[Shiny]

Looking for ideas and hand-holding.

After getting my car idling decent for the 1st time in about 15 years (thanks again to all), I started to check the timing and I find the vacuum is low: 12" to 13" at idle.

What are the likely causes and how do I chase it?

The carb was set up by Jeff (shaker455) and I've left it alone other than idle speed. I'm 4000 or 5000 ft higher than him, so I guess it's possible I need to adjust mixture screws leaner, but I know he has it set well and I don't want to touch it.

I have a vacuum tree in the carb base and started with all ports capped except for the vacuum gauge. I systematically connected the transmission, power brake, and vent lines and none changed the idle or vacuum so they all seem tight.

The timing is stable with no mechanical advance below ~1000 rpm and the vac advance is not yet connected. I am NOT using the TCS system.

The engine is starting and idling pretty well, but adjusting base timing does not seem to affect the vacuum level at all, at least between 8 and 14 degrees.

In the past, the vacuum has been ~18", and that is what I was expecting. It's a stock 1970 350 with #11 heads and a Summit 2800 cam.

What should the vacuum be?

Where do I start?

I guess I might have missed a big leak somewhere, like an open port on manifold so will look again.

Spray carb cleaner around the carb and intake?

Could the PCV valve be an issue?

After that, I guess its time to measure compression?


.....
Mike

[Joe's Garage]

Borrow one from a friend and compare readings. That will rule out a gauge failure.

Are you pulling the vacuum for the gauge from the vacuum tree or another vacuum port?

If it idles nicely and has decent throttle response, there probably isn't a vacuum leak. Carb cleaner is the go-to to check for gasket leakage.

Good luck!

[Shiny]

Thanks for the quick reply.

I will check the gauge. I have a MityVac I can use as a reference.

Yes, I'm checking the vacuum at the tree. Is there a better location?

[Formulajones]

In case you're not aware, it's normal for an engine to produce less vacuum at higher elevations.

I'm also at 5000 feet and routinely drive down to sea level. The difference in vacuum is about 3-4 inches. That's just the nature of the beast. As long as it's stable there really isn't anything to worry about. Put timing where it needs to be for a particular engine, dial in the fuel curve, and then forget about the vacuum drop that occurs at higher elevations.

[Kenth]

You will have higher vacuum readings with the vacuum advance connected to full manifold vacuum.
This will reduce throttle blade opening and risk for engine dieseling (run-on) at shut-off.

[steve25]

If you have your carb idling at a good rpm ( not too high) and it’s 1) a stable rpm and 2) you have no off idle stumble then there a real good chance that applying vacuum advance at idle will ruin those two main good things, especially if you pick up enough idle rpm to then need to back down on the idle speed adjustment you have.

Try it out, but I don’t think it will help, BHDT!

The only time I have seen such help is when you have poor sealing cylinders, or way to low of a compression for a given performance cam.

Go with what’s said in post 4 of this string.

[Shiny]

Originally Posted by Formulajones

In case you're not aware, it's normal for an engine to produce less vacuum at higher elevations.

I'm also at 5000 feet and routinely drive down to sea level. The difference in vacuum is about 3-4 inches. That's just the nature of the beast. As long as it's stable there really isn't anything to worry about. Put timing where it needs to be for a particular engine, dial in the fuel curve, and then forget about the vacuum drop that occurs at higher elevations.

I am definitely not calibrated, so thank you.

I'd love to know all is good! I can't drive it yet, but it does seem to be idling well and at least revs up without stuttering.

I am at 5800' so maybe 13" is what I should expect, even with the near-stock config?

I first had it running with this carb and cam in Fort Collins, which was maybe 5000' but don't remember the vacuum level. I do know it was ~18" after moving to WA at near sea level. Now I'm back at 5800 and don't know what to expect.

Originally Posted by Kenth

You will have higher vacuum readings with the vacuum advance connected to full manifold vacuum.
This will reduce throttle blade opening and risk for engine dieseling (run-on) at shut-off.

I have not yet connected the vac advance. I am planning to use ported vacuum and limit the vac advance to about 10 degrees. I think you are suggesting I add the vac advance at idle, which will increase the idle speed. If I follow you, I will have to dial the idle back, closing the blade, and increasing manifold vacuum.

Jeff has the carb set up well and it is clearly running off the idle circuit now, which is a breakthrough.

But I can try adding the vac advance at idle and see what happens after I measure the mechanical curve.

Mike

[Shiny]

Originally Posted by steve25

If you have your carb idling at a good rpm ( not too high) and it’s 1) a stable rpm and 2) you have no off idle stumble then there a real good chance that applying vacuum advance at idle will ruin those two main good things, especially if you pick up enough idle rpm to then need to back down on the idle speed adjustment you have.

Try it out, but I don’t think it will help, BHDT!

The only time I have seen such help is when you have poor sealing cylinders, or way to low of a compression for a given performance cam.

Go with what’s said in post 4 of this string.

Good input, thank you.

Yes, it's idling well between 650 and 700 and drops in and out of gear without stumbling. I won't know about off-idle behavior until I drive it but it definitely revs up fine without a load on it.

My plan is still to use ported vacuum until I have a reason to change it.

