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

[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’.

[SRT8-X]

Originally Posted by geezer#99

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

Thanks again geezer#99, I've read a few threads on here and others regarding setting the valve / lash adjustment. This was on my list but I was saving it in hopes of not making a huge mess but I'm pretty sure my dad had an old small block stamped steel valve cover with the top center cut out for setting valves. I'll be able to reproduce if not....If anyone wants to reference a link to a proper valve setting instruction thread I'd appreciate it.

The idle does drop approx. 250-300 RPM going into gear so we'll see if I can set the idle low and readjust the timing to see what happens. I'll check the floats first just to verify they're set properly while I'm doing the timing and leave it alone beyond that.

A quick question, will the timing being off / too far advanced / not optimized clear up at higher RPMs because the advance and vacuum at those engine speeds or is it something else? Again, that's what has always baffled me, the certain specific times when this occurs.

Thanks again!

[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.

[paul blair]

Hi Keith, Do a compression check and see if that tells you some thing! I had a broken valve spring on one cylinder and chased the ignition and fuel systems and did not find the problem until a compression test and a visual on the valve springs. At least you will know if you have a tight engine!

[SRT8-X]

Originally Posted by 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.

Thanks again! I'll get back on this after checking it out, looking at the valves and compression.

[SRT8-X]

Originally Posted by paul blair

Hi Keith, Do a compression check and see if that tells you some thing! I had a broken valve spring on one cylinder and chased the ignition and fuel systems and did not find the problem until a compression test and a visual on the valve springs. At least you will know if you have a tight engine!

Thanks Paul! I'm headed this route after a few suggestions. I'm going to take a look at the valve lash and springs too. I appreciate it.

[geezer#99]

Check for uniform rocker movement.
I’ve used tall pieces of cardboard as oil deflectors when running the valves.
When you set them just start with #1 and back off each nut until they click and then tighten just enough to be quiet. Do all that way. Then shut the motor off and adjust all to 1/2 turn.
Sometimes if you’re adjusting the preload too quick you can cause the motor to stumble and die.
Half a turn gets you close depending on whose lifters are in it. Factory setting is likely 1 turn but not all lifters are created the same. Some design differences from cam company to cam company.
Then do a compression check.

[68 P.O.S.]

Your HEI specs sound fine, just have to verify it’s working properly. After your rewiring job, it should be getting the full 12 volts, so just verify that to be sure. Is the vacuum advance adjustable? Normal HEI spark plug gap is .045, but if DUI suggests .050 for their unit, keep it there.

Since you’re a little confused on timing, here’s the skinny. Initial timing is just the engine by itself without any mechanical or vacuum advance added in. Rotating the distributor adjusts it and gets the engine in the ballpark to ensure its working well. Basically a coarse adjustment. Total timing is the engine’s initial timing, plus however many degrees of mechanical advance the distributor has. This fine tunes the initial and mechanical advances together. To adjust it, the engine is rev’d to high rpm (3-4K + rpm, depending on the spring strength) until it stops advancing, which is when full mechanical advance is reached. Personally I just take the springs out (don’t lose them!) and rev it between 1500-2000 rpm to set it because, #1, I’m not too fond of the engine screaming at 4K rpm while my head is under the hood, and #2, full mechanical advance is achieved at a much lower rpm without the springs holding it back. Using your engine and 36 degrees total timing an example….24 degrees mechanical advance, plus 12 degrees initial timing, equals 36 degrees total timing. 24+12=36. That’s your total timing. Then there’s the vacuum advance. The vac advance is used to add an extra amount of timing to increase the engine’s efficiency during low load situations; idle and cruise. Rich mixtures burn quickly, lean mixtures do not. Idle and cruise are lean operating conditions and need the extra timing from the vac advance to burn the lean mixture efficiently. So to achieve this, your vac advance must be plugged in to manifold vacuum. Timed, or ported vacuum, is used for emissions equipment your engine may be required to run, such as EGR valves, smog stuff, etc. Our trucks aren’t encumbered with that, so we can use the vac advance to its full advantage. Adding too much timing will cause detonation, so 10-15 degrees vac advance is plenty. Once hooked up to manifold vac, the extra timing will increase the idle, so you can lower it back down to 750 rpm and you should be able to adjust your idle mixture screws as outlined in the manual. Here’s a timing article written by a GM engineer…more than you want to know I bet, ha, but invaluable info: http://www.camaros.org/pdf/timing101.pdf

Valve adjustment is a great idea as well. Use Comp Cams’ EOIC method for adjusting them dead on.

