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914World.com _ 914World Garage _ Intermittent Spark - '73 2.0L

Posted by: CantConfirmOrDeny Feb 13 2010, 06:40 PM

Hi all,

My son's '73 914 (drastically modified 2.3L rat motor, Weber carbs), which had been running "fine", now won't start after being parked for a week. Long story short: the spark is very intermittent - only sparks about every 3rd or 4th time that it should. It had a Crane optical pickup, so I put the points/condenser back in - same symptom! Put the Crane back in, then put a scope on the coil (-), and while cranking, I see a clean, 13.5V square pulse with a 240ms period and 180ms of dwell (Crane varies the dwell as it deems necessary, I think). If my math is right, this works out to a cranking speed of 117 RPM and a 69 deg. dwell angle, which oughta work. It always has before!

We've replaced the coil, we tried grounding the dizzy directly to the battery, we tried hotwiring the battery directly to the coil. Same symptom - only sparks every 3rd or 4th pulse. Plugs and wires are next, although why they would simply fail like this whilst parked is beyond me.

If that doesn't work, what's left?

Cheers,
"CCoD"

Posted by: ME733 Feb 14 2010, 08:19 AM

check the tachometer wiring, especially in the engine bay.....MM

Posted by: CantConfirmOrDeny Feb 14 2010, 07:14 PM

Well - I took the tach lead off the coil ... no joy - same symptom. Thanks for the idea. Plugs and wires should be in tomorrow.

Posted by: 904svo Feb 14 2010, 07:31 PM

Don't forget to check the cap and rotor.

Posted by: ME733 Feb 15 2010, 10:08 AM

.....well you got a hellish problem that can really P--- you off . SO try these checks. 1) make sure the battery is charged , and does not have a bad "cell"', and the alternator is charging..AND make sure you have FUEL...2). you have not moved the distributor, altering the TIMING. 3) the c.d. module is good/ not defective....and the "breaker Plate "is secure,and there are no frayed wires....and that the points actually fit without shorting /rubbing/internally in the distributor housing .and the condenser is new.4)... If you pulled the distributor,(check inside the case)that the small spring is in place. it helps the distributor drive gear from jumpping around.5)... The coil wires are not frayed and grounding,AND the large coil wire has not pulled out of It,s connector-at eighter end.6)... if using "resistor wires" suspect all of them, they can and will break internally.(causing misfiring).If it,s not just one cylinder it,s the coil wire. 8. ensure the c/d box is grounded where and if required....see if any of this helps. check everything twice....Murray

Posted by: CantConfirmOrDeny Mar 5 2010, 06:03 PM

All,

Thanks for your suggestions. The problem turned out to be the spark wires. What made it so baffling was that it had been running just fine, then suddenly (in effect) all the wires failed simultaneously! This makes no sense, but after replacing them, I now have good, strong, consistent sparks on the scope. And there you have it.

Cheers,
-pmb-

Posted by: kwales Mar 5 2010, 07:04 PM

Piss somebody off that knows cars?????

Posted by: Rand Mar 5 2010, 07:11 PM

Can't imagine the wires themselves suddenly failing, but a loose connection would sure do it. Especially the one between coil and dizzy center.

I ran into that... First occurrence it died in an intersection. Fortunately it was at WCR (Seattle) and JeffH hopped out of another teener behind me and we pushed it out of the way. It was a relief to discover it was only the middle wire coming loose. Pushed it back in and it fired back up. It acted up again later and I replaced with better plug wires for a permanent fix.

Posted by: McMark Mar 5 2010, 10:50 PM

Perhaps your wire connections just got corroded.

Posted by: pbanders Mar 6 2010, 09:54 AM

IMO, it's always a good thing to have a full set of ignition components on hand for swapping. Finding faults can be very hard, it's often faster to systematically swap components.