Got a chance to start troubleshooting the skip on my 69 with the L6 250

Car has been running rough since I got it.

Changed:
Points / condenser
Plugs
cap
rotor
Still rough
Vacuum rapid bouncing at close to normal
Dwell 31Timing 4 BDC
Pulled wires 5 and 6 no change in roughness or rpm
compression test 145 150 150 165 150 150
Pulled the valve cover and movement looks normal best I can tell.
Plugs completely dry on 5 and 6
Sprayed carb cleaner by carb and intake gasket no change in RPM

I am starting to think it might be a clogged intake runner seeing how cyl 5 and 6 are served by the same runner?
Thoughts?

 

Rapid bouncing vacuum sounds like intake valve problems, but mostly even compression??? that doesn't add up.... Plug wires are good?? How about a vaccum leak at the intake that feeds #5 and 6??

 

Did you change IGN coil?
Does the misfire produce any backfire?

 

Changed the wires ... forgot to list.
Coil is on order I wanted an AC delco... not in stock.

 

Firing order correct? Try swapping positions of known good wires and plugs individually to see if the symptoms stay with the cylinder or follow the ignition parts. Erratic vacuum could be a valve spring problem, a compression test may not detect this.

L6 carbs often came loose between the throttle plate and the fuel bowl. Try twisting the carb in place and watch the throttle plate.

Yes, a blocked intake runner is possible. Pull the carb and look into the intake runner with a borescope. Intake gaskets were a common failure but you checked that. What does it do if you restrict some of the air intake with your hand over the carb?

Put a timing light on plug wire 5 and 6 and watch the blinking to see if the blinking is intermittent following the misfire.

Good luck!!!

 

Understand you want AC Delco, I’ve had better performance with Accel. A weak ignition coil field will produce misfiring and sluggish engine performance.

 

A weak coil doesn't kill two cylinders while leaving the others running fine...unless those two cylinders have the ground straps broken off the plugs, or the plug wires have excess resistance. TEST the coil, what you have is probably better than what you can buy new, now. For that matter, TEST the plug wires for resistance. Also, run the engine in a dark garage, or at night with the hood open. Look for glowing plug wires (corona effect) where your spark energy is leaking past the insulation.

Does that 5/6 intake runner have a vacuum fitting on it? Pinch the hose, see if those cylinders come back to life.

I had a distorted intake gasket on my 5.7L; 1 and 3 were dead at idle.

 

Schurkey has held a wrench once or twice. I'll add to it... backing out even simpler...read the plugs. Swap them after numbering the plug to see if the bad plug follows along. Or try another set. Clean the head seat for good ground. Work back on the ign system from there.

Verify no vacuum leaks exist to compound/mask other problems. An intake or carb leak can bounce the gauge as well. Troubleshoot vs exchanging parts.

 

Had a Olds 350 that seemed to have a dead cylinder, I think it was #7, Whole car would miss and shake at idle. I thought I had checked everything and was all set to remove heads and get valve job done. Then I noticed #7 cylinder intake runner had a vacuum fitting that went to the distributor vacuum advance. I plugged that fitting and it ran great- had a bad vacuum advance unit that leaked . After fixing, engine ran smooth as can be until I sold the car. My bet is it has to do with vacuum. When you sprayed carb cleaner did you get any change around the 5-6 intake branch to head? Does car smooth out if you hold choke plate 1/2 or so closed with it idling? I had a 68 F85 with a 250 6 and it was too lean- it just drove crappy but when I held choke it smoothed out. Since #5 and 6 are furthest from carb( well so is 1 and 2 I guess) I wonder if those tend to be leaner??? Good luck I've been following you along...

 

The plot thickens

Replaced the coil 5 seems to fire at higher rpms 6 still dead.
Put an inline spark tester 5 and 6 are fine
Pulled the carb and scoped the runner and all is clear.
Even changed the new plug 6 no change (plug looks like it never fired.)
I am thinking it must be an undetected vacuum leak?
Not sure what else it could be Thoughts?

 

Carburetor/Fuel Delivery.
Measure your vacuum with a gauge. Adjust each A/F mixture screw to the highest achievable vacuum.

 

Photo of this "inline spark tester"?
You've been asked about vacuum ports on that manifold runner, and on the carb. Have you verified that?
You've been asked if the firing order is correct. Easy to swap 6 and 5 plug wires.

