70 240Z runs on...

[prook]

Any idea what would cause the engine to keep running after it is turned off? It finally quits. Seems to be worse if the car sits at idle for a while and then the key is turned off vs turning it off immediately after driving it for any length of time.

I just got the carbs professionaly rebuilt and tuned on the car. Timing is factory set. Points, plugs, wires, condensor, cap, rotor are all new. Dwell set to factory spec's. All factory ignition parts used.

[2ManyZs]

Well, it worked on one other car so I'll suggest this again. Check your ignition switch. If the car is continuing to run at a somewhat normal idle it is possible the ignition switch is worn out and is not cutting off the current to the ignition.

Since you seem to have covered most of the other causes this is a distindt possibility.

Hope it works.....If it doesn't let us know and we'll try to come up with some other ideas.

Oops, it worked on two cars that I know of. My first Z and one since I joined here.

[justaZcarguy]

Well, this may seem weird, and it is, but I had a similar problem with my Suburban after I put a toggle switch to switch the blower fan on and off. (YES, lazy way out I know, nut I was in a hurry if and was going to fix it right and that was two years ago and the stupid toggle switch is still there.) ANYWAY, if the switch is on then is runs for about two to three seconds after key off, this may have nuthin' to do woth your porblem but I thought if you added a recent switch it may help out.

Zyal8r.

and good luck.

[jcdozier]

To follow up on 2ManyZs post, if the car is NOT running at a normal idle, you may have a dieseling problem and not an electrical/ingnition problem. (Although the worse after idle than after driving isn't a typical dieseling symptom).

Is the run-on smooth or rough? What grade of gasoline do you use? Do you use a fuel additive, such as Techron?

[prook]

The cars hadn't run for 5-6 years before last Sunday. I had the gas tank boiled and replaced all the rubber. I do notice that the ignition key is sticky and plan to replace the locks with MSA's kit. The run on is definitely NOT smooth. It's almost like a belated mechanical sneeze coming from the engine. Thanks for all the replies... Oh the gas is low grade (87) Unleaded - no additives but plan on adding something to compensate for the Lead I think it requires. And recommendations for lead additives?

[jcdozier]

Hmm, "...a belated mechanical sneeze..." isn't one I'm familiar with. If you hear pinging or a clatter, it could be dieseling. Since you're using the lower grade fuel, you could confirm or eliminate dieseling easily - fill the tank with the expensive premium grade. The higher-octane fuel won't auto-ignite and the engine won't run on without spark from the plugs. If it is dieseling (commonly called spark-knock), we can get it weaned off of the premium fuel over time by "dosing" the fuel with Techron.

I don't use a lead additive in my '71 240-Z. However I do add the detergent additive Techron, (a concentrated form of the mouse milk blended into Chevron gasoline) every two or three fill-ups because I don't have ready access to Chevron Gasoline here in the hinterlands :mad: .

To be open and honest here, I worked for Chevron (now ChevronTexaco) for 25 years. I took my retirement in a lump sum and rolled it over into an IRA, so I don't get any financial benefit from folks buying Chevron products. I use Techron because it is arguably the best intake system cleaner on the market. (I use either Texaco or Shell motor oil.)

[beandip]

Prook, It sounds to me like the problem is dieseling, and not electrical. the key is that it the idle . The engine is stumbeling and running rough and usually also knocking. The advise to switch to hich grade fuel is right on also check that the choke is completely off when when you move the lever to the off position. Another cause is the engine running rich, I know you said that you just had them done but they may have set them a little rich. I am assuming that you are running , round tops . Alot depends on the condition of the engine , one with good compression is more likely to have this happen also one that is driven on short runs and the engine isn't properly warmed up . Carbon builds up and some times it will glow in the combustion chamber and cause pinging , also called pre detination, which is not good. Now that the weather is warming if you can take the car out and drive it for 20 or 30 miles on the hiway speeds that should help greatly . definatly fill the tank with high octain fuel for the next three or four times and then use pluss . If you want instructions on how to set up SU round tops just e-mail me .

[2ManyZs]

The two main causes for "dieseling" are idle speed set too high and timing that is too far advanced. Since you say the timing is set according to factory spec we can assume that isn't the problem. You did not say however, what your idle speed is. It should be between 700 and 800 rpm with the accepted 750 as the norm.

The gas issue shouldn't be a major cause as the timing also plays a factor in that. That is the main reason I suggested it is an electrical problem associated with a worn out ignition switch. It is possible the switch is shorting out the contacts and it could be only giving partial voltage to the ignition, thereby giving you the sensation of "dieseling".

With all the new parts you have installed if it isn't the idle speed it could certainly point to the ignition. Let us know if you find the problem, or if you need more options we will be glad to come up with some.