[notsolo]

Just changed the gap in my spark plugs, old ac plugs I was told .045-.050.....went back to .035...runs alot better...Just my 2 cents I know nothing about high voltage coil...etc.

[AussieinNC]

As I mentioned I suspected the distributor and or module is not working correctly...

Your mentioning dropping the plug gap down to .035 and it runs better, confirms a lack of high voltage for the spark system...

To check the system, drop in a standard HEI distributor setup and retry your test drive...dont be surprised if it fixes the off idle stumble and misfire...

[SRT8-X]

Originally Posted by AussieinNC

As I mentioned I suspected the distributor and or module is not working correctly...

Your mentioning dropping the plug gap down to .035 and it runs better, confirms a lack of high voltage for the spark system...

To check the system, drop in a standard HEI distributor setup and retry your test drive...dont be surprised if it fixes the off idle stumble and misfire...

Thanks AussieinNC, however that was another individual posting about the gaps. I put out a lot of information so if it was missed, I had an original OEM HEI set-up on this engine when setting it up initially. It had the same issue then and after just replacing the cap, rotor and coil. Then I replaced the entire distributor and the module, still the same.

Same story with the plugs, originally gapped at 0.035 and A86's then went to AP86's gapped at 0.050. Same result. I tried Performance Distributors on the phone all day today and got a busy signal so I sent a message. They do offer to have customers send in the units for testing and repair and if they don't find anything, they charge a $25 service fee so that's a viable option if I can't rule ignition out.

[AussieinNC]

Thx for the update...

This may sound silly,,, but there is a good ground strap in place between the engine block and the cab of the truck?

I have witnessed a simple missing ground strap drive guys insane for weeks....

[SRT8-X]

Originally Posted by 68 P.O.S.

Your HEI specs sound fine, just have to verify it’s working properly. After your rewiring job, it should be getting the full 12 volts, so just verify that to be sure. Is the vacuum advance adjustable? Normal HEI spark plug gap is .045, but if DUI suggests .050 for their unit, keep it there.

Since you’re a little confused on timing, here’s the skinny. Initial timing is just the engine by itself without any mechanical or vacuum advance added in. Rotating the distributor adjusts it and gets the engine in the ballpark to ensure its working well. Basically a coarse adjustment. Total timing is the engine’s initial timing, plus however many degrees of mechanical advance the distributor has. This fine tunes the initial and mechanical advances together. To adjust it, the engine is rev’d to high rpm (3-4K + rpm, depending on the spring strength) until it stops advancing, which is when full mechanical advance is reached. Personally I just take the springs out (don’t lose them!) and rev it between 1500-2000 rpm to set it because, #1, I’m not too fond of the engine screaming at 4K rpm while my head is under the hood, and #2, full mechanical advance is achieved at a much lower rpm without the springs holding it back. Using your engine and 36 degrees total timing an example….24 degrees mechanical advance, plus 12 degrees initial timing, equals 36 degrees total timing. 24+12=36. That’s your total timing. Then there’s the vacuum advance. The vac advance is used to add an extra amount of timing to increase the engine’s efficiency during low load situations; idle and cruise. Rich mixtures burn quickly, lean mixtures do not. Idle and cruise are lean operating conditions and need the extra timing from the vac advance to burn the lean mixture efficiently. So to achieve this, your vac advance must be plugged in to manifold vacuum. Timed, or ported vacuum, is used for emissions equipment your engine may be required to run, such as EGR valves, smog stuff, etc. Our trucks aren’t encumbered with that, so we can use the vac advance to its full advantage. Adding too much timing will cause detonation, so 10-15 degrees vac advance is plenty. Once hooked up to manifold vac, the extra timing will increase the idle, so you can lower it back down to 750 rpm and you should be able to adjust your idle mixture screws as outlined in the manual. Here’s a timing article written by a GM engineer…more than you want to know I bet, ha, but invaluable info: http://www.camaros.org/pdf/timing101.pdf

Valve adjustment is a great idea as well. Use Comp Cams’ EOIC method for adjusting them dead on.

Thanks 68 POS, I appreciate the informative response. I'm good with the timing now, thanks for the refresher, I just wasn't sure regarding the original calibration they put on the distributor.

[SRT8-X]

Originally Posted by AussieinNC

Thx for the update...

This may sound silly,,, but there is a good ground strap in place between the engine block and the cab of the truck?

I have witnessed a simple missing ground strap drive guys insane for weeks....

Yessir, I have ground straps at the frame to the block on both sides, the heads to the firewall on both sides, frame to cab in (4) places and same with the bed to frame. I'm confident the wiring and the grounds are good.

