Occasional Weber Ignition Problem?
 

Today on the way to work the car ran like *****. It was studdering and stumbling at anything over 20% throttle and 2500RPM. My wideband 02 sensor was reading lean. After I stopped and got gas everything was fine. Ran like a champ the rest of the way to work and on the way home.

I think it was the cycling of the ignition, not the gas that did the trick. This is not the first time this has happened, although it was the worse. The other times it corrected itself, no refueling was involved.

Lately, I've also noticed that the pitch of the fuel pump changes when I turn on the headlights and the fuel pressure runs .5 - 1.0 psi less. I'm using the PMO clamp regulator.

Any ideas?

My engine is an '83 3.0 with the following ignition setup:

stock re-curved distributor
MSD 6AL and blaster coil
Magnecor wires and Bosch W8DC plugs

wild guess is that slip rings and brushes are worn.
ie: intermittent early stages of kaput.

The headlights always load the alt. At idle it'll be noticeable. Stock lamps is about 9amps. 100w low beams load about 17amps.

could easily be an ign. box going bad - how old is it?

have points? if so are they dirty?

then there's bad gas

no idea how the above could affect the pump pitch

MSD box and coil are basically brand new. No points

Alternator is original as far as I know. Are there any repair options other than replacement.

Yes. you would just take it to a rebuild shop -- they will have a tester also.

But I think you have some other problem (or problems).

What are the ignition wires like?

I think Ron hit the nail on the head.

MSD's are VERY voltage sensitive and will not work unless they get adequate voltage and current.

I would be doing some thorough checks on your alternator, regulator, battery, battery cables & grounds.

Lastly, make sure that the MSD box and its black wires are VERY well grounded to a nice bright shiny chassis ground. These draw a lot of current. :)

Huh - I'll file that one away... didn't know they were that sensitive.

An old timey volt meter with a needle could be handy - it will swing and you can watch that.

i've had some problems where the MSD didn't switch from multiple spark to single spark when the RPMs got higher, like around 3K. ran like all the wires were cross-firing. tossed the red box and went back to stock. there's really nothing wrong with the stock bosch unit, you know. very good setup.

don't count on a volt meter when the alternator is creating "noise" in your system. My attitude is that any IC board is voltage sensitive. Be it from connections or the alternator.

IMO another issue is that voltage ground placement can be an issue with aftermarket systems. For example my latest engine monitoring gizmo is grounded at the engine case, not the tub. My personal prob with our 911's is that the alternator is grounded at the engine, not directly to the battery. Another issue is tub ground placement of systems that are engaged at engine start-up. The system may be crashed by the starter seeking a ground thru the add-on system such as audio amps. There is a design reason that the factory used specific tub ground areas.. whatever.

This brings up a question I've had for a while. The PO installed the MSD box and grounded it on the engine. I gather that is OK?

http://forums.pelicanparts.com/uploa...1116556252.jpg

in my non-pro opinion the engine can be the worst place to install a ground except under special circumstances. In the MSD, its power source is noted in their directions to be used at the battery. [Our 911 elec system is a compromise.]

There is a factory tub ground stud in the engine comp on the left wall behind the CIS fuel filter&accumulator. There may be other grounds in this location. I would remove and clean all ground connections and install the MSD as the first ground against the tub stud. The MSD may routinely draw 10amps, possibly more when it's very hot. There is definately resistance when placing many connectors on to of each other. I use a max of 4 connectors under any situation.

I would use the starter main power as the attachment point for the MSD's main hot. I attached the large starter power supply lug on first against the starter then attach the MSD power on top of the power supply terminal. Again, clean all connections. Dielectric paste is the desired anti corrosion treatment, especially on critical electrics. I would silver solder all wire connectors. Gold plated connectors is a bit over kill but doable.

Hi:

Everyone has their favorite installation techniques so its a matter of opinion, but I'd not ground anything like that. :)

I have several things I do for grounds:

1) Ground the MSD case as well as the black leads to a clean chassis mount. The stud behind the left side engine compartment panel works great but make sure thats its clear of any oxidation or corrosion.

2) Run a braided ground strap from the engine to the same chassis ground point.

3) Make sure that both ends of the main ground strap at the transmission mount/chassis are VERY clean and tight.

4) MSD main power comes from the battery lead at the starter.

5) MSD switched power from the original CD box wiring

Installed properly, these ignitions have been quite reliable but they do not suffer fools in this area,....:) :)

http://forums.pelicanparts.com/porsche-911-technical-forum/188429-cd-ignition-msd.html?highlight=racers

"creating "noise" in your system."

The needle will not respond very fast so it will damp out rather than indicate any high freq. noise.

To fix up your grounds, search up the vinegar posts by Grady Clay.

Re: Occasional Weber Ignition Problem?
 

how's it running today?

I went for a nice long drive this weekend. It ran fine most of the time. Although, a few brief times it ran like crap. I put it in neutral and gave a quick blip of the throttle and it seemed to correct the problem.