Engine Vaccum readings
- Thread starter Mongo
- Start date
Kurt in NJ
Well-known member
were is the spark advance ---too retarded and vacuum will be low
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.
Bill Pursel
Member
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.
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.