[SRT8-X]

Quick update (sure, as you all can tell, I'm not quick)...

I lowered the idle to 650 RPM or so, set initial to 12° (it was confirmed only the 4° off from my initial 16° setting so timing was accurate anyway) and 36° at 3000-3200 RPM.

I ran the engine at various RPM in the garage while working on this and getting to operating temps. This was likely 25-30 minutes as I was doing a few other things, looking at vacuum (dropped about 8-10" due to idle speed) and then adjusting the idle mixture screws after timing. I also tested the accelerator pump with the engine off and its squirting plenty on the front barrels of the carb. I was checking plug wires in the dark, underneath the truck, etc. and no arcs were noticed. It could be my head but it did seem the misfire at idle changed when moving some of them although I couldn't see any arcing.

I left the vacuum advance unplugged for the test drive as I wanted to see how it drove based on the total timing alone. I also left the air cleaner off. Oddly enough, here's where it gets funny, I backed out of the driveway, then put it in drive and went down the street for a few stop and go take offs. It did absolutely wonderful with mid-full throttle off the line starts as well as minimal throttle off the line. Ran like a champ, smooth and misfire free....for about a mile. As soon as I took a curve, it began to act up bogging and hesitating. It did this a few times but for the most part was running better. To me it seemed like curves or turns were worse than straight take offs. Not sure if this means anything. About 5 minutes later and a few more turns, it was bogging and hesitating like it's been doing. Coming home after a 15 minute drive in traffic, it would hesitate intermittently at off idle speeds. As noted previously, it always ran fine when the kick down was activated. Some turns and take offs whether light or mid throttle were OK, others were not. Full throttle did nothing more than light throttle. Not sure if any of this means more or not, just thought I'd report it.

Still going to look at the valve adjustment, the float settings and the jets. I found the .101 jets in my kit so no need to buy one just to test the richer settings.

After thinking about it more while driving and listening to feedback, its seeming more and more like a lean setting and starving for fuel unless in the mid to upper power band. I'm not ruling anything out yet however.

Thanks again for everyone's input. I'll keep trying.

[AussieinNC]

PM to you on the way

[Mike C]

Something is drastically wrong with your carb if you can bottom out or remove the idle mixture screws with ZERO net change.

If you have not bought a book on tuning an AFB/Edelbrock carb, I'd be looking at Amazon. Something is really wrong with yours. I have a 750 cfm Edelbrock on my boat and a 600 Carter on my CJ5 and they don't behave that way at all.

[geezer#99]

Sounds like a float problem.
When you check the float level also check the float drop. It has to be set right too.

[Fbks_Cruiser]

Quote: “I lowered the idle to 650 RPM or so, set initial to 12° (it was confirmed only the 4° off from my initial 16° setting so timing was accurate anyway) and 36° at 3000-3200 RPM.

I ran the engine at various RPM in the garage while working on this and getting to operating temps. This was likely 25-30 minutes as I was doing a few other things, looking at vacuum (dropped about 8-10" due to idle speed)” Quote

Your initial post said you had about 20” of vacuum at idle. Does this mean you now only have 10” vacuum at idle? That is way low. Barring a vacuum leak somewhere, such low vacuum indicates possible valve issues as discussed in other posts

[HO455]

A couple of items that may help. It sounds like backing the timing off helped. Have you verified that your timing marks for TDC are correct?

Misfires while turning does suggest a float level problem. When you check the float level make sure that the floats are not fuel soaked or if brass that they have no fuel inside of them. Also check that they move freely when holding the air horn level and at angles to mimic the forces felt when turning.

Your plug wires could have high internal resistance that may be changing as you move the wires. Here is what I use to test plug wires and spark power.
https://www.amazon.com/Lisle-20700-C.../dp/B00IXMA1RY

Have you tried wiring the choke open to take it out of the equation?

It is quite possible that you have more than one problem contributing to your symptoms.

Here's a link to a valve adjustment procedure.

https://www.crankshaftcoalition.com/...raulic_lifters

[SRT8-X]

Originally Posted by Fbks_Cruiser

Quote: “I lowered the idle to 650 RPM or so, set initial to 12° (it was confirmed only the 4° off from my initial 16° setting so timing was accurate anyway) and 36° at 3000-3200 RPM.

