I tried to work on the project after 3 months of being in a "PTSD SEMI-COMA".

Details:
Project-Car has been "de-bugged"/electrical upgrades made within the last year and ran perfectly for previous 6 months
Car sat in garage for 3 months untouched
1968 convertible
1970 350 engine with new aftermarket HEI Cable-Drive Distributor
New 130 amp alternator (14.9V max output)
New "heavy duty" charging wire @ alternator
Firewall wiring in good condition (maybe?)
New dash wiring harness
New fuse block (original fuse block was corroded internally)
New 800 amp battery
New Positive cable from battery to starter
New Negative cable from battery to frame
New starter
Edlebrock carb rebuilt/has worked perfectly/mechanical fuel pump @ 6 PSI
Engine idles perfectly at 700 RPM ...... Timing set at 12* TDC

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Symptoms:
Engine backfires/loses power above 700RPM

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Troubleshooting revealed this with digital multimeter:

Alternator works perfectly 13.0 to 14.9 V with NO fluctuation

13.0 to 14.9 volts at horn relay with NO fluctuation

13.0 t0 14.9 V at battery with NO fluctuation

HEI (BAT terminal) shows 0.2 to 13.0 V FLUCTUATION ABOVE 700 RPM.

ALL FUSES SHOW 0.0 TO 13.0 V FLUCTUATION WHEN ENGINE IS REVVED ABOVE 700 RPM

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I will lift the car this week and trace/probe main wiring at the starter and at the firewall with the engine running to see if voltage fluctuation is happening BEFORE wiring enters the new fuse block.

Did I miss something.........Does anyone have an idea as to what might cause the fluctuation?

 

with all of the wiring work and replacements, I would look at the dash harness and fuse block connections for problems first. you could also bypass the key switched power to the distributor and jump it straight from the battery and it should run just fine, pointing to the run circuit. of course, you'll have to yank the wire off by hand to kill the engine.

 

If the above doesn't work then it points to an issue with the ground.

 

Thanks Men.....The battery ground is new and shows NO fluctuation at the battery post. Is there another ground I should check?

I'll also check the ignition switch positive terminal for fluctuation before I take the new fuse block off to open it up and look for a loose connection that supplies voltage to it.

I remembered that for the last year there has been a 1 second voltage drop that would cause the engine to miss for a split second....

I guess the problem has worsened to the point that I must find the cause

 

I dealt with a frayed ground wire at starter (on my '80, it attaches to one of the bellhousing bolts to engine block). It runs in the same harness to the starter area with the 2-3 wires thatconnect to starter. The wire feeding into the grounding ring was frayed and weak. I snipped it off and replaced with a new ring. Now, my C3 spins over hot or cold without issues. Also, it holds charge for months without noticeable loss.

NOTE: I have disconnected the OEM clock and I also went with a newer radio (with memory) that no longer appears to drain the (2-year old) car battery. New starter and battery installed together also (reman starter).

 

Correct me if I am wrong, but it sounds like connections to the fuse box are at issue. Are you always measuring using the same grounding point for your meter? Do the always hot connections on the fuse box have this fluctuation?

 

I am wondering what is changing above 700rpm? Could this be when the engine gets enough momentum to shift towards the passenger side and cause a bad connection - probably ground?

I had a similar problem on a 67 Chevelle years ago, where I could turn a corner and it would die - electrical. Then it would crank right back up and be fine. Took me a while to find the + cable was missing some insulation and shorting just enough to shut down when I made a turn.

Good Luck

 

Thanks Men....I'll check those areas by connecting the multimeter and positioning it so I can see it as I move each wire separately (I unwrapped the firewall harness to check for damaged/pinched wires...the purple wire had been repaired 3 times/it's getting replaced too!)

It seems to have gone from an intermittent slight miss and has gotten much worse when I replaced the starter yesterday....Hmmmmmmmm!

47 year old wiring...about time it failed...LOL

I did manage to put about 30 miles on those new tires before I limped home.

 

After studying the wiring diagrams it seems the main supply wire to the fuse block must be damaged.

