Vacuum at dist. advance - how much?

G

GMorse

Member
Hello again. Perhaps someone will be able to tell me how much vacuum is available at the dist. vacuum connection.
According to my shop manual, from 0 vac. advance to full advance will give approx. 10 degrees of advance when you apply 10in. vac. My'39 (248) will only pull approx. 6 in. of vac., so I do not get full advance. Has anyone measured their vac. from the fitting at the base of the carb? I think this is called "ported vacuum". Any info appreciated. My normal full manifold vac. at idle is 20 in. vac.
Thanks,
Gary
 
"Ported" vacuum should not give you any vacuum at idle. 6lbs of vacuum at idle could mean that your butterflies are not sitting low enough in their venturis. Common problem with a motor which is running a more performance oriented cam.
 
Thanks Bob. My mistake, the 5 to 6 in. of vacuum is at 1500 rpm, when I should get enough to advance 10 degrees.
If needed, I will slightly shorten the spring in the vacuum unit.
Gary
 
These days i have shortened my spring too, because of less reaction while accelerating.
Also i have renew the old vacuumtube, because of too small and grow thight inside.
Now i build me a new one out of a breakline tube.
Now i feel a better acceleration, the small "hole" are gone.
 
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