Author Topic: At a loss - engine stalls under load  (Read 9869 times)

R68GTO

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At a loss - engine stalls under load
« on: July 04, 2022, 05:57:29 PM »
Looking for some insight on troubleshooting my engine troubles.  Car starts, idles, drives out fine until you give it any heavy throttle.  I'm not talking mash to the floor, just hard acceleration, say 1/2 to 3/4 throttle.  When you do that, engine stumbles, then dies.  Quickly restarts and runs fine at light/partial throttle.  Car will rev in neutral without issue.  Car also seems to be able to handle heavier throttle when I first take it out.  After about 1 mile of running through the gears a few times, it can't handle heavy throttle.  I started to troubleshoot, then fell into the black hole of replacing parts one at a time to no avail.  Here's what I've done/checked:
1) checked plugs - fine
2) check resistance on plug wires - fine
3) checked coil - fine
4) replaced points with electronic module / along with coil
5) checked timing - fine
6) plugged vacuum advance - no change other than difficult idling
7) changed plug wires
8) changed fuel pump

What am I missing?  BTW, it's a solid lifter 427 built to stock specs - heads, cam, carb, distributor are all OE stuff.
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

Steve68

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Re: At a loss - engine stalls under load
« Reply #1 on: July 04, 2022, 06:53:56 PM »
Have you checked the float levels and other carb adjustments such as the accelerator pump?

jwbavalon

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Re: At a loss - engine stalls under load
« Reply #2 on: July 04, 2022, 07:23:00 PM »
I'm with Steve on the accelerator pump.  On the "first take off" the choke is on sop lots of extra fuel.

R68GTO

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Re: At a loss - engine stalls under load
« Reply #3 on: July 04, 2022, 08:09:40 PM »
Float level is good in both bowls. 
How do you tell if accel pump is functioning correctly?
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

GMAD_Van Nuys

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Re: At a loss - engine stalls under load
« Reply #4 on: July 04, 2022, 08:25:06 PM »
The fuel inlet in your gas tank could be clogged with debris.

Steve68

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Re: At a loss - engine stalls under load
« Reply #5 on: July 04, 2022, 09:10:09 PM »
Float level is good in both bowls. 
How do you tell if accel pump is functioning correctly?

Engine off. Remove carb cleaner. Look down carb throat. Use a light to light up area in throat if you can’t see clearly. Hold the throttle linkage in you hand and as you look down carb move the throttle as if accelerating, there should be a discharge (squirt) of gas into the throttle body area.  If you don’t see gas coming out during the throttle movement then there is an issue and likely your problem.

Use a service manual or Google to see how to set the correct spacing between linkage and pump lever.

R68GTO

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Re: At a loss - engine stalls under load
« Reply #6 on: July 04, 2022, 11:36:53 PM »
Checked the access pump per Steve's suggestion....carb received a shot of fuel as soon as I moved the throttle lever.
Fuel tank is new as are the lines all the way to the carb.  I did retain the original sending unit just because it was original to the car.  It was in perfect shape and the filter sock was still pliable and not deteriorating.
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

Steve68

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Re: At a loss - engine stalls under load
« Reply #7 on: July 05, 2022, 12:28:02 AM »
How did you check the fuel bowl levels?

maroman

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Re: At a loss - engine stalls under load
« Reply #8 on: July 05, 2022, 01:25:23 AM »
What kind of carb?
Doug  '67 RS/SS 396 auto I know the car since new

R68GTO

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Re: At a loss - engine stalls under load
« Reply #9 on: July 05, 2022, 01:30:03 AM »
4346 GM Holley - all original parts, Erik Jackson went through it before I installed it.
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David K

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Re: At a loss - engine stalls under load
« Reply #10 on: July 05, 2022, 04:10:07 AM »
Did the problem occur out of the blue?

Stingr69

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Re: At a loss - engine stalls under load
« Reply #11 on: July 05, 2022, 01:39:04 PM »
Fuel starvation sounds likely.  Filter inside carb fuel inlet needs to be looked at first.  Also, currently available fuel pumps seem to be pretty bad.

