Very odd engine symptoms

[Mitchyb]

Ok my vintage friends, I have been chasing this issue for some time now and have taken steps that thought would counter but here you go. I seem to have a low ( under load ) misfire as soon as It goes above 2000rpm all symptoms are gone pulls well and feel confident but if it shifts a gear and I try to accelerate the mis is back till I can get it to drop back down into 2nd and bring up the rpms. I have replaced cap,rotor,and plugs with no luck. The plugs were quite saturated in fuel, so I am thinking that I may have a vacuum leak or possibly a faulty EGR valve? What do you guys think?
My memory eludes me but if its a vacuum issue does the engine create more or less vacuum at higher rpm? Hence at a higher rpm, higher vacuum flow will over take a small vacuum leak??

Now that I have rambled what do you think
I should add that it cruises at 55mph without issue

[Sammie]

Plug Wires?

Sammie

[Mitchyb]

My thoughts are if I had a faulty wire tre misfire would be consistent through out the rpm range wouldn't you think,

[gio8856]

more rpm , more vac. what engine, fi, carb,???? points , electronic, sounds like timing, but may be lots of things..

[gio8856]

The more i think on it , chk. the distributor weights, may be stuck or rusted. Seams like no one ever oils the inner workings of it.. If it's a GM engine, replace it with a HEI distributer. First thing i do to all my old cars, they run a 100 times better.

[stink]

If you have the 460cid you might need plug wires. Numbers 7and 8 are next to each other in the firing order and if the wires are parallel for just a few inches they will crossfire. Vacuum leaks and egr should not effect running at speed under load unless you have a real bad leak and then no idle. You may have carb troubles but I dought it. I agree with Sammie, plug wires.

[Chowboy]

The misfire is at high load. No need to make excuses for the plug wires if they are not silicon and reasonably new put on new ones. As mentioned check the weights, major problem. I am guessing you have electronic ignition, have you changed out the module?
Engine is low on vac at full throttle, high at idle coasting.

[Mikeflyme]

Originally Posted by stink

If you have the 460cid you might need plug wires. Numbers 7and 8 are next to each other in the firing order and if the wires are parallel for just a few inches they will crossfire. Vacuum leaks and egr should not effect running at speed under load unless you have a real bad leak and then no idle. You may have carb troubles but I dought it. I agree with Sammie, plug wires.

I found similar problems on an old Dodge 413 engine had plug wires cross-wired...at least 2-4 wires...recheck the firing order and the wires sequence.
Took me a month or so to figure that out after doing all the usual R/R on the engine.

Mikeflyme

[TeJay]

Plug wires as suggested seems likely. I can't just give an answer without explaining why. That way you can know why they failed.

It takes a certain amount of voltage to overcome the resistance inherent in the wires, rotor gap in the cap, spark plug gap and the harsh environment within the cylinder. Lets say at idle all of that built in resistance requires 3-6,000 volts of pressure to jump the gap and ignite the fuel air mixture. The coil has the potential of producing upwards of 30-40,000 or more volts. It won't unless it needs to. Your plug wires are designed with good insulation to keep the voltage from leaking or jumping to ground so it can jump the plug gap. As those wires get older they loose their ability to keep the voltage inside. As you accelerate combustion pressures increase, the time to ignite the mixture gets shorter and this makes the need for a higher voltage to jump the gap and ignite the fuel go up. It could go up as high as 6-14,000 volts or higher. When that happens those plug wires that worked at idle and lower demands did their job. As the demand became greater they failed because the voltage found an easier path to ground through the weakened plug wire insulation and you have a misfire.

If you took off the dog house at night started the engine and lightly sprayed some water around the wires and cap you'd see a nice electrical spark show. There could be some other issues but if you have not changed those wires and they are stiff and old that's what I'd do.

JM2CW
TeJay

[Muddypaws]

Goes for intake manifold leaks.

[Mitchyb]

Hey thanks guys ! Ill pick up a set of wires tomorrow and give that a shot! I should have explained its a 90 fuel injected 460 but the plu wires are original quite a reasonable prices place to start! Ill let you know tomorrow evening

[Gorlininc]

Could be wires. Good to change anyway.

If you changed plugs, points and condenser.
It's probably a cracked distributor cap or your distributor. Either weight springs are loose in the distributor or vacuum advance is sticking or a vacuum leak in the vacuum line.
I guess you already changed your fuel filters.

[Franka548]

Mitchyb,
Don't skimp on the wires, go for quality over price, an MSD high performance elctronic module and MSD Blaster coil will up your power and also give you a jump in fuel economy. I agree with the others, plug wires is a good place to start.
Frank

[vyndance]

A vacuum leak creates a lean condition. Wet plugs say a rich condition. I would check the float level.