Shavarsh
Shavarsh Reader
6/26/22 6:52 p.m.

Hello All, 

I'm looking for some ideas on what would cause timing advance to drop during throttle tip in. To clarify right off the bat, this happens with the vacuum can unhooked. The setup is Summit brand 500cfm carb, Pertronix ignitor 3 stock appearance distributor, Pertronix flame thrower 3 coil on a mild heads/cam/intake 302. Voltage is above 14v at the coil (although my meter is a bit slow so if there is instability at tip in I probably can't measure it). The reason I am looking into this at all is a rich bog at tip in. After removing the accelerator pump cam I still had a rich condition on light acceleration for a moment before the mixture would lean out. I am tuning with a timing light, and wideband O2 gauge. While setting mechanical advance I noticed that the timing drops for a moment during tip in, then steadys out at the intended advance. 

 

Any thoughts of what to test next from the hive? Am I simply expecting too much from an old school setup?

NOHOME
NOHOME MegaDork
6/26/22 7:03 p.m.

What do you define as a rich bog? Under 12/1?

I chased what I thought was a fuel bog for a long time and it turned out to be the reluctor wheel hitting some of the magnetic pickups  in the HEI style distributor when the distributor shaft was accelerated. It was a very light touch but enough to kill some spark for a small amount of time before the shaft re-centered. Felt like a carb bog to me.

ShawnG
ShawnG MegaDork
6/26/22 7:34 p.m.

I've had nothing but troubles from Pertronix. They're junk these days.

That said, perhaps you're idling right at the rpm the weights begin to swing out. Then when you open the throttle and have just a little hesitation, the weights swing back in and your timing retards and makes the whole thing worse.

Shavarsh
Shavarsh Reader
6/26/22 10:19 p.m.

Thanks for your responses, the gauge shows as low as 9:1 and usually clears up to around 12:1 where it will start pulling. I checked the clearance to the magnetic pickup and there is a bit of play there but I don't think enough play to cause contact. I'll try removing a spring to see if that changes the behavior at tip in.

Paul_VR6 (Forum Supporter)
Paul_VR6 (Forum Supporter) SuperDork
6/27/22 7:38 a.m.

I would try and lock the weights and set to a static ~20 ish and see how much it changes. I have seen timing change afr but not as much as 3 points. 

NOHOME
NOHOME MegaDork
6/27/22 8:02 a.m.

That is a big leap in fuel ratio. 

You know there is an observed timing change, so I am more inclined to pursue the distributor guessing that you are also losing spark to some or all cylinders. 

Do you have a timing light? You can test on each plug lead as you replicate the timing change and observe if the strobe effect changes by dropping out or missing a beat. Next best thing to having an 8 channel oscilloscope. Start with the coil wire.

SkinnyG (Forum Supporter)
SkinnyG (Forum Supporter) PowerDork
6/27/22 10:36 a.m.

Do you have vacuum advance tapped into manifold, or ported?

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
6/27/22 10:49 a.m.

He said it was not attached to any vacuum.

Aftermarket dizzys are hit or miss.  I had a brand new Mallory HEI that dropped 4 degrees at 4000 rpm (tested on a Sun machine) because the shaft was so weak it twisted under the friction of the bushing.  I have also had situations where the distributor's oil pump drive pin hit the bottom of the slot before the distributor seated on the manifold and allowed excessive play in the drive gear.   Pertronix may have also used a crap gear on the distributor allowing excessive play.

Take the cap off and try to spin the advance plate.  If you get anything more than "I think it might have moved," it's no good.  It should be planted with no perceivable play.

How good is the timing chain?  Play in the timing chain would mean that under added cylinder pressure (throttle tip in) the cam could retard a bit against the pressure on the valves.

Edit:  you should be able to check the timing chain by pulling the fuel pump.  Stick a telescoping magnet in on the chain and push/pull on the chain.  Do this at a bunch of positions of the crank and try it after spinning the crank both ways.  There should be no real discernable difference in chain play at any position forward/backward of the crank.

APEowner
APEowner GRM+ Memberand UltraDork
6/27/22 12:35 p.m.

When I see ignition issues with a Pertronix system my first thought is that the pickup is defective up but in this case you've got enough other stuff going on that I'd want to to do a little more digging before condemn it.

When you say throttle tip in are you talking about no load from idle?  Same question on the A/F ratios, is that idle vs part throttle steady state no load?

Is this a fresh build or something that was running well and is now acting up?

 

Shavarsh
Shavarsh Reader
6/28/22 2:51 p.m.

Thanks all for the responses, I'll need to do a bit more testing before being able to intelligently reply to all of them. I think its very possible I am expecting too much from the timing setup. The comments on timing chain play and gear lash at the distributor are a good reminder that this is still a system with clearances that must be taken up, slowing reaction time. I suspect the timing is dropping on tip in from observing the direction the timing mark moves with a blip of the throttle while timing with a light in the driveway. I'll need to do this test again with the weights locked out to be sure. There is no doubt I still have too much accelerator pump shot and possibly too much jet in one of the other circuits. I think I'll decide on a timing curve (and stop changing it), then tune the carb to suit so I am not chasing so many variables. I have had the car tuned up quite nicely a couple of times but have a hard time leaving it alone and not chasing the last 5% of optimization. 

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
6/28/22 4:04 p.m.

With vacuum disconnected you should never have a retarding situation on tip-in.  Proper, functioning timing chains, cam timing, and dizzy gears make for very accurate timing.  Not ultimately optimized timing like an ECM and COP would be, but very durable and workable.

In fact, it's pretty much all I do.  I'm dragging my feet on my latest build because it's EFI and learning curves are tough.

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
6/28/22 4:18 p.m.

Really the only "lash" you should have is at the timing chain, and with the engine spinning forward against the drag on the cam that lash is mostly non-existent... provided the chain isn't worn.  Total lash in a properly operating system is likely less than 1/4 of a degree.  OEMs still used cam-driven distributors for decades even after the tech was available to go fully computer controlled.  There isn't anything wrong with them.

Jerry From LA
Jerry From LA SuperDork
6/28/22 4:27 p.m.

Anybody else thinking 14v at the coil is too high? Is that before or after the ballast resistor? Is there a ballast resistor?

APEowner
APEowner GRM+ Memberand UltraDork
6/28/22 5:37 p.m.
Jerry From LA said:

Anybody else thinking 14v at the coil is too high? Is that before or after the ballast resistor? Is there a ballast resistor?

That's slightly high but within reason.  If the voltage doesn't go down after the car is run for a while then there's likely a  charging system issue but it shouldn't effect the ignition system.  That coil is intended to be used without a ballast resistor.

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
6/28/22 8:28 p.m.

In reply to APEowner :

Is the Pertronix intended to be used without a ballast resistor?  The coil is one thing, the electronics are another.

APEowner
APEowner GRM+ Memberand UltraDork
6/28/22 10:23 p.m.
Pete. (l33t FS) said:

In reply to APEowner :

Is the Pertronix intended to be used without a ballast resistor?  The coil is one thing, the electronics are another.

The Pertronix ignitor 3 can be used with or without a ballast resistor but Pertronix recommends without for best performance.

Shavarsh
Shavarsh Reader
6/29/22 7:05 p.m.

The pertronix behaves quite badly with the ballast resistor in place. I had an issue during the initial driving phase of the build where a geyser of fuel would shoot up out of the carb when trying to start the engine. This was only fixed after installing a relay triggered by the ballast wire from the ignition to clean battery voltage.

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