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

[SRT8-X]

I'm posting the following picture of the lifter valley just to determine if anyone sees any lifters which look out of position or sequence. I know I need to verify / adjust the valves but figured one of you may be able to visually see something wrong here. I don't know what position the engine is at this point, it was when I was installing the new intake and carb. Just figured it may help identify an issue with the valves if present.

[geezer#99]

Remember that when you do the valves again, if you do it cold and use the eoic method, wiggle the slack out of the rocker. Twisting the rod is harder to get zero lash right on.

[68 P.O.S.]

Originally Posted by SRT8-X

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 sounds like the float levels and fuel pressure to me. Especially since it's doing it during turns and curves.

[68 P.O.S.]

Another thing to look at is the accelerator pump assembly. I once had the rubber plunger un-seat itself from the pump rod and was floating around freely. It sort of worked still, but it obviously wasn't doing it's job right. I pulled it out and re-seated it, but it did it again a couple days later. So I bought the following accelerator pump assembly and that did the trick: https://www.edelbrock.com/carburetor...mbly-1470.html. The blue plunger at the end of the assembly in the pic is what I'm talking about.

[Gromit]

Noticed the crankcase breather hose is plugged in the photo - maybe that is just because you have it apart so just a reminder to have some means of venting the crankcase.. no venting could definitely cause tuning issues.

My 1970 350/350 truck has the stumble issue but not often enough to be a problem - I believe on mine it is the stock torque converter not very well matched to the torque curve of the cam. So the posters who have asked about your cam I would second their questions - your cam specs are important to have documented in solving your issue.

I can affect the stumble by adjusting my vacuum secondary rate but that adjustment seems to change with the weather..

[72c10deluxe]

Hello, A few years ago I had a 67 Corvette roadster with the 327-350hp
engine. I also had an off-idle stumble that I was able to eventually resolve.
I played with the Holley carb and eliminated fuel delivery issues. I installed a set of Autolite #295 spark plugs and the car responded well with those plugs.

[SRT8-X]

Originally Posted by Gromit

Noticed the crankcase breather hose is plugged in the photo - maybe that is just because you have it apart so just a reminder to have some means of venting the crankcase.. no venting could definitely cause tuning issues.

My 1970 350/350 truck has the stumble issue but not often enough to be a problem - I believe on mine it is the stock torque converter not very well matched to the torque curve of the cam. So the posters who have asked about your cam I would second their questions - your cam specs are important to have documented in solving your issue.

I can affect the stumble by adjusting my vacuum secondary rate but that adjustment seems to change with the weather..

Gromit, that's correct, I have a breather with integral PCV valve in the passenger valve cover typically. It's plugged for the intake replacement only.

As noted, the cam is likely a stock or extremely mild grind, it has a smooth idle and no aggressiveness so I'd suspect a low end power range. I will probably be replacing the cam and valvetrain, possible aluminum free flowing heads too, after everything is ruled out. Thanks for your comment.

[SRT8-X]

Originally Posted by 72c10deluxe

Hello, A few years ago I had a 67 Corvette roadster with the 327-350hp
engine. I also had an off-idle stumble that I was able to eventually resolve.
I played with the Holley carb and eliminated fuel delivery issues. I installed a set of Autolite #295 spark plugs and the car responded well with those plugs.

Thanks for the reply, I plan to work on the carb (check floats, look for debris / trash, check filters, adjust idle mixture screws, adjust choke, check accelerator pump, etc.) this weekend and reduce the timing advance just a few degrees. I want to see what that does then move on to other suggestions. It "feels" like a fuel delivery problem and I had not checked the floats upon delivery / installation. It makes sense to rule that out first.

[paul blair]

Have you done a compression test yet? Has this engine ever run good since you have owned it?

[SRT8-X]

Originally Posted by paul blair

Have you done a compression test yet?

Paul, looking to do this over the weekend as well.

Off idle it has run better than it is now but never what I would expect from a 327 in a light truck. My '71 that had a mild 327 would run OK but never to my expectations, I know they're short on torque but these trucks are light and I've seen plenty of them that run well.

[geezer#99]

Have you checked the valve lash yet?

That’s the first thing you should do.

[SRT8-X]

Originally Posted by geezer#99

Have you checked the valve lash yet?

That’s the first thing you should do.

