Ignition timing with Performer Combo

Torque Junkie · October 15, 2001

What exactly should the idle timing be set at with the Edelbrock performer intake/cam/carb package set? If I go after about 12 BTDC, I get a majorly rough idle. When I bought the truck, the thing was waaaaay advanced - like 25 deg. BTDC.

Now, in my carb's owner's manual, it says that use with an agressive cam may require alot of ignition advance at idle.

So I'm confused as to what to set it. I have it at about 12 deg BTDC (there's no numbers etched into my timing dial - just a white mark and lines) now. As some of you know, I put the new 600 CFM carb on last week, if that makes a difference.

Also, I should note that w/ a vacuum guage I show little to no vacuum while idling, but as soon as I move the throttle even the slightest bit WHAM, the vacuum drops! I have the idle speed set right about 750

I'm gonna get this truck running in tip top shape if it kills me

Thanks

76BBSub · October 16, 2001

There are two types of vacuum ports on the carb, ported vacuum and manifold vacuum. Manifold vacuum is straight off the intake tract, so it's high at idle and drops under load, such as when you blip the throttle. Ported vacuum is the opposite, little or no vacuum at idle but increasing when the throttle is opened.

Usually the vacuum advance canister on your distributor should be hooked to manifold vacuum, but I believe that some later distributors (emissions stuff) may be reversed. The best way to find out is to test. Using a vacuum gauge, find a manifold vacuum outlet and a ported vacuum outlet. Disconnect the vacuum advance hose from your distributor and use the timing light to note your base timing with no vacuum advance. Now hook up the distributor canister to ported vacuum, and note the timing. Next, do the same with manifold vacuum and note the timing. Whichever causes the base timing increases (usually about 12-14 degrees or so) that's where you want to hook up the hose.

As far as a base timing setting, remember to check that with the vacuum advance disconnected. Also remember to hook the hose back up before you make any other adjustments (such as idle speed, idle mixture, etc) because the idle speed will increase a few RPMs when the vacuum advance is connected.

If you are running stock heads your truck will most likely want a little more timing than what is shown on the tune-up sticker. Stock heads are a less efficient than newer or most aftermarket heads, so it takes longer for combustion to complete. My suggestion would be to set the timing where the sticker says, I think you said it was 12 degrees BTDC. Hook everything back up, take a little spin. Now try setting it at 15 degrees and take it for a spin. There is definitely such a thing as too much advance, even if your engine doesn't knock, it can hurt your performance. With larger tires you increase the load on the engine over stock, so it probably won't require a whole lot more timing than stock. I wouldn't go more than 18 degrees initial, unless you really feel a seat-of-the-pants difference.

The other thing you might want to try is changing the timing curve in the distributor. You can get a kit from Accel or other ignition companies at most parts stores. It has three sets of springs and some new weights. You probably don't have to change the weights, but you might try some lighter springs and see if bringing the timing in a little earlier helps. Right now I think I have mine coming in a little bit too early, but I can quickly swap springs if need be.

This reply is getting long, but do one more thing before you change the base timing. Disconnect the vacuum advance and rev the engine to about 3000 or 3500 RPM while watching the timing with the timing light. Your advance should increase as you rev it, about 20-30 degrees or so above base timing. If it doesn't increase that may explain why the timing was so far advanced, the previous owner was compensating for lack of mechanical advance. This happened with my Suburban. If you find that you don't have any, or enough, mechanical advance I'll step you through fixing it.

Hope this isn't too long!!

Jeff

Torque Junkie · October 16, 2001

Wow Jeff! Thanks! I'll give those 'preliminary' steps a try tonight after work (if it's warmer than the 40 degrees it is outside now) and let you know.

When I said "I'm gonna get this truck running in tip-top shape if it kills me" i didn't mean for it to be subtitled that it'll kill you too

Thanks again for your help

-Mike

Torque Junkie · October 17, 2001

One thing I forgot to mention, Jeff, is that it idles at around 900 rpms when it's warm warm, but when it's only partially warm, with the choke off, it idles at 500. Soooo..... if I go any lower w/ the idle screw, it stalls when it's lukewarm, but it diesels sometimes for up to 5 seconds, sometimes not at all when it's fully warm where it's set now.

