hei question

[jjsticks]

hey guy! i have a question.i have just purchased my first 27 t .what fun!the question is with a chevy 350 engine does the hei dist.hav vac all the time or is it ported?having hesitation problems,carbs just rebuilt but same problem thanks in advance jj.

 

[Rick]

You say "Carbs" tell more detail and post a pic of the engine.

 

[RPM]

A hesitaition is usually caused by low timing or not enough accelerator pump shot.
To answer your question they came both ways. I like manifold vac on a performance street engine. All mechanical on a race type motor. You'll probably need about 18 degrees initial on the crank and another 18-20 in the distributor mechanical advance and then another 20 vac advance. A total of about 60 degress, now at full throttle that vac advance won't work, no vac.

When you set or check the timing are you unhooking the vac advance? That will really screw things up if ya don't.

 

[Wild Mango]

Hey JJ, ported vacuum is sourced at the vacuum port on the carb, straight vacuum on the manifold same as the brake booster. Mild cam I would stick with the ported vacuum which means the vacuum is on the distributor at idle but when you crack the throttle the advance disappears. Lumpy bumpstick you may well find a better idle and off-idle performance by using manifold vacuum, so when you just crack the throttle to move off in a dignified and respectful manner the motor doesn't stumble or hesitate (flat spot) as it suddenly retards when the vacuum signal vanishes. As RPM says, flat spot can come from not enough pump shot too, if its really pronounced it'll backfire which isn't fun either, as your air cleaners attempt escape velocity. For cruisin' you need all the advance RPM talks about (60deg) both for gas mileage and also to keep the guts of your engine clean by ensuring the "heavy ends" of the fuel are burnt instead of tarring up the chambers and slug crowns.
Happy tinkering!!!!!!!!