Choke, Mix, Idle.....

[ddezso]

I am the proud new owner of a 1970 240Z HLS3003137 but am not the most mechanical person yet (but working on it).

The car does not run too good yet and I think one issue is the carb tuning. I have 2 questions:

1) I changed spark plugs and the old ones were covered in black soot. When I start the car soot shoots out the tailpipe and you can barely detect that the exhaust is a bit black. Is this simply a too-rich fuel/air mixture or are there other possible issues?

2) I plan to get an airflow meter and tune the carbs. Before I do that I need to set the proper idle speed. My question is on the default position of the choke because that seems to be influencing my idle speed and mix. Currently, the choke is pressed all the way forward. If I pull it back at all, the idle speed plummets and the car dies. Should the choke be adjusted so it is in the middle of it's range, then the idle speed increased to compensate? I always thought the choke was there to increase idle speed at warm up, then be pulled back to normal speed to drive - but mine is all the way forward so it's of no value to me.

Thanks - I've got a lot to learn!

D

[MikeW]

I always thought the choke was there to increase idle speed at warm up, then be pulled back to normal speed to drive - but mine is all the way forward so it's of no value to me.

The choke should only be pulled back to start the car and drive while it's still cold and then pushed all the way forward once the car has warmed up.

[Arne]

See my reply to your other thread in the Engines forum.

Probably best not to start the same thread in more than one place.

[240ZX]

Derek,

If you intend to tune your Z, also include doing the other basic tune-up items....setting valve clearance, setting timing, etc. Also, new plugs, plug wires, condenser, rotor, rotor cap, and maybe even the coil. And as you have planned, breath on the carbs. Consider the condition of your fuel, especially if the car has been sitting a long while. The list goes on but, if your going to do it, do it right as each system effects the other for overall performance. Good luck and congrates on the early Z find....3137 is a pretty low number!

[26th-Z]

In all my experience in all my years of fooling with SUs, the engine electronics is 75% of the problem. The rest of it is timing and carburetor adjustment. Make sure your electrics are in top performance meaning brand new. The service life is far gone for original equipment. Make note that original spark plug wires were dated. If you a set of dated wires, keep them. If the old black distributor cap has copper contacts, keep it. My suggestion would be to replace all the electrics, set the timing and valve clearance before I looked at the carbs. Now, they may have some varnish build-up and I'll go along with the suggestion of getting a book or video before you take them apart and clean them.

I'll bet they look better than this!

[Arne]

All the way forward is supposed to be the 'off' position. Pull back to activate the choke.

Yes, it sounds like your car is running way rich. Tuning your SUs is more involved than just using the gauge and synchronizing them. There are mixture settings as well. If you are not familiar with the workings of your SUs, get the video from ZTherapy and/or the Power Tune your SUs book before you start.

[ddezso]

Thanks all and sorry for the duplicate post. I'm new here, posted in the engine section, then scrolled down and saw the carb section. My question was more logical there.

[Bruce Palmer]

The black sooty stuff is rich. Try that again after making sure the chokes are off and not stuck part way on. A build up of varnish can cause them to stick thus making all the other settings off. The JUST SUs video and DVD is a real handy tool for understanding the whats and wherefores of these really simple carbs.

You may want to check out the tune up video for the other engine related issues.

[Arne]

A final thought - many years of dealing with British cars with SUs have led me to the conclusion that people mess with their SUs far more often than they should. Once you get them set, leave them alone. Typically, the SUs are the first thing people start messing with if the car starts running badly. But in the vast majority of cases, the carbs are fine and something else is the problem. Just because they have SUs, and people who know nothing about them have given SUs a bad reputation, people always blame everything on the carbs, and generally just make things worse.

[Bruce Palmer]

Well said, When you stop by the shop I'll buy ya a Grape NEHI.......

Wonder what happens if I crank on this little gizzy?

Don't lose your head to save a minute.

You need your head, your brains are in it.

Burma Shave 1947