Severe engine mis/stumble, much worse when cold

[toms68cst]

Hi folks,

I’ve been having some severe stumbling/mis on my 68 I can’t figure out.

When cold, or even before it reaches operating temperature on a warmish day, it will hardly rev up under load. I can accelerate out of first but in second or third gear it will stumble severely. It's better when warmed up but running on the highway or letting it rev hard under load, it will still stumble severely.

It's been very cold blooded in recent years exhibiting the same stumble but previously only when it's very cold outside.

Things I’ve done recently:

Replaced spark plugs (The old ones looked good), Replaced fuel filter at the carb. The sending unit is newish so I think the sock filter there is OK.

The choke pull-off seems to work ok and the secondaries will kick in when it's at operating temp.

My mechanic checked the timing and the fuel mixture recently.

It’s the original 327/q-jet. The q-jet was rebuilt about 4-5 years ago by my mechanic. It has a 10yo stock-look Pertronix distributer. (I have a new ignition coil coming.) My mechanic said the distributer cap looked OK.

I can’t say for sure if it’s the carburetor/fuel or ignition at this point. Any ideas for troubleshooting?

TIA, Tom

[Rust_never_sleeps]

I'd want to verify that the choke is behaving itself first

[Steeveedee]

Make certain that the choke closes all the way when cold, and that the choke pull-off is set to specs, both for the current ambient temperature(s). We get used to the late-model fuel-injected, computer-controlled vehicles, and lose sight of these settings. Used to be that the divorced choke rods that closed the choke were adjusted seasonally. Same for the internal chokes- there are even indicators on the choke housing for this.

['68OrangeSunshine]

I suspect the Pertronix gear.
Could be the module. Ten years is a long time for those components.
Granted, in my situation, the Set had been on the shelf a few years before I tried it, but it burned out after 7 years of active duty as a DD.

I had a Pertronix Ignitor II, Flamethrower II on my 292 L6, in a '68 C/10 Stepside. It worked fine for about 7 years, then it was OK, at city speeds [ 25 - 50 MPH], but when I got over 50 or 60 MPH, ~3000RPM -- the Ignition would flat cut out.
[This is the RPM that it was supposed to boost performance.]
On the freeway at speed, the ignition would kill at 3000, and I was deadmilling, engine turning but not firing.
Kick in Clutch, IGN OFF then ON and popstart at 45 or 50 in 3rd or 4th.
Didn't make me popular with the commuter right behind me.
I switched back to Points for a year, and it ran well. Later I found an L6 HEI. Six years later it's still working fine.
Ditch the Pertronix.

[toms68cst]

Wow, that almost sounds exactly like my symptoms. I still have my original distributer on hand. I might have to throw it back in.

Originally Posted by '68OrangeSunshine

I suspect the Pertronix gear.
Could be the module. Ten years is a long time for those components.
Granted, in my situation, the Set had been on the shelf a few years before I tried it, but it burned out after 7 years of active duty as a DD.

I had a Pertronix Ignitor II, Flamethrower II on my 292 L6, in a '68 C/10 Stepside. It worked fine for about 7 years, then it was OK, at city speeds [ 25 - 50 MPH], but when I got over 50 or 60 MPH, ~3000RPM -- the Ignition would flat cut out.
[This is the RPM that it was supposed to boost performance.]
On the freeway at speed, the ignition would kill at 3000, and I was deadmilling, engine turning but not firing.
Kick in Clutch, IGN OFF then ON and popstart at 45 or 50 in 3rd or 4th.
Didn't make me popular with the commuter right behind me.
I switched back to Points for a year, and it ran well. Later I found an L6 HEI. Six years later it's still working fine.
Ditch the Pertronix.

[Sheepdip]

Pull your distributor cap and rotor and check the play on the shaft, worn bearings will cause this issue....ask me how I know.

[toms68cst]

Thanks everyone for the help. I'll have to do some checking tonight.

I was wondering if there are any good alternatives to the Pertronix stock-look electronic distributer. I don't want any big ugly thing in there. Also, I wonder if clearance for installation is an issue with the bigger HEI units.

[geezer#99]

To fit an hei you need 3 inches from the centre of the distributor to the firewall.
If you have room then buy a cheap hei from eBay.
If that doesn’t help then you’ll know your pertronix is good.


https://www.ebay.com/itm/18622880613...BlBMUIjwjNDWZA

[Sheepdip]

Originally Posted by Sheepdip

Pull your distributor cap and rotor and check the play on the shaft, worn bearings will cause this issue....ask me how I know.

Just to add to this when your distributor shaft wiggles out of/off center it throws the dwell required to properly charge your coil into a tizzy and creates the stumbling, popping lack of RPM or..."E" all the above

['68OrangeSunshine]

Originally Posted by toms68cst

Thanks everyone for the help. I'll have to do some checking tonight.

I was wondering if there are any good alternatives to the Pertronix stock-look electronic distributer. I don't want any big ugly thing in there. Also, I wonder if clearance for installation is an issue with the bigger HEI units.

I got a Remote Coil HEI. Works just as well as the High Hat one. Mine's on a 292 L6, but you get the idea.
Bottom right.

[toms68cst]

Well, I've done some further testing. I tried a new coil with the pertronix and it made no difference. I tried the old distributor with the new coil, and it made no difference.