You might want to mount a fuel pressure guage at least temporarily to see if you loose pressure. 

169INDY

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Re: At a loss - engine stalls under load
« Reply #12 on: July 05, 2022, 03:20:37 PM »
 Car will rev in neutral without issue. 

I had a problem like this on a quadrajet car, turnout to be missing that darn thin metal separator gasket.

Check the vacuum values?

Might NOT apply to your Holley situ
Jim
68 SS/RS L35 Th-400 LOS
69 Pace Car L48 Th-350 LOS
68 Z28 M21 LOS

R68GTO

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Re: At a loss - engine stalls under load
« Reply #13 on: July 05, 2022, 11:31:09 PM »
Did the problem occur out of the blue?

I have been trying to recall the sequence of events.  I believe this condition started after I installed new 8.5 power valves in the carb.  I did check them before installing.
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

R68GTO

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Re: At a loss - engine stalls under load
« Reply #14 on: July 05, 2022, 11:32:56 PM »
Car will rev in neutral without issue. 

I had a problem like this on a quadrajet car, turnout to be missing that darn thin metal separator gasket.

Check the vacuum values?

Might NOT apply to your Holley situ

I'm not familiar with the thin metal separator gasket.  Where is it on a quadrate? Vacuum is right at 14.5 inches
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

Tinkerr

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Re: At a loss - engine stalls under load
« Reply #15 on: July 06, 2022, 12:29:23 AM »
Is the tank venting properly? If not, could be creating a vacuum situation with the fuel system after a few hard pulls. As a side note you have a solid lifter BB pulling 17" of vacuum at idle? Factory were at PV's were 6.5 on solid lifter BB's. Idle vacuum divided by 2 is the formula for PV.

David K

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Re: At a loss - engine stalls under load
« Reply #16 on: July 06, 2022, 01:03:30 AM »
Go back to the last thing you changed…see what happens.

R68GTO

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Re: At a loss - engine stalls under load
« Reply #17 on: July 06, 2022, 09:46:52 AM »
Thanks for all the input guys.  The engine pulls 14.5" of vacuum t 900 rpm idle.  I'll try to crack the gas cap on the tank to eliminate that potential cause.  Per suggestion, I'll pull the carb again and check my PV installation.  Will also make sure everything else on the carb looks ok.
Is there any possibility that I have an intermittent short in the electrical circuit between the ignition and the coil?  I dunno....if it was that, it would not just do it under harder access correct?  Totally grasping at straws these days.
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

janobyte

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Re: At a loss - engine stalls under load
« Reply #18 on: July 06, 2022, 04:53:08 PM »
Just throwing it out there: have somebody ride behind you to see what, if any, color smoke out the exhaust. When it dies, don't try to start, pull and read the plugs,,lean vs. rich condition. Does it die or fall on it's face pulling a holeshot?

Normal operating temp?
68 Z/28  born with: 302, drive line, etc..

R68GTO

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Re: At a loss - engine stalls under load
« Reply #19 on: July 07, 2022, 12:15:15 AM »
Car runs cool.....roughly 180 degrees.  I'll pull the plugs and take a look, might just change them to eliminate that potential cause though I would think if I had a plug issue, it would run rough all the time.
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

David K

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Re: At a loss - engine stalls under load
« Reply #20 on: July 07, 2022, 03:57:02 AM »
Did the problem occur out of the blue?

I have been trying to recall the sequence of events.  I believe this condition started after I installed new 8.5 power valves in the carb.  I did check them before installing.
[/quote
Go backwards to fix what started this problem.

janobyte

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Re: At a loss - engine stalls under load
« Reply #21 on: July 07, 2022, 02:57:44 PM »
Checking plugs to see if you have a rich or lean condition asap after the issue.
68 Z/28  born with: 302, drive line, etc..

