Tune up help needed?

[Sweeet67]

Alright, I am getting so frustrated with this thing. I have put new plugs, wires, distributor(HEI). Rebuilt the quadrajet per Cliffs book. Replaced vacuum hoses, and the fuel pump. When I put the hei in I took out the inline resister to get a good 12 volts to the distributor.(sorry 71cheyennesuperlongb)

When I drive this truck, it will have multiple small misses. You can feel it not running smoothly in the pedal and in the seat. Seems to be worse at lower to mid rpm ranges. At higher rpms it seem to smooth out.
I appreciate any and all suggestions you might have. Thanks
By the way it’s my 69 GMC K10.

[71cheyennesuperlongb]

More info needed.Stock HEI or junk ass white box no name model.By the way it is a distributor. NOT A DIZZY! What is the plug gap? Type of plugs?

[Sweeet67]

I have tried a gm hei and a cheap version that I have, the problem stayed with both. The plugs are ac-44t, gapped at .035.

[Grumpy old man]

Post a picture of your engine , Auto/Manual ? Cam ? Nice looking truck but need more info than it's not running smoothly ?

HEI gap at .45-.55

[Sweeet67]

It’s bone stock 350 with the sm465 tranny. I’ll get a pic of the engine tomorrow. Thanks for your willingness to help.

[Coley]

Ok, job 1: What is your timing?....this is first before any carb adjustments.
Next, job 2....do a complete front to back, port by port, gasket by gasket, hose by hose check to see if you have any vacuum leaks.

Start there...and we can keep going.

Coley

[mike16]

cliffy's book? well something is certainly missing. not just yer engine. A good vacuum guage will tell us alot too but its a bit of a lost art reading it. HEI coil wires installed correctly? is it smokin black? PCV hooked up.

[Boog]

Compression test?
And here is something useful when diagnosing.
Intermittent miss can be caused by several things. New spark plug wires can be bad. Leaking spark through the boots to the exhaust manifold. Use a spray bottle with water to mist over the wires while idling in darkness and look for sparks. Crossed plug wires? 1&3, 5&7.
Just some areas to explore.

[Killer Bee]

as a mechanic, I gotta ask;

what was it doing before you changed everything?

when was the last time it was running well?

what changed between then and now?

why did you change what you did?

you have a lot going on there and when you change multiple items it gets tough to tell what had what affect on performance..

the guys above hit on a lot of things to get you started and may land on the issue..

but you will need to start gathering pertinent data to be able to diagnose it without guessing..

good luck and keep us posted, like to hear those success stories from guys that figure it out on their own..

[tomshortwide69]

you could have #5 and #7 plug wires crossed

[Sweeet67]

Thanks for all the replies. All great suggestions, I will look into them all. I am determined to find this issue as I really love driving my truck. Especially since spring is trying to arrive. I will report back soon. Thanks

[Benjamin]

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[Oldironside]

I had exactly the same problem that you have described in your posting - after doing exactly the same things to my truck that you did. The only exception: I left my carburetor (Edelbrock) untouched. I installed a Petronix Flamethrower hei distributor - after that, the engine ran like a bag of nuts, no matter what I tried. I connected the new HEI distributor to the same carburetor port where the old one was formerly connected to - but after some research on the Internet, I found out, that I had used the wrong vacuum port at the carburetor. I simply switched the vacuum ports and the problem was solved immediately and now she is running like she's supposed to (very smooth).

Unfortunately I have no knowledge about quadrajets, but if you have two vacuum ports like the Edelbrock, I'd suggest to give it a try and switch the ports for testing . If this doesn't solve the problem, no damage is done.

Just wanted to to tell you because your problem sounds very similar to the one I had and I know how annoying it can be

[71cheyennesuperlongb]

Try increasing your plug gap to .45 or .50 HEI needs more gap than points.See if intake vacuum works better.You should get your advance vacuum from below the throttle plates.

[Sweeet67]

A quick progress report, I regapped the plugs to .45, checked for crossed wires, looked over all the vac hoses. The timing is at 14 degrees btdc. It’s still got a cut out or miss to it. It seems to be in the lower range of rpm under load. I also have new fuel filters as well.
I will try the different port for the vacuum advance. Thanks for all the suggestions, I will keep after it.

[demian5]

Dude - Answer these questions and you can be better helped.

Originally Posted by Killer Bee

as a mechanic, I gotta ask;

what was it doing before you changed everything?

when was the last time it was running well?

what changed between then and now?

why did you change what you did?

you have a lot going on there and when you change multiple items it gets tough to tell what had what affect on performance..

the guys above hit on a lot of things to get you started and may land on the issue..

but you will need to start gathering pertinent data to be able to diagnose it without guessing..

good luck and keep us posted, like to hear those success stories from guys that figure it out on their own..

[Sweeet67]

I bought the truck about a year and a half ago. It was running but very poorly. I dove into a complete tune up right off the bat. Quickly carb rebuild, plugs and wires, Hei, new fluids, etc. I guess what I’m saying is, I didn’t have a good base line to start from. I’ve been battling since then to get it right.
I’m gonna try a different set of wires tomorrow to see if that helps. I’m wondering if they are sparking off to the manifolds or something else. It just feels more like a ignition issue than carb.

[fotyfobravo]

Hey Buddy I feel you pain. I had a similar low rpm skip. Ran great as long as your foot was in it but spit and missed at low power and a skip at cruise speed. I did all you did PLUS rebuilt my old Edelbrock twice. THere must have been a tiny port somewhere that I never unclogged. I finally just bought a new Edelbrock 1406 and it now runs like a new truck. Good luck...you will get it fixed. I usually found timing to be the problem with most problems but this one was the carb.

