Timing - idle rpm?

[sdyck]

1972 240Z - dual round top SU's

My new '72 is running rough, I thought I'd start with checking the timing. I am borrowing my neighbours basic timing ligth tonight.

The FSM manual says teh timing should be set to 17 degrees @ 550 rpm. When warm my car idles around 850-900 rpm, do I adjust simply adjust the idle down on the carbs first? 550 seems really low.

Thanks,

Steve

[siteunseen]

Use the upper middle adjustment on the throttle rod to raise or lower the rpms. When I first started fooling with mine I mistakenly adjusted the ones on each carb. Another thing I didn't know about the 240s were the pulley marks are different than my 280. The bottom one is TDC, then 5 degrees as it goes up. I'm sure your pulley is in better shape than mine but if it's kinda rusty/dirty sand it a little and put some kind of paint in the notches, White Out works good. http://www.classiczcars.com/forums/h...on-issues.html. Post #4 & 5

It's the one by the top plastic marble to the left of the valve cover hose in this picture.

I think the suggested rpm when you balance the carbs is 650, that's with the linkage disconnected.

[zKars]

I beg to differ about idle adjustment. The top center idle adjust is used only during the high-RPM (3K or so) carb balance check after setting idle RPM and balance using the idle screws on the individual carbs and inter-acting linkage.

It does make it tougher to set your idle when that's all you want to do, as adjusting each carb's idle also affects balance. Whatever works for you though.

And I see 750 RPM @ 5 deg as the idle spec in the '72 FSM for USA and Canada spec cars.

[sdyck]

And I see 750 RPM @ 5 deg as the idle spec in the '72 FSM for USA and Canada spec cars.

Thanks, when I look at the manual on line ( I can't cut and paste the section), it shows L24 SU Carb. 17degress/550 RPM. Then it also shows SU Carb (Emission control) 5degrees/750 rpm). I may have made a mistake in assuming that the 1972 was pre emission control. How do I determine what is correct for my car?

Thanks.

[Jarvo2]

You'll need to look at the model # of the distributor that you have. Another tell-tail sign would be all of the emissions gear on the balance tube & carbs, but given the age of these cars, who knows what has happened over the past 40+ years.

[zKars]

The good news here is that it really doesn't matter if your timing is exactly to spec or your idle sf 750 or 785. Tune it so it runs nice for you (starts easy, idles nice, gets the power and drivability you like). There are plenty of interacting parameters that make getting eaxact factory specs quite tricky with 40+ year old parts.

What matters more than anything is that your distributor advance mechanism works well and as expected, which is a another way of saying "to your liking and engine performance design". You'd be hard pressed to tell if your car had 5 or 10 initial advance, but you will loose power if you don't get all the advance you can handle (engine design and specifics of your gasoline determined, but 35deg total-ish) by 3000RPM -ish and that it comes in a nice linear-ish and consistent manner from idle up to that RPM, with load compensation via the vacuum input, which of course requires a car with no vacuum 'irregulaties'. That's up to the dizzy mechanical and vacuum advance and the overall condition of the distributor and engine.


There are many options to get the advance curve you want/desire. Most involve money.

[JR Ohio]

I found that a PO had changed the distributor on mine. I had it out to clean things up so that the weights and vacuum advance pieces could move again. They were pretty gummed up. The dist. is a D612-53. It has 12 degree weights giving it 24 degrees advance (actually was 25 when we put a light on it but only 1 degree). Also since it has a L28 motor and L24 pulley, we had to locate TDC with a dial indicator then put the appropriate mark on the pulley and indicator.

Pulled and blocked off the vacuum advance hose then set the timing at 10-12 degrees at 550 rpm. Give me a total of 35-37 degrees at high rpm.

Re-attached the vacuum and she's done.

Note that all of the above should be done before balancing and adjusting the carbs for best results.

Now with just a little choke she starts on 1st try and can take the choke off almost right away. Screams like a raped ape going down the road.

My $0.02
John

[siteunseen]

I apologize for the misinformation. That middle adjustment isn't even touching the linkage at idle, sorry.
I'm just starting to learn about these carbs, had the car about 3 months now and it runs better every time I crank it. When I first bought it I fiddled with each carb without a syncrometer and got them all fouled up. http://www.classiczcars.com/forums/s...su-tuning.html

[sdyck]

Thanks for the tuning link, one line in the Blue's tuing descrption leads to another questions

"3) Take the car for a run and listen closely for popping in the exhaust (rich), popping in the intake (lean) and note power"

I have a brake booster leak, when I step on the brakes, the engine revs go up a couple of hundred RPM. Doesn't a vacume leak result in a lean condition. I get increased poping, when I'm on the brakes and decelerating, but it sounds like its coming from the exhasut which the above quote suggests is rich.

My exhuast is very loud, is there a way to confirm poping is coming from intake or exhaust. As I type that it sounds like a dumb question, but I guess I'll ask it anyway.

Thanks! Steve

[siteunseen]

I say this from my 280's lean experiences and it may not be true for the carbureted cars BUT sitting still in my garage if I revved it by pulling the throttle rod in the engine bay it would "pop" through the intake when I had a vacuum leak.
It also has a header and 2.5" exhaust and is quite loud too, oxymoron there. When I'm decelerating it has a "pop", so I'd think when you're on the gas, accelerating, and it pops through the exhaust it's rich.

So roll down the windows and stomp it, no brakes.