'69 C10 327 SBC Off Idle Stumble / Misfire Help - I feel like I've tried EVERYTHING

[SRT8-X]

I'll start with the basics; I have a 1969 C10 that has been going through a frame-off / nut and bolt restoration for several years. Below this introduction, please find the specs for the engine and drivetrain. I apologize in advance but my summary and narrative will be lengthy as I want to provide as much information and detail in hopes that someone has an idea of what may help.

Engine:

1965 Chevy 327 CID
Stock internals
Stock double hump “462 fuelie heads”
NEW Edelbrock Performer EPS Intake
NEW Edelbrock Performer 14063 600 CFM carb with electric choke
NEW Edelbrock Perfromer RPM 1721 Fuel Pump (reads 4.5-5.5 PSI on gauge)
NEW Edelbrock billet fuel filter and fuel line
NEW AC Delco GF61P in line fuel filter & steel fuel lines from tank
NEW Hamburgers 3225 1” aluminum spacer
NEW Performance Distributors / DUI HEI vacuum advance distributor with 50k volt coil and high dwell Dyna-Module, advance from timed side of the carb
NEW Performance Distributors / DUI C9051BK Live Wires under header 8mm wires with heat shield
NEW Autolite AP86 Platinum Plugs
NEW Ceramic Coated Shorty Headers
NEW 2-1/2” stainless exhaust with Flowmaster 40-series mufflers turned down at rear axle
NEW Billet Specialties breather with integrated PCV valve
Original GM in cab tank with sender
NEW Derale 16927 4000 CFM dual 13” puller cooling fans and shroud
NEW Full Painless wiring chassis wiring harness, electric fan harnesses fans on 195 / off 175
NEW 180 degree thermostat
NEW Lokar throttle cable conversion and kick-down cable
NEW Optima battery and true 0 gauge oxygen free copper cables
All gaskets are FelPro

Transmission / Drivetrain:

Mildly built TH350 trans with stock stall converter, B&M Trick Shift fluid
NEW Derale 14200 3-1/4” deep pan with cooling tubes
NEW Derale 13900 650 CFM Transmission cooler mounted under passenger side of cab
NEW B&M modulator with steel modulator vacuum line sourced from T-fitting at rear of intake
NEW aftermarket CPP Power Steering
NEW aftermarket Wilwood Master Cylinder / CPP Booster Power Brakes
3:73 stock rear differential with one-piece driveshaft, NEW Moser 30-spline axles and Bendix drum brakes

The problem is once the truck is warmed up, it begins to stumble or surge off idle, often misfiring from off idle to about 2500 RPM making it frustrating cruising around town. It runs OK cold but not great. Once it reaches 2500-3000 RPM, the misfire clears and it runs great. The kick-down works great and the truck is a blast 3000 RPM and up. However, off the line, even if taking off slowly with minimal throttle or flooring it, the truck does not go as it should, sputtering and popping, hesitating, etc. It has never been bad enough where it stalls though. WOT from standstill does not do much more than limited throttle off the line. Cruising and WOT is different however as it's further up the RPM range resulting in good performance. As noted, it runs great cool, idles great both cool and hot. It also does have an intermittent small miss at idle at times as well known by a vibration and a vacuum gauge. There is not enough power to take off well much less to spin the tires off the line or power-brake (under load).

The engine has not had any internal work or inspection since installing a few years ago even though we've only put about 400 miles on the truck since getting her roadworthy. The engine oil is clean and has Royal Purple 10W30 oil installed, K&N screw on filter and a touch of Marvel Mystery Oil. It does have a small transmission leak at the front seal / pump but shifts fine and does not show any signs of internal damage. The truck has an “under load” intermittent vibration at 40 MPH + that goes away if put in neutral leading me to believe there could be a converter balancing issue. We run 93 octane pump gas with Lucas ethanol treatment and Marvel Mystery oil at every fill up. I started using the Mystery Oil at the suggestion of a buddy who believed we may have a valve sticking. There is some valve train noise at times but it’s not significant beyond anything I’ve heard with other SBC engines.