KurtS

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Re: At a loss - engine stalls under load
« Reply #22 on: July 09, 2022, 03:46:44 AM »
It doesn't sound electrical / spark, sounds more fuel related.
Do you have a spare carb laying around to try? I'd also ask Erik, he is good at troubleshooting.
Kurt S
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R68GTO

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Re: At a loss - engine stalls under load
« Reply #23 on: July 10, 2022, 06:53:07 PM »
Update....borrowed a carb from my brother and swapped it onto the car.  Good news......stall condition under load is gone.  Bad news, I have almost no vacuum (5 inches) and the engine has a slight miss in it.  I adjusted the idle mixture to try to bring the vacuum up with no luck. 
One of the other things I noticed is that the new carb puts my car in fast idle when cold, my old carb never did that.  Could that be an indicator of the cause of my past problem?
Going to go back through my wires and plugs, adjust timing, etc. to see if I can find the source of the miss. Feel like I'm headed in the right direction, just not there yet.
Thanks again for all the ideas!
Jim
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

David K

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Re: At a loss - engine stalls under load
« Reply #24 on: July 10, 2022, 08:52:05 PM »
So the carb is the problem. The carb is supposed to be at fast idle when engine is cold, and idles down as it warms up. Is the borrowed car close to your carb specs?

R68GTO

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Re: At a loss - engine stalls under load
« Reply #25 on: July 11, 2022, 09:33:27 AM »
Carb is essentially the same, just a newer version
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

R68GTO

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Re: At a loss - engine stalls under load
« Reply #26 on: July 12, 2022, 01:57:01 AM »
Did more testing tonight.  Set initial timing at 4 degrees BTDC, adjusted idle mixture screws (2 turns out) and set the idle at 1000 rpm.  Seems to run pretty good with no cutout or stalling under hard acceleration.  But just have 5 inches of vacuum?  What about a carb change takes me from 14.5 inches of vacuum to 5?  Nothing I tried seemed to bring the vacuum up other than rpm.  pulled about 12 inches at 2,000 rpm
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cook_dw

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Re: At a loss - engine stalls under load
« Reply #27 on: July 12, 2022, 02:16:32 PM »
Is fuel coming out of the boosters at idle?

R68GTO

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Re: At a loss - engine stalls under load
« Reply #28 on: July 12, 2022, 05:27:29 PM »
Need some help understanding the question…..fuel spraying out at normal idle looking down into the primaries?
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

cook_dw

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Re: At a loss - engine stalls under load
« Reply #29 on: July 12, 2022, 06:06:36 PM »
Yes is there any fuel (at idle) coming out (spraying, dripping etc) of the boosters of the carb?  There shouldn't be at idle.  Idle circuit of the carb is below the primary throttle plates so if you see fuel on top of the plates or dripping from the boosters you have a power valve failure or sized incorrectly.  Did you ever measure your idle vacuum when you first got the car running?  Assuming you have a stock L78/L72 cam then you should be around 16-20"hg.  I'm making a few assumptions as I know Eric would set this up correctly.  Have you not reached out to him on your issues?

« Last Edit: July 12, 2022, 06:39:32 PM by cook_dw »

Kelley W King

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Re: At a loss - engine stalls under load
« Reply #30 on: July 12, 2022, 09:40:27 PM »
To me it sounds like you have a vacuum leak. Hard to hear with a BB at 1,000 RPM. I think you might have caused it during your carb change. Does the different carb have a port not used?
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R68GTO

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Re: At a loss - engine stalls under load
« Reply #31 on: July 13, 2022, 12:36:43 AM »
primaries are bone dry at idle.  With old carb that Eric worked on, I had 14.5" vacuum.  New carb is 5 inches of vacuum.  I haven't reached out to Eric yet but probably will once I get done with Camaro Nationals and Supercar reunion.
KK, I'll double-check on the unused port thing in the morning.
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

R68GTO

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Re: At a loss - engine stalls under load
« Reply #32 on: July 19, 2022, 02:17:45 AM »
Update....I sent the carb to Eric who found a few minor issues plus found the rear float was set near the bottom of the bowl.  He said that would definitely cause my stalling at heavy throttle.  At one point I had checked the float level in both bowls and set them to the bottom of the sight screw hole but obviously jacked something up.  I won't have the carb back in time for the SuperCar reunion but think I have it running good enough to do a few smoky burnouts and take a couple of easy passes with it.  It's been roughly 50 years since this car made its debut on the strip as Rat's Nest 8)
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