[Coley]

Originally Posted by Sweeet67

I bought the truck about a year and a half ago. It was running but very poorly. I dove into a complete tune up right off the bat. Quickly carb rebuild, plugs and wires, Hei, new fluids, etc. I guess what I’m saying is, I didn’t have a good base line to start from. I’ve been battling since then to get it right.
I’m gonna try a different set of wires tomorrow to see if that helps. I’m wondering if they are sparking off to the manifolds or something else. It just feels more like a ignition issue than carb.

Ok...for me it would be 'full stop'.
I would shut this process down and start with a complete compression test to make sure I didn't have a dead or weak cylinder, ...or two for that matter.
If it wasn't running right to begin with you might be spinning your wheels here and buying parts etc that won't help rectify this.

I seriously recommend a compression test to start.

My two bits..
Coley

[Sweeet67]

Ok Coley, compression test it is. I’m hoping it’s not that for obvious reasons. I guess it would motivate me to get the other 350 that’s been clogging my shop up for years built.

[davepl]

If you haven't done one in a while, here's a reminder to block the throttle OPEN when you're doing it, people often forget. And take all of the plugs out so it can spin at some reasonable speed with the starter.

[HO455]

It sounds like a light throttle cruise miss fire. Steady RPM with small throttle opening is the most difficult time to ignite the air fuel mixture. There is lots of empty space in the combustion chamber that the flame front must jump across to ensure complete combustion. This is why there is a vacuum advance in the ignition system. Start the fire early so it can burn all the way across. You should check to see how much vacuum advance is in the distributor. You can do this by disconnecting the hose to the distributor (plug the open port on the carb) and pull vacuum on the distributor with a separate vacuum source, measure your timing (an adjustable timing light makes this easier), and record the number. Getting a vacuum reading at speed when the symptom is noticed will be helpful with this kind of problem. Compare the amount of advance to the vacuum reading at speed.
Ignition weaknesses will show up during this condtion. You may be able to detect the cylinder affected by inspecting the plugs. But to do this you need to shut the engine off during the misfiring and coast to a stop and pull the plugs. Pulling over with the engine running is likely to recolor the plugs for idle conditions. New plugs will help make it easier to read the plug.
Anything that causes one cylinder to have an improper air/fuel ratio can cause this symptom. What intake manifold are you using? Some aftermarket manifolds are poorly designed and can cause this type of problem. What style of plugs are installed? Changing to a longer reach plug may help. Doing the above recommended vacuum readings may indicate a bad valve seal which could be pulling exhaust gases back into the combustion chamber and leaning the mixture out. A vacuum leak into one intake runner can be detected by spraying carburetor cleaner around the intake gaskets. Some intakes have vacuum ports into individual runners these should be carefully inspected for leaks. Preferably with a hand vacuum pump with a guage to check the rate the vacuum bleeds off. Dont forget the vacuum advance pull off itself may be leaking.
Up to this point I have concentrated on lean misfiring. It is possible you getting unatomized fuel to the combustion chamber (i.e. drops of raw fuel flowing into the chamber). You may be able to see this in one or two plugs looking sooty. Looking at the floor of the intake runners may show signs of fuel puddling.
Sorry for the long post but this a difficult symptom to find the source of. For me I need to take notes and make a concerted effort to only make one change at a time to make progress on the solution.
Good luck and keep us posted.

[Killer Bee]

Originally Posted by Sweeet67

I bought the truck about a year and a half ago. It was running but very poorly. I dove into a complete tune up right off the bat. Quickly carb rebuild, plugs and wires, Hei, new fluids, etc. I guess what I’m saying is, I didn’t have a good base line to start from. I’ve been battling since then to get it right.
I’m gonna try a different set of wires tomorrow to see if that helps. I’m wondering if they are sparking off to the manifolds or something else. It just feels more like a ignition issue than carb.

that is a sticky wicket then.. and agree with Coley, get down to the basics and work your way up.. this will ensure a more solid tune-up strategy and save time and money replacing things that may be serviceable and unrelated to the cause of your symptoms.. especially if this engine is somewhat new to you and never has run correctly since you've owned it.. there's no telling how many actual miles or engine hours are on it, what's been done to that engine or how it was maintained until you verify all of the components you can, short of a tear down.. I'd bet a cold beer the previous owner probably had this issue and it contributed to their decision to sell..

along with what Coley said checking compression, I would pull the rocker covers and inspect the valvetrain components, timing chain to valve stem, and run the overhead adjustments.. then start at the very beginning of the ignition system, working your way from the distributor to the plugs, then fuel system fuel tank to carburetor..

I'll insert this here if you're not sure how to check timing chain slack..
How To Check Timing Chain Slack -EricTheCarGuy
this video is decent and illustrates the procedure pretty well.. of course they use a f o r d since the distributor is in the front but concept is same.. and this test of course is dependent on the distributor base gear to cam lash so take that into consideration.. you can take up that slack prior to testing the timing chain slack so it doesn't affect your measurement..

may seem a little time consuming but by running through all of the systems front to back, it minimizes the time spent guessing and retracing steps over and over again and possibly skipping over the very cause or contributing factor causing the symptoms.. this also allows you to identify and correct any other issues whether contributing to your current symptoms or not, that may lead to future problems..

sounds like you have a good grasp on how to inspect and repair as necessary, just need to get your diagnostic strategy organized and start at the beginning..

good luck and please keep us posted!

[Sweeet67]

Thanks guys for all the help. I’m gonna follow your advise and start over.
I’m just trying to decide now if I wanna put all that time into a tired engine or just replace it and be done. And have a known quantity to go forward with. A few more checks at least then make a decision.

[harpo231]