unsteady timing

The timing on the rear bank of my 308 is unsteady....the timing mark jumps back and forth over about 5 degrees. The engine starts and runs but less than optimally. The front bank's timing is rock steady. I've triple checked the timing light connections to ensure there was no crossover interference. What can cause this?

78 USA version dual coil/dist, one set of points per dist.

Thanks in advance!
Mike

 

Distributor advance mechanism is screwed up (worn bushing, broken spring, etc.)

 

I bet that first advance spring needs a shim in it. It's the weakest of the springs and is often the first one which requires shimming.

 

shim it or replace it? http://superformance.co.uk/a-308/ignition.htm

Check the first item on the page.

 

Not a bad idea to rebuild the whole thing with new parts but either way you should check the advance curve on a distributor machine before installing it back on the car.

 

No, don't use the kit. Shim it with a small thin metal washer which fits behind the spring and in the spring cup holder, then regrease the rubbing points on the advance mechanism. The first advance spring is also the softest to push down (obviously) and almost always do they already have a shim or two in there from the factory. There is a great thread buried in the archives somewhere showing which springs do what. It is best to have it set up on the dist machine though.

 

how do I get to the advance mechanism? From the top via the points or from the bottom?

 

There are 8 screws on the bottom holding the mechanism in place. You have to pull everything off the distributor shaft (carefully!) and slide the entire shaft down through the opening which then provides access to the weights and springs etc. If you have never messed with one of these before, I highly suggest you have someone who knows these things well have a look at it or be prepared to take it apart and put it back together many times to get the advance correct.

 

Good advice.

 

Thanks. I got it apart. There wasn't as much grease as I expected inside but it had the appearance of old axle grease that was pretty sticky. I can imagine how stiff it was in cold weather. Otherwise the springs were in good shape but not one shim...

While there, I noticed two things wrong. The inside facing seal of the top dist bearing had failed. There was a collection of very dark grease spun out in a ring pattern with a heavy concentration of it on the lowest part of the dist as it sits on the car. Unfortunately this is also where the points connect to the coil. Might the expelled bearing grease embedded with metallic particles be conductive? The points themselves had almost 25% of the surface area pitted. I cleaned it up, replaced the bearing, points, and condenser, refilled the wick, set the gap. I will test it out this afternoon.

The worn bearing allowed the dist shaft to wobble slightly, making a noise I had spent far too long trying to locate previously. It works nice and quietly now.

 

It worked well. The dist runs more quietly, the engine runs smoothly once timing was set, idle timing was steady, and checking at 5000 rpms the yellow dot was right on the mark. Since 5000 rpms was my only data point, I'm not sure of the curve's accuracy. But the engine runs a heck of a lot better.... I couldn't be happier

So, I started the other bank ( 5-8 ). I found and put PM 5-8 on the flywheel to TDC. Removed the dist cap and noted the rotor position as pointing toward the red notch in the dist ( firing # 5 ). Given this, where would I expect to find the cam markings?

 

The cam markings (on all of the cams) would all be 45 deg past their reference marks (when the engine is in position to fire cyl #5) -- i.e., the cam marks (even on the 5-8 cams) only have meaning when the flywheel is at PM1-4 (and #1 at the end of its compression stroke).

Good find/fix with the bad bearing. Since you have a points-based ignition, a dwell meter might be able to help you diagnose whether an unsteady timing is the result of a bad bearing (both dwell and timing would be unsteady) vs. something wacky in the advance mechanism (dwell steady, but timing unsteady) -- although one can argue, if the timing is unsteady, the distributor is coming out regardless

 

Thanks, it's been fun learning while doing.

What strikes me as odd is, whomever timed the car previously had idle timing set at roughly 5 degrees BTDC with only one set of points! The locals held a dyno day about 18 months ago and three runs ranged from 168 to 174 at the rear wheels. With driveline loss that put it very darn close to the factory spec of 205. I wonder what I'll get without such a retarded engine?

 

Scroll down to post 232 for some info/pictures/specs on the advance mechanism innards:
http://www.ferrarichat.com/forum/showthread.php?t=105874&page=12

 

Oh I've followed that thread for quite some time... In fact it helped give me the confidence to undertake this and similar projects. I want to emphasize the importance of replacing the o-ring in the "spider" and/or the bearing it holds. Together they add significant stability to the distributor shaft and therefore consistant timing.

Also, I didn't go to the effort of mapping my advance curve. I set each dist first at idle but the final adjustment was made at 5K rpms. Both banks are spot on together at speed. Which is where i intend to keep it now that it's properly timed.

As it is now, the engine has fresh extenders and coils, new plugs, wires, condensors, and points. Proper gap set on points and plugs with most importantly correct timing. I can't wait for daylight to take it for a spin.

 

Went for a brief drive this AM. It starts easily and runs much better than before. It's more responsive off idle with what feels like more low end torque. I still get high rpm stumbling though... it starts to break up around 6K. Any ideas?

The WSM warns against running the car with only the retarded points. My R1s were essentially timed as if they were R2. It claims damage can result. What would it do to the engine?

 

One set of points does not do harm to the engine. The second set is for closed throttle only, to improve emission readings at idle.

Do you use a new set of points? Are they installed correctly (not bend)?

Regards
Harry

 

Just a wild thought: Make sure the rotor is aligned with the nibs on the inside of the cap when it is fully advanced. Because of the width of the rotor, there is some leeway as the mechanism advances. But if you are on the "other" points the rotor could be getting too far from the nib at high RPM.

