Distributor advance question

I am chasing a problem with my Jalpa, the engine cuts / seems to lose spark or fuel at higher RPM's. Carbs have been rebuilt, new leads, new cap new rotor, new MSD6AL, good ground, plugs, compression/leakdown ok, carb linkage set up properly.

No points, mechanical and vacuum retard.

I thought the distributor mechanical advance was working properly - when i hold the base of the distributor shaft and turn the rotor head, its advances cleanly (by hand) and doesn't seem to bind.

I put it on a Sun machine today, and it looks like the advance is ok at 1750 (5 deg) and 2300 (10 degrees), but it doesn't advance beyond that.

the advance curve shows 15 degrees at full advance, so i am assuming there is a problem with the mechanical advance, but before i do anything I thought i would ask someone more knowledgable.

I swear the first time we ran it, it went to 15 degrees, then we tested the vacuum retard, then i respun it and it only went to 10, but i am not sure if i read it right the first time. The high end spark cut is erratic an only seems to rear its head when the car is hot. Other than the mechanical advance, could it be the sensor that has failed? Sorry if a dumb question.

Thanks in advance (ha!)

Geno

http://www.youtube.com/edit?o=U&feature=vm&video_id=CBsl_sDTD2E
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That timing curve data in the post above is from the WSM for the 3.0 engine, there was no separate WSM for the 3.5 Jalpa, but this chart appears in the Jalpa drivers manual.
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Cutting out? Like a misfire above a certain rpm? Timing will make it run poor, and retarded timing at hi rev's makes the exhaust get really hot, but i wouldn't suspect a misfire. Does the mechanism feel sloppy? Like it can bounce around. If bushings are shot, the spark can bounce around whch could exhibit some weird performance.

Again, I wouldn't think a misfire, or rev's being limited. Just MHO.

How did it run before new cap & wires?

 

Because the distributor is running at half the speed of the crankshaft, 10 degrees advance on the distributor machine is the same as 20 degrees at the crankshaft.

 

Thanks for the insight, very helpful.

I think i have it traced down to a shot alternator. I am running an MSD 6AL, and have read of problems with spark with the alternator is putting out a solid 12V.

 

Try removing the MSD out of the circuit, I had 2 fail on me. And others I talked to report problems with it sometimes.


Ago

 

Is the problem spark or fuel? A scope test (that "old school" thingie before OBD2) should tell you if the spark is crapping out. Even a timing light connected while you rev the engine should tell you plenty. I've had MSDs fail but usually it is all or nothing.

If (and only if) ignition is working, check other things.

Are you sure the carbs are set up right? Floats too low? Fuel pump not keeping up?

No brain surgery here but you have a few variables to check. Be systematic and don't change too many things at the same time.

Do not assume that new parts mean that something is fixed.

 

That fundamental law of the cosmos should be carved on a rock somewhere...

One easy thing you might rule in-or-out is the phasing of the dist rotor to the dist cap. Your symptom seems to occur after all of the dist advance has already happened, but a (bad) rotor position that still quasi-works at moderate RPM might get more unhappy at higher RPM -- just a thought (and something that takes $0 to get off the list of possibles).

 

Not an uncommon condition with converted distributors!

 

Thanks Steve, what is the best way to check this? I set the #1 on TDC in the compression strong, remove the drive belt for the distributor, roll the distributor so the timing marks line up on the body of the distributor, and that is how I set initial timing. Beyond that, how can i check for out of phase.

Also, thank you Tim. Is the test you are referring to putting a timing light on the flywheel marks and checking for smooth timing mark movement?

the MSD box is new, and well grounded.

Also carbs rebuilt so I don't think its fuel.

 

Now that makes tons of sense (to me), as the MSD draws more current as a function of spark rate or RPM's. I recall something like 7 amps at 5k rev's for a 6AL and more for the 7AL. Anyway, this would be a very crude way to limit RPM's if your electrical system can't keep up -OR- bad connections to supply or ground.

Knock wood - Those MSD units have realy impressed me with their reliability.

 

Distributor "phasing" refers to the alignment of the rotor tip to the cap terminals relative to the actual spark timing. In most cases with stock equipment this isn't even something most guys need to consider since the distributor is set up at the factory to be "in phase". If the distributor has ever been apart or especially if it's been converted from points to an electronic trigger, phasing needs to be considered. There is usually a fairly wide range of movement where the rotor and cap are close enough for the spark to make the jump due to the relative width of the 2 electrodes. If there is misalignment and the rotor is at an advanced position relative to the cap when you start out, it's possible that when it advances more while running that the gap will widen and quit firing until it slows down a bit. Something to check. I've converted a few V12 Jaguar Lucas distributors over the years and I have a cap with a hole drilled in it so I can easily observe rotor position when installing the sensor.

As far as the timing light, I'm only suggesting it as a simple way of observing spark. If you rev the engine and it misfires you should see the timing light stop flashing accordingly. If necessary move the inductive pickup around to different wires if the misfire isn't on every cylinder.

