Cranks and cranks, but won't start... sometimes

[RJ80]

So I picked up a carbed '79 Spider about a month and a half ago. It ran great for three days after I bought it and then refused to start after that. Would crank but not catch. A couple times it would finally catch, run for about half a minute, and then die again... even with a steady throttle.

Took it in to a local, well-regarded repair shop and they said it was starting and running well (couldn't replicate the issue), but reccomended a full tune-up and I drove it home running and starting fine. Drove it another day without issue (though it seemed like it began to stumble slightly on light throttle but not full throttle -- but not enough to cause serious driveability problems). Next morning, it wouldn't start again. Same old thing -- starter cranks the engine, but it won't catch. Tried unsucessfully throughout the week to start it and it still won't start.

At this point I'm thinking there must be a loose electrical connection that is prohibiting spark occasionally. Does this sound accurate to anyone? Advice on a good "starting" point for diagnosis? (pun intended)

Thanks in advance,
Rory

[So Cal Mark]

have you removed the dist cap to inspect the wires on the electronic pickup? Frayed wires there can cause intermittent trouble. The next step would be to use a spark checker when it won't start to verify ign spark. One quick test is to watch the tach needle when cranking. With no spark the tach needle shouldn't fluctuate at all

[mdrburchette]

It may also be a fuel problem. When my fuel pump went out, it acted the same way until it eventually died.

[kristoj]

First thing I would do is check for spark. I usually just pull one of the wires off a plug and hold it near a ground source while the engine is cranking to see if you have spark. If you do not, then you can focus on the igition system. If you do have spark, then you can shift focus to your fuel system and go from there.

[TulsaSpider]

Mine acted like that and it was a bad condenser, then later it did the same and it was a loose condenser!
Yep I agree when it happens check for spark! Replace the wires going to the distributor! They may look good but I'll bet they are failing! Cheap insurance!

[RJ80]

Thanks to each one of you for your posts. Regarding the frayed wires in the cap comment, I assume they would have resolved that issue at the shop when they did the tune-up... they supposedly replaced the cap. Maybe I should ensure that they actually did! I did mention my rather erratic tachometer needle (not very accurate, jumps around quite a bit), but they told me my tachometer was broken. Maybe there is more to it then that?

Thank you for the rest of the comments, I will check first for spark as I have not seriously attempted to diagnose the problem myself yet (I was happy to pay to make the problem "just go away" so I could enjoy the car in my initial weeks of ownership before I got down to my typical DYI attitude).

Once I know if it's electrical or fuel, I'll dive deeper. Should I not have spark, the condensor will get some attention. If I do have spark, I'll look into the fuel system/pump.

Thanks again, gentlemen! Your comments are much appreciated. Updates to follow later in the week.

Best,
Rory

[TulsaSpider]

Rory,
Look at the wires coming out of the dizzy you will be able to tell if they have been replaced. The wires may look fine but are broken inside. Your needle bouncing is a tell tale sign of an electrical problem I would say! Easy to fix hard to locate sometimes!

[4babycar]

Granted I am new here and my knowledge is quite limited but the one time I had my tach going crazy, I had a cracked magnet in the magnetic pick-up, I have also read several of other posts where the pick-up wires inside the distributor were all messed up so I would check those as well. - Good luck.

[RJ80]

Wanted to update this in hopes that it might help somebody. After letting the car sit for a month (took a while to get the motivation up to start tinkering) I went out to the garage to see if it would start randomly. No go. Same thing, cranked for a while but wouldn't catch.

Got out the trusty Volt-o-Meter and started removing connections and checking individual components. Coil appeared to give the proper readings, so did the magnetic pickup and control module. When I went to test the coil, I noticed that the +B lead was on its terminal very loosely and connected it tightly when all was done -- not sure if I did this while removing the coil cover though. After all the connections were back together, I thought I'd just check and see if per chance, she'd fire up. Almost caught on the first attempt, then fired right up the second attempt!

Drove it around a few blocks after packing up my tools and cleaning up and the car ran just fine and didn't die on me. So... maybe that lead really was just loose the whole time. Maybe I tightened up something else that needed it. Maybe I got rid of some terminal or contact corrosion during the whole half-hour process of testing components. But it seems to be running just fine in the end! I figure even if the problem happens again, at least I know where to start my search, as the primary problem was that the car would run fine one day and not the next. I'll be seeing how consistently it starts over the next few weeks.

Guess this is a good lesson to always check the basics first. Can't believe that all it took was a half hour to get it going again. It's a shame to think of all those weeks of wasted driving. Of course we got our first rain of the season the weekend I fixed the car...