Tuning a '67 Model 396/325
I am turning my attention to tuning this customer's 396. By identifying casting numbers I have identified it as a vintage '66 or '67 engine from a Chevelle. It seems stock as there is no evidence of a high lift cam (in sound or performance) and has cast iron heads. It has a Quadrajet 4-barrel carb from a mid '70s truck, based on identification numbers. There are no stampings in front of #2 cylinder, maybe removed during decking.
It runs real nicely, smooth and all that, and will easily rev toward 5K. It has a terrible time starting after sitting for more than a couple days. I end up splashing some gasoline into the carb to facilitate starting. I don't know if adjusting timing will make any difference there but if so that is a plus. My goal is to maximize performance and road manners.
I read on another thread I should shoot for 18 initial and 36 total timing with the mechanical advance all in by 2,500 rpm.
I find the vacuum advance can is all in at ~ 15 in. The mechanical advance is all in at 2,000 rpm. The timing now is 31 advanced when above 2,000 rpm.
This tells me I need to add about 5 more to 36. Okay with that. I checked the engine's vacuum based on where the vacuum advance was connected at the base of the carb and it never gets above about 5 inches there. That cannot be good.
I check other ports on the front of the carb and cannot locate one with 15+ lbs at idle and reduced at higher rpm's. Although the PCV port at the base of the carb sucks to the point the engine stalls out if the hose is removed. I know I need manifold vacuum at idle for proper function of the vac advance.
What does this tell me about this carb? I know I want the higher vacuum at idle and less vacuum at higher rpm's.
Okay, left to right on the attached photo:
1. Black hose is connected to vacuum gauge and it pulls about 19 lbs throughout the rpm range. The exception is when I blip the throttle it will drop to about 5 then maintains the higher vacuum at higher rpm's.
2. PCV valve connection at base of carb. Lots of vacuum! I suspect that is where the PCV should be attached.
3. White capped port, where the vacuum advance was attached and it pulls maybe 5 lbs with little change as the engine revs up.
4. Port with black hose and golf tee, similar to #3.
What to do here? I wish this was a Holly carb. I suspect this is from the era of not using manifold vacuum to control your vacuum advance. With no vacuum can connected I can hit 19 initial and 35 total. Connecting the can to any of the available ports provides negative results.
302DZ, M22W Muncie, GV overdrive,12-Bolt Rear 3.73
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Last edited by Andrew69; Jun 10th, 19 at 06:10 PM.