Geezer, I have not had a chance to work on the '69 since Monday. Will be resuming this evening and Saturday. I'm taking all suggestions into account and will verify everything but I wanted to look at the carb to verify since this is recommended on new units anyways. I'll be pulling the covers to check the valve lash regardless. Thanks.

[paul blair]

Has this engine ever run good since you have owned it?

[SRT8-X]

Originally Posted by paul blair

Has this engine ever run good since you have owned it?

Off idle it has run better than it is running now but never what I would expect from a 327 in a light truck. My '71 that had a mild 327 would run OK but never to my expectations however it was over carbed and had a 3 spd manual behind it while waiting for a built 700R4, not much expectations from that one. I know they're short on torque but these trucks are light and I've seen plenty of them that run well with a 327.

[SRT8-X]

Just wanted to reply to all those that offered help and suggestions. Unfortunately we had a death in the family and have been dealing with funerals, preparations, etc. and maintaining the normal home life so I have not had much time to work on the '69. I'm posting an update so you guys know you're appreciated and I haven't forgotten. Time off is coming next week so I'll get some wrenches turning and provide some updates then....It snowed here NW of Houston today so we're not exactly in the ideal tuning weather. When this thread was started, it was in the 90's still!

Thanks again and if you guys don't hear from me, Happy Thanksgiving and a day-late "Never Forget" to all you veterans out there!

[SRT8-X]

Just wanted to update those interested. I was running through timing again, adjusting the idle mixture screws and playing with step-up springs after checking the rockers / valves and carb floats. Both checked out leaving me stumped still. At least they're confirmed.

I was able to get Performance Distributor's tech dept. on the line and had a lengthy conversation. There were a few things they asked me to check, (EXCELLENT human communication and tech support by real people who know their products in and out and aren't afraid to talk on a normal level to customers) including the rotor casting, pick-up, stuck weights / springs, coil, voltage to the dist. (as many have suggested), ohm reading at wires, etc.

When I let them know I specifically bought the Live Wires with their 1200° sheathing for protection against headers *under header design*, he stated to make absolutely certain the wires were not coming into contact behind the primary tubes regardless of temperature and other shielding (I have installed boots near 7 & 8 which turned up tightly against the header).

Needless to say the wires were touching in spots and even though there was no arching apparent in the dark, looking from top or bottom, there was some white residue on the plug wires. I decided to go away from the clean aesthetics and give rerouting a few of the plug wires that concerned me over the valve cover and others that were tie wrapped together, I added a tie wrap spacer on each individual wire. He said this was an old trick to make sure the wires don't transfer spark to one another.

After doing this, I have successfully driven the truck for a several hours and over 100 miles without experiencing the misfire or off idle hesitation. She runs great now, it's a 327 so nothing spectacular out of the hole but fun nevertheless....now I can get to fine tuning the carb and idle mixture screws to precise settings to get all I can out of her. Note the idle is set around 650 now and idles smooth, starts with a simple key turn in all temps and has a little more lope than was noticed before. Still thinking it's a stock cam and internals but definitely more fun to cruise.

I'm ordering custom length plug wires from Performance Distributors (can't say enough good about these guys) so that none are tight and I can still run them cleanly under the header. I'll be using 6" boot shields on each plug and the Made For You Vertical Wire Looms 50-710-11 separators with attachments to the block / oil pan so they are routed away from the heat.

I am still having a bit of run-on even with the timing / advance correctly set. I'll continue to play with this and see what happens.

Thanks to all that helped, it ended up being a very simple issue.

Now I do have a dilemma, my new fuel pump is leaking oil at the mount so I'm pulling it off and hoping it's just the plate or the gasket that is a problem.

Again, Happy Thanksgiving to all and I appreciate your help once again.

[Coley]

...so it appears it was an electrical or spark related problem?

ie: you were experiencing some shorting of the spark across the headers etc when running...hence the misfire and stumbling.

Do I have this correct?

C.

[SRT8-X]

Originally Posted by Coley

...so it appears it was an electrical or spark related problem?

ie: you were experiencing some shorting of the spark across the headers etc when running...hence the misfire and stumbling.

Do I have this correct?

C.

Absolutely, it was definitely the spark issue causing the hesitation and such. It’s quite possible that the other tuning, timing, carb adjustments helped along the way but this was the only solution that eliminated this.