Low idle after hard stop ???? 240Z

[inline6]

Hi,

I have an annoying problem. Whenever I come to a really hard stop, the engine idle drops to about 400 rpm for 4 or 5 seconds and then bounces up to normal idling speed (650-700). While it is doing this it idles like the mixture is rich.

I have the stock 71 SU's, floats are perfectly adjusted to the high side of spec (14mm clearence instead of 15). Brake booster seems to be functioning properly. Anybody have any ideas?

I have had the problem for a couple of years, but can't relate it back to anything I've changed. Seems like it has to be fuel related. I unhooked the fuel chamber vent lines to air filter, but no change.

Is it just the fuel sloshing to the front of the fuel chamber, allowing the float to drop and bring in a little too much fuel temporarily until the car has stopped? If this is the case, I can see that the fuel level rising too far would cause an overflow situation at the nozzle, causing an extreme rich condition until the motor sucks up the excess fuel. The longer I am at threashold braking, the longer it seems the car takes to return to normal idle after stopping.

But if this is it, then I would expect others to have experienced it. Anybody????

[The Sandgroper]

Hi

I have the same problem too, I have a 70 zed which has just had a complete resto job including carb rebuild using a Z thereapy kit, carbs were completely rebuilt by a specialist, not only do I get the problem when I brake hard but I can't run the car on choke for more than a few seconds when I first start her up otherwise she coughs and splutters because of carbon deposits on the plugs, the carb guy insists that everything is set up right so I am at a loss, I have spent sooooo much money trying to get this sorted but have now resigned myself to the morning routine of starting the car with enough choke to get her going, then shutting the choke down completely and keeping her running using gentle pressure on the accelerator until she warms up a bit.

If anyone has any suggestions to fixing either of these problems it would be much appreciated.

Lee

[Gav240z]

Ok i think it happens under hard braking because of teh vacumm assist. My car does the same I have not cured it but it doesn't seem to be a huge problem.

The Sandgroper your mechanic or carb guy might say it's set fine but doesn't mean it is. When setting up your chokes make sure you attach the cables when you have fully turned the choke levers so they are <b>completely</b> off then attach the cables the normal way.

Also the choke should be pulled to full when starting and once started it should be set to around 1200rpm for a little you may drive straight away just adjust the choke as you go down the road. You'll find the engine revs will seem higher each time you stop as the engine warms keep the choke at 1000rpm after about 2mins driving. It should not foul your plugs if your plugs are fouling still check and make sure it's not valve stem seal's leaking and causing oil build up .

Hope that helps!

[The Sandgroper]

Thanks Gav

I will look at the area's suggested, I'll be spewing if it's leaking valve stem seals as the head has just been completely rebuilt.

Thanks

Lee

[halz]

Hi there Lee - why the name change?

How did you go at the Z-day in September? I'm interested in what you said about getting your head rebuilt... who did it? Was it expensive?

I'm thinking that I'll need to pull the head off mine sometime down the track and get it cleaned up...might also consider getting bigger valves. Just wondering if this will cost $100s or $1000s...

Hayden.

[halz]

...thought I was typing a pm - sorry to hijack the thread! Lee can you reply via email or PM?

Hayden.

[inline6]

Hi Lee and Gav,

My head was rebuilt ~40k miles ago and the bottom end only has about 75k on it. This motor is in great condition.

I have the same situation on start up Lee. My motor is a F54 L-28 with an E-88 head. I only have to pull the starting circuit out 1/2 way, crank for about 6 seconds and it fires right up. I have to shut them all the way within 10 seconds though or it starts running really rough. Amazingly, I can drive like normal after about a minute with the chokes in all the way even if it is pretty cold out. I have had different needles before that provided different behavior. I am currently running the SM needles and not the N-27's that came from the factory. The N-27's needed to have choke while driving until warm. All this varies depending on you initial mixture setting of course.

Anyway, it is interesting that a few others have the same problem. I am thinking of lowering the fuel level in the bowls a little bit to see if that helps.

Garrett

[alternativez]

Back when these were new there was a dashpot, and ugly vacumn operated motor mounted on the balance tube, which acted to prevent the su pistons from dropping too quickly and choking the engine. Sounds like it's missing. Possibly the check valve in the booster hose may be bad. Very cheap fix. Could also make your brakes feel like they are fading. Otherwise, loose or defective vac lines will cause this.