It seems all is good and 13" of vacuum may be just fine... seems weird but I don't need another problem even if I'm good at creating them.

I will check with Jeff to see what power piston spring he installed. He may not have planned on the vacuum being this low... I may also have to revisit the vacuum can... but these are NOT problems!!!

Mike

[Goatracer1]

Why don't you use vacuum advance? It's there to improve idle and low speed driveability .

[Formulajones]

Originally Posted by Shiny

I am definitely not calibrated, so thank you.

I'd love to know all is good! I can't drive it yet, but it does seem to be idling well and at least revs up without stuttering.

I am at 5800' so maybe 13" is what I should expect, even with the near-stock config?

I first had it running with this carb and cam in Fort Collins, which was maybe 5000' but don't remember the vacuum level. I do know it was ~18" after moving to WA at near sea level. Now I'm back at 5800 and don't know what to expect.

When you're a mile up like that 13 inches of vacuum with a stock engine is normal. My stock 400 Pontiac makes about the same 13 inches at 5000 feet and will push over 16 when I'm down in phoenix which averages about 1000 feet or less depending on where I go.

Still plenty to work the brakes just fine. My Z only makes 10 inches up here and still works the brakes fine. It goes to about 14 inches when I'm off the mountain.

One thing I'll mention with vacuum advance if you decide to run it to manifold you may run into an idle situation where it will idle higher at sea level, as the engine makes more vacuum it pulls on the vacuum advance harder, so if it's not fully engaged at the lower vacuum at elevation, it will kick the idle up when you drop elevation.
My solution to that is to use the adjustable crane advance cans and set the spring tension very light to where it starts about 6 inches and is usually all done by 8-9 inches, that way no matter what elevation I'm at the vacuum advance isnt changing the idle. I then do as you mentioned and weld the slot to limit it to about 10 degrees at the crank.

[Heybuck]

I'd pull the vacuum pipe out of the brake booster and plug it then test the vacuum level.

Had a similar problem to what you describe on a 327 Chev engine in my '65 Parisienne. Incredibly frustrating.

Eventually traced it to a bad booster (which worked very well, but it had the smallest leak internally causing my issue).

Ian

[ta man]

You need to adjust your mixture screws...no carb builder can set them for you.

[Radman]

I just looked up the Summit 2800 camshaft. With that duration your reading at idle is normal. If you are running and idling well, you are fine.

[Shiny]

Originally Posted by Goatracer1

Why don't you use vacuum advance? It's there to improve idle and low speed driveability .

Sorry I wasn't clear. I have vac advance can set up to apply 12 max and plan to use ported vacuum. I don't expect it will have any effect on idle, given it won't kick in until cruising. I just don't have it hooked up yet, as I am just getting it put back together and being cautious.

Originally Posted by Heybuck

I'd pull the vacuum pipe out of the brake booster and plug it then test the vacuum level.

Had a similar problem to what you describe on a 327 Chev engine in my '65 Parisienne. Incredibly frustrating.

Eventually traced it to a bad booster (which worked very well, but it had the smallest leak internally causing my issue).

Ian

Thanks, this is good to know. I actually had all vacuum ports capped with exception of the gauge. By installing one line at a time, I could tell there was no change in idle when the vac lines were installed to booster, transmission, and the vent controls. My booster and check valve are new but have sat for many years so I'll note your warning.

[Shiny]

Originally Posted by Formulajones

When you're a mile up like that 13 inches of vacuum with a stock engine is normal. My stock 400 Pontiac makes about the same 13 inches at 5000 feet and will push over 16 when I'm down in phoenix which averages about 1000 feet or less depending on where I go.

Still plenty to work the brakes just fine. My Z only makes 10 inches up here and still works the brakes fine. It goes to about 14 inches when I'm off the mountain.

Thanks again, expecially for sharing real-life experience. This makes me feel a lot better!

One thing I'll mention with vacuum advance if you decide to run it to manifold you may run into an idle situation where it will idle higher at sea level, as the engine makes more vacuum it pulls on the vacuum advance harder, so if it's not fully engaged at the lower vacuum at elevation, it will kick the idle up when you drop elevation.
My solution to that is to use the adjustable crane advance cans and set the spring tension very light to where it starts about 6 inches and is usually all done by 8-9 inches, that way no matter what elevation I'm at the vacuum advance isnt changing the idle. I then do as you mentioned and weld the slot to limit it to about 10 degrees at the crank.

This is smart, thank you. My can kicks in at 7 inches and is maxed out (12 degrees) at 15 inches. It is about 9 degrees at 13 inches. I may need to revisit.. I have an ajustable stop on it so I can at least change the max.

Mike

[Shiny]

Originally Posted by ta man

You need to adjust your mixture screws...no carb builder can set them for you.

Thanks for the suggestion.

I don't know what Jeff can or can't do but he worked a small miracle on my Frankencarb.

It does make sense to me to adjust but if there is an underlying issue, I am highly skilled at chasing my tail so I am being cautious.

Mike

[Shiny]

Originally Posted by Radman

I just looked up the Summit 2800 camshaft. With that duration your reading at idle is normal. If you are running and idling well, you are fine.

Thank you! I certainly like the sound of "you are fine" way better than the alternatives.

Mike