If I move the harness around with the engine running the voltage will stabilize at the distributor 12V connection...so the harness must have been pinched in the past during engine removal, which explains several repairs on smaller wires.

Rather than continuously dealing with failing harness wires/splices from previous owners.....I ordered a harness from Willcox to connect to the new dash harness (original was spliced/repaired in 5-6 places)

 

Check the fusible links that feed the fuse box and the lights. They (2)connect at the starter terminal (stud) and run to the fuse box and the headlight switch.

 

Thanks.......That's what I thought when I read the wiring diagram last night. After unwrapping the complete engine harness, it's so "messed up" that I ordered a new harness (And I;m one who replaces original parts as a last resort)

 

I re-tested everything......same results.

I revved the engine a few times while testing the alternator to make sure it wasn't failing.....it's good.......no fluctuation....14.9V at start-up which levels off at 13.5V after a few minutes.......

The popping/backfiring is still consistent

The new harness should be here in a couple days.

The good news is.....I wasn't on the planned 350 mile road-trip when the backfiring began!

 

If you still have the points ignition system, check the coil feed wire , its a resistance wire that limits the current to the coil. This failure is quite common. Sounds like your is on its way out. Usually it has enough to power the ignition at idle , but when put under a load such as part throttle or heavy throttle , the wire can't deliver the amperage required to feed the coil and ultimately fire the plugs. What you can try is to hook a 12V feed from the battery to the coil, but not for long as you will burn up the coil.

Sorry just read you have an HEI Dist, forget what I just said. Look into the HEI. Check ign. module, pick-up coil . I hope your not re-using the original resistance wire to feed the new Dist. The HEI needs a 10 Gauge wire feed from the fuse box . The old ignition wire must be removed completly back to the fuse box and a new 10 Ga. wire run to the dist. Thats all I have. Good Luck!

 

Thanks.............

I ran a new wire to the HEI, but I'll do the 10ga. (I think I used 14 ga.) All the HEI stuff tests good....it's just the whole fuse block/HEL feed wire have wild voltage fluctuations...Forum members are thinking the harness that branches off and feeds the fuse block is damaged/fried since the alternator/starter/horn relay have a steady 13.5 V going to them.

 

I'm still waiting on the new engine harness......

I checked voltage again at the fuse block.......

The fuse block IGN terminal now has NO voltage.....I disconnected the HEI lead from that IGN terminal and temporarily connected it to the Heater fuse

It seems like the members are correct----the harness connection at the firewall to the fuse block is failing for some reason (I cleaned it thoroughly when I installed the new fuse block-dash harness).

 

I'm assuming you are removing the bulkhead connector at the firewall. It connects to the back of the fuse panel inside the cabin. (this bulkhead connector has a bolt in the center of the connector) Yes? This connector has to be separated from the back of the fuse panel to make the new 10 Ga. wire connection. Do not splice into the existing wire, you have to remove it completely. If I remember correctly its a blade terminal at the bulkhead connector.

 

Sounds good to me.....That's the first thing I'll do when the harness arrives/with 10 ga.wire.

 

I removed the original firewall harness. The waterproof "grease" that the factory coated the inside of the connector with had dried up and was allowing rain/moisture to soak the terminals between the two halves of the fuse block.

Apparently I forgot to clean/inspect the firewall connector when I replaced the dash harness/fuse block

Back to the garage...............................

 

3 things to buy in big jugs/tubes at Napa: Anti-seize, Brake grease, and Die-electric grease!

work on enough old cars and having those 3 around makes life a lot easier...I put the di-electric grease on every terminal I take off, headlights, a little on the alternator, battery, you name it...same with anti-seize, if it unscrews, it gets some...and brake grease is great on lots of rubber grommets and other stuff as well...

 

3 things to buy in big jugs/tubes at Napa: Anti-seize, Brake grease, and Die-electric grease!


Thanks for the info............

I've always been a fan of anti-seize and rubber conditioner.........

I just added dielectric grease to my list/connections yesterday......after being stuck on the side of the road 4 times this year and spending $200 on new wiring last week......I want everything to last!