Other than the new parts, I’ve tried tuning the carb for rich / lean settings, tried several variations of metering rods and springs, several choke settings, several idle settings, all (3) accelerator pump rod settings. I have not replaced or checked the accelerator pump itself or the floats in the carb, as it was new in the box prior to install. Vacuum registers about 20” Hg at ide but does have a -1-2” fluctuation at idle sporadically. The engine timing is set at 16° initial at idle and 36° total timing @ 3200 RPM. This has been verified several times and reset, adjusted, reset, etc. I have put heat insulation boots on the two rear plug wires due to their proximity to the header tubes and noticing an arc on cylinder #7 even with the live wires. The new live wires have white residue from resting on the headers but they are supposed to be rated to 1200 degrees. I have not seen any arcs here under darkness but some plug wires are very tight against portions of the headers due to the run being short on this custom set. Compression seems good based on feel but I have not tested each cylinder. I have not pulled the valve covers to set the lash but have physically observed the rockers and springs.

No parts, other work or tuning have made a significant change for better or worse. I’ve done all the work on the truck myself with the exception of the upholstery on door panels and dash pad so I’m opposed to trusting a local shop or others to try diagnosing this situation and don’t have an open checkbook to do so. My father was my mentor, this truck was his daily driver for nearly a decade in the 90’s and I’ve built it as a tribute to him after his passing in 2016. I’d really like to complete this and have a solid running truck as it’s been meticulously built otherwise. Please help if you can and if you have any ideas. Thanks in advance for your time if you’re able to reply with any suggestions or comments.

[AussieinNC]

Nice to see a beautiful truck...great job!

Performance Distributors / DUI HEI vacuum advance distributor ?

Did you have the distributor advance curve tailored to the truck, gearing etc?
Sounds like its a bit slow to bring in the advance....

Misfire under load could also be a plug lead arcing across to a ground...maybe to a header pipe....in Park try looking under the hood in the dark and blipping the throttle....darkness makes the spark more visible...

I would be looking more at the distributor first...including the module...

[SRT8-X]

Originally Posted by AussieinNC

Nice to see a beautiful truck...great job!

Performance Distributors / DUI HEI vacuum advance distributor ?

Did you have the distributor advance curve tailored to the truck, gearing etc?
Sounds like its a bit slow to bring in the advance....

Misfire under load could also be a plug lead arcing across to a ground...maybe to a header pipe....in Park try looking under the hood in the dark and blipping the throttle....darkness makes the spark more visible...

I would be looking more at the distributor first...including the module...

Thanks for the comments and I appreciate your response. I can’t say enough about the forum and how it’s allowed me to silently research but this has gone too far. The distributor was calibrated to 24 degrees at 3000 RPM per the build specs and for street applications. I set full timing at 36 not accounting for this spec. Do I need to offset the timing for this spec or is 36 degrees 36 degrees regardless. It does idle on or “diesel” sometimes too.
Posted via Mobile Device

[SRT8-X]

Originally Posted by SRT8-X

Thanks for the comments and I appreciate your response. I can’t say enough about the forum and how it’s allowed me to silently research but this has gone too far. The distributor was calibrated to 24 degrees at 3000 RPM per the build specs and for street applications. I set full timing at 36 not accounting for this spec. Do I need to offset the timing for this spec or is 36 degrees 36 degrees regardless. It does idle on or “diesel” sometimes too. I’ve tried looking at the wires under dark but will try again with some help and get an underneath view since they’re so tight.
Posted via Mobile Device

Posted via Mobile Device

[Steeveedee]

If you have the initial timing correct, the first thing I would suspect is that the carb is jetted too lean. I had a 1406 on my 305 CI in my Skylark, and with headers you will have to feed it a little more fuel. I think that I went 4 numbers richer on the jets but don't remember what I did with the metering rods. You will need to buy the kit. It isn't that expensive, and should give you what you need.

Mine would stumble off idle, but if I went heavier into the throttle, the accelerator pump would cover the lean condition just enough to start laying rubber. Does yours cover the stumble if you stab the throttle harder? Or does it just backfire?

