Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
Hello! My name is Gary.Welcome to JustAnswer.I'm reviewing your question now, and will post back with a reply shortly.
What is the firing order you are using?
If it only these two cylinders then it, will not be a carb or manifold issue.
Either the plugs are faulty, (measure with an ohmmeter, seen this many times), or the valves are too tight.
How much lifter preload did you set at?
Have you done a compression test as yet?
The spark is firing from each plug lead to ground when you remove it from the spark plug?
Is the camshaft made for this firing order?
I want you to verify the No. 5 and No. 8 leads are sparking to ground, if these are the cylinders that are misfiring.
Your Edelbrock is a 500, a little small for me, but they still run good out of the box.
Are all the lifters quiet now?
Ok, let's see you do have spark from the 5 and 8 leads, and the compression test results.
The 500 will be fine to 5000 rpm or so, perfect for stock street car.
Ok, good luck for tomorrow.
With your intake manifold, cylinders 1,4,6 and 7 are fed from one half of the carb.
The other half, primary and secondary feeds 2,3, 5 and 8.
How have gone with some further testing?
There first thing I would do is check the resistance of those two plug leads.
Even a new lead could be open circuit.
Do you have a digital multimeter?
If they test ok, then move on to compression test.
I'll hear from you when you know some results.
Ok, take off one at a time, so you don't get lost.
Set your meter to 20k ohms, and put the meter leads together. Meter should read zero.
Probe each end of a good known lead, meter should read about 5k ohms, 5000 ohms.
Hopefully no 5 and no 8 give an open reading.
Some leads are as low as 50 ohms per foot.
An open reading would be whatever you meter says while disconnected.
While the meter leads are shorted together, the meter should read zero.
That all seems fairly normal.
Can you read the spark plug from the terminal to the centre electrode?
And do no. 5 and 8 look different to the rest?
Ok, with your meter the plug should either read zero ohms or about 4.5k ohms for a resistor plug. From the terminal end to the centre electrode.
The no. 5 being white certainly sounds lean.
I would say you have no fuel entering the cylinder, it can still generate a compression reading, so please don't be mislead by this.
If you are quite sure it is only these two cylinders 5 and 8, then is there any chance of some abnormality in the intake manifold for that runner?
Or possibly the inlet lobes have been wiped off the cam?
Can you run the engine with the valve covers off to check the valve operation?
This is certainly an interesting issue.
If you have spark from both 5 and 8 wires to ground, the valves appear to be working, then pull the carb and see if there is a lump of wood in the runner?
Something stopping airflow from the carb into those 2 cylinders.
Or have you got a power brake hose plumbed to that runner?
Is the carb a bit lean maybe?
Ok while you have carb off, I would verify the float levels are correct.
I have yet to see any carburetor that I have used, Edelbrock, Carter, Holley, BG, QFT, etc be correct out of the box.
The floats take a beating with no fuel in the bowls.
The edelbrock is a good carb, and the owners manual is a good tuning aid.
The inlet manifold you have on there could have some fault and that's why the seller sold it on ebay?
Come back when you have the carb off the manifold.
Hopefully something stands out.
Just another thought, did you have the intake manifold surface ground to match the angle of your heads?
You could have a vacuum leak across the gaskets?
How far out from seated are the mixture screws?
Did you have the throttle wide open while you did the compression test?
Are all the valve heights still the same when each valve is shut?
I've had one engine with a valve not seating, still had 160 psi, but misfiring.
Nothing odd as yet?
Have you retorqued the intake manifold?
Yes, sounds like you have found something now.
V8 engines are prone to leaks of different types.
Good luck on the next step, I feel your pain.
I use a bead of sealant along the block rails, no gaskets.
Only gaskets on the intake side of the heads.
I use a tiny bead around the water ports only.
Prior to running the rail beads, I already know the necessary height from a trial run.
Run the rail beads, fit the intake gaskets with a tiny bead as nec for water only.
Fit the manifold with no moving around etc, get all bolts in, pull down gently in many stages.
Clean up the front and back as necessary, like a bathroom sealant job.
Let set overnight.
I'll leave it with you now, and we'll talk in a day or two.
Have a good look over that manifold and the new carb as above.
Just another thought.
Would you contemplate changing to a lowrise single plane intake?
I ran one on one of my V8s with an edelbrock 600, went really well from off idle to 6600, even with the cam I had.
I would do some measuring to see what room you have under bonnet, then check at your local speed shop to see what is available to fit.
Now that you are taking the manifold off, it is worth an experiment.
I think you will be pleasantly surprised at the performance.
I would like to think that it would.
As long as we are sure that those two have the 4 stroke cycle happening, and spark.
When you get the manifold off, have a good look at that shared runner, top and bottom etc for vacuum leak etc.
You may find that the other pair at the front are being fed just enough to not misfire.
Does the misfire only occur at idle?
Ok, I would richen the idle mixture screws, 3/4 turn out is not enough normally.
Try it at 2000rpm, which will prove either way.
Try it on good cylinders as well for comparison.
I should have asked earlier, what distributor do you have?
And what body is this combo in?
So you don't have a bonnet issue, like a Camaro might have.
Are you happy to stay with the Edelbrock, or would you like to try another carb if you go to a single plane manifold?
Have you got another carb to try now, before you pull the manifold for the coolant weep?
Ok, I would ensure there are no vacuum leaks.
Then adjust each idle mixture screw slightly richer to the point of highest idle speed.
You should be able to find the happy point, further out should make it slow down from being rich.
You will need to lower the curb idle speed screw a bit as you do this.
It would be great if this is your issue.
That's sounding like good news for now.
If you now have all 8 cylinders, will you be going for a little drive soon?
How about the little coolant leak?
That's great, bet you are excited for a small maiden drive.
Hopefully hear from you in a day or two.