Is this also related to the timing?

Thanks

-Mike

76BBSub · October 17, 2001

I know the dieseling does. What happens is that the timing is too far retarded. This causes the idle speed screw to be turned in farther to compensate for the retarded timing. The effect is that the throttle blades are too far open, so when you shut the engine off it continues to suck air and diesels for a few strokes. The other problem this causes is that at idle you are running in your primary metering circuit, making your idle mixture screws basically ineffective. You definitely need more initial advance. My guess (based on your two posts in this thread) is that your vacuum advance hose is connected to the wrong port on the carb, causing your timing to be very retarded, leading to your dieseling problem and possibly mileage problems as well.

Do you have thermostatically controlled timing? If you have a vacuum tee coming out of your thermostat housing, the intake manifold water passages, or the side of the engine somewhere, then you have thermostatically controlled timing. If these hoses are hooked up incorrectly or the switch is malfunctioning, this could cause these symptoms.

If you get a chance post your base timing with everything hooked up, then with the vacuum advance disconnected, then rev the engine with the vacuum advance disconnected and tell us if the timing advances and approximately how many degrees.

You might also note the base timing when the engine is cold, lukewarm, and fully warm. This sounds like either timing or choke setup.

Let us know what ya find!

Jeff

Torque Junkie · October 18, 2001

I do have that "tee" you mentioned on the thermostat housing. It has 4 vacuum looking ports on the passenger side, and an almost electrical looking connection on the other side. However, there is NO vacuum hoses hooked to any of the 4 ports, nor is there any wires going to or from it. The thing is totally naked.

Does it need wiring hooked up to it? What is it and how does it work?

FYI, the vacuum advance is currently hooked up to the passenger-side small vacuum port on front of the carb. With the light, I see no difference in timing at idle with the vacuum advance hooked or unhookoed.

My girlfriend hates when I work on my truck when she's at my house, so I'm gonna have to wait til Saturday morning to get to do anything to it. She's off both tonight and tomorrow night.

Again, Thanks for all your help. You're a life (and potentially a gas) saver!

Mike

76BBSub · October 18, 2001

The thing on your thermostat housing is a thermostatically controlled switch. When all the hoses are properly hooked up it controls the amount of vacuum advance given to the engine. When the truck is cold it needs a different advance curve from when fully warmed up, although I forget exactly which conditions warrant more or less advance. Regardless, you can live without it, for many years prior to emissions controls engines never had it, including my '76 Sub. The trick is to find out whether you need ported or manifold vacuum to make your vacuum advance work. Refer to my first response, start hooking that vacuum hose to difference ports on the carb until you get some additional advance at idle. You may even have to hook it to a port directly on the manifold, if one is available.

Once you've found that we'll go from there.

Jeff

P.S. I understand about the gf thing, my wife gets tired of me constantly having my head under the hood of one of my "toys".

Jeff

Torque Junkie · October 20, 2001

OK, Here's what I've come up with.

Driverside vacuum port on the carb is the manifold port, while the passenger side is the ported. The vacuum advance was hooked up to the ported side.

At Fully warm idle & ~800 rpms - I plug a vacuum guage into the ported side, and it's little to no vacuum until I blip the throttle, at which point it drops to instantly to ~10". On the manifold side, the vacuum runs anywhere between 10 and 14" at idle, and then jumps up to ~7" breifly before dropping to 16-18" with the blip of the throttle.

With the advance plugged into manifold port, I see NO difference in idle advance compared to the ported port, nor do I see any difference with it disconnected completely. When I rev to 2000-2500 or so, the timing advances gradually in the same way - with no noticable difference in advance between the ported and manifold. Becuase of this, I notice no difference in idle smoothness or performance with it on the ported vs the manifold vacuum port.

I have and have had for a few days had my baseline set at around 12 BTDC. (there's no markings on my dial, just "teeth", and i have it just before the foremost tooth.

Any ideas now?

