Low RPM miss

[Stanley]

Have a miss and occasional backfire through the carbs under about 2600 RPM. 73 240Z with 3-screw SU's and 6 year old euro distributor. Stock head and cam from C211. Runs strong mid-range and top end. Plugs look ok but I'm no expert at reading them. New needles and nozzles fixed top end miss and improved the idle. Not sure when it started since there other carb problems, too rich, then too lean.

Thinking it's ignition. Feels like it's too lean but richening with the choke lever doesn't help. The chokes aren't sticking.

New advance timing light on order - should be here next week. Will try retarding it a little but don't want to. Top end is good and no ping. Something else to check?

[madkaw]

You need to take some time and learn about reading your plugs. Lots of pics and info on the interweb:)

Does the miss happen under load or just random below 2600?

You say you have other carb issues going on at the same time?

Need more info?

[djwarner]

Since you are in the land of fruits and nuts, do you still have your air pump? Part of the early smog systems was an anti backfire valve that injected fresh air into the intake manifold on deceleration. Seems the overly rich mixture with the throttle closed could be ignited due to the OEM valve timing.

Since you have a non standard cam, this might point you in the right place to look.

[Stanley]

The miss happens only under load, worst at about 2000 rpm. Been driving the AT like a stick to keep the revs over 2600. Plugs are about 5 months old and show light brown/red/yellow color, no wet or dry black, no whitish, gaps all 33-35 thou. No smog stuff, BSPT plugs in all the balance tube holes. No vac leaks. The carbs are balanced with each other, maybe slightly lean but pulling choke lever after it's warmed up doesn't help. Float bowls set to spec with sight glass. The previous carb problems were sticky choke causing rich condition, and too lean condition caused by my tune-up, both fixed. It backfires through the carbs once in a while accelerating at low rpm when it's not fully warmed up, or on the freeway when I take my foot off the gas due to traffic ahead and then step on it again. Had it on the freeway Saturday but it didn't backfire, ran great until back on the surface streets.

It ran ok before with the same head assembly. I'll set it a little richer today and see if it helps. Work is interfering with my "messing with the car" time.

[Stanley]

Tried lowering the nozzles about 1/4 turn - maybe try a real mix adjustment tomorrow. The idle dropped slightly so maybe it's too rich now. Seemed to do a little better but it backfired at about 2000 RPM when floored after a stop sign. Fuel pressure steady at 3.9 psi while driving, drops to 3.8 psi at idle. Seems about right. Very inconclusive.

Thought I'd gotten past the beginner stage up to intermediate with my tuning but apparently not.

[madkaw]

What oil are you running in the SUs?

[Stanley]

Always used 3-in-one oil (20 wt). Put some in 2 weeks ago.

[Captain Obvious]

Very broad brush, but often... Low RPM miss is ignition. High RPM miss is fuel.

Your experience with the choke lever and nozzle level seems to support that broad brush. I'd be looking for something electrical.

[Stanley]

After setting carbs 1/4 turn richer yesterday with no improvement, set them 1/2 turn leaner today and drove to the store. Didn't make a noticeable difference. New timing light won't be here for a few more days so I checked the carbs, rear one was low on oil, and fuel level in the bowl was about 5 mm too low on the front one. I had them set correctly last winter. Don't know if it's part of the problem but it needs to be fixed anyway, so I'll check the valve and set the float tomorrow. Hope the float's ok. Read an article couple months ago in Hot Rod IIRC that ethanol is shredding the floats, even the new ones that are supposed to resist it.

[madkaw]

Low float level is a problem. I also agree with the captain that low rpm heavy loading lends to coil or electronic issue

[bartsscooterservice]

wait for the timing light and check that first

[Captain Obvious]

Would be interesting to use the timing light while the problem is occurring, but since it only occurs under load, that's a little tricky. Would add value to see if the timing light was skipping beats at 2K when under load, but might not be feasible.

If the wires are long enough, maybe run the timing light wires out of the engine compartment and into the pass side window and take it for a drive while a passenger mans the gun? :laugh:

You wouldn't be as concerned with the absolute value, just that it was consistent, steady and not skipping beats.