Timing Oddity

[Steve Simmons]

Any thoughts on what would make a particular car want a different advance curve over another nearly identical one? Lets just pretend it's early MGB. One "pings" at mid RPMS under light load (clears under heavy load) and another doesn't? Lets also pretend the same distributor is used in both cars so that isn't an issue. Intake issue? Thoughts?

[Jimmy]

I think the key word is "nearly" identical. No two engines really are.
Then there's the crap load of variables in all the external parts, including, but not limited to, the friction in the advance weights, spring tension, etc. (I'm pretending it's two different distributors).
Heck, it could be the fuel.
Or that the one that doesn't ping has to transport a heavy roof, so it's always under load.

[Steve Simmons]

Same distributor. Works fine with the vacuum advance disconnected, but I have to greatly retard the timing to prevent pinging between 2500-3000 RPMs under light load. I'm down to about 8-degrees@1000 which probably gives around 21@3000. I wonder if it's leaning out. Maybe different needles are in order. Hmmm...

[malcolmr18zoy]

Hi Steve,
I think that what you are hearing is not pinging. If it were, it wouldn't go away under load. It's probably piston slap.
Malcolm

[Jimmy]

Meanwhile, I think that if you just disconnected the vacuum advance, life would be good.
It's not as if you NEED to have it, and it obviously forces you to retard the timing a lot.
Not that this answered your original question...

Then, would somebody please slap Malcolm with a piston?
Obviously the pinging goes away under load since the vacuum advance backs off then.

[Steve Simmons]

You could be right Malcolm, and I've feared worn bores, rings, etc for a while because of burning oil and some smoke from the exhaust upon acceleration after freeway exit. But it didn't make sense to me that retarding the timing would eliminate the noise if it were piston slap. I will admit that it doesn't sound like "normal" pinging / pinking, if there is such a thing. The engine actually runs quite well without the advance connected at all, but does run stronger with it attached and timing reset.

I haven't run a compression check in a while but I know the numbers are a bit on the low side, around 140-150 IIRC. Vacuum is low at idle but that's probably from the timing being retarded so far. I haven't really played with that. The engine is so strong and nice that I don't want to tear into it. But my god that noise is annoying!

Jimmy, the funny thing is that it only "pinks" in a small RPM range (yeah I know, sounds like piston slap) under certain throttle. Neutral to very light throttle, no problem. Light throttle, noise. Anything beyond, fine. Obviously that makes it a timing thing but it's just SO sensitive to it.

[malcolmr18zoy]

No Jimmy, the piston slap goes away under load because the vac advance turns off. If it were pinging, it would increase under load irrespective of the small amount of vacuum advance that is required for fuel economy.
Malcolm

[malcolmr18zoy]

You should probably do a leakdown test on it. Oil burning can cause pinging, but the symptoms you describe fit piston slap perfectly. It probably runs strong because it's so loose. you should hear my Jag, it has lots of piston slap, but runs so well.
Malcolm

[Steve Simmons]

Yeah, I've been meaning to do that for over a year. Then again I've been meaning to tune-up the car for two years and finally just did it yesterday. I guess that means I'll get around to the leak-down test next year.

Realistically I probably don't need to perform the test because I already know what the outcome will be! Ultimately I'll probably just put the timing back advanced and ignore it if I can. If not, I'll just run with the advance line disconnected. Or maybe I can find a different vacuum unit that has less mid range advance.

[malcolmr18zoy]

You should be able to disconnect the vac advance without affecting the performance. The vac advance only comes into play on a light throttle to improve fuel economy in those circumstances. That would also give you your piston rattle on a light throttle.
Malcolm

[Steve Simmons]

It does seem to run a little stronger in the low end with the vac advance connected but it's hard to say since I'm comparing two different timing settings against one another with only my butt dyno to tell what's best. Not much scientific about it. Then again, I don't care about numbers anyway. Thanks for the insight, I'll probably just forget about this for another year.

