Timing a 350

[79dentside]

Hey guys, been tinkering with the farm truck while she awaits other repairs and I realized her timing was too retarded. I advanced it and tried to get good vacuum, but could only get to about 16 on my vacuum gauge. She runs so much better, she had a bad bump in idle that has worked it’s way out. A 350 was put in the truck in 2007 by a previous owner, not sure if its stock. She sounds okay, but is 16 too low?

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Thanks

 

[78C10BigTen]

Steady 16?

 

[legopnuematic]

Low vacuum means one of a few things, either a worn out timing set causing retarded valve timing (while also retarding ignition timing), or a large cam with a low lobe separation angle, the final big one is carb adjustment with the idle mixture screws. Most 350's are happy with 10-14 degrees advance with anywhere from 32-38 degrees total.

An easy way to get an idea on timing chain play is to disconnect the battery, pop the cap off the distributor and then bring the engine to tdc (or where the balancer reads 0), then slowly rotate the crank back the other direction until the rotor moves, look at how many degrees the balancer moved and divide by two to get an approximate amount of play. There shouldn't be a hole lot of play in the timing set, but if there is excessive movement before the rotor begins to move, worn timing set.

 

[78C10BigTen]

Low vacuum means one of a few things, either a worn out timing set causing retarded valve timing (while also retarding ignition timing), or a large cam with a low lobe separation angle, the final big one is carb adjustment with the idle mixture screws. Most 350's are happy with 10-14 degrees advance with anywhere from 32-38 degrees total.

An easy way to get an idea on timing chain play is to disconnect the battery, pop the cap off the distributor and then bring the engine to tdc (or where the balancer reads 0), then slowly rotate the crank back the other direction until the rotor moves, look at how many degrees the balancer moved and divide by two to get an approximate amount of play. There shouldn't be a hole lot of play in the timing set, but if there is excessive movement before the rotor begins to move, worn timing set.

Damn you and your knowledge again!! You just learned me

 

[Goldie Driver]

Low vacuum means one of a few things, either a worn out timing set causing retarded valve timing (while also retarding ignition timing), or a large cam with a low lobe separation angle, the final big one is carb adjustment with the idle mixture screws. Most 350's are happy with 10-14 degrees advance with anywhere from 32-38 degrees total.

An easy way to get an idea on timing chain play is to disconnect the battery, pop the cap off the distributor and then bring the engine to tdc (or where the balancer reads 0), then slowly rotate the crank back the other direction until the rotor moves, look at how many degrees the balancer moved and divide by two to get an approximate amount of play. There shouldn't be a hole lot of play in the timing set, but if there is excessive movement before the rotor begins to move, worn timing set.

Good to know on the timing chain.

If the chain was sloppy, wouldn't the timing mark jump around as the light flashed on the timing gun ?

 

[78C10BigTen]

Held on to this thinkin it would help me someday.

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[legopnuematic]

Good to know on the timing chain.

If the chain was sloppy, wouldn't the timing mark jump around as the light flashed on the timing gun ?

I cant say for certain, but i would think due to the drag involved that it wouldn't, at least at idle or under load. I could see on deceleration it would flutter.

 

[79dentside]

The truck had a 350 put in, it has about 25,000 miles on the engine as we speak. Has not been used a lot since the motor and rear end were done in it.

The 16 varies a little, I believe I got another vacuum leak. I have so many loose or old lines on an old Quadra jet. There is an Edelbrock performer rpm intake that doesn’t quite match up with the carb that’s on it, I want to change the gasket anyways.

thanks for the tips!!!

 

[79dentside]

My needle flutters from about 15-16, maybe even 15.5-16.

Also, does engine rpm matter? I idled it down when I did the timing to about 600 rpm. I have no idea if you are supposed to do that or not, but that’s what I did.

 

[88 diesel]

With the engine idling slowly restrict the air flow. Does the vacuum come up?
Look for a leak or carb adjustment.
The previous answer about checking timing chains is right on the money and is really the best way to check chain wear.
An aftermarket cam will affect idle vacuum also.
Another thing that's overlooked are distributor bushings. So many things just don't have them now.
Dollar bill on the tailpipe can give lots of info too.
Bill

 

[88 diesel]

600 is to low an idle speed for most engines to maintain valve train lubrication.

 

[79dentside]

With the engine idling slowly restrict the air flow. Does the vacuum come up?
Look for a leak or carb adjustment.
The previous answer about checking timing chains is right on the money and is really the best way to check chain wear.
An aftermarket cam will affect idle vacuum also.
Another thing that's overlooked are distributor bushings. So many things just don't have them now.
Dollar bill on the tailpipe can give lots of info too.
Bill

Good stuff, thank you! It kinda sounds like someone put a cam in it. It is a GM remanufactured engine. The carb needs An adapter plate, I found one that I will end up getting for the Qauadrajet.

I can raise the idle, I idled it down for timing purposes. It is slightly about 600 warm idle now. At 600 or 700 it still maintains over 30 oil psi HOT, by 1,000 rpm it is 60 oil PSI. I didn’t realize 600 was not optimal for drivetrain lubrication.

Let me do some checking on some possible leaks. When you say timing chains, do you mean stretch?

thanks again guys

 

[75gmck25]

How much time do you want to spend on figuring out the timing of this engine? The best first step is to buy a piston stop, take out all the plugs, and verify TDC. Match actual TDC to the pointer on the engine so that you know the zero point, and then use a timing light to find out the base timing.

Set base timing to about 12 degrees BTDC with vacuum advance disconnected and plugged, and then set the idle in Park to about 700-750. Then check your vacuum. With a good stock engine it will usually be about 18-19 inches, maybe more. Also verify that as you rev up the engine the mechanical will add about 18-20 degrees of timing. If you don't have a dialback timing light it will quickly go off the scale, but at least look for the advance. Performance will suck if that mechanical advance is not working.

Then reconnect the vacuum advance and adjust the idle if needed. If its on ported vacuum it may not change the idle, but manifold vacuum should raise the idle. Most stock vacuum units will add as much as 20 degrees advance, so you may have to set the idle back down if its on manifold vacuum (my preference). You want the vacuum advance to work, since it helps gas mileage quite a bit on the highway.

Once you finish all that work, then check for vacuum leaks and/or bad vacuum hoses. PCV valve is also a simple check (shake it to make sure it moves inside). Also check the big hose going from the carburetor base to the brake booster.

Bruce

 

[73c20jim]

What is your elevation. All readings are developed at sea level. Optimum vacuum is less as you go up. I have a post here about tuning and elevation.

https://www.gmsquarebody.com/threads/high-elevation-tuning.27605/#post-527057

 

[79dentside]

Thanks for the info guys. Bought some new vacuum lines and spark plugs, but it got too cold out.

I did buy a new carb gasket and wanted to address this first considering all 4 sides of the intake manifold; where the carburetor base meets the intake, has tarnish running down. As soon as I stuck my socket on the carburetor bolts, I knew I had a problem. All 4 bolts were loose, the front 2 were no more than finger tight. I tightened them all down and took it for a spin, didn’t bother hooking a vacuum gauge up, too cold outside today. Runs a little smoother I suppose. I couldn’t get it to kick down before, now it does. Still hard to get it to kick down as it always has been. Oh well.

elevation is 1,000 ish.