[SRT8-X]

Originally Posted by Steeveedee

If you have the initial timing correct, the first thing I would suspect is that the carb is jetted too lean. I had a 1406 on my 305 CI in my Skylark, and with headers you will have to feed it a little more fuel. I think that I went 4 numbers richer on the jets but don't remember what I did with the metering rods. You will need to buy the kit. It isn't that expensive, and should give you what you need.

Mine would stumble off idle, but if I went heavier into the throttle, the accelerator pump would cover the lean condition just enough to start laying rubber. Does yours cover the stumble if you stab the throttle harder? Or does it just backfire?

Thanks, I have a kit but it’s for the older Carter AFB series and it seems all the part numbers changed on the jets and rods. I’ll grab a new Edelbrock kit and try to focus on the jets too if the timing situation isn’t the issue.

Mine stumbles off the line regardless of throte input. The only variable is that it does not run nearly as bad when cool. When cruising 30 MPH plus, it will downshift and run great when I stick it.
Posted via Mobile Device

[paul blair]

Have you done a compression test? How do the spark plugs look?

[SRT8-X]

Before and after changing the plugs, they tend to be brownish and dry at the electrode however there is some oil on the threads. I have not done a compression test with a gauge but the compression while bumping over with the finger was significant.

[68 P.O.S.]

First, have you verified top dead center with your balancer/zero timing mark?

Your timing sounds right on; 36 total is perfect. All in at 3200 rpm isn't bad. 2800-3000 may be a little better, but nothing of concern. You said the distributor is "calibrated to 24 degrees at 3000 RPM per the build specs and for street applications". Does that mean the mechanical advance is 24 degrees? How many degrees does the vacuum advance provide?

Do you still have the manual for the carb? You need it to setup the carb up right and calibrate it. You're gonna have to open your carb and set the float levels. They're always off out the box due to shipping and whatever. Since you have 20" vacuum, your step up springs should be the plain silver ones that come in the cal kit. Silver is the strongest/stiffest set.

What cam and specs? Fuel pump install ok? Any degraded or twisted fuel lines? Sounds like you have enough fuel pressure. Hmmmm.....

[SRT8-X]

[QUOTE=68 P.O.S.;8373603]First, have you verified top dead center with your balancer/zero timing mark?

Thanks for the reply. Yes, TDC was verified with the #1 screw in tool and was previously done with the compression as well.

Your timing sounds right on; 36 total is perfect. All in at 3200 rpm isn't bad. 2800-3000 may be a little better, but nothing of concern. You said the distributor is "calibrated to 24 degrees at 3000 RPM per the build specs and for street applications". Does that mean the mechanical advance is 24 degrees? How many degrees does the vacuum advance provide?

I’m going to call Performance Distributor tomorrow to ask about this and if it changes anything. I’m assuming the 24 degrees is all mechanical as they instruct purchasers to set timing with the vacuum advance disconnected which I followed. I have not timed with the advance connected but I can do this to verify what the vacuum advance is adding.

Do you still have the manual for the carb? You need it to setup the carb up right and calibrate it. You're gonna have to open your carb and set the float levels. They're always off out the box due to shipping and whatever. Since you have 20" vacuum, your step up springs should be the plain silver ones that come in the cal kit. Silver is the strongest/stiffest set.

Yes, I have the manual. I’ll definitely take a look at the floats and set them as well as trying the plain silver step up springs.

What cam and specs? Fuel pump install ok? Any degraded or twisted fuel lines? Sounds like you have enough fuel pressure.

Funny thing is the fuel pressure was consistent 5.5 psi when installing the fuel pump but now reads 4.5 or so a month later. This install went smooth but I did notice an oil leak at the pump base I need to take care of. The fuel lines are all new and I used all high end aluminum fittings along with the Edelbrock braided hose hit to the metal filter and carb fuel line.

Does the fact that this only occurs consistently when hot make a difference with some of these suggestions? Thanks again for the responses. These are helpful.

