Hi everyone I been trying to get my timing/carb set after I installed the motor in my 77 the motor is a 383 stroker with roller cam and vortec heads (not sure on cam spec) it's 400hp/440tq at flywheel. I bought a quick fuel 735 vacuum secondary how the question I got the timing set at 12 degree with the vacuum plugged off on the right side of the carb got the vacuum gauge and plug it in manifold port set vacuum at 16 hooked up the vacuum line to the distributor did not idle up any in fact it was idling rough I moved the vacuum line from the distributor to the manifold port and it idle up adjust the idle on the carb idling good Do I need to keep vacuum to the distributor or is something else wrong I always thought that you hook it up to the right side of the carb 16 is the best I could do on the vacuum when setting the carb am doing something wrong thanks

 

Why arent you setting the mechanical advance to a total of 36 degrees with the appropriate springs to have all in by about 2,800 RPM. Once that is set you connect the vacuum advance to the manifold or a port on the carburetor that has vacuum at idle. Once that is established you adjust the carburetor speed and mixture screws for best idle.

 

I'm not really sure how to do that do you set the idle up to 2800rpm's and adjust the distributor to 36 degree I never seen it done like that

 

Do you have a stock distributor or aftermarket? If aftermarket what manufacturer and part number? Did the 383 come with this distributor? If so did the seller tell you to set initial timing to 12 degrees?

 

It's a acdellco distributor I bought new for this motor the man that built the motor said 12 to 13 degree would be fine

 

If you set mechanical to 36 total I do not think you want any more than that ??
Normally set initial timing with vacuum advance disconnected then hook advance hose to a ported vacuum on carb so as you open throttle and mechanical and vacuum advance timing together up to about 32 to 36 degrees >if you do not have a timing light with a advance **** get a degree tape and stick it to the balancer .
Wes

 

Manifold vacuum is only available from the manifold or the cast aluminum base plate of the carburetor. Any vacuum above the base plate is "ported" vacuum that only comes on at above-idle speeds like 1500 rpm or more. And yes, your distributor needs constant vacuum to it all the time. As far as total timing goes I always recommend around 31 to 32 degrees in at 3000 rpm or more for 87 to 91 octane gasoline because 91 octane will ping under a real heavy load at 36 degrees.

 

If you have Vacuum all the time full to advance then why have any vacuum advance ?? Object of vacuum advance was for better economy
Looking at on original Factory service manual it say's hook to ported advance .that's the way I was taught GM school years back .
Wes

 

Maybe it going to idle a little rough or maybe with a little lope just want to get it hooked up right thanks for the replys

 

Because the engine idles better and cooler than if it was connected to ported vacuum.

 

If you read through this thread, you will have a complete understanding of the timing and how to adjust it for performance. It's a long read, but well worth the time.

https://www.corvetteforum.com/forums...long-post.html

BTW, vortec heads wont need more that 34 deg all in.

 

You need vac advance can connected to full vacuum, not ported.
Search the forum as this has been discussed a zillion times.

 

Although there are a few who like constant vacuum to the distributor, it is not how the system was intended to function. You need ported vacuum to the distributor. There should be no vacuum at idle to the vacuum advance, only at part-throttle and above.

 

Actually, ported vacuum advance came in in the 70's as a smog device. This was combined with the AIR systems designed to increase exhaust gas temperature. It made engines run hotter at idle to help burn off hydro-carbons.
Vacuum advance controls the curve by adding advance at low load. Idle is low load...
Idle mixtures are lean, requiring more lead.

 

The actual reason for vacuum advance was to improve low-load fuel economy at speed. As far as I know, that was the only reason. Because of this it should be attached to ported vacuum.

 

No.

 

OK back to the posters original question.....

It's a bit difficult to read your original post. Breaking it up into smaller chunks makes it easier to read, and helps you make your point.

It sounds to me like you base timing is off if it runs rough when you hook up the vacuum advance.

The stock GM method of setting timing and sometimes using ported vacuum sources were for emmissons compliance, not performance.

With your 383....I suspect you'd like a little performance. Use this method for some performance...

-pull vacuum advance line from carb and plug the carb port, also ensure all other vacuum ports on the carb are plugged.
-set engine RPM to 3000
-Adjust timing to 36 degrees by moving the distributor (use a dial back timing light, or a timing tape on the harmonic balancer.)http://www.harborfreight.com/timing-...nce-40963.html
-adjust engine RPM back to idle
-connect vacuum line to a manifold vacuum source at the carb....not a ported vacuum source.

Once this is done you can make a graph on how your MECHANICAL advance comes in, but that is another discussion, get this setup done first.

 

Interesting, I remember my high school teacher and my tech school instructor saying just that...and this was back when these were still being produced. So, what is is wrong with my post?

 

I got it set to 36 degree at 28 00 rpm so I set the idle hooked up vacuum advance line to the porte vacuum on the carb performance wise is about the same if I hook it up to manifold vacuum just the idling is different

 

400 hp engine. What is your goal. Off the line drag racing all the time or a street cruiser with a helluva punch.

If your racing then no ported vacuum, use manifold or disconnect it.

Cruising and you want some gas mileage and drivability use the ported. Ive read and reread posts here, other performance sites and racing sites. That is how the majority will tell you to set it up.

I did the following on a 72 lt1 motor solid lifter and 600 cfm carb with headers. Roughly 330 horse. No dyno numbers yet. The performance jump from stock 4* to 36* was ridiculous.

Go buy a good timing light and read the reviews. Craftsman has lousy reviews.. sears, autozone ,summit racing all sell a $90 light that has advance and a built in tach that works great. I lke it cause I dont have to buy the right timing tape, didn't have to set up a seperate tach, and you can see everythng right where your pointing the gun.

For an old school 350 use 36*, newer use 32*- 34* I believe. If you get pinging retard or advance til it goes away, that point is your max

Close off the vacuum.

Set your timing light to 36* advance and work up the throttle to see where it comes in.

Adjust the 36* to match tdc on the timing mark at the point where yours comes in.
It will depend on the distributor. Mine is all in at 2300 rpm.

You can go higher to see if it advances higher but mine didn't change from 2300 to 3000. If you cant reach 36* you may need to disconnect a mechnical advance spring to make it come in easier. This means you need to buy a lighter spring setup

Reset idle and reset timing light to 0 and recnnect the spring.

Check timing at zero at idle. It will be between 12 and 16 probably. Right where your mechanic wants it. This is your new idle timing mark. Reconnect the vacuum advance to the port you want. Mine is on ported and idles down to 900 rpm easily.

This is from information I researched and found and from personal experience on a car for the street not the strip. It will get out of its own way easily.

I would love to feel 400hp in a vette. I know in my GTO it was powerful, and that was a car almost 800lbs heavier.