erratic miss

[sailpanel]

As I posted back in Nov. I finally fired off the 400 in our 66. I've spent alot of free time since( excluding the holiday season ) working out bugs, but I've come up against one that has me pulling my hair out. I have a pronounced erratic miss on #1. I also seem to have one on the right bank that might possibly  be #2  but it's a little harder to pin done since it's not as pronounced. Since its so erratic I was leaning towards ign. So far I've tried replacing the pertronix with points, changed out the wire , plug, different cap & rotor, and coil but nothing has helped. Vacuum reading is pretty much rock steady. I haven't done a comp. check, but with it erratic it might not tell me anything. Short off pulling the valve cover and seeing if I've got something going on with the valve train ( and even if it was why would it be erratic ) am I missing something? 

[firefighter3931]

It could be a sticky valve....try running a dry compression test and see what you've got. How does the ignition spark look ? Ground out the plugwire and look for some color as you turn the engine over. A bright blue spark means you have a good quality fire....dull orange means it's weak.

Let us know what you find. 



Ron

[sailpanel]

Spark is nice and blue. Porcelain plug color is barely off white. I have the Sealed mixture/left hand threaded air bleed screw AVS. Would a lean mix cause a miss more on one cly. more than another?

[firefighter3931]

Would a lean mix cause a miss more on one cly. more that another?

Yes that is possible, especially with a dual plane manifold....fuel distribution will be unequal with some DP manifolds. Make sure the bleeds aren't clogged and spray some carb cleaner in them just for good measure.

The spark quality sounds good so you can rule that out. 

What compression are you running in this motor ? How hot a plug are you using ?


Ron

[sailpanel]

Running Holley Street Dominator with 1/2" 4 hole spacer and Champion RJ12YC s, Kieth Black KB240 pistons with just enough taken off  heads and block to true it up. They say 9.5 but.... thats with 0 deck, mine... maybe some where between 8.5 and 9.00 also put .010 shims under rocker shafts.

[firefighter3931]

The fuel distribution is pretty good with the SD. I would look at the carb....sounds like a lean miss to me. 

Plugs are the right heat range for that compression.  I do prefer to run an NGK plug though...i've had excellent results with the XR5's on an iron head street motor.

What plug wires are you using ? How old are they ?


Ron

[sailpanel]

Plug wires, are new cut to length Standard brand. I like NGK s,and will most likely make the switch, guess I went with the CPs out of sentiment or something. I've allready been thinking about switching to a new 650 (maybe) thunder AVS because of the readily available metering rods and jets. Thanks as allways, guess the next purchase will be a new carb. I just wish I could be sure before I lay down the green. Think I'll call it a night and sleep on it. ( sailpanel)

[firefighter3931]

The Thunder carbs are nice....definately the best that Edelbrock makes. 

Run the comp test and see what you come up with. Maybe pull a valvecover and make sure that all the valves are moving up and down freely while cranking the engine over. Try to eliminate all the mechanical stuff first. 

I would also run a resistance test on those wires just to ease your mind. I've seen brand new wires that were junk right out of the box. You can also try running the engine in the dark and look for some arcing. I've also seen brand new plugs that were defective....especially Champions. 

I'm sure you'll find it....just be patient. 


Ron

[Steve P.]

I would also run a resistance test on those wires just to ease your mind. I've seen brand new wires that were junk right out of the box. You can also try running the engine in the dark and look for some arcing. I've also seen brand new plugs that were defective....especially Champions. 

This was my first thought.   Especially since you think you may have a second miss.
It does sound like resistance in a wire or even a crack in a plug.