WaltZ

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Re: At a loss - engine stalls under load
« Reply #33 on: July 19, 2022, 05:07:44 PM »
Just throwing this out there.
After the floats checked, try increasing the squirter size +2 and test for any difference in stumble,
if it's better, then try the next accelerator cam up, try again and if the stumble is lessening, try the next couple of cams.
Had this stumbling and sometimes stalling issue with my stock 302 and when bumping up the accelerator cam, it took care of the stumble and stalling, therefore it was starving for fuel during load and acceleration by the pump cam curve not being enough.
It may be a good idea read and understand how these cams can affect your performance, for better or for worse.
Been thru it and I know your frustrations. Sometimes it just the simple checks we overlook.
Good Luck

R68GTO

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Re: At a loss - engine stalls under load
« Reply #34 on: July 26, 2022, 11:37:55 PM »
Got the carb re-installed and it is 100% better so I guess I can confirm that I "jacked it up" somewhere along the line.
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David K

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Re: At a loss - engine stalls under load
« Reply #35 on: August 05, 2022, 01:23:02 AM »
Glad it’s fixed. Refer to post #13 for the possible issue cause.

R68GTO

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Re: At a loss - engine stalls under load
« Reply #36 on: November 04, 2022, 11:18:21 PM »
One more update....periodic stalling problem under moderate to heavy load continued.  Took it to a chassis dyno so we could troubleshoot under load.  Dyno guy has been tuning engines and using a dyno for over 20 years - after 3 hours he's scratching his head.  Because the stalling was so immediate (like turning the key off), he decided to temporarily install an MSD distributor and MSD box to effectively bypassed my entire ignition system.  Viola!  problem disappeared.  Made another jet adjustment and some other tweaks and the car has "woken up" pulling well to 5,500 rpm.
So the problem is on the inbound side of my coil.  Ultimately found a melted wire that goes to the positive side of the coil.  I'm not sure I understand why that condition caused the symptoms to be what they were as a melted wire is melted at both low and heavy throttle.  What I do know is when we took it out of the circuit, the problem went with it!
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

David K

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Re: At a loss - engine stalls under load
« Reply #37 on: November 05, 2022, 04:44:44 PM »
Awesome. Where was the coil wire melted? Near a heat source?

R68GTO

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Re: At a loss - engine stalls under load
« Reply #38 on: November 06, 2022, 07:35:10 PM »
It was melted from the coil back into the taped bundle of wires that runs along the firewall.  I haven't opened the tape up to see how far back it's bad
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David K

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Re: At a loss - engine stalls under load
« Reply #39 on: November 07, 2022, 06:03:12 PM »
Do we get pictures?

R68GTO

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Re: At a loss - engine stalls under load
« Reply #40 on: November 12, 2022, 06:15:44 PM »
Here's a couple of pics....melted at the coil and at the firewall box, so assume it's melted in between as well.  I'm bummed that my original harness is toast.  New harness has differences despite claims of being factory correct.  Oh well, some parts don't last forever and on the bright side I didn't start my car on fire!
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

David K

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Re: At a loss - engine stalls under load
« Reply #41 on: November 12, 2022, 06:23:37 PM »
Well that’s not pretty, BUT, you might be able to unpin the wire from the block and replace it. There’s enough used harness parts so finding an original is easy.

169INDY

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Re: At a loss - engine stalls under load
« Reply #42 on: November 12, 2022, 10:57:56 PM »
My Electrical Apprentice Instructor reminded us.
P=I2R
Power Equals Amps Squared times Resistance.

We all know the function of that wire is to voltage drop (IR) by way of resistance some of the available voltage to the primary winding and the points to preserve point life.
The cost is Heat that is dissipated by the length of resistance wire.