[Fbks_Cruiser]

Might want to get rid of those autolite plugs and install some AC Delcos. I have heard of folks having issues with autolites

[SRT8-X]

Originally Posted by Fbks_Cruiser

Might want to get rid of those autolite plugs and install some AC Delcos. I have heard of folks having issues with autolites

Thanks, I may go that route as well, I've heard the Gen I small blocks can be partial to AC Delco for some reason. Please note I ran regular Autolites originally and just switched to the platinum Autolites trying to rid this issue. Just another note, they are gapped at 0.50" +/- per Performance Distributors instruction due to the high voltage coil.

[geezer#99]

Your timing numbers don’t add up.
24 in the mechanical with a total of 36 gives you 12 initial not 16.
Is the esp intake an air gap type?
What cam?

Try a dry fuel pressure gauge. Oil filled ones don’t like heat.

[Mike C]

It sounds like once the carb transitions off the idle and primary circuit that it clears up. I too would be looking at jetting up and 4 sizes seems to be a good start.

Before that, try adding two full turns on the mixture screws to richen up the idle mixture and see if that doesn’t improve things.

I assume that all of these issues are at a steady throttle application and not when you are “punching it”? Sudden application of throttle and response issues are usually because of insufficient accelerator pump shot. But Edelbrocks are not as affected usually because of the secondary air door that keeps from giving too much air that the accelerator pump can’t make up for it.

Great looking truck BTW.

[HO455]

Try putting your timing light on each plug wire and running the the engine up to the rpm with misfire and see if the light flashes indicate a misfire. Does the flashing show smooth movement as the timing advances and retards on each cylinder?. This should help you decide if the problem is fuel or ignition. My first thought was that you have a bad ground inside the distributor at low RMP (advance) and once the advance mechanism moves the plate enough the ground is restored and the engine runs as it should. Have you tried disconnecting the vacuum line to the vacuum advance and running it?
I would concentrate on the ignition side before diving into the carburetion side. It would be easier to swap the distributor with a simple point style to prove the ignition is good or bad than swapping carbs. The fact that the problem gets better with RPM leads me to believe that you don't have a cap or plug wire problem.

[Coley]

Ok I'll weigh in with a possibility....you might not be getting full needed voltage to the HEI/coil (equivalent) at lower/idle speeds, noting that you list an HEI unit as the distributor.
If you aren't....then the engine will run like it misses at lower speeds and can be hard to start.
Anything above 2500-2800 rpm starts to smooth things out as the alternator puts out more voltage to compensate for the low end, low speed voltage deficiency.

You can test this by running a 'hot' 12 volt wire direct from the battery to your hei or coil....then start up the truck and see how it runs.
This will essentially bypass any original wiring that might be deficient or failing to provide the voltage lower down.

All good
Coley

[68 P.O.S.]

I think it’s a lost cause and you should give up so I can take that sweet truck off your hands

Yes, your distributor should have 24 degrees….was just trying to verify. Keep your total timing at 36, it should run very well there. Yes, you can plug your vac advance into the manifold vac port on the carb to see how much advance it provides. It should have no more than 10-15 degrees in it and be plugged in to manifold vac for it to work right. Another possibility, did you add the HEI and are you using the original power wire? The stock power wire has a resistor in it and won’t give the HEI the full 12 volts it needs. You can de-pin the wire from the harness and replace it with an un-resisted power wire.

Glad you kept the manual; it has lots of helpful info. Get the 1487 calibration kit, and together with the manual, you’ll have everything you need to set it up correctly.

Geezer is correct on the pressure gauge. The heat under the hood doesn’t agree with the liquid filled gauges and makes them read incorrectly. Maybe your Delco inline filter is making the fuel pump work a bit harder, reducing pressure? He’s also correct on your timing numbers too. I let that go since your total timing of 36 is more important than the initial.

I agree with HO455, make sure your ignition system is good to go, then move on to the carb. Test the distributor to make sure you didn’t get a bad unit. It happens.

[SRT8-X]

Originally Posted by Coley

Ok I'll weigh in with a possibility....you might not be getting full needed voltage to the HEI/coil (equivalent) at lower/idle speeds, noting that you list an HEI unit as the distributor.
If you aren't....then the engine will run like it misses at lower speeds and can be hard to start.
Anything above 2500-2800 rpm starts to smooth things out as the alternator puts out more voltage to compensate for the low end, low speed voltage deficiency.

