64 with low engine vacuum

[Ted K.]

I rebuilt my AFB carb on 64 because I was having hesitation whenever I accelerated. Was sure it was the accelerator pump. Car still hesitates. Everything appears to be working and adjusted properly including accelerator pump. Problem seems to be low vacuum about 8-10 inches when normal would be about 20 inches. Engine timing is about 6degrees advanced. Timing advance and vacuum advance all working.
What am I missing to cause low vacuum. I believe the low vaacuum is contributing to the hesitation when accelerating?
Thanks, Ted

[William C.]

Re: 64 with low engine vacuum

Well let's start with some basics, have you done a comperssion test? If so, what were the results? Secondly, have you verified the location of the timing mark on your balancer is at TDC of the piston travel? I would call that a minimum baseline for starters.

[Timothy B.]

Re: 64 with low engine vacuum

Ted, I also believe the low vacuum is causing your problem with hesitation. Vacuum leaks with AFB can be a warped or damaged base of the carburetor due to the heat slot, also the choke hot air tube works with vacuum from a slot in the carburetor base. Also check the vacuum advance can for a small leak, plug the line and advance the timing to about 25 degrees BTDC and see if the car idles with better vacuum. This setting simulates the idle timing if the can were hooked up.
I am sure you did this but just to double check, 6 degrees advanced is BTDC. Do you have a vacuum guage and is the reading steady at 8-10"? Tell us more about the engine, do you know if someone changed the cam or had the intake or heads off?

[Ted K.]

Re: 64 with low engine vacuum

Some good ideas to check out including compression, tomorrow.
Vacumm guage flutters between 8-10, not steady.
Engine is 65 327 block with original heads and intake. Cam is not a performance cam. Story with car is engine blew up in 1965 and dealer replaced short block using everything else off original engine.
Will report back tomorrow.
Ted

[Duke W.]

Re: 64 with low engine vacuum

Just as a reminder, manifold vacuum should always be stated with full context, which includes idle speed. Also, a statement about idle quality is useful. Is the idle very smooth like a 300 HP engine should be or is it lumpy like a SHP engine? If the vacuum gage flutters, I would expect a pretty lumpy idle.

A vacuum leak or burned valve can cause low idle vacuum, but is usually accompanied by poor idle quality including a noticeable miss and poor top end power. Check all vacuum systems for leaks.

If there is no obvious miss at idle and power is othewise what is expected through the rev range, I would suspect an aftermarket cam.

I've recently been discussing an L-79 engine restoration with the owner. It's an original L-79, but the idle is about 12"@700-750 versus the typical 14-15" @750 expected for an OE L-79. In addition it only makes about 190 SAE corrected RWHP at 4000-4500, which is about what a decent 300 HP engine will do rather than the expected 220@5500 or thereabouts for a decent L-79.

We suspect it has an "RV cam" - high overlap, short duration aftermarket cam, which yields poor low end torque and mediocre top end power. What's left - a narrow bandwidth of decent torque/power in the midrange. Even an RV ought to have better torque bandwidth than that.

And guys wonder why I just laugh at the mention of aftermarket cams.

Verifying timing as suggested is a good idea. Does it have a correct pre-'69 balancer? How do you know it doesn't have some mongrel cam? All kinds of terrible things can happen to engines in forty years. Bubba is everywhere.

Duke