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Old 07-21-2013, 09:18 PM
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Vacuum gauge reading

I have a question concerning vacuum readings. Have a 327 with a mild cam (Corvette hydraulic), bored 30 over. Timing set at 10 BTDC. At idle the vacuum gauge reads between 15-17 inches of mercury, which I understand is good. I have increased the vacuum more by setting the timing higher, but trying to start it is almost impossible. When doing highway speeds (60mph), foot set just right to get this speed (also used cruise control) and I have a reading of 7-10 inches of mercury. I know that the vacuum does drop, but isn't this a little too much? Am I too far advanced or are the valves too tight or? Car runs well except when I shut it off and try to start it, the motor is firing against itself as if it is too far advanced. The only thing that was changed is that the carb (quadrajet) was rebuilt. Starting cold is no problem. Any suggestions?

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Old 07-21-2013, 10:16 PM
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Quote:
Originally Posted by 27Tall T View Post
I have a question concerning vacuum readings. Have a 327 with a mild cam (Corvette hydraulic), bored 30 over. Timing set at 10 BTDC. At idle the vacuum gauge reads between 15-17 inches of mercury, which I understand is good. I have increased the vacuum more by setting the timing higher, but trying to start it is almost impossible. When doing highway speeds (60mph), foot set just right to get this speed (also used cruise control) and I have a reading of 7-10 inches of mercury. I know that the vacuum does drop, but isn't this a little too much? Am I too far advanced or are the valves too tight or? Car runs well except when I shut it off and try to start it, the motor is firing against itself as if it is too far advanced. The only thing that was changed is that the carb (quadrajet) was rebuilt. Starting cold is no problem. Any suggestions?
What's the total? And 10* is too low as a base, if its hard starting hot at only 10* initial, look at all your elec for something amiss, or is the the original 40yr old starter? At only 10* you should never have hot start problems. Get the starter issue solved. Why are you chasing a vacuum number at cruise if the car runs right?
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Old 07-21-2013, 10:33 PM
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The total is 36 degrees and the starter was rebuilt 30,000 miles ago. Think it's time to replace it with a mini one. Not really chasing a vacuum, but my mileage should be a lot better than what it is (10 mpg). This thing is hooked up to a 200 4R trans as well, rear gear ratio 3.54 and tires 28". Hence my question.
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Old 07-22-2013, 06:01 AM
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Make sure the vacuum advance is hooked to a ported source. If it is hooked to a manifold vacuum, it will pull timing in as the motor is cranked.
That would advance the timing as you try to start. Just a thought
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Old 07-22-2013, 06:21 AM
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After sorting out the distributor advance curve, use an interrupter switch if you're still having trouble w/hot restarts.

Good luck!
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Old 07-22-2013, 12:09 PM
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Be sure the GM starter motor rear brace bracket is installed on your GM starter.
if yours is missing, get one (GM parts dealer)

Slow cranking with advanced initial timing: get and install a simple spark power interupt toggle switch.
Crank it over with no spark and hit the spark and it will fire without drama.

Adding a fender mounted Ford type starter solenoid and adding a starter motor heat wrap shield helps a GM starter too.
To run agressive idle timing yet avoid excessive full advance recurve the distributor and restrickt the mechanical advance travel stop limit. try maybe 16-18deg initial and 34-36deg at max advance.
The more agressive the camshaft, the motor the motor will like agressive idle timing.

re crusing in OD with a cammed motor:
This thing is hooked up to a 200 4R trans as well, rear gear ratio 3.54 and tires 28". Hence my question.

A cammeed up motor wants to rev. It wants to cruise at a fair rpm. On this motor unlike a stock cammed motor designed for very low rpm cruising in OD, the slowest cruise rpm is not the most efficient cruise rpm.

Your hiway cruise rpm In OD with 3.54 gears / 28" tires is probabily too low. Should have put more gear in it.

EG 4.10 4.30:1 gears. Now you are taking advantage of the OD transmission and will ahve a better overall cruise rpm
not to mention great off the line acceleration with your "corvette" cammed 327. It wants to rev.
The right rear gear ratio makes a big big difference.

Find some 4.30's for it.

Last edited by F-BIRD'88; 07-22-2013 at 12:22 PM.
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Old 07-22-2013, 12:38 PM
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If you do want a low rpm hiway cruiser 327 using 3.54:1 gears- 28" tires and the OD trans and get cruising efficiency
you need to change the camshaft to something a lot shorter duration and less valve overlap .

Low rpm overdrive cruise efficiency ='s small camshaft. 195 to 210 duration @.050"

Three cams for you to look at
Isky cams #201256 #201258 #201256/262 If you want to stay with the 3.54:1 gears.
Adding 1.6:1 rockers gives higher valve lift yet keeps the valve duration short.
It will be a whole different motor.
ISKY Racing Cams - Do It Right. Race with the Legend. Camshafts, Connecting Rods, Valve Springs, Lifters

Having the right camshaft makes a big difference.

