Thought I'd come to my favorite place for some help!

1974 302 fresh rebuild w/ ~7000 miles. Only thing not replaced is distributor (original) and ignitor (~15 years old).

Drove my truck home yesterday, and threw the vacuum gauge on the vacuum tree off the manifold. Interested to get opinions on what the gauge is telling me. A link to a good vacuum chart would be good too. The way I see it, the vacuum increases ~1 in-hg irregularly. Can't find that on any vacuum charts. I'm leaning towards ignition/distributor, but this is my first time running diagnostics off vacuum.

Performance Issues - The only issue is a shift happy (1-2, 2-1, 1-2...) transmission between 1&2 at rolling speed (no throttle). I am wondering if this may be the source.

None of this is a new development in the engine or transmission. Has been like this since rebuild.

Video's have idling in park and in drive as well as goosing it. The off idle vacuum starting ~1000 rpm and up is steady. I didn't have enough hands to take a video.

Idling in Park:

Idling in Drive:

Goosing in Park:

 

I'd check the trans vac. modulator. If you have a suction device you can make sure it holds a vacuum. It affect shift points and could be a vacuum leak source. Otherwise you might need to fine tune the carb idle mix.

 

Do yourself a solid and replace the distributor and vacuum can. Set it up before installation so there's only about 10° of mechanical advance (20° at the crank) and some light springs. Limiting the internal advance will give you lots more room to play with setting the initial advance. Most V8 are going to like between 10° and 15°, often can't get away with that much on a stock distributor. The end goal is to have about 34° to 38° total mechanical advance, not counting vacuum advance, before 3000 RPM.

Swap the Ignitor over to the new one, with a good coil the ignition will be about as hot as it gets.

Make sure the idle RPM when checking with a gauge is 550 - 650 RPM. A fast idle might give an artificially high reading. Want to see 18" to 20" steady at sea level for a stock motor in good condition. A performance cam will lower the vacuum quite a bit. Subtract about 1" for every thousand feet above sea level for correction factor.

 

your idle speed is too high, resulting in a falsely high vacuum reading. idle speed in drive should be on your yellow sticker, or maybe about 650. Idle speed in park might be 800. At 800, in park, you should be 17-18, but steadier than what you have.

how thorough was the rebuild, and how was the break-in? You might check the compression to make sure the rings are all in good shape.

 

Thanks for the quick responses. Tried to respond to each question in paragraphs.

Idle speed is easy enough for me to drop, I can do that and see where it lands me. I suspect same story just with lower vacuum though right? FWIW, the Edelbrock cam specs (2122 Part #) say 16 in-Hg at 1000 rpm. I am at 300' elevation too.

The distributor story is annoying, it involves two pertronix brand distributors sent back because they were creating hesitation anywhere above mid throttle. No matter what I did with timing. So I put the original back in since it had no hesitation and tabled the matter until I could figure a different route to go. I am thinking I will try to find another stock replacement distributor with a new ignitor... Coil is already new. I think this should be first on the list since it needs to be replaced anyway.

Compression was the next place I was going to check. May take me a week or so before I have time to do so. Wouldn't I be seeing other performance related symptoms if this were the problem? Should I check compression w/ engine hot or cold?

I was interested about the timing. So the recommendation is to time the truck in drive and idling at 650? When I go to park, the RPM increases roughly 200 RPM. Typically, I will let the truck time itself where it seems happiest and then I check to make sure it lands somewhat close to the recommended specs.

The rebuild was a complete redo with a good machine shop checking the block and putting the heads together for me. New Pistons bored .030 over, Rings, Cam, Intake, Carb. Crank was turned. I put it back together myself. It was my second rebuild but also used a step by step book as a guide and the forum here along the way. Was very careful about break-in. 2000-2500 RPM for 30 minutes not holding at any given RPM. Truck is basically stock otherwise using Edelbrock "stock replacement" components (cam, heads, intake, carb).

