15 inches of vacuum

[Rusty Nail]

Lol ...

 

[vkh]

I vote try advancing the timing. Since you had a shop do it I bet they just used the factory spec which is often retarded for emissions. Early on I ran my truck at 4° advance as the spec was until our neighbor suggested advancing to around 10° initial and that really woke it up. Gained an mpg and could do burnouts with just a stab to the throttle after that. 10° is fairly safe for an initial setting, if your at a high altitude you can push it a touch farther, long as you don't have detonation your timing is safe.

 

[Trippin3030]

87 is right I the only "upgrades" I made to this engine were the intake and a double rotor timing chain. I got lucky when I was cleaning the cylinders and saw all the tops were stamped .03 and there was barely any miles on this thing. So its definitely a rebuilt but I have no idea on the cam I'm going to assume it's stock because everything else in and around it was.

 

[Trippin3030]

Got fed up an Went through replaced a few vac lines and clamped or cut the ends off the rest now I'm pulling 18" without touching the timing that's a next weekend thing. It's running fine the only thing I notice is it misfired on 2 corners I took during hard acceleration. Once again thank you for the input from everyone especially 87 I know your studying for finals I really appreciate you taking time out to help a rookie.

 

[1987 GMC Jimmy]

Got fed up an Went through replaced a few vac lines and clamped or cut the ends off the rest now I'm pulling 18" without touching the timing that's a next weekend thing. It's running fine the only thing I notice is it misfired on 2 corners I took during hard acceleration. Once again thank you for the input from everyone especially 87 I know your studying for finals I really appreciate you taking time out to help a rookie.

That misfiring while taking corners sounds like a carb issue exclusively. If you punched the brakes at any time while taking them, that would be a good reason to take a look at the check valve or the booster itself, but if not, then likely not the problem. I know you said you just rebuilt the carb, but there's not much else it could be there. A stable 18" is a great idle vacuum. I'd still advance that timing, as it'll put more pep in the truck's step. If you can do 10* initial, that'd be great. If it starts to ping, dial it back to 8 and see what happens, and if it's not pinging at 10, you can try 12, but I wouldn't do more. You're welcome, by the way. If any updates or more questions, just post 'em up!

 

[Rusty Nail]

[yt]YZ1vAYjylc8[/yt]

Oh dang. I think that noob is working up a rad bromance with my boy '87.
What's that Lady Gaga song? There has got to be a parody! Ohyouuuutuuuuube!






































For teh lulz, don't trip.

 

[Trippin3030]

Lol fair enough rusty no hard feelings
Just making sure the credit goes where's it's due

 

[Rusty Nail]

That video is hilarious!

I can't get that song out of my head, guess we're even!

Welcome to the GMSB, noob. Thanks for being a good sport!

 

[1987 GMC Jimmy]

Lol fair enough rusty no hard feelings
Just making sure the credit goes where's it's due

After your issue, went and checked my car for vacuum leaks, and the line going to my AIR diverter was leaking, as was an old cracked piece of hose. You can never be too sure with vacuum hose.

 

[73c20jim]

Does high altitude have an effect on engine vacuum???

 

[Joshua Keith]

Does high altitude have an effect on engine vacuum???

I’m late to the party too. Lol My stock 350 with around 90k miles pulls around 15.5 right now. I have the idle set to 600 RPMs and my idle screw definitely changes the vacuum. The higher the idle the more vac, but I’m happy with the idle at the moment. I’ve read from some that 15 is low, and I’ve read that it’s perfectly fine as well just depends on the mechanic I suppose. I started at 13.5, changed all the vac lines,(one was too soft and would collapse from the manifold vac), tuned the Holley carb, and installed a valve cover breather. (PO has 2 PCV valves) ‍ It brought it up to 15.5. It runs so much better, but thinking maybe a timing adjustment could bring it up a little higher.

