Question on setting my timing

[Michael L.]

Guys,

I'm working on my 69 4spd L-46 restoration and at this point I'm working on getting the timing set on the car. When I rebuilt the engine I went with a mild hydraulic roller for the camshaft. Everything else is stock. I'm guessing this has probably changed the ignition timing I should set the car at. I was told by the guy who rebuilt/restored the carb that given the non-stock configuration of the motor I should use a vacuum gauge to set the timing. I've looked at a couple of youtube videos of how to do this but they're not very good at explaining things. Can someone tell me how to use a vacuum gauge to set my timing? Once I get the initial timing set using the vacuum gauge, how can I re-curve the factory distributor to keep the total timing at the proper level? Thanks for the help.

Mike

[Bill B.]

Re: Question on setting my timing

Hmmmm ... Using a vacuum gage to adjust the carburetor's idle mixture screws, YES! Using a vacuum gage to set ignition timing?? Never heard of. A timing light connected to the #1 spark plug is the only way to properly set ignition timing in my book of 1969/70 experience.

[Joseph L.]

Re: Question on setting my timing

You want to set maximum timing while heavily accelerating with no ping. I like to use a hill. When you get a ping, back off a couple of degrees and try again.
You cannot use initial timing since you do not know the characteristics of that engine/ cam combination.

Joe

[Michael L.]

Re: Question on setting my timing

Yeah that's been my experience too but once you swap out the factory camshaft for aftermarket you can throw the factory timing out the window, right? The timing light will tell you where the timing is but how do you know where to set it when you don't have timing specs to follow? should I stick with the factory settings, or go with 15 degrees initial, or maybe even 20? While the total timing is usually a ballpark number you can guess at as long as the engine isn't crazy modified and can listen for pinging to see if you have too much, I'm guessing the initial timing needs of an engine can change a bunch if you swap out the camshaft. I'm just looking for a way to figure out how to set that initial timing.

[Leif A.]

Re: Question on setting my timing

Michael,
Has the engine been run/cam broken in since the rebuild?? If not, here's what I do. Bring number #1 cylinder to TDC on the compression stroke. Be sure the adjustment window on the distributor is facing the front of the engine. If so, remove the distributor cap and note where the tip of the rotor is facing. Is it facing the #1 terminal on the distributor cap (first hole clockwise of the adjustment window)? If so, you're good to go for initial startup. If not, you need to pull the distributor and align everything. There is a very simple way to do this called "walking" the distributor. If not familiar with this, we can explain. Snug down the distributor hold down bolt but leave loose enough to be able to rotate the distributor.
You say the carburetor was rebuilt. Air and fuel adjustment screws should be set at 1 1/2 turns out from all the ways closed for an initial setting.
When you fire off the engine for cam break in, you'll be able to slowly turn the distributor clockwise until the engine starts to stumble, then turn the distributor counter clockwise until it starts to stumble. Place the distributor between these two points, snug down the hold down bolt and run the engine in at 2500 RPM for 20-25 minutes, varying the RPM slightly up and down during this process.
Once the cam is broken in, the fine tuning of the distributor and carburetor can begin. This is where a vacuum gauge and dial back timing light will come into play.
Hope this helps.

[David H.]

Re: Question on setting my timing

... throw the factory timing out the window, right? ...

Michael,

Initial timing is for ease of start. Once things are moving, centrifugal and vacuum advance take over.

For lack of a cam manufacturer's starting point, I'd just use GM specs - adjust from there.

Search this site for Duke Williams' many posts re timing.

Dave

[Robert P.]

Re: Question on setting my timing

There is no need for camshaft break-in he stated it had a roller camshaft
Bob Peckham

[Duke W.]

Re: Question on setting my timing

A vacuum gage won't help you set timing, but you need to use one to determine idle vacuum.

Do a Web search:

"tuning vintage corvette engines for maximum performance and fuel economy."

Study it thoroughly particularly the page on emission controlled engines, converting the OE ported vacuum advance to full time, and using the Two-Inch Rule to select a proper VAC.

You have a steep learning curve to climb. Your carb guy will be of no help because he sounds clueless when it comes to setting up a spark advance map for an engine.

The hydraulic roller cam did not change the ignition timing, but it changes what will be the optimum spark advance map.

Due

[Michael L.]

Re: Question on setting my timing

Thanks for the input guys. Yes the motor has been running for some time now. Initial start up was on the dyno a few years ago, then installed into my rolling chassis. The car is now about 90% together and I'm looking to nail down the fine tuning for drivability, ease of starting, getting the choke and cold start sorted out, etc. Just looking for the best way to optimally time it when the camshaft is no longer factory. So, IIRC, if factory initial timing is supposed to be around 8, with this new camshaft I'm thinking of going to like 12-14 or even 15. The reason why I'm asking is the guy who rebuilt/restored the carb told me that is the best way to figure out optimal timing on an engine that has been modified from factory. The problem with the current set up is I set the timing initially on the factory specs but then I had to crank up the idle screw to get the car to idle properly. This caused the idle mixture screws to become ineffective. I thought it was a problem with the carb so I sent it back to the rebuilder who said the carb was fine but he noticed the idle screw was set too high. He set the idle screw where it should be based on the exposure of the transfer slots and sent it back and told me that I shouldn't use the idle screw to set the idle but I should use the timing to raise the idle and the best way to do this was a vacuum gauge. Without using the vacuum gauge I just turned the distributor and the car is running well now with the initial timing up around 15-16. I just wanted to try the vacuum method to further fine tune it but the videos I've seen are light on details and info.

