Been dealing with the excitement (aka frustration) of carburation the past few days. It got hot here in the North Bay Area and I started noticing very rough idling to the point of dying at stops with my '72 240z (L24). I was also getting strange lag every 15 seconds or so, more at some rpms than others and some rough backfiring. I thought it might be a cylinder misfiring or bad plugs at first. Beyond being in contact with Bud from Redline Weber about the sad state of my current 32/36 linkage setup and making the included notes regarding mixture and idle settings.

Then, this afternoon my car was running significantly smoother in all gears through the full rpm range above 1500 ~ the catch?

I had my manual choke slightly engaged. Without the rpm dropping much below 1000 I was experiencing a much smoother ride, no backfiring, and only one or two places where getting on the gas quick lead to almost killing the engine (as if I were flooding it or something?).

Anyway, the down side, other than running with the choke partially engaged was that when I stopped at the bank to deposit a check my engine tried to refuse being shut off, for at least 10 seconds. Dieseling I think I hear it called sometimes?

So, I'm not positive that this is a carburation issue - any ideas? I also thought that maybe the afternoon had cooled off enough from the mid-day heat... although that could just be my mind creating the connection there.

Thanks!

By the way:

Here are my findings from today's tooling around~

Mixture Screw settings:

Front (left) Carb & Rear (right - closest to driver's seat)

seated - no run, fires up then dies

1/2 back - doesn't run

1 full turn - no run

1 1/2 turn - fires up then dies after a few seconds

2 turns out - runs, rough @ 750 rpm, fluctuating between almost dying and 900 rpm - sometimes completely dies after a few minutes of fluctuation

2 1/4 turns - runs, still rough

2 1/2 turns - runs very rough at about 500 rpm

2 3/4 turns - barely runs, very very rough

Idle Screw

Tightened (counter-clockwise) idle 1/4 turn and went from 650 +/- rpm to abut 750

Tightened again another 1/8th and now idles at 850 - still fluctuation from 900 rpms

DGV Webers? Well there's your problem!

I don't mess with DGVs but I have 2 sets of round-top SUs and will be taking them to the Datsun swap meet in Manteca on Saturday.

There's definitely not much love for the DGV Webers.

If mine looked and were sync'd the way they're supposed to I really wouldn't mind them.

I'd seriously LOVE to make that meet but I have work all day. I have a couple buddies who are headed out there in a slammed 510. How much are you selling SUs for, and what kind of shape are they in?

Based on what you said, your car is running too lean, at least at idle. Since the idle mixture screws are not having any affect the idle circuits are likely blocked with dirt or varnish. You should take the carbs off the car and clean/rebuild them. Be particularly attentive to the idle circuit jets and passages to makesure they are clean and open.

If mine looked and were sync'd the way they're supposed to I really wouldn't mind them.

I'd seriously LOVE to make that meet but I have work all day. I have a couple buddies who are headed out there in a slammed 510. How much are you selling SUs for, and what kind of shape are they in?

$150 a set (carbs, manifold, linkage) for 4-screw round tops. Don't know the condition of the throttle shafts though, hence I sell them for ZT's core price. I've got heat shields and air cleaner housings for a little extra.

At this point, I don't see myself owning a Z without triples, so I have no need for the SUs.

Sounds reasonable. Did you sell them at the meet or do you still have them?

I've got one set left, but no more heat shields. I thought I'd sold all my air-boxes but I just found one in my parts storage a few days ago.

Actually, when looking at my last set I remembered that I did run them for a short while before converting to triples.

I have a set of lightly used (and working!) DGV's available if you want some spare parts. No one seems to want them.... Wonder why.... PM me.