 

My apologies for the lack of clarity.

My car has one set of points per bank - R1.

The timing for R1 seems to generally accepted as 6 ATDC with some suggesting up to 10 ATDC. The timing for US spec cars with a second set of points R2 which were engaged only at idle was 5 BTDC. R2 was never intended to be the only set points throughout the rev range. As soon as the throttle was applied, R2 was disabled. This immediately advanced the timing from 5 BTDC to 6+ ATDC while further advance came from the centrifugal mechanism as the revs increased.

My R1 was timed to R2 specs. My dist advance may have been working fine but they had to start at 5 BTDC instead of 6+ ATDC. They never had a prayer of reaching full advance according to the specs.

This is the basis of my question:
The WSM states the imperative nature of disabling R2 AS SOON AS the throttle is applied to avoid possible engine damage.
What can running an engine in a retarded state do?

It is now set to 6 ATDC and both dist have equal advance at 5K.

Also I have a 5500 + rpm engine stumble. It acts as if there is a rev limiter. This also occured previously and was something of an impetus for me working on the dist in the first place. I had high hopes of my efforts on the dist, solving the problem. It did not.

60 idle, 140 main, F36, 195 Air Corrector: Once I completed the timing adjustments I went back to adjust carburetion. It occurred to me that the stumble acted like a lean cylinder coughing through the carbs. The difference being at such a high rpm the vacuum created by the engine breathing kept the cough on the inside. I had a set of 190 air correctors that I installed to mixed results. Perhaps it wasn't significant enough of a change....

I understand the issue could be many things besides carburation.

improper dwell?
voltage regulator?
cam timing?
improper valve clearance?

Are there any ways I can confirm or elliminate the above? Maybe it's something else?

A frustrated tifosi as the weather cools crying for a carbed Ferrari,
Mike

 

A minor point Mike, but you've got some of your nomenclature backwards -- the timing value for the R2 points is ATDC (meaning "after top dead center" -- not "advanced of..."), and the timing value for the R1 points is BTDC ("before top dead center" -- i.e., the more advanced location).

With regard to your troubles, I liked Beta's point about ensuring that the dist rotors are well aimed (especially if you are using the stock "narrow end" rotors -- the later QV rotors, which you could use if they weren't so darned expensive , have a much larger end so they aren't so fussy about their alignment) -- it's free, so I'd do that first. It can also give the symptom that you have as the rotor's position, relative to the cap, does change with RPM so it can be OKish at low RPM, but move (advance) into a position that is not OK at high RPM.

You should be able to use a fairly inexpensive dwell meter to confirm/deny if the dwell in either distributor is changing, or unstable, at high RPM. And I wouldn't suspect cam timing, as, even if it's wrong, it really can't cause a problem that occurs "suddenly" vs RPM. Additionally, a "weak" fuel pump, or an obstructed fuel filter, or a kinked fuel line, or a bad over-pressure regulator, (i.e., anything that would limit the amount of fuel delivered) would result in "It acts as if there is a rev limiter", so you might do a quickie volume delivery test (compared to the 308GT4 WSM specs) by removing the fuel supply line from the 5/6 carb inlet and manually running the fuel pump just to rule that in or out (and it's free too ). JMOs -- Good Hunting!

 

Ha! yes you're right.

Great, how do I check and adjust this?

How do I adjust the dwell?

 

When the flywheel is in the position to fire cylinder #1 (~7 deg BTDC and cyl #1 is at the end of the compression stroke), the 1-4 distributor rotor in the 1-4 distributor should be pointed at the red notch in the rim of the 1-4 distributor housing, as shown in this jpeg (which shows the "narrow end" stock carb rotor):
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And likewise, for the 5-8 side -- when cyl #5 is ready to fire, the 5-8 distributor rotor in the 5-8 distributor should be pointed at the red notch in the rim of the 5-8 distributor housing. Just remove the distributor caps and rotate the flywheel to those two positions and see if things line up or not.

For reference, here's the "wide end" QV rotor 124288:
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Dwell is directly related to the physical opening distance of the points: bigger gap = less dwell, and smaller gap = more dwell. But you should just measure it before deciding if it needs to be adjusted (any autoparts store, or JC Whitney, should have an inexpensive dwell meter available).

 

If they need adjusting, how is this done? Rotate the dist body or remove the dist and move the rotor a spline over and move the breaker plate?

 

I put the timing light back on and like before rev'd it to 5K - looks good - on the yellow dot. Then I took it up towards redline and the yellow dot quickly moved (advanced) out of sight. It is my understanding advance reaches a maximum then holds steady by about 5k. But mine keeps going and going.....

The high rpm miss I experience occurs after 5k as the advance continues.
The timing light doesn't fire when the miss occurs.

What's the best way to halt advance? Is this where a shim might come in handy?

 

No, although the "spec" is given at 5K engine RPM, the advance should continue to increase up to 6K engine RPM and then stay constant -- see the graph in your OM Section 3 (which shows the advance increasing up to 3K distributor RPM which = 6K crankshaft RPM)

Always good when you can "confirm" by another measurement method that a problem is occurring, but, unfortunately, this could be caused by either 1) the points floating (i.e., the dwell reducing to 0), or 2) rotor misalignment.

You can't rotate the dist body, nor move over another spline, to "fix" any rotor-to-red mark misalignment as this would change the timing -- so you'd have to move the points relative to the dist body so that they just open (fire) when the rotor is pointing at the red mark.