Just put a vacuummeter in my 1977 F150 351M, C6, 4X4.
It shows 15 inHg when idling. Under normal acceleration it shows 12 inHg. Hard accelerations sends it down to 9-10 inHg. When I let of the gas pedal and let it roll it shows 20 inHg. Under normal driving, no load, flat road it shows around 15 inHg. Any others out there running a vacuummeter who can tell me if these numbers are ok?

 

Those are good numbers. You can adjust carb to get highest vacuum reading. Then carb will be set correctly. While Driving keep the vacuum as high as possible to keep speed you want to get the best fuel mileage.

 

I'd say those are low numbers for a stock engine but it depends a lot on your altitude.

 

Numbers look Ok to me except the hard accel number.

At WOT manifold should be near zero.

Take care,

 

This is going to be useful many times, I think. Tried to loosen the distributor to play a little with the timing, and then see what would happen to the vacuum. But it was stuck of course. Soaked it with some "salvation oil". Have to try later. In time I am going to get a sraight up timing kit, 4 barrel intake and a new carb.

 

Hi,

Are you really running a Boost gauge? Do you have an induction setup on this motor?

If not,... you should NEVER see a + indication on the meter.

You need a true vacuum gauge, not a boost gauge.

 

Using the boost-gauge because no dealer around here had a vacuumgauge :-)
Only using the part of the scale from 0 to -1

 

Those readings would be tough to read on a boost gauge, so I agree, find a true vacuum gauge online.

Definitely tune the carb to get the highest vacuum reading. And a ditto on all the numbers looking good, but at full throttle you should be at or near 0. 15 at idle isn't bad, but you should be able to get closer to 18 with proper tuning if the engine is in good condition. It all depends on the condition.

It's great that you have the gauge and have a base on the numbers, you'll see problems coming ahead of time if you see changes in the vacuum.

 

'77 F150, 35M (with low compression option), cam, hard acceleration drops it to near 0. Cruising varies with grade, I try to keep it above 10 as much as possible as that was above the opening point of the power valve in my old Holley but now I have an Edelbrock 600, just old habit. Closed throttle coast down it'll go near 25 inches or mercury.

You can improve your gas mileage by driving with highest readings and steadiest foot in mind, every time you hit brakes you kill energy that you will have to mash the gas pedal and drop that vacuum and operate that accelerator pump to make up for some place. Driving by that gage will train you to anticipate as much as possible.

I agree, that boost / vac gage is not the thing needed for use on a NA engine. I use a SunPro Vacuum gage on my '77, it's a good 30 years young. It's told me things like back when I burnt a valve in the late '80s chasing gas mileage by leaning the main jets in my then Holley 600, learned a lesson and got a great set of heads though. We have them in stock still at Advance usually.

Picture below is me a some years ago just holding steady rpm warmed up, in neutral, no load, 20" looks like?



Back in the '50s my dad had a Gas Saver Gage which was a vacuum gage with a color coded ring and in later years after he passed away, I found it in a tool box and used it on my '69 Dart Swinger 340 and '65 GTO and some others ..... I still have it, just put it up in a box as a keepsake. The face looks a lot like the one below.

 

Thanks for sharing useful info. But as you see from the Picture here, it is also a lot of Things that can be related to vacuum-Readings beside the carburetor.

 

UHaul uses those same gas saver gauges in their moving trucks.