inches at 40mph is not too far off based on my measurements.