As far as modulator valve, I can adjust the point where the "shiftiness" occurs with the valve, but I wanted to make sure the valve was getting rock steady vacuum before messing around with it any further.

Tinkering w/ Idle mixture doesn't seem to help but it is easy enough to try it again.

 

With the engine warm, tires chocked, and transmission in park:

• connect a vacuum gauge to manifold (not ported) vacuum.
• connect a tach or timing light with tach.
• adjust the idle mixture to maximum vacuum, while keeping idle speed below 800 rpm. I say 800, but that's an artificial constraint - it's low enough to keep the centrifugal advance from kicking in and not so high as to artificially inflate the vacuum reading.

If the idle speed exceeds 800 rpm, back off the idle speed and start over.


Once the idle mixture is set, don't mess with it any more.

After the idle mixture is set, adjust the ignition timing:

• engine warm, tires chocked, emergency brake set, idling in drive
• adjust idle speed to 650 (or as specified by the yellow sticker)
• disconnect and plug the vacuum line going to the vacuum advance can on the distributor
• hook up the timing light, and adjust the distributer to factory time.
• rotate the distributor to maximum vacuum
• if the idle speed increases, back off the curb idle screw and start this section over

The timing will probably be a bit more advanced than factory sticker

• lock it down and take it for a drive.

If it pings under load, back off the distributor a hair.

 

Perfect - will follow this timing procedure to the tee and report back here. Looks like there are a couple of things I've typically not done. Will do this and report back. Going to borrow my dad's compression tester when I see him next weekend. I should have that info for future troubleshooting anyway.

 

In general, vacuum looks a little low vs stock specs. Things that can make vacuum low are 1) Performance cam. It sounds like you have one. Can you post specs? At least intake duration @.050 2) Ignition timing. More cam needs more advance. Inadequate advance will reduce vacuum. Increasing advance at idle will probably help idle vacuum... but may be too much at speed. Distributor may need re-curved for less total mechanical advance with higher idle settings 3) Carb settings. Tune for maximum vacuum and back off a little works for me. 4) vacuum leaks... as mentioned. Leaks would also mess up idle mixture settings.

 

Here is the Cam Card. Again an Edelbrock Cam Flat-Tappet. Heads are E-street Edelbrock heads, but the springs were replaced w/ Comp Cam springs - 109 seat pressure, 300 psi @ 472 lift.

As for recurving the dizzy - It would be a new journey for me but not something I'm not willing to do.

Went ahead and ordered a Cardone dist. w/ new pertronix last night. I've read mixed reviews, but like the idea of being able to keep a spare set of points in the glove box if the pertronix quits. Maybe I'll go HEI down the road a bit.

 

That cam is very mild and would not cause a low vacuum.

I don't agree with the 16'' at 1,000rpm !!

For reference, I have a similar spec cam, and my vacuum at, and since, I rebuilt the engine, is 18 in drive (600rpm) and 21 in park (800rpm), both with steady needles.

Those readings are with full manifold vacuum, and idle timing at 14 degrees.

I tried ported vacuum for a while, and the readings were something like 16 and 19, steady needle.

I don't favour ported vacuum, but each to his own.

Also for reference, I bought a Pertronix dizzy a while back, and the timing curve could not be set.

After a handful of technical emails to and from Pertronix, they sent me a new one, no hesitation, and after setting up the curve etc, it's been perfect since installation (+/-5 years I think).

I'd seriouly reconsider the cardone dizzy !!!

The 300' (is that feet?) above sea level is not too significant.

As for recurving a dizzy, sounds like you wouldn't have a problem at all.

I get a similar needle pattern when I start the engine in the cold without applying choke. (I.E. too lean for a while.)

Not sure if the lean mixture applies to you, but PBY's setup procedure for the carb is a must.

Mind you, your engine sounds too smooth to be lean, but as mentioned already, the high rpm could be misleading.