[Jimmy]

So what's with this sudden rash of engines behaving badly at light throttle, anyway?
Could it be that Mark's, Steve's and my problems are one and the same, except that Steve's motor is sloppier than the others? Then again, Steve didn't complain about surging or hesitation.

[Steve Simmons]

Mine has done this for the past year. I've just ignored it by plugging the vac advance.

[JerryB]

maybe I read this wrong but the same dist in 2 different cars ?
And one car works fine but the other rattles ?

Check the timing mark on the crank dampener. It should show "0" with the #1 piston at TDC...this is a check to see if possibly the outer ring of the dampener has spun or moved in position, or the timing marker is not in the correct place .

Plan "B" is to also check the vacuum line to the vac advance cannister. Most early cars w/ vac advance have a single diafram can and the line goes to PORTED spark and not manifold vacuum. Make sure your using the same (or right) place to apply vacuum to the can.

If all seems well it might be that the rattling motor needs attention.

[Jimmy]

Mine started more recently, and the dumb dual-point distributor doesn't have a vacuum advance.
Oddly enough, Malcolm suggested that my problem could be too much timing advance, but my engine doesn't ping, as far as I can tell. Actually, I've considered advancing it some more since I never kept tinkering once it ran well enough.
First I'll put the timing light on it, though, to have a "baseline" of sorts, since I don't trust factory timing marks one bit and simply tweak the distributor until the car runs well.

[JerryB]

yes check the factory marks by rotating by hand clockwise and sticking a wire or rod down the plug hole. You will get close but maybe off 1-2 degrees because of the piston dwell at t.d.c. ....buyt close enuff for what U r doing.

Measure the circumfrence of the dampener and divede by 360 and multiply by 32 and make that mark on the dampener on the before TDC side of "0" and use a timing lite at 3500 rpm and set the dist at the 32 mark. Thats your total. The initial will then show depending on the advance that is built in to the dist.....prolly 4-5-6-7 *

[Steve Simmons]

Interesting thought about where the vacuum is pulled from, but to the best of my knowledge it's all correct. The port is on the top of the manifold behind carb #2.

Like Jimmy I don't trust numbers either, so I usually tune my cars by feel and sound. I can tell that the timing marks are at least reasonably accurate. Setting them with a timing light and then by feel results in nearly an identical result. I do know that they were accurate when the engine was rebuilt, so if they're off now then they would have to have slipped.

[Jimmy]

Interesting thought about where the vacuum is pulled from, but to the best of my knowledge it's all correct. The port is on the top of the manifold behind carb #2.

That would mean regular, unported vacuum.
If I understand it correctly, using ported vacuum would keep the vacuum advance off at idle, which makes sense, and then allow it to work once the throttle is opened.
But either way, it still shouldn't matter in your case.
Time to start chasing vacuum leaks, guys. Good thing I have lots of carb cleaner.

[max71]

I have that lovely Snap On leak down tester I offered to lend you.

[Steve Simmons]

I have a snap-on Compression set also. I just have to reconfigure it to do leakdowns. I've forgotten how I did it before, and probably no longer have the parts, but if I did it once then I can do it again! I appreciate the offer though.

I don't really need to do a leakdown at this point anyway. I'm sure the rings are leaky and have been for a while.

[Jimmy]

What you might need is carb cleaner so that you can start spraying around the carbs' bases and such to chase down any vacuum leaks. Listening through a hose of some sort works, too, but it's easier to first find the general area with spraying. Gets stuff clean under the hood in the process as a bonus.

Oh, and while under the hood, connect your line to ported vacuum.

[Steve Simmons]

I have no leaks in the intake. I tested that a year ago when the problem surfaced. Good thought though.

Why would I want ported vacuum?

[Jimmy]

Because otherwise you'll have full vacuum advance just sitting there at idle.
Ported, on the other hand, only activates the advance when you're actually driving, under light throttle.

[Steve Simmons]

Color me red, I misspoke. My vac advanced IS ported. It's the vacuum gauge that is on the manifold. Sorry, brain fade. That's what happens when I post on car forums while I'm "working".