Ok, this is not something that I've played with for quite some time. I am trying to get my timing straightened out. I've got a 351 with a holley on it and my mechanical advance seems to be all I need. I'm not sure where to go from here. I have 6 degrees initial advance. As I take it up the rpm scale, it seems that I gain 2 degrees every 100 rpms, until I reach 3800 where I have a total mech advance of 32 degrees. If I plug in the vacuum advance, I have 18 deegrees at 1100 rpms.....Where did that come frrom? Am I plugging in to the wrong port? I put it in the top port on the front of the carb. My issue is that under light throttle , I get a lot of pinging. I can see why, but How do I fix it. Should I use the lower port, which should be manifold vacuum

 

Well, first of all, no, you should probably not use the manifold vacuum port for the vacuum advance. Exceptions can be made, but we'll leave that alone for now.

The VA should be giving full advance at 1100rpm under no load when plugged into a ported vacuum source, so it sounds like it's working. You're not idling at 1100, are you?

Your initial advance of 6* is on the low side. I'd recommend 8-10* initial @ idle speed (which should be 650-800, if possible).

Your total advance is right, but it's coming in way late. Ideally you want about 32* @ 2800-3000rpm. It's possible your advance weights are sticking and need to be cleaned. Either that, or the springs are simply too stiff. Pick up an advance recurve kit from your local chain parts store and try some lighter springs.

As far as detonation...have you done anything to the motor? What's the compression ratio? What kind/size holley do you have on it? Have you tuned it for the motor? What do the plugs look like? What octane gas are you running?

If anything, the advance is on the low side and shouldn't ping at all, but a lean mixture, high compression, bad gas, etc could cause detonation. It's also possible that the outer ring on your balancer has slipped and is giving you an inaccurate reading.

 

Thanks star for the response. That's my hole thing. I don't under stand why I get the ping. This is a fairly new motor. I built it last year (Jan) and it is mostly stock but has a bit bigger cam. (not sure exactly as the first cam tore up my dist )it was like a 274? and I went down a step to my origonal lunati...if that makes any sense. I'm working with a 69 with flat tops so compressionn is like....11/1? Burning 91 octane as that's the best we get here.

Ii started with 10 degrees but it pinged like hell. I kept backing off on it til it was drivable and that got me to the 6 that I am at today. If I hook it up to man vac, it seems to have better throttle response.

Am I just running too lean? The carb is a holley....I think 600.

Oh...and the dampner is new..with the motor.

Ii almost forgot..My idle right nnow is 650, advance starts cocming i9in at 1500.


Don't know what's up with my keyboard either..

 

I agree with Tad, although the more radical the cam the higher your initial timing and idle should be. It does concern me that you don't know the compression ratio. THis is critical in determining the octane rating you should be running. Since it is mostly stock, I assume that means iron heads? If the CR is truly 11:1, with an iron head you need high octane fuel, in fact race gas betwen 98 and 101 octane is probably the best. You can't compute the CR without disassembling the engine. You also can't trust that your timing marker is necessarily in the right position.

If it was me, I would put race gas in the car and try to get the timing up to about 12 initial and shoot for 34 in by 3000 rpm with the vacuum advance disconnected and plugged. This may mean with a bigger cam that your idle ends up between 800 and 900. If it runs well with race gas and you want to get back to pump gas, you will need to lower the compression ratio. That means some radical changes to the engine, with the easiest, but expensive solution being to upgrade to aluminum heads.

 

O.K....let me start over. I rebuilt my motor last Dec,Jan...I used a 69 block, 69 heads, had everything ground, balanced and used all new parts. Everything was balanced, new dampner, flywheel ground....had the heads (69) redone, the whole nine yards but to "stock" spec's....then added a little cam. I believe that the comp ratio for a stock 351 was 11/1...This is my daily driver and I don't want to (or believe that I should have to) run racing gas in it. I just want to get rid of the ping.

If I baby it around, I can drive it O.K. It is only once it gets up to operating temp and I start to put my foot in it that it pings. It really sux to know that I can drive around all day without an issuee but, can't put my foot in it.

Could it be that I'm just running lean....and maybe need to rejet the carb....or is it somehow a timing issue.

 

CR will be 10.7:1 for a 4V 351W and 9.5:1 for a 2V. Do you know if you used 2V or 4V Heads?

 

The heads are the same either way, but it is and always was a 4v.

 

Compression ratio in your case would depend on the pistons. Do you remember which specific pistons you put in the engine? If so, we may be able to figure out the CR. If you're using flat tops (w/ or w/o valve reliefs?), it's likely you're pretty close to that stock 10.7:1 ratio, which means you very well might need to be running race gas (or octane booster).

You also need to check your plugs to see how the engine's running. White or grey deposits indicate a lean mixture, dark brown or black (dry) deposits indicate a rich mixture, wet black indicates burning oil, and light tan deposits indicate a good mixture.

 

Well, that's not what I wanted to hear. It is what I was thinking though. I am running flat top w/relief, hyperu's I'll pull the plugs a little later and take a look at them . Maybe even pick up some boost and see what that does.

This could get expensive since I drive 50 miles a day. I might have to turn this into a weekend car and find something else to drive.

 

Hypereutectics? Drop the timing, now. Hypereutectic pistons are nice (i have them in my 331), but they're very brittle and can shatter even with light detonation.

Drive the car to the parts store, pick up the octane boost, then advance the timing and see if it helps any.