Vacuum Reading During Acceleration?

[Slow Left]

I'm wondering what a 'good' vacuum reading should be during acceleration (under load)? While under 'load', say, driving up hill at 55 mph in 4th gear; consistent pressure on the gas pedal.

According the vacuum diagnostic chart below, my engine is performing normally. I think.
I'm getting excellent vacuum at idle (21 inHg).
Driving uphill in 4th gear at 55 mph, the vacuum falls to no less than 5 inHg.
On the top of the hill where it flattens out but still lightly giving gas, vacuum increases to 10 inHg (if I focus).
Going down the hill, the vacuum goes up to 25 inHg.

My question is, under load when I am going up the hill with steady pressure on the pedal, shouldn't the vacuum be climbing quickly to 25 inHg for peak performance while still under load? Or am I understanding the chart wrong. My vacuum only recovers to 25 inHg (beyond the 20 inHg at idle) after I'm over the top and going back down hill!
My vacuum is always below 10 inHg under load; below 5 inHg under heavy load, and only recovers to 25 inHg when the load is relieved (when I've gone over the hill and am on the way back down...)

According to the chart below, if I read it correctly, the expected operation of a normally functioning system is that under load, vacuum drops to no less than 2 inHg and then recovers to 25 inHg (above the 20 inHg seen at idle for a properly running engine), but it doesn't specify if how quickly that is supposed to happen; whether the engine is still under load or the load has been lifted (if you don't mind the poetic licensing...).

Any one have a 'dialed in' rig out there with vacuum readings under load as a comparison of what I should expect?

Thx.

 

[Elbert]

what performance or drivability issue are you chasing?

 

[hank14]

Based on how mine read when I had a vacuum gauge hooked up temporarily, I think what you're describing is perfectly normal. The vacuum when the engine is under load will be low (5), then jump above normal (25) when the load is gone (downhill).

 

[Slow Left]

I'm trying to get the best performance out the stock carb.

I'm not sure that the secondary is opening properly. I should add that my understanding of how carb works is that it is part mechanical and part vac regulated. Mechanical in that when the primary throttle linkage is fully open an arm attached to the primary shaft actuates the secondary shaft to turn the secondary butterfly to open ~2mm; after that point, vacuum on both the primary and secondary barrels regulates the secondary butterfly via the secondary diaphragm.
I'm wondering if the vacuum I'm currently getting under load is enough to get that secondary to operate correctly.
Then the next step for me to do is to check/adjust the linkage on the primary throttle shaft so that when the pedal is floored, the butterfly on the primary throttle is fully open and the secondary is opening 2 mm.

But before I do that, I'm finishing up making sure the emissions (fully stock 12/1981 FJ60) components are working correctly (so I get best vacuum).

I'm looking to the vacuum reading under load as an indicator to the overall performance of the system.

I know that I need to replace the VCV on the distributor side (see image - whether or not I apply vacuum to S on my VCV, air flows thru Y to Z...so it is not working correctly) but I'm not sure how this failed VCV is affecting spark advance. From what I can tell, when vacuum is applied under load on port S (S connects to a BVSV?), it allows air from the air filter to enter under the distributor cap. Why? Is that to provide positive pressure to help move the vac advance diaphragm on the distributor side at the same time that vac is applied on the other side? Is a failed VCV enough to prevent the advancing of the timing?
I was hoping that vac reading under acceleration/load would give me an indication of that.

Also, once I have convinced myself that the spark advance is working correctly (and replaced that VCV), then I was wondering if measuring the vac under load might help me set the idle mixture and idle speed screw for optimal performance...
then see if I need to adjust the linkages in order to get the secondary to function properly...

 

[klinetime574]

Your readings sound similar to mine.

Under load is low readings. Idle is 21. Coasting in gear is higher than 21.

Can't help you on the carb though. I have a Weber

 

[TobyB]

"A carburetor is merely a (poorly) controlled vacuum leak".

I don't remember who said that, but it's mostly true. Given a small- carbed low- flow motor
like the 2F, having the vacuum drop to mid single digits under load seems about right.
It's working pretty hard to keep all that friction and drag moving, so it needs a noticable amount of
throttle, and thus, a pretty high throttle plate opening.

If you had an 8 liter V10, then it'd just be loafing along at cruise, and you'd see better vacuum readings.
And possibly (but not likely) better economy.

Since the vacuum will vary so widely with a relatively small throttle plate angle change, it's pretty hard
to do much fine- tuning with it under load- it works better on industrial engines where the throttle can be
fixed and repeated...
But I find it DOES help me keep a light touch on the accelerator, and thus, at least, aware of how
heavy my right foot is!

t