You can test this by running a 'hot' 12 volt wire direct from the battery to your hei or coil....then start up the truck and see how it runs.
This will essentially bypass any original wiring that might be deficient or failing to provide the voltage lower down.

All good
Coley

Coley, good suggestions, I'll check this out. I had some electrical issues with the original harness last year which prompted me to rewire the complete truck with Painless Wiring's chassis kit so every wire on the truck is new, including homemade custom heavy braided stainless grounding straps in over a dozen locations.

My dad was an electrician and gear head so I know all about bad grounds and the issues they can cause in 12V systems. All the wiring has been installed meticulously and very clean but there still could be an issue with the coil wire when it comes through the Painless bulkhead. With that said, when I first set up the wiring harness, I ran a return 8 gauge wire "loop" per the instructions if running a 65amp + alternator but then removed it with some modifications as I had a parasitic draw that kept running the battery down. Not sure if the two were related but it has not had any issues since with the battery since.

I'll try running a dedicated wire from the battery to the coil direct and see if that helps. Thanks again for the suggestion.

[SRT8-X]

Originally Posted by HO455

Try putting your timing light on each plug wire and running the the engine up to the rpm with misfire and see if the light flashes indicate a misfire. Does the flashing show smooth movement as the timing advances and retards on each cylinder?. This should help you decide if the problem is fuel or ignition. My first thought was that you have a bad ground inside the distributor at low RMP (advance) and once the advance mechanism moves the plate enough the ground is restored and the engine runs as it should. Have you tried disconnecting the vacuum line to the vacuum advance and running it?
I would concentrate on the ignition side before diving into the carburetion side. It would be easier to swap the distributor with a simple point style to prove the ignition is good or bad than swapping carbs. The fact that the problem gets better with RPM leads me to believe that you don't have a cap or plug wire problem.

HO455, thank you for the suggestions. I'll give the timing light process you mentioned a shot. One thing to note, as I mentioned I am running Live Wires which are an 8mm wire with 2mm heat resistant cover. I noticed when setting the timing, the light would get odd RPM readings, sometimes half of what it should be or zero, when on #1. If I jiggle the induction pick-up it comes back where it should be but it "lost" the RPM several times each time I used this light.

Regarding the distributor, I started with a stock Delco HEI which had an Accel High Energy coil, switched that out for a new DUI cap, rotor and high energy coil, swapped that out for the entire new DUI drop in unit with same specs but an upgraded Dyna Module. The problem never changed with these upgrades and was present before I started "hunting". I doubt the distributor could be the issue since the various combinations produced the same issue, but its possible. I also ran a set of 8mm Taylor wires prior to the Live Wires with the same results.

I have run the truck with the advance disconnected under load but not since the carb / distributor were changed. I did this prior to the new carb and prior to the distributor upgrade but it was the same before and after.

Thanks again for your replies.

[SRT8-X]

Originally Posted by Mike C

It sounds like once the carb transitions off the idle and primary circuit that it clears up. I too would be looking at jetting up and 4 sizes seems to be a good start.

Before that, try adding two full turns on the mixture screws to richen up the idle mixture and see if that doesn’t improve things.

I assume that all of these issues are at a steady throttle application and not when you are “punching it”? Sudden application of throttle and response issues are usually because of insufficient accelerator pump shot. But Edelbrocks are not as affected usually because of the secondary air door that keeps from giving too much air that the accelerator pump can’t make up for it.

Great looking truck BTW.

Thanks for the suggestions. I've decided to get a new Edelbrock tuning kit, the one I have is an older Carter AFB which does not seem to interchange numbers with the Performer carbs. I've played with the step up springs and metering rods but will try jetting the carb when I get the new kit.

I have adjusted the idle mixture screws every which way and until they're bottomed out or removed completely, there is not much change at idle.