 

[Havasublue]

Hello...
I'm new here...
I say...
Get your ignition right !
Has distributor ever been out...?
Easy for a distributor to be installed a tooth or two out...
You could try to tune it to spec but you would be off...right...?
Rotor phasing...
Say yes.
Get initial timing right.
Check the springs on the centrifugal advance...
They could be too strong...
Coming in too late...
Usually the case.
Does it have adjustable vacuum advance...?
When you get the ignition right...
Go to the fuel system...
Start with the fuel transfer slots...
Exposed the right amount...
Learn how to use or use a vacuum guage...
Operating temp...
Small adjustments.

 

[75gmck25]

You've spent very productive time finding vacuum leaks - now you need to verify timing.

- If you are sure the TDC and pointer are correct, then just leave it there. Otherwise should buy a piston stop and use it to verify the pointer and actual TDC do match. Its a lot easier to turn over the engine by hand if all the plugs are out, and you can use the opportunity to check the plug gap and condition.

Buy an inexpensive dialback timing light. I have one from Harbor Freight and it seems to work fine.
- Disconnect and plug the vacuum advance line to the distributor and set the idle down to about 650-700 so there is no mechanical advance. Then set base timing to about 10-12 degrees, which should be fine for a relatively stock SBC. The factory used 4-8 degrees, but that was mostly for emissions.
- Measure timing as you increase engine speed and see how fast it advances. You will probably see about 18-20 degrees more from the mechanical advance. Its best if it comes in by about 2000 RPM, but a stock distributor might bring it in at a higher RPM. Just make sure its working, and write down the numbers.
- Let it go back to idle, and reconnect the vacuum advance to manifold vacuum. Measure timing again and you should see about 18-20 degrees from the vacuum advance. I prefer to use the manifold vacuum port on the carburetor all the time since it smooths out the idle. The factory probably connected it to ported vacuum on the carburetor.

Now take it out for a drive and see how it works. Listen carefully for light throttle pinging to see if you may have base timing set too high, or need to change the vacuum can. One general rule of thumb is that base+mechanical advance should not exceed 34-36 degrees, and base+mechanical+vacuum should not exceed 54 degrees.

I have aluminum heads with about 9.4 compression and a 213/217 @ .050 cam, and I am using about 15-16 degrees base timing, 18 more from mechanical by 2000 RPM, and 15 from vacuum advance. However, aluminum heads have more tolerance for advanced timing, so I don't thnk you want your base timing that high. I also switched vacuum advance cans from a 20 degree can to a 15 degree can to bring the cruise advance down a little.

Bruce

 

[MikeB]

Does high altitude have an effect on engine vacuum???

Yes. That's because engine vacuum is relative to atmospheric pressure. So, the lower the atmospheric pressure, the smaller the difference between that and engine vacuum. Engine vacuum decreases about 1" hg for every 1,000 ft above sea level. So, a 350 that makes 20" hg in Houston would make around 15" hg in Denver and only 10" in Breckenridge!

Cylinder pressure is also much lower at altitude. I lived in the Colorado mountains for a few years. Had I stayed there, I would have bumped up the compression in my 69 C10's 355 by milling the heads .030" and going to a .028" head gasket. 10:1 up there, even with old school combustion chambers, still would not have required premium gasoline.

 

[MikeB]

I also switched vacuum advance cans from a 20 degree can to a 15 degree can to bring the cruise advance down a little.

Bruce

That's a good idea. So many of the vacuum cans have 20" or more, which is "clatter" waiting to happen as you lightly roll on the throttle. Most guys will simply back off initial timing a bit to get rid of the clatter, but that is NOT the answer because it hurts performance.

I found a Borg-Warner vacuum can for HEI distributors that has around 10-11 degrees max. Can't find the P/N, but it was used in 75-76 Buick 350. A GM part would have been stamped "AR4" and the NAPA number at one time was VC1828. I have also limited the pin travel on stock cans using a home made limiter plate.

Here's a great article for anyone who wants to learn more: https://www.460ford.com/threads/timing-vacuum-advance-101-good-article-by-gm-engineer.117504/

Go down about 1/3 of the way to see listings and specs for performance-oriented canisters for GM non-HEI and HEI distributors.