Last edited by F-BIRD'88; 07-22-2013 at 12:48 PM.
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Old 07-22-2013, 04:13 PM
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Quote:
Originally Posted by F-BIRD'88 View Post
To run agressive idle timing yet avoid excessive full advance recurve the distributor and restrickt the mechanical advance travel stop limit.
Can you provide some instructions or part numbers here? I have been trying to figure out the correct way to do this. It seems as if MSD has some bushing you can use...will those work on all HEI distributors MSD and OEM? Or MSD only?
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Old 07-22-2013, 06:15 PM
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For what its worth... How to Use and Interpret a Vacuum Gauge
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Old 07-22-2013, 09:19 PM
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Quote:
Originally Posted by Silver Surfer View Post
Can you provide some instructions or part numbers here? I have been trying to figure out the correct way to do this. It seems as if MSD has some bushing you can use...will those work on all HEI distributors MSD and OEM? Or MSD only?
Some info on timing curves, etc. here.

MSD supplies:
Red (allows the least amount of initial timing) - 28 degrees
Silver - 25 degrees
Blue - 21 degrees
Black (allows most initial timing) - 18 degrees

If you need even more initial timing you can use aftermarket 10 and 14 degree bushing$, found here.

You can also download the MSD instructions at the link below to look get a full explanation of the bushing and how it affects the total timing.
http://www.setyourtiming.com/

Last edited by cobalt327; 07-22-2013 at 09:35 PM.
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Old 07-23-2013, 11:18 AM
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Quote:
Originally Posted by cobalt327 View Post
Some info on timing curves, etc. here.

MSD supplies:
Red (allows the least amount of initial timing) - 28 degrees
Silver - 25 degrees
Blue - 21 degrees
Black (allows most initial timing) - 18 degrees

If you need even more initial timing you can use aftermarket 10 and 14 degree bushing$, found here.

You can also download the MSD instructions at the link below to look get a full explanation of the bushing and how it affects the total timing.
Set your timing - for more performance!
So those busing are MSD specific? What about limiting mechanical advance on an OEM HEI?
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Old 07-23-2013, 11:55 AM
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Quote:
Originally Posted by Silver Surfer View Post
So those busing are MSD specific?p
That's correct.

Quote:
What about limiting mechanical advance on an OEM HEI?
Refer to the link above in post #10 where it says "Some info on timing curves, etc." Down a ways from where the link first places you on the page, you'll see a photo and description on limiting the mechanical advance on the GM HEI distributor.
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Old 07-28-2013, 06:10 AM
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Have you verified the vacuum advance is working at cruise? One reason its there is to provide better fuel economy.

From what I recall, this is the typical starting point recommendation for a stock GM HEI if you are interested in economy.
-- 8-18 degrees initial - you should verify TDC manually to make sure you get the right reading. More cam usually requires more initial advance to help low end. Most stock engines will take about 12-14 degrees or more initial without any problem. Adjust by turning the distributor.
-- 18-20 degrees mechanical. Works best if its all in by about 2000-2500 rpm. Adjust total advance using a mechanical stop; adjust rate of advance by changing springs.
-- 15-20 degrees vacuum. Stock GM canisters come in different ratings (both max increase of timing and minimum vacuum to activate) . Aftermarket vacuum canisters are often adjustable. You can run without vacuum advance, but cruising fuel mileage will suffer.
-- Initial plus mechanical should not exceed 36 degrees.
-- Initial plus mechanical plus vacuum should not exceed 54 degrees.

Economy cams usually run a duration of about 195-205 at .050, or for a little more power you can use an RV cam that runs up to about 210-214 at .050. Most of the GM cams will be dual pattern (e.g., 204/214) is a common RV cam.
If you want a wider power band you usually want a cam with a larger LSA (112-114). A lower LSA (108-110) may work fine for a particular application, but will be generally more RPM & speed sensitive.

Bruce
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Old 07-28-2013, 10:33 AM
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You said you set the timing to raise the vacuum, did you actually put a light on it to see what the initial is. You cant set timing with a vacuum gauge. Sounds like your initial timing is too advanced. Anything past 12-14 BTDC can give you potential hot cranking issues.
is this vehicle a t-bucket, just a guess considering your screen name, if it is that tall flat windshield is your poor mileage issue if you have one. I have the same issue.
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Old 07-28-2013, 09:40 PM
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My Initial timing is set at 11 degrees; centrifugal is 21 degrees, at 2800 rpm. Vacuum is still at 15" at 700rpm and even lower at cruise speed. My next step is to change the vacuum advance (replace it with an AR12 or AR10). Hopefully this will make a change. Even at cold temp. the starter (at times) sounds like it is firing against itself. Right now it is on manifold vacuum, before on ported vacuum.
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