Run on after shutdown
 

When I shut off my engine it tries to run on a few revs and dies with a light metallic clunk ar ping. Is this a timing issue. The motor has just been rebuilt so I'm trying to set everything up right. (2.0L with Webers)

This happens when there is something real hot in the combustion chamber. Probably the spark plug electrodes. They're lighting the gas with or without a spark. This can cause damage, including bent rods potentially. Often it is caused by an over=lean mixture, advanced timing of spark plugs with the wrong heat range.

I doubt that timimg is the problem, since there isn't any spark being generated after you turn off the ignition switch! On carbureted engines, this often happens if the idle speed is set too high.

Also, as indicated above, if the spark plug's heat range is too hot for summer conditions, that can cause an overrun condition, too.

Do you have the phenolic spacer blocks installed between the heads and manifolds?

I'm thinking I might need to change the plugs. I have the timing slightly retarded because I cannot get the idle down below 1500 rpm by adjusting carbs.( even though I did a rebuild). I did pull the plugs a few night ago and 4 out of 6 were a nice tan colour, the other 2 slightly blacker so I leaned out the idle a little on those. I really need to be able to set the timing more advanced but I can't yet. Maybe I should richen up the mixtures all round to cool off the burn and consider changing the plugs. I don't want to wreck the engine after all this hard work. Any thoughts?

Warren, I don't have phenolic spacers btn heads and manifold. What would they be for?

its been a while since i came across this problem, but i would swear it is a timing issue. imho

anyways, start with base line. you gotta set the timing correct, and sort out the idle problem properly.

This happened on my first car (1984 mini). It was the timing and after adjustment ran fine!

Steve
Try running a different brand of gas, and maybe higher octane. What plugs are you using?? Perhaps they are too hot a heat range. I would avoid playing with one or two of the mixture screws based on black soot on the plugs. If you are going to adjust mixture, do it the right way and do it for ALL of the cylinders. My plugs are always black and sooty. This is pretty common with today's gas and Webers. The tan color on the plug tips may actually be indicating a lean condition, which can contribute to run-on.

Good luck.

I think you need to look at your idle speed problem before anything else. Get your timing set right and then find out why it is idling high. Vacuum leaks, bent carb shafts, butterflys not centered? The high idle will cause running on - don't go chasing the plugs and octane when you have a known problem somewhere else. Attack the problem not the symptom!

Yep, you're right, I'm probabley trying to compensate for a root cause problem I can't find. I re-adjusted the timing last night and it seems to run smoother, the idle is up but I'll have to deal with it. Being up here in Canada the driving season is short so if you don't fix a problem in the winter you hate like hell to take the car off the road even for a few days to fix something during the summer so you try to compensate. False economy.

Steve,

The phenolic spacers were used on the 914/6 ... from what I have been told, because it tended to run more than 20°F hotter than the 911T model with the same engine, i.e., your car!

The spacers are pretty much a standard fix in hotter regions on carbed engines. I suspect that your gasoline quality is a contributor to your problem. I don't think altering your ignition timing is such a good idea. If air leaks are causing your higher idle speed, then perhaps installation of phenolic spacers and new gaskets, along with inspecting your manifolds and carb. throttles and shafts while you have them off would be a good idea.

with the timing set correctly, i know it idles high, but does it run on when you switch off??

It didn't run on last night but I never got a chance to get it really warmed up, I'm going to give it a run tonight or tomorrow, also I don't even know what plugs I have in it, still the same as when I got the car. They were brand new so I just left them in. Is there a link somewhere that tells me what plugs are for what heat ranges etc?

Steve
I use Bosch platinum WR8DP. I haven't had any problems with these plugs in my 2.4. If I recall correctly, the NGK plug recommended by PMO for Webers is BP6ES.

Have you refueled since the engine rebuild? I wonder because stale gas may give you these symptoms as well.

On the elevated idle speed--how retarded is the timing? What is the factory setting? Have you checked idle speed with a reliable instrument? The dash tach may be a little optimistic (mine is), so 1500 RPM on the tach may not be 1500 RPM. Just some thoughts.

Good luck.

John, Thanks for the plug recommendations, I'll look at those, I did remove and clean the tank so I know the gas is fresh. Maybe I should try a higher octane. Funny you should mention the installed tach cause there is a thread inside me that questioned it as sometimes the revs don't sound like they are really that high. Great input thanks
The timing was back to about 10 ATDC at an indicated 1000rpm, but now I don't even know what that rpm really is. The factory decal says 30 BTDC at 6000RPM but I've got to ask "who sets up at 6000RPM on an engine you've just rebuilt and are running in?" After re-ajustment it runs up to about 20 BTDC at an indicated 3000RPM. I'm even wondering if my distributor is not advancing all the way.

Steve,

Set timing at 6000 rpm ... the factory DID, after warming up the oil to 180° F, at least back in the 'good old days' when nothing left their factory with a water mixture for coolant!

There is no reason to fear 6000 rpm ... your engine isn't going to get any stronger with age! If the rebuild was proper, it is ready for 6000 rpm, now, and later. Break-in was completed on the cams and rockers after the first hour of running, and though the rings may still be wearing-in a bit ... high rpms isn't going to hurt them, either!

Assuming you have a stock distributor ... set your timing at 5° to 10° BTDC at idle, balance the carbs and adjust for 900 rpm (with a dwell tach) idle ... an do a check of the advance (record the results) at 500 rpm inctrements to 4000 rpm ,then 5000 rpm, and 6000 rpm. Let us know your results!