I ran the engine at various RPM in the garage while working on this and getting to operating temps. This was likely 25-30 minutes as I was doing a few other things, looking at vacuum (dropped about 8-10" due to idle speed)” Quote

Your initial post said you had about 20” of vacuum at idle. Does this mean you now only have 10” vacuum at idle? That is way low. Barring a vacuum leak somewhere, such low vacuum indicates possible valve issues as discussed in other posts

Sorry for the mistake, I meant to write 8-10%. The vacuum at the low idle was around 18-18.5" Hg. It still had a minor drop / fluctuation at idle at times and was steady at times as well during this tuning.

[SRT8-X]

Originally Posted by geezer#99

Sounds like a float problem.
When you check the float level also check the float drop. It has to be set right too.

Thanks again geezer, I will be checking the float and float drop this week.

[SRT8-X]

Originally Posted by HO455

A couple of items that may help. It sounds like backing the timing off helped. Have you verified that your timing marks for TDC are correct?

Misfires while turning does suggest a float level problem. When you check the float level make sure that the floats are not fuel soaked or if brass that they have no fuel inside of them. Also check that they move freely when holding the air horn level and at angles to mimic the forces felt when turning.

Your plug wires could have high internal resistance that may be changing as you move the wires. Here is what I use to test plug wires and spark power.
https://www.amazon.com/Lisle-20700-C.../dp/B00IXMA1RY

Have you tried wiring the choke open to take it out of the equation?

It is quite possible that you have more than one problem contributing to your symptoms.

Here's a link to a valve adjustment procedure.

https://www.crankshaftcoalition.com/...raulic_lifters

HO455, thanks for the updated response. Yes, I've verified TDC and the marks several times. My new mark is about 4° behind that of the original mark on the balancer.

Noted on the floats, I appreciate this information, I will check this as well.

Regarding the choke, I have not wired it open but have run several settings. I may give this a try as well but the choke seems to function properly and visibly open when it's supposed to be. Another note, the same situation was apparent with the old Carter AFB as well, it had the same or similar problems with the old carb and intake set-up.

Thanks for the link on valve adjustment, I'll plan to tackle this on the upcoming weekend and visually inspect the springs as well.

[Ironangel]

Run your valves again to verify you dont have one or more hanging open. Use the EOIC method, never mind cutting a valve cover. Secondly, maybe I missed it but what cam & lifters are you running? Thirdly, find a proven functional carb off another small block with comparable cam and heads just to verify a carb problem. Surely a friend or board member, perhaps a local speed shop or drag racer has a carb laying around to loan for a couple of test hits?...Lastly, read up on mods to the accelerator pumps on those AFB carbs. With those heads and a comparable semi race cam a stock 600 cfm AFB squirter aint going to cut it. I'm thinking either more volume of the squirt, and/or a longer duration of the squirt will make your 327 snap to attention...I'd loan you a proven Cliff Ruggles Quadrajet if you were closer and running a spread bore intake...Good luck, your well on your way to becoming an expert AFB guy!

[SRT8-X]

Originally Posted by Ironangel

Run your valves again to verify you dont have one or more hanging open. Use the EOIC method, never mind cutting a valve cover. Secondly, maybe I missed it but what cam & lifters are you running? Thirdly, find a proven functional carb off another small block with comparable cam and heads just to verify a carb problem. Surely a friend or board member, perhaps a local speed shop or drag racer has a carb laying around to loan for a couple of test hits?...Lastly, read up on mods to the accelerator pumps on those AFB carbs. With those heads and a comparable semi race cam a stock 600 cfm AFB squirter aint going to cut it. I'm thinking either more volume of the squirt, and/or a longer duration of the squirt will make your 327 snap to attention...I'd loan you a proven Cliff Ruggles Quadrajet if you were closer and running a spread bore intake...Good luck, your well on your way to becoming an expert AFB guy!

Thanks Ironangel! I appreciate the advice. I was thinking old school with the valve adjustment when I suggested that but have since been turned on to the EOIC method and watched a few good videos on this. I just remember seeing my dad do this running a few decades ago and haven't needed to adjust an SBC valves until now.

Regarding the cam, it's mild at best, no lope or choppy idle but I cannot be certain what the specs are on the cam and lifters. I'd say its similar to what may have been in this 327 stock and I doubt it was one of the HP 'Vette units. My knowledge is that it was from a '65 Nova but even this could have been 275HP+.

Noting again that I had the previous Carter AFB 600 CFM with similar issues, this makes me believe it's not the carb since my dad was a carb guru and rebuilt AFB, Holley and Q-Jet's for a hobby. I'll definitely take a look at a borrowed carb if the float adjustment doesn't do the trick as well as accelerator pump alternates.

Thanks again for the comments and encouragement!