Was the valve lash setup 100% ?
Subject to correction, if I remember correctly, 'too tight' valve lash can drop the vacuum.

 

Thanks FMJ - Good numbers for me to use as comparison.

I've only run the truck on ported vacuum to date but not necessarily opposed to trying manifold.

I will say I am weary of using the Cardone. You can understand why the Pertronix has left a bad taste in my mouth. I could try a third one. The issue, is when I read the message boards everyone has an opinion and some negative experience with every option. I will ponder on it today though.

I'll go find the PBY procedure for setting up the carb. I used the instruction with my edelbrock carb previously but it did involve going to max vacuum and then in ~1/2 turn.

Valve Lash was set by me following the Comp procedure that came with my pushrods/rocker arms (same procedure that is all over Youtube videos). Involved a lot of engine turning , tighten down until slight drag then give it an extra half turn. I can add that to the list of things to check.

 

Hi bsraborn,

Your're welcome.

By all means try the manifold vacuum connection, and decide for yourself which one you like better.

I remember there was a very slight difference (maybe in my head only !), but these days I run manifold vacuum for one main reason.

I have a vacuum gauge in my dash, and a while back the rubber tore in the advance cannister.

The needle immediately dropped from 18 to 16 so I was able to identify the problem very quickly.

Had I not had a gauge, I'd be wondering forever why my fuel consumption was crap.

If the Cardone dizzy is basically a remanufactured OE dizzy, then setting a decent timing curve is either a p.i.t.a or is impossible.

I also had an MSD dizzy that was not 100% and I returned it.

I found the customer service and sencerity at Pertronix to be way ahead of MSD.

The Ignitor 1, 2, and 3 all seem to be good.

MSD and Pertronix are good products, with bad apples of course.

Once again though, opinions will vary.

Sounds like you have your checklist in order.

I'm curious to know the cause of the low vacuum, and slight fluctuation.

Good luck.

 

Agree that cam is not real aggressive. Also agree near stock timing specs should be OK... maybe a couple degrees more (vacuum advance disconnected and plugged).

Also agree use ported vacuum for the distributor. Beyond the theoretical Ford/Chevy debate, the basic Ford setup was designed for ported vacuum. Many Chevy's ran manifold vacuum.

The other variable is timing mark accuracy. May not be great from the factory. If parts have been changed marks may be off. Also, if the vibration damper has slipped, marks may be off. Recommend you check with the "positive stop method" (google it).

 

Okay - I have some preliminary results, but first...

• I've received a billet pertronix distributor but waiting on a cast dizzy gear before installation. I am NOT going to install the Cardone. This distributor can be re-curved as well.
• The first video below shows the new vacuum after fiddling with the carb (following PBY's procedure posted above). I really just wanted to play with things before switching the dizzy out. I think this actually improved the fluctuation ~80-90%. I think my vacuum gauge is to blame for the vibration. Thoughts?
• I have a positive stop tool coming to ensure my balancer is correct and hasn't slipped at all. At $10 bucks, it wasn't worth my time to fabricate one.
• I performed a compression check w/ engine cold and hot. Results are posted below, but I don't know about the trustworthiness. I rented one of those Autozone testers and it wouldn't hold pressure well. It also seems like these readings are really low. I've ordered one from Summit and will reperform checks. In the meantime, what compression pressure should I be expecting? Anything above 100psi okay? I've also read the all cylinders need to be within 25% of one another.

Will continue to update as I go, but may be a few days until I get more work done


Vacuum reading after carb adjustment (in park at ~740 rpm):

Spark Plugs 1-4 (5-8) looked no different:

 

Compression numbers are pretty low. Should see 150 to 170 psi say, when hot, for a V8

Videos say they are private. Want to see 18" to 20" of steady vacuum at factory idle RPM on a stock engine at sea level. It's unlikely the gauge is defective, they are very simple devices.