I'm thinking about replacing the accelerator pump as well just to make sure. Punching the throttle does not do much off idle, it bogs and sputters however at a 30 MPH roll, punching it seems to be fine. That's what is baffling to me, still in a lower RPM cruise range punching it will allow good take off, just not from a really low speed or take off from a dead stop.

Thank you again for the comments, much appreciated.

[SRT8-X]

Originally Posted by geezer#99

Your timing numbers don’t add up.
24 in the mechanical with a total of 36 gives you 12 initial not 16.
Is the esp intake an air gap type?
What cam?

Try a dry fuel pressure gauge. Oil filled ones don’t like heat.

Thanks for the reply. I did not account for the calibration in the distributor in setting the timing. I found TDC, verified, set initial to 16° at idle (950 - 1000 RPM) at the zero mark on the timing tab, then set total timing to 36° at 3200 RPM. I will be calling Performance Distributors to verify this but their instructions for setting timing comply with the method I used, not referencing the calibration specs.

The intake is the traditional non-air gap type, it's rated from idle to 5500 RPM.

I do not have the cam specs but it's pretty basic, nothing radical, no significant lope, idle is smooth.

[geezer#99]

Originally Posted by SRT8-X

Thanks for the reply. I did not account for the calibration in the distributor in setting the timing. I found TDC, verified, set initial to 16° at idle (950 - 1000 RPM) at the zero mark on the timing tab, then set total timing to 36° at 3200 RPM. I will be calling Performance Distributors to verify this but their instructions for setting timing comply with the method I used, not referencing the calibration specs.

The intake is the traditional non-air gap type, it's rated from idle to 5500 RPM.

I do not have the cam specs but it's pretty basic, nothing radical, no significant lope, idle is smooth.

That’s why your numbers don’t add up!
At 950-1000 rpm you’re getting into the mechanical advance in the distributor.
You need an idle lower near 700 to set initial.

Did you build the motor?
Who set the preload on the lifters?

[SRT8-X]

Originally Posted by geezer#99

That’s why your numbers don’t add up!
At 950-1000 rpm you’re getting into the mechanical advance in the distributor.
You need an idle lower near 700 to set initial.

Did you build the motor?
Who set the preload on the lifters?

Geezer#99, I'll give it a shot setting the timing at idle numbers lower per your suggestion. I appreciate you catching this!

I did not build the motor, it was purchased as a built long block which I went through and properly torqued all the main and rotating assembly connections. My father may have adjusted the valvetrain on the long block when we installed but I cannot recall. As far as I know, no valvetrain adjustments have been made. We were working on this project together for years and I can't be certain of everything he did prior to his passing 2 years ago.

[geezer#99]

Originally Posted by SRT8-X

Geezer#99, I'll give it a shot setting the timing at idle numbers lower per your suggestion. I appreciate you catching this!

I did not build the motor, it was purchased as a built long block which I went through and properly torqued all the main and rotating assembly connections. My father may have adjusted the valvetrain on the long block when we installed but I cannot recall. As far as I know, no valvetrain adjustments have been made. We were working on this project together for years and I can't be certain of everything he did prior to his passing 2 years ago.

Best to keep your idle down near 700 all the time. That way you keep the idle transfer slot covered correctly (your mix screws work then) and you’re not idling on the power circuit like you are at 900 or so rpm.
If it wants to die going into gear at 700 then you need more initial.
I mentioned lifter preload because it’s possible all your problems are related to a valve or two not seating fully.
Just another thing to look at.
If you rerun the valves just ask here for guidance here first.
And leave the carb alone until you’ve optimized your timing.
After all the motto is ‘Timing first, carb second’.

And your run on isn’t only timing related. It’s also caused by too much air which you’re feeding it with your primary throttle position.

And the other one is ‘99% of carb problems are actually timing problems’.

[geezer#99]

Timing can do that.
Too much fuel pressure can do that.
A valve hung open can do that.
Lots of possibilities.

You gotta start over.
Rerun your valves.
Optimize your timing.
Verify your fuel pressure at all rpm’s.
Adjust your carb.
See what it does then.

If your timing light is wacky like you mentioned just replace #1 wire with a normal stock type wire.