The Stovebolt Site Forums The Shop Area Engine Shop Timing on 283

I am getting ready to install a new summit vacuum advance distributor on my 57. The truck has a '65ish 283 bored over to about 302 with a cam from a 327 'vette motor. My shop manual lists the timing to 8 degrees before top dead center at idle.

Now, I think you have to set the vacuum advance at a certain, higher rpm. My friends crate 383 is set to 32 deg at 4000 rpm. Any idea what it should be for my motor?

Am I making any sense? Thanks!

set it to 8 at idle for starters, the distributor vacuum and mechanical advance take care of higher RPM - you might need to tweek it up a bit, mine bored 60 over likes 10-12

Bill

Thanks, Bill.

Ok, so that's 8 before tdc correct? My motor is .125" over btw. The summit vacuum advance distributor adjusts with a 3/32 Allen key screw in the vacuum advance apparatus, so how do i "tweak it up a bit"?

Hy dalyman72, you set your intitial timing at idle with the vacuum disconnected, then when you connect up your vacuum advance it will either advance it further [manifold vacuum] or do nothing until the throttle is opened up [ported vacuum]. Which vacuum source did the instructions say to use?

yes 8-12 btdc - not familiar with the Summit setup, maybe an HEI? I set mine with a light and the tab on the balancer, should be the same AFAIK

Bill

It is a HEI, the instructions say to use the port on the carb that registers no vacuum at idle. I do not know what port that is, there are two ports on the front side that are plugged off by the PO.

(I'm not sure what ported or manifold means)

you do not want full, unregulated vacuum direct off the manifold - check the plugged carb ports with a vacuum gauge for one of them with 0 vacuum at idle [and some vacuum under acceleration], but as 3B sez, you want that disconnected when you set the inital advance

Bill

To set the total spark advance you'll need either a delay-flash timing light, or a balancer with degree markings all the way to 45 degrees or so before top dead center. Some places like Summit or JEGS sell a piece of stick-on tape that goes on the balancer to read total spark advance with a conventional timing light. Total advance is a combination of initial timing, centrifugal advance, and vacuum advance. Set the initial advance at 8-10 degrees, leave the vacuum line disconnected, and rev the engine up to about 2500-3000 RPM or so. The combination of centrifugal and initial advance should be around 36-37 degrees. If it's not correct, adjust the distributor housing to get the right reading, and let the initial advance fall wherever it will. Sometimes it will be necessary to modify the centrifugal system to get good starting and midrange power and still stay below 37 degrees of centrifugal advance. Connect the vacuum line, and rev it up again. With high RPM, low load, (not pulling a hill, passing, etc.) the vacuum advance should add 8-10 degrees of advance for good fuel economy. As soon as you tip the throttle in to pull a hill or pass, etc., the vacuum advance will go away and you're back to the 36-37 degree maximum setting. Adjust the vacuum chamber to get the desired additional advance with vacuum applied.
Jerry

HRL gave a good analyles of it. I will ad, If it kicks back when you try to start it you have to much initial advance. If it runs rough & ragged in neutral at about 2-3 thous. RPM you have too much vacuum advance. If it pings on hard acceleration you have to much total advance or you need higher octain gas if you have a high comp. ratio.

Ok, I want to say first thanks for all the help! I have not been able to get into setting up the vacuum advance yet, but I was able to play with it as is and get a few more questions. First of all, the PO said they could not get it to hold timing. Now, I have heard that that could be because of to low and idle, and I may have to raise the idle to get the lope out. Make any sense? My other question is that when it was ideling, I could floor it quickly and it would kill the motor. Is this because of the timing not advanced, or am I just not used to a carbureted engine lol. Also, my harmonic balcencer does have timing marks around it all the way to 60 deg.

Thanks again guys

What's the total advance right now, measured at 2500-3000 RPM with the vacuum disconnected? If it's around 36-37 degrees, you won't be able to dial in any more initial advance to help it accelerate without stalling. If not, set it there before trying anything else. It sounds like you've got a pretty radical cam, but the 302's like lots of cam and RPM if you've got a standard transmission to go with it. Not so much if you're running an automatic, unless you've also got a high stall speed torque converter.

You've obviously got a race-prepared engine. What kind of racing do you intend to do with it? It's going to be a total PITA if you intend to street-drive it!
Jerry

So I should try to set the initial (mechanical?) timing WITHOUT the vacuum advance to 36-37 deg, then plug the vacuum advance up, which will do its job automatically?

Like I said, its a 283 bored to 302ish (.125 over from what I can figure). This is the story on the motor:

The PO got it from a guy cheap because he couldn't get it to run right. The PO took it to a machine shop and was informed that it was camed WAY to much for the 283. So they put a cam from a 327 vette motor (machine shop said it was good all-around cam) and set the heads to it. It is paired to the origional(?) 3 speed on the column.

I will not be doing any racing with it, just cruising. Guess I'm going to have alot of work just to have that lope lol...

The most popular "327" factory high performance cam was the Duntov "098" (the last 3 digits of the part number). It was a fairly radical-grind solid lifter cam with a pronounced idle lope. It was fast, and fairly streetable, but it required constant attention to the valve lash because the reccomended clearance was .030" on both intake and exhaust. It was a VERY noisy cam! To get a little bit of idle lope and still have a pretty well-mannered engine on the street, I like to use a Comp Cams 268H High Energy hydraulic cam. It will idle pretty smoothly at around 1200 RPM, and still have steady enough manifold vacuum to use a power brake booster. If you've got to go to 1500 RPM or faster to get the idle to smooth out and the timing to get steady, you're probably still over-cammed a little.
Jerry

I'll see what I have to hold it at to get it smooth once I get a tach on it. Is my understanding of your other directions correct?

Yes, the only function of the vacuum advance is for fuel economy at light-throttle cruise. As soon as any noticeable load goes on the engine, the vacuum advance goes away, so the initial and centrifugal advance combination is the critical setting. Once you've got the 36-37 degree advance set at 2500 RPM with the vacuum disconnected, plug the hose onto the vacuum chamber. It should advance about 8 to 10 more crankshaft degrees at the same speed. (45-47 degrees total) If not, tinker with the screw adjustment inside the vacuum nipple to add or subtract a few degrees of vacuum advance. Have fun!
Jerry

Thank you sir. I sure will!

I got the timing nailed down last night. It's set at 30 deg at 4000 rpm, and 12 at 450 rpm. I know I could have advanced it a little more like you said. But it already have sooo much better than it was.

By the way, it would not hold timing becasue the distributor clamp was just a crappy piece of stiff wire... $11 at O'Rieleys fixed that.

I believe I probably gained about 75 hp over the other timing. Now the question is the Vacuum advance. Is it going to give me any power under load? If not, I think I might leave it plugged.

The vacuum advance will go away almost as soon as the manifold vacuum drops as a load comes onto the engine. Its only real function is to squeeze a little more fuel economy out of the engine when it's running at light throttle cruise. You can see this by jacking up the rear wheels and getting them spinning at highway cruise speed, then ride the brake a little while watching the timing mark. As the vacuum starts to drop as the load comes on, the timing will drop back to your "power" setting. I used to do that demonstration for my students pretty often.
Jerry

Ok, gotcha. I think I will leave it off unless I'm getting some godawful mpg numbers. Thanks for all your help Lincoln.