Also, once I set the initial based on vacuum i'll need to recurve the distributor to keep the total from going over. I looked for a few videos on this but again didn't find one that really shows how to do it.

Mike

[Timothy B.]

Re: Question on setting my timing

Sounds like the camshaft has lots of duration, can you post the specs. IMO, using a vacuum gauge you will probably over advance the initial timing and make the engine too efficient (lean) and maybe have some low speed (launch) driving issues.

I would try the 8* initial with a 12* vacuum advance connected to a manifold vacuum source and see how it runs. Make sure to read up on Duke's two inch rule as that will allow the V/A to be pulled in full at engine idle and retard the moment you open the throttle blades. That will give you approx 20* idle timing, you want to be careful not to make the engine to efficient at idle (too much timing) because the lower vacuum causes a poor fuel signal to the carburetor.

The more initial timing you give the engine the leaner it will run and that may be fine at idle speeds but you don't drive at idle speed so the engine can be too lean when you put it under load. Keeping the primary transfer slots at approx. .020 you will probably need to give the engine another source of idle air because of the camshaft duration. Maybe drill the primary throttle plates .060 (or start smaller) and see how it idles, this will allow a correct adjustment of the emulsion screws while maintaining the proper transfer slot, that's very important.

Have you called the camshaft manufacturer to see if they have a recommendation for initial timing. After all is said and done you should target total wide open throttle centrifugal timing 36-38* @ approx 3000rpm.

[Duke W.]

Re: Question on setting my timing

I guess bubba knows how to make a YouTube video, but he doesn't know squat about setting up a fuel or spark advance map.

Duke

[Bob I.]

[Michael L.]

Re: Question on setting my timing

Here's the cam specs:

Duration at 050 inch Lift:
224 int./224 exh.
Advertised Duration:
280 int./280 exh.
Valve Lift with Factory Rocker Arm Ratio:
0.525 int./0.525 exh.
Lobe Separation (degrees):
110
Computer-Controlled Compatible:
No
Grind Number:
CS 280H-R10

Compression is whatever the factory L-46 came with, which I think is advertised as 11:1. Dizzy and carb are the factory pieces, original to the car. It's a 4spd car.

Here's the dyno sheet:

dyno results.jpg

It ran well on the dyno, but that's a totally different scenario because it's a brief period of warm up at 2,000-2,500 followed by several dyno runs at WOT. It really didn't idle for any period of time. Also, although I ran this carb and dizzy, it was hooked up to a different source of ignition with an MSD set up and the fuel pump was supplemented (IIRC) with an electric pump. Otherwise it was the same as now.

Mike

[Michael L.]

Re: Question on setting my timing

A vacuum gage won't help you set timing, but you need to use one to determine idle vacuum.

Do a Web search:

"tuning vintage corvette engines for maximum performance and fuel economy."

Study it thoroughly particularly the page on emission controlled engines, converting the OE ported vacuum advance to full time, and using the Two-Inch Rule to select a proper VAC.

You have a steep learning curve to climb. Your carb guy will be of no help because he sounds clueless when it comes to setting up a spark advance map for an engine.

The hydraulic roller cam did not change the ignition timing, but it changes what will be the optimum spark advance map.

Due

Thanks Duke. I did look at that presentation but I found it of limited use. It looks like a powerpoint presentation that was given so there was a lot said to fill and color what is on the slides. As you correctly state this is a steep learning curve for someone like me with limited diagnostic and troubleshooting experience. The problem is, as I've mentioned in previous posts, I've haven't been impressed with the local shops in my area and have little faith that they would be able to do a better job than I would ultimately. Nevertheless, I may just have to pick the least bad shop and drop it off there and see what they can do.

[Duke W.]

Re: Question on setting my timing

That cam has the same .050" lifter rise duration as the OE L-46 cam, but the narrower LSA, 110 compared to 114 means a lot more effective overlap. Assuming the distributor is OE and not modified, it needs to be, and if you provide some information I can help get you get it optimized.

First, give me the centrifugal and vacuum advance specs from the CSM or AMA specs from the GM Heritage Web site, and there are two other things I need.

1. Idle vacuum in neutral: report as xx" Hg @ xxx RPM.

2. The data stamped on the VAC bracket. You may need to loosen and move the dist. cap out of the way to see.

Duke