 

I once had a '69 Chubby pickup that I bought from a friend. He was never able to get it to run right and I was pretty cocky that I could! Did a full tune-up on it with plugs, wires, points, condenser, carb and timing adjust. The whole thing. Still had a misfire. Changed to an HEI distributor, still a misfire. Put on a freshly rebuilt set of heads, new timing set, cam and lifters, intake and carburetor, still a misfire. I was sitting in my living room floor mumbling curses to myself about SBC engines and my loving wife asks me if I changed the plugs? Of course I reply, that was the first thing I did. "Well, they're cheap", she says, "change them again!" What the hell, I bought a better set of spark plugs (AC Rapid Fire) and damned if that didn't fix it! I was putting the new plugs back in it after every change and my friend had changed plugs several times as well. I ultimately ended up with a very nice engine, but there was nothing wrong with it initially except the AC Delco plugs.

That is the long version of put in GOOD plugs! They do make a difference!

 

Yes I checked the firing order... rechecked for the 100th time see photos.
No ports on that runner see photos.
Inline tester attached to #6 see photos.


But I may have found the issue take a look at the carb gasket could it be that simple?









gasket leaking on the side that is supplying the runner feeding 5 and 6?

 

Brake booster and PCV hoses may be reversed. PCV should enter manifold plenum, so fumes can be distributed to all six cylinders; brake booster can go to one or two cylinders since the vacuum leak happens at closed throttle.

This diagram shows the PCV hose, but not the brake booster. I don't have a diagram for your specific vehicle.
https://www.autozone.com/znetrgs/rep...528007db68.gif

But I doubt this is the root cause of the two dead cylinders.

 

That intake valve looks abnormally clean/no carbon as in it isn't getting fuel due to a vacuum leak. How do the other intake valves compare? Plug the brake booster line and PCV as suggested.

Those intake manifolds were notorious for vacuum leaks at the intake gasket.

Good luck!!!

 

One of the first things I did when I got the car was to plug all vacuum lines including the brake booster

Here are the pics for the other valves.







Gonna take the carb apart and soak it overnight and give a rebuild tomorrow will see what happens from there.

 

Taking the carb apart I may have found something
The vacuum line going from the back of the carb looks fine, in my tests it was easiest just to pinch the hose off with vice grips. (live and learn)
Take a look at the bottom of that hose. It's nice and crispy I am thinking that might be part if not all of the problem.

 

 

 

After too many hrs rebuilding the carb and finally putting it back together correctly (note to self don't remove the throttle and choke plates)
The skip remains.
Pulling the spark plug wires on 5 and 6 has no effect on rpm at idle as well as at around 2000
I plugged all ports on the carb.
The strange thing is when the vacuum line going to the canister on the firewall is disconnected the RPMs






increase slightly (not sure if my idle mixture screw is set correctly would be a factor on this one)

 

Swap #5 and #6 spark plugs with cylinders that run at idle. See if the misfire moves with the plugs.

 

head gasket going bad? compression check?

 

From the first post compression test 145 150 150 165 150 150

 

Well this is crazy. You sure deserve to have this fixed, with all the time and effort you've invested. It must be something simple, but what? I'm sure the fuel mixture is "lazy" and wont go into the #5and6 intake runner unless it is pulled there, but you have checked all places for vacuum leaks.... OK I just re read your entire thread and you mention rapidly bouncing vacuum readings. My guess is you have a intake valve in #5 or 6 that doesn't seal properly and allows compression to go back into that runner when engine is running., therefore not under vacuum so doesn't pull mixture into either 5 or 6 so no fire from both? Could it be a really weak intake valve spring that will close off the intake valve well enough to give a good compression reading when cranking over and doing a compression test but allows it to leak when running?? I don't know but i think if it were mine at this point I would pull intake off and see if you find elusive intake leak and if not go ahead and pull head. That car with a 6 is so unusual and cool I hope you don't give up and switch to a V-8. Keep us posted. I am really interested in what ends up being the problem!!

 

Is it possible that I can still have proper compression with a bad head gasket?
Would a leak between cylinders 5 and 6 allow the test to seem ok pressure wise but cause the misfire?

 

Do a dry then wet compression check. Note any significant delta P.
Spray WD40 or propane or starter fluid around the intake & carb areas and note the vac gauge needle and rpm changes.
Have you isolated and plugged off all vacuum lines & fittings while the engines running?
Does it smooth out with your hand partially to completely blocking off the air horn?
Apparently you have access to a scope. Look at the valve heads then the seats. Get a snake light and feed it down the intake to get light behind the valve while the scopes in the cylinder.

 

So after doing a leak down test that checked out fine like all the other test.
I set the #5 and #6 valves to zero lash (a little noisy) but all cylinders are now firing and the misfire is gone.

Not sure if somehow I transferred the original problem to a different one.
I did at one point try to get rid of clacking rockers and maybe over tighten and caused the valve to open wrong( but compression was good)
There was a definite miss when I got it just whether it was caused by something other than the valve adjustment.
At least it is running smoothly on all 6 now (not perfect but close)

Thanks for all the input

​​​​​​​Tim

 

Good, I hope it is all set. Thanks for the wrap up.