Low RPM miss

[madkaw]

Do an Ohm check on your wires and coil for now

[FastWoman]

A weak ignition will sometimes reveal itself under load, so refreshing the ignition components wouldn't be a bad thing.

That still wouldn't explain the backfire. I think that would only happen with a very lean condition. Perhaps the pistons (correct term?) in your carbs are hanging or aren't responding quickly enough?

[Stanley]

Took 2 hours to adjust the front float, even with the sightglass. Make a tiny adjustment and it doesn't do anything, make another tiny adjustment and it floods out the vent. Finally about sundown had it slightly high. Would rather it be slightly low than high for safety, so I decided to do one more - if it was too low hell with it - but it came up perfect. Too late to set a mix.

Took a drive, still sputtering at low RPM. Midrange and top end seem stronger than ever. Watching the tach it sputters up to about 2100, then it's like you flipped a switch and it instantly zips up to 2300 and starts winding nicely.

[Stanley]

Well, everyone that said it was ignition was right on the money. Was going to set the mix today but had a load in the laundry so decided to do a 20 minute (I thought) job while it was drying. Had a new Nissan condenser for the distributor so started to install it. What a nightmare getting to old one off - the upper radiator hose and the thermostat housing are in the way, even the little stubby screwdriver was no good. Got the 90 degree screwdriver but it didn't work either, finally got it off with a screwdriver bit in the needle nose vicegrips.

Same thing with the upper screw where the wire connects.

Cleaned off the terminals, got the condenser in place, but couldn't tighten the upper screw. The square nut inside had fallen into the guts of the distributor. Next time I'll take off the cap and hold the square nut. Had to remove the vac advance and the plate under the points, was intimidated by the task but it was easy. Glad I didn't take the points out first. Found the nut, put everything back together, and installed the new condenser.

Noticed the rotor end wasn't shiny and there was some corrosion on the contacts in the cap, brushed them with a small wire brush. The point, cap and rotor are only 6 months old, the condenser was probably 40 year old. The points looked a little played so I check the dwell next time.

Took a drive, no more sputtering. So it was either the condenser, corrosion at the condenser's connection, and/or the corrosion on the cap and rotor. Wonder why it didn't affect mid-range and top end.

[madkaw]

Not sure if I have an exact answer for that-but in MOST cases that I have seen where there is an ignition break down, it seems to be most prevelant under low rpm and heavy loads. Buick V6s would buck like the trans was going out under load , but it was weak ignition coils breaking down.

Glad you found it.

[Captain Obvious]

Well, everyone that said it was ignition was right on the money.

Excellent! From what you changed, I'm not sure I can pinpoint a smoking gun, but if it stays fixed, the point is moot.

Loose or dirty connection somewhere on the low voltage side of the coil? I doubt it was the dirt on the rotor or the posts inside the distributor cap. I don't like the sound of a 40 year old condenser, but I can't come up with a failure mode that wouldn't have had an impact on high RPM's as well. All they can do is leak, go open, or short.

Short wouldn't run at all. Open wouldn't have any affect until your points burned up, and leaky would weaken spark at all RPM's not just low. Maybe the rich mixture at low RPMs combine with a weak spark from a leaky condenser? Just looking for a solution that fits the findings...

[FastWoman]

One more thing to check, even though it's not backfiring anymore: The breaker plates on these distributors are notorious for sticking and freezing. (I understand that MOST old OEM distributors in running Z's stick and freeze, including my own when I bought the car.) Take your distributor cap off, and try rotating the breaker assembly by hand. It should turn freely, under spring tension, with no binding. If it sticks, your timing could be almost anything at any given time. Your having taken the plate apart and reassembling it could have been what did the trick -- somewhat. It's a fragile and poorly designed mechanism.

Also check that your vacuum advance works and that the vacuum tubing to the advance is intact and leak-free. While this wouldn't be "the problem," it's certainly a common problem (in most Z's that haven't been gone over).

Edited November 10, 2013 by FastWoman

[Stanley]

Took drove about 25 miles yesterday on the freeway and surface streets, no sputtering or backfires so I guess it's really fixed. Still not quite right though, pretty sure it's the mix/timing. Someone that doesn't drive like I do wouldn't know there was anything wrong with the car.

Yes, the carbs are probably off, I haven't set mix since putting new needles and nozzles a couple weeks ago. Done so many 30 second adjustments I don't remember how many turns down they are. Also, the timing isn't advanced enough. The Innovate came today, didn't have time to do much but if I pushed the right buttons it's 10 deg advance at 1300 rpm, supposed to be 17 deg at 700 RPM (euro dist with automatic trans), so it's way off. Doubt if it will run at the stock setting with CA gas, but will get it as close as possible with no pinging on the hill.

The Innovate light seems ok, BTW. Strobe is rock steady - with the other cheapie light the marks on the damper jumped around. Should be better, it cost 4 times as much. Will make it much easier to document settings, and record and graph the advance curve. Nice to check dwell, rpm, battery and alternator with just one gadget.

The distributor is ok, it was new 6 years ago, only 6000 miles on it. I didn't dissemble the plate, just removed it with the 2 screws and put it back like it was. Everything looked new inside.

[Stanley]

Just to close out this thread, should say that putting a new condenser and cleaning the contacts at condenser, cap, and rotor only improved it about 75%. When The Innova light got here I checked the dwell with it first. It was 44 degrees, supposed to be 35 to 41. Started to adjust them but they looked shot (after only 6 months, due to a bad condenser?) so I put in some new Nissan points and finally got the dwell to 36 deg. Then I cleaned off the balancer and checked the timing, it was 10 deg at 1000 rpm. (supposed to be 17 deg at 700 for the D-606-52 but use CA gas). Set it to 15 deg at 1000.

It's running the best it ever has and I still haven't done a full carb adjustment since putting the new needles and nozzles. No misses or backfires in any conditions. Previously I had to shift at 4800 because it wasn't pulling after that. Now it's pulling hard to 5000 and maybe higher.

So timing also had at least something to do with it. Always figured condensers didn't really do anything, now I know better.

[madkaw]

Put a Pertronix Unit in there an ditch the points- you won't regret it

[Stanley]

Have a Pertronix in the old Chevy truck. 15+ years and zero maintenance, always starts right up. Guess it's paid for itself in points, caps and rotors.

[Zed Head]

Always figured condensers didn't really do anything, now I know better.

My understanding is that the condenser was originally added to damp excessive arcing across the points to make them last longer. Then some electrical wizard discovered that they also made for a stronger spark through some sort of electrical oscillations,allowing the coil to charge faster. Read up on Kettering ignition systems, it's pretty interesting stuff. So when your condenser went bad, you got a weak spark that also wore your points out faster. Probably why a set of points usually comes with a condenser (capacitor) included.