 

Yes, that generally sounds fine (if you mean that you ensure the dist rotor is pointing at its #1 cyl reference mark on the dist housing when the crank and valves are where they should be when #1 should be firing) -- but it assumes that the gizmo triggering the actual spark is correctly positioned inside the distributor housing too (and the various marks are correct). If, after doing this, you had to rotate the dist housing a bunch to get the idle timing right when the engine was actually running = a sign the triggering device isn't in the right place. Also, never hurts to do your own simple "visual" confirmation by looking at the rotor end when doing this procedure, and then looking inside the dist cap at the actual corresponding #1 terminal and ensuring that they would seem to line up OKish when the cap is in place.

If you don't have points, what is triggering the MSD box?

 

Steve and Tim, thank you for your help. A few other data points

1) I can not get the engine to miss while stationary. In neutral, it runs up to redline smoothly without hesitation. Also, while driving, as long as i gently apply accelerator it runs up to redline fine. Its under heavy load there is the miss, 2k rpm cruising at freeway speeds, if you apply the throttle it misses. Doesnt seems like a fuel miss, but a spark miss (just a feeling).

2) I ran a dedicated lead from the MSD to the positive battery terminal, and spent a lot of time getting a good ground on the frame.

3) Most pronounced at full engine temperature.

4) the distributor has two wires coming out the base, so there is some sort of optical trigger, but I am not that familiar with this distributor (bosch unit, same as MB 450).

The P300 manual (there isn't one for the P350 Jalpa) says to put the flywheel mark at 13 degrees (compression stroke on #1 cylinder), and line up the rotor with the distributor mark, which is the #1 cylinder. Confirmed rotor is pointed at #1 distributor terminal.

With this configuration, it wont start. To your point Steve, I needed to turn the distributor significantly to get it to run. The distributor is not like the 308 where it has limited movement with in the slotted bolts, but this distributor will turn 360 around. I am going to redo this operation in the morning and report exactly how much, and what the relationship between the running timing and the position of the rotor to the mark on the distributor body is.

 

NG -- this mis-phases the rotor-to-cap (but you now know what needs fixing). It also matches your symptom as the voltage required to fire increases as the engine load increases (so under light load, the spark can jump a huge gap and work OKish with a huge rotor end to cap terminal gap; whereas, under heavy load, it can't jump the same huge gap).

Good. Alternatively (with the dist housing still mounted as it is now), you can just recreate the #1 firing position, and see how much the dist rotor to #1 terminal in the cap relationship is mucked up. Bottom line is that you'll need to move the triggering stuff inside the distributor (either the pick-up or the shutter) until you only need to make a small adjustment of the dist housing from the static #1 firing set-up position to the position that gives the right idle timing with the engine running.

 

I had a 246 come to me years ago with a very similar set of symptoms. Turned out it was a CW distributor in a CCW application. Any chance of that here?

Another possibility. Is there any chance the wires are plugged in 1 position off all the way around? Not sure what style cap you have but in the Marelli caps many floating around marked for one application being sold for another with markings for one but not all. In the Lambos some have different firing order/cylinder numbering.

Also what drives the distributor? Some Ferrari motors have a type of drive with a shaft with a helical gear. The shaft can have a lot of end float that you can't feel by hand. While running timing can change radically.

If you have had a timing light on it what do the marks do when you raise RPM?

 

Brian, thank you for chiming in.

One more thing: I took the car to Monterey, and it ran perfectly the whole weekend. Unloaded it from the trailer, and the miss appeared, so it was sudden change in performance.

1) I will check the wires/cap, I think they are on correct, but will double check

2) The front cam runs a toothed belt that turns the alternator and a shaft that sits in the valley of the V, which turns the water pump, and the distributor (90 degree gear). Since the problem isn't apparent with the engine cool, i thought it was belt stretch changing the timing, which is reported with many Jalpa's, but i havent checked timing at hot condition. Will do that next. I did replace the belt which didnt help. But it runs much better cool than warm. Gears could be worn as well I supposed.

3) I did put a timing light on the flywheel, and it does advance, but not much....but i dont have enough experience to know if its advancing enough or not.

I guess one of my next steps is to see if the timing is different hot vs cold.

thanks for the help guys.

 

not following you Steve, can you explain?

I was going to get it running, time it at idle, and see the position of the rotor to distributor housing mark. Is there an easier to see how much i need to move the trigger?

 

Sorry if I wasn't clear -- you've got the plan right...

 

Thanks Steve, if anyone doesnt know what they are doing its me.

I will report back shortly.

Geno

 

One more question: the distributor has a vacuum retard, do i set timing with the hooked up or not?

 

I've no information for that. Do you even have a port that supplies a representative constant common intake vacuum? Is this distributor a total transplant from another species, or is it in the "stock" family?

 

To my knowledge, its the stock distributor. Pretty sure of that. The vacuum source comes from below the throttle plates (manifold vacuum?).

 

Usually a manufacturer would then clarify "with vacuum" or "without vacuum" in the timing specs, but L was a small outfit.

Yes, but does it come from a small volume only under the influence of a single cylinder, or is there a larger common plenum that gets vacuum from all 8 cylinders or maybe 4 cylinders?

 

The only shop manual is for the 3.0 Silhouette, nothing for the Jalpa and the drivers manual is laughable. There is a pretty good parts manual, but there is no mention of this topic at all.

the vacuum source comes from 4 cylinders.