I'm wondering if it's not a carb or fuel delivery issue. It's weird, I can floor it in first gear and it will rev up no problem. I can floor it in second and it will cut out on high revs if it's accelerating hard. It will cut out at moderate RPM's in third. When it cuts out, it takes a second or two to recover. Right now, I can hardly get it past about 50MPH.

Could the fuel pump be marginal? Would something with the secondaries be malfunctioning?

[Steeveedee]

That has all the earmarks of a fuel delivery issue. There's enough in the float bowl even with a Q-Jet to get you through first and into second. If you haven't already, verify that all the soft lines are good.

[sixty8HDC30]

^ Agreed, could be a weak fuel pump too. sounds like its running out of fuel under load at higher RPM

[toms68cst]

I think I found a smoking gun. I went to change out the fuel pump and noticed a pretty bad kink in the little loop back rubber line to the suction of the pump. It was also getting pretty brittle. I'll get things back together tomorrow morning.

[toms68cst]

Problem solved! Replaced the fuel pump, (The old one was probably Okay) and the brittle hose. Reves up and pulls nice now.

Lessons learned:

Look for simple/obvious issues first.

Don't let the fuel pump push rod drop and jam.

The truck runs better with the distributer cap on.

['68OrangeSunshine]

Good to hear.

[RichardJ]

Off Topic for 68orangesunshine.

The large cap HEI with external coil was only made for the inline six and only for '75 and '76.
Looks like you're missing the bracket and the two stud bolts to mount the coil properly.

[Stocker]

Originally Posted by toms68cst

Problem solved! Replaced the fuel pump, (The old one was probably Okay) and the brittle hose. Reves up and pulls nice now.

Lessons learned:

Look for simple/obvious issues first.

Don't let the fuel pump push rod drop and jam.

The truck runs better with the distributer cap on.

Glad it's all good now!

[RichardJ]

>>Don't let the fuel pump push rod drop and jam.<<

Short bolt on front of block can be removed and temporarily replaced with longer bolt before old pump is removed. Finger tight, it will hold pump rod in the up position.

Not mine, this video on YouTube will show you this 60 year old trick.

https://www.youtube.com/watch?v=pIvrCgZIlzQ&t=69s

[RustyPile]

Well, this just proves that old adage... 70% of the ignition problems are in the fuel system...

['68OrangeSunshine]

Originally Posted by RichardJ

Off Topic for 68orangesunshine.

The large cap HEI with external coil was only made for the inline six and only for '75 and '76.
Looks like you're missing the bracket and the two stud bolts to mount the coil properly.

Richard J: Thanks for that fact. I was unaware the Remote Coil HEI was for the L6-Only... I had assumed GM also made a remote coil for V8s. In fact, I vaguely remember seeing them in the aftermarket.
[I used to have one, but it's a plastic cap with a long spring from a plugwire stud on top to cover the distance an inside coil would take up, allowing use of any preferred high performance coil externally. May not have the diminished height desired for a low clearance. ]
When updating electrical systems for my 67/72 trucks, I often scan the parts available for the next decade and modify accordingly.
On the Flying Coil set up -- as I was unable to source the OEM Remote Coil mount and its stud-head bolts, (forged from purest unobtainium), we adapted, improvised and overcame with cutting down some industrial structural extrusion into a steel arm to hold the coil.
Works fine, should last a long time.

['68OrangeSunshine]

Originally Posted by RichardJ

>>Don't let the fuel pump push rod drop and jam.<<

Short bolt on front of block can be removed and temporarily replaced with longer bolt before old pump is removed. Finger tight, it will hold pump rod in the up position.

Not mine, this video on YouTube will show you this 60 year old trick.

https://www.youtube.com/watch?v=pIvrCgZIlzQ&t=69s

My 1998 MrGoodwrench 350 has that fuel pump push rod Stop Bolt provision, but I'm not sure all the SBC blocks did. Especiallt early ones.
Sure beats trying thick Red Lube, though. Doesn't work in th Tucson Summer.

[geezer#99]

Originally Posted by '68OrangeSunshine

My 1998 MrGoodwrench 350 has that fuel pump push rod Stop Bolt provision, but I'm not sure all the SBC blocks did. Especiallt early ones.
:

Those 2 bolt holes have been there ever since the first 265 came out in 55.
They were used for the front motor mount brackets.

['68OrangeSunshine]

Originally Posted by geezer#99

Those 2 bolt holes have been there ever since the first 265 came out in 55.
They were used for the front motor mount brackets.

As an inveterate Inliner, there is much about the mysterious SBC V8 I don't know.
L6s have a direct cam connection to a shoe on the bottom of the fuel pump.

When I did my first Crate Engine swap into my '71 Jimmy in 1999, I wondered why the motor was fuel starved after install. It started on ether OK. Then some one asked if I'd put in the Push Rod? I found one still in the old 307 that had sucked 2 valves, and tried it. SOB! -- it worked!

[toms68cst]

Yes, I looked at a video showing the front bolt hold trick AFTER I removed the fuel pump. This old man isn't used to consulting Youtube. I did get pretty good at finding #1 TDC and installing the distributor though!

Appreciated all the help here too!

Originally Posted by RichardJ

>>Don't let the fuel pump push rod drop and jam.<<

Short bolt on front of block can be removed and temporarily replaced with longer bolt before old pump is removed. Finger tight, it will hold pump rod in the up position.

Not mine, this video on YouTube will show you this 60 year old trick.

https://www.youtube.com/watch?v=pIvrCgZIlzQ&t=69s