I have not found a vendor source that is selling raw lengths of GM applicable resistance wire & the Harness vendors plainly state they do not sell "Parts" i.e. various connectors, terminals and wire. So Harvesting a Donor harness to obtain a used acceptable chunk of wire to repair your existing harness might be in order.

This is Where I obtain terminals, connectors and tooling for Harness repairs.

http://www.texasindustrialelectric.com/_packard_terminals.asp
Jim
68 SS/RS L35 Th-400 LOS
69 Pace Car L48 Th-350 LOS
68 Z28 M21 LOS

R68GTO

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Re: At a loss - engine stalls under load
« Reply #43 on: November 13, 2022, 10:49:21 PM »
My frustration has peaked.....installed a new harness to replace the melted coil wire and the symptoms still exist.  I need someone with a logical mind to help me out.  The symptoms disappeared on the dyno once we swapped out to an MSD distributor and box..  Found the melted wire as stated in the above posts so assumed that was the culprit - apparently not.
Does this mean my root cause issue is "upstream of the firewall connection?
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

maroman

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Re: At a loss - engine stalls under load
« Reply #44 on: November 13, 2022, 11:37:42 PM »
I THINK you need to find out why the wire melted.
Doug  '67 RS/SS 396 auto I know the car since new

R68GTO

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Re: At a loss - engine stalls under load
« Reply #45 on: November 14, 2022, 11:39:01 PM »
I agree Maroman.....does anyone have any tips on where to look next?  Could this be a problem under the dash?  In the steering column harness?  What likely suspects would cause the power to the coil to be interrupted under load?  What I do know is that with a brand new wire, the same symptoms exist, so it likely is not the wire.
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

Chuck-68-Cam

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Re: At a loss - engine stalls under load
« Reply #46 on: November 14, 2022, 11:47:01 PM »
I agree with someone who said earlier...It sounds fuel related? I see where it said you changed out the fuel pump, did you replace filters. It just sounds like not enough fuel when you punch it under load.
Chuck

68 Camaro SST 350/TH400

R68GTO

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Re: At a loss - engine stalls under load
« Reply #47 on: November 15, 2022, 12:34:47 AM »
Multiple times on the dyno when it stalled, we check the float bowls on the carb.  Each time the fuel level was at the bottom of the sight screw.
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

rich69rs

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Re: At a loss - engine stalls under load
« Reply #48 on: November 15, 2022, 03:44:49 AM »
What electrically is “in the circuit” now that wasn’t when you used the MSD distributor & box?

If the car truly ran correctly on the chassis dyno with your ignition system not used/bypassed then methinks you have isolated the problem.

Richard
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David K

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Re: At a loss - engine stalls under load
« Reply #49 on: November 15, 2022, 05:24:41 AM »
If possible, I would have a voltmeter connected to the coil to see if it stays put during the power loss.

R68GTO

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Re: At a loss - engine stalls under load
« Reply #50 on: November 15, 2022, 10:16:18 AM »
When the problem disappeared, we were running an MSD distributor and ignition box wired directly to the battery.  In prior attempts to solve, I had replaced both the distributor and coil with no change in symptoms.  That leaves the wiring from the positive side of the coil back to the ignition switch on the column.  Wire from firewall block to coil just replaced, so that leaves the stuff from the fuse box back to the column switch, and I guess the power from the battery to the column switch.  What could go wrong in those areas that only shows up with the engine under load?
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles

Tinkerr

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Re: At a loss - engine stalls under load
« Reply #51 on: November 15, 2022, 05:12:32 PM »
Was the replacement  Distributor points and condenser unit? If so you could use a ballast resistor run 12 volts direct to bypass the factory wiring for testing purposes. If that corrects your issue as it did with the MSD you know the issue is the car wiring.

R68GTO

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Re: At a loss - engine stalls under load
« Reply #52 on: November 15, 2022, 11:58:35 PM »
Tinkerr....to bypass the car wiring would you run a wire from the battery to the positive side of the coil with a ballast resistor in line to reduce voltage to 9.6V?  Would I then also have to run a jumper to the starter wire?
1969 427 COPO Camaro Lemans Blue/Black, M22 4 speed, 15,500 original miles