-Mike

76BBSub · October 22, 2001

There should have been a pretty noticeable amount of advance added when the hose was connected to one of the ports, kinda strange if it didn't.

Here's another one for ya to try. Hook the vacuum advance hose to a vacuum pump (the little brake bleeding hand pumps are available at parts stores pretty cheap). Start the engine, check the timing, pull a vacuum on the canister and see if the timing changes. If it doesn't change, your canister is bad and needs to be replaced. Should be a pretty cheap part, I think you can still get them from the dealer, but any of the auto parts stores should have them too. I have a feeling that your canister is bad. Seeing the timing advance slowly as you rev it up shows that your mechanical advance is working. If you end up replacing the canister you might consider installing a re-curve kit for your mechanical advance. The kit is cheap ($12-$15) and you can get more advance earlier to improve the power curve. Comes with three sets of springs so you can make the curve as steep or flat as you need for your engine.

Jeff

76BBSub · October 22, 2001

You might also consider an adjustable vacuum advance canister, they are available from performance parts houses, or maybe your local parts store, depending on whether they carry speed parts or not.

Jeff

Torque Junkie · October 23, 2001

I honestly think that using the vacuum pump is kinda redundant if I can detirmine that the hose isn't leaking, becuase withing seconds of pulling the guage (which was reading 12" of vacuum) off the port I put the hose to the canister on the port. But, what I will do is detirmine if the hose is leaking. If it is not, then I will replace the canister. Most of the reason for this is I don't have the tool handy and really can't afford to be buying more tools - especially with that wish list so big and unexpected things popping up every day.

I'm curious as to why I should recurve the mechanical timing? This is something I know nothing of and have never messed with. The ignition system is a bear I've never had to deal with until recently, and I'm not to fond of it. You have helped me emensely And the strange thing is, the research I've done on the internet has led me nowhere! You've been my only reliable source of information.

I will look into the adjustable canister when I go to NAPA tomorrow. There's no speed shops up here in the sticks, and the parts stores cary very very limited performance stuff.

Thanks again

-Mike

76BBSub · October 23, 2001

Adjusting the mechanical advance is not a requirement by any means. It's just another little tweak that you can do if you are so inclined, but most people don't bother with it and come out just fine. It's more of a hot-rod tweak every possible 1/2HP out of the engine kind of thing. If you are mostly stock and dial in a little more initial advance you should get good results without messing with the mechanical. I'm one of those sick individuals who enjoys altering the timing curve by 2 degrees just to see what happens...

I agree with the redundancy of testing with the vacuum pump, kind of a belt and suspenders thing I guess. Let us know how the new vacuum advance canister works out.

Torque Junkie · October 23, 2001

Ok, good. I'm glad it's not necessary, becuase I want to keep this as simple as possible. I'm having a hard enough time understanding what to ajust and how much as it is - and it makes it exceptionally difficult when components are bad

Well, I took a break from work and went out and tinkered and come to find out that my timing has been about 10deg ATDC retarted for the last week and I still don't know why. So I set it back to about 20deg BTDC where it was before I started tinkering, becuase it runs much much smoother there.

I plan on replacing the canister tonight after work, beucase I've made plans to tow my friend's car with a dead clutch 50 miles back home, so I need it running better than it has been.

I'll let you know the outcome of the new canister

-Mike

Torque Junkie · October 24, 2001

Well I didn't get back from towing the Honda until 9 pm and by then it was too late to start working on my truck... My girlfriend doesn't work tonight, so I won't be doing it until Tomorrow night or Saturday morning....

I'll still keep you posted

Torque Junkie · October 26, 2001

Whelp, I got the canister changed, and it was quite a bit easier than I thought it was gonna be. It only took me about 20 minutes, which is suprising to me.

I re-timed it, set the base at 12 BTDC. I then held the timing light and popped the vacuum advance hose onto the port and WHAM, an immediate increase of about 10-12 degrees! SWEET! The old canister was definately bad.

Lets see how the mileage is next fillup! Since I re-advanced it to about 20BTDC on Tuesday, I've already seen a noticable improvement in mileage. I'll let you know the results when I fill up next.

Thanks for all your help, Jeff!