68 911L Ignition timing

[jfd]

I am still missing something???

1968 911L Prof. rebuilt 2 yrs ago to orig. specs. about 2000 mi on engine ran fine.

Equip. MSD with tach adapter, New Pertronix and coil, Distrib. rebuilt by PartsKlassik, Distrib. graph shows: 1000 rpm with 2deg adv., 5500 rpm with 18 deg advance.

Actual running: Static 3-5 deg adv. BTDC to start. Running at 1100-1500 rpm creates 20 to 30 deg adv. Static check for TDC at No 1 is verified. This condition existed before the distrib. rebuild. Using ccw distrib. rotation to retard, and cw to advance, I am not able to replicate the distrib graph? I am afraid to try anything near 5500 rpm. Carbs were balanced front to back and side to side at 7 on the synchrometer before the current condition, now require 10.

What am I Missing? Any help is appreciated?

[edmayo]

Rotating the distributor CCW will advance spark, and CW retards it. At 5500 engine rpm the distributor should be at about 30* advance, so added to 5* initial advance total advance would be 35* Is your graph showing engine rpm or distributor rpm , which is half engine rpm. Hasn't this been going on for a couple years now? Might be time to seek professional help.

[jfd]

For the record, I consider any response to my calls for help from Ed Mayo to be "professional help".

My source for distributor rotation for CW/CCW, is apparently wrong.

Setting basic ignition timing is not rocket science. But, for some reason I am unable to achieve the specs for advance at idle and at 5500 rpm. Would any malfunction of the chain tensioners have any effect on the ability achieve the specified timing requirements?

Any help is appreciated.

[edmayo]

Lets start with engine running at idle timing light hooked up. What is the idle speed shown on the tachometer? Adjust the idle speed screws on the throttle arms to achieve a 1000 RPM idle, now what does the timing light show the timing to be? Answer those and then we'll go to the next step.

[jfd]

Hi ED
Thanks for your willingness to help.

When I start the engine it shows 1100 rpm and 20-25deg advance. It wants to stall at 1000 rpm. A slight increase in throttle revs to 2500 and 30 deg advance. The throttle does not BLIP and return quickly to idle. There is minor choking and backfire.
The rebuilt Distrib. graph shows at 1000rpm the advance 3deg +/-, and at 5500 rpm at 32 deg.

If Texas was next to NC, this otherwise perfect 911L would be in your shop!

Any help is appreciated.

[edmayo]

You first need to stabilize the idle. I hope you have one of those airflow devices to measure airflow. Disconnect both vertical drop links that are attached to the throttle arms, just pop off the end that goes to the cross bar. Now what is the air flow for each side, assuming they are unequal use the screw adjustment at each throttle arm to set the idle speed keeping both sides equal in air draw and lowering the idle speed as much as possible without the engine dying. Hopefully you're around 1000 RPM. Now adjust each idle mixture screw, one at a time, turn screw in until idle drops off then slowly turn back out until idle speed picks up again. If they were out of adjustment and you've improved things the idle speed may have increased, if so lower back to about 1000 RPM. Let me know how this goes before I give you more to do.

[jfd]

Hi Ed
From a cold start it will idle. I can do 1000 rpm with 15-18 deg advance. After warm up a slight increase in throttle goes to 1300 -1600 and 25 deg advance. IT will not come back down to 1000 rpm. On warm restart it goes to 2000 rpm and 25 deg adv. and will not come back to 1000 rpm, have to shut it off. Carbs are balanced front to back and side to side at 10 on the synchrometer. This seems like a distrib. issue to me, but I had this condition before the rebuild?

The distrib is newly rebuilt to 911L specs. MSD is 10 yrs old, Pertronix and 3.0 0hm coil are new.

I cannot stabilize the idle, only on cold engine? The carbs are balanced. After warm up it wants to stay at 2000 rpm.

Any help is appreciated!

[1QuickS]

Idle mixture is rich. This is indicated by STE reading of 10. Typical STE reading at idle is 5 to 5 1/2. A 10 reading occurs when mixture is rich, this requires more air to burn the rich mixture. This is why I like STE Synchrometers, they balance air flow but they also indicate basic Lean Best idle mixture adjustment. A rich mixture will cause a lazy return to idle. I suspect your idle jets are not 55 which is what you need for a 2.0 engine. They may be marked 55 but have been re-sized to a larger orifice. A jet gauge set will verify jets for being 55 (0.55mm) or you can buy a 0.55 mm drill bit and use the shank as a gauge. Get a 0.60 mm drill to check that it will not insert into the jet.

Also, a high STE reading indicates the throttle valves may be opened enough to begin to expose the first progression hole which also richens the mixture.

Ed described Lean Best as screwing mixture screw closed until engine stumbles and then opening until engine smooths out. I open 1/2 turn from the stumble which results in a slightly rich idle mixture and seems to be what the engine wants. This must be performed after idle air flow balance on all six cylinders has been achieved. As Ed said, this Lean Best mixture adjustment will probably result in an increased idle speed so the process will need to be reiterated.

[Richy]

I certainly hope Jfd understands and appreciates ED's and Pauls help , the best !

[edmayo]

Now once it has warmed up with both the timing light and your synchrometer at hand loosen the distributor clamp bolt (takes a 10mm wrench) and slightly turn the distributor clockwise (retards the timing) and see if you can bring the timing back to about 5* advance, the idle should also come down, you may have to readjust the idle speed screws to come back to 1000 RPM idle. ( both drop links still disconnected so each bank is independent of the other) I want you to repeat process until each time you rev the engine the timing will come back to 5* and the idle will return to 1000RPM. Tell me what that does.