Engine Misfire - The Saga continues

[Terry Stapley]

This spring when I started the engine for the first time cylinders # 4 and # 8 were VERY intermittent in firing, headers pipes were warm but not hot. I changed the spark plugs and it seemed to correct the problem, at least at idle. When I took the Cobra out for the first time to bed the new brake pads the engine was not running correctly and had a definite misfire at about 3K rpm!
Last night I was trying to see what the issue was with the misfire and cylinders # 1 and # 8 have headers pipes that are hot, but not as hot as the other 6 cylinders. Water sprayed on the other 6 immediately puffs off and water sprayed on #1 and # 8 steams but does not puff off like on the other header pipes so they are not as hot. I pulled the spark plug wire from # 1 with engine running and could here no noticeable change in engine RPM. Holding the wire close to the spark plug I could hear the spark jumping but it made little difference if any in the way the engine ran versus being away from the plug.

418W - MSD 6AL - MSD Pro Billet Distributor - MSD Blaster Coil - Holley 750

I had a spare magnetic pickup for the dist. that I changed last evening and also a spare Blaster Coil that I changed to see if those were the culprits but no change in the engine misfire.
Last year before putting the car away for the winter I had done the following:
New Spark Plugs
New Spark Plug & Coil Wires
New Distributor Cap and Rotor
New Fuel Pump
The engine was running fine in November when put away for the winter.
There is no fuel leaking from the carb and fuel pressure is steady at 6 lbs.
Last night it was running so rich I upset my wife because even with both garage doors open the smell of unburnt hydrocarbons was wafting into the house.
Any ideas would be greatly appreciated to help me track down this gremlin!!

[Silverback51]

I had this same problem and replaced the entire ignition system. Still had the misfire. Then a friend of mine said it could be the alternator. I removed the fan belt and started it up. The misfire was gone. Bought a new alternator and the misfire returned. Ended up being the voltage regulator. Appearently the MSD boxes are very sensitive to voltage spikes, and that was causing my problems.

Good luck.

[Bobcat]

MSD makes a surge/voltage spike protector , which is really just a large capacitior that goes across the input terminals to the MSD box. Sells for about $30 to $40 at Summit or Jegs and can be found on the MSD website . You are correct about the boxes being very sensitive to voltage spikes .... and this little device appears to solve , or reduce that problem .

[Danr55]

Have you called the local 12 year old consultant from down the street? I only asked because after chasing a very similar problem for about 2 months and spending many $$ on different proposed solutions, the kid from down the street asked if the distributor cap was cracked. Having just put a new cap on, I said NO!... but then recanted and decided to check it. Sure enough there was a pin hole on the split line of the cap and it was discharging to the clip that held it in place.

[Rick Parker]

You may want to try spraying some carb cleaner into the Brass Air Bleeds (2 for each venturi). One of them could be clogged (hi speed).
They can become clogged with varnish and prevent fuel being pulled during transition from idle to main circuit, or make it run exceedingly rich because it is blocked. I would also consider replacing the aluminum or chromed plastic wire seperators that are in the pictures in your gallery. They form a perfect path for crossfire.

[xlr8or]

You might want to try a different dist. cap. Maybe it's a phasing problem in the cap.

[vettestr]

Down and dirty ignition inspection would be checking for visual signs. Look inside the distrib. cap for fine carbon tracking or fine black lines from any point to another, check center coil post as they are known to wear to nothing, check rotor. Verify all wires are connected firmly and no discolored or burnt signs.

Verify battery voltage is above 12.5 VDC at rest and voltage above 13.7 when running. With engine running pull a good plug wire from plug and look at spark color (be careful) it should be neon blue = yellowish is bad condenser-points or cap and orange is a bad coil or low voltage. Other causes possible but neon blue is what you need and a quick check can be done just cranking engine but running is best test.

Check wires for leakage or arcing by starting engine in the dark and looking for sparks. Move them around a bit with wooden hammer handle but do all this with common sense and be safe. Double check for proper firing order and plug wire routing if anything was worked on before problem started. If 2 or more cylinders are at fault are they side by side or in the firing order next to each other. Will problem follow a wire moved from hole to hole. If none of this works it is my professional opinion you are screwed and need a big block or a bigger big block.

[Three Peaks]

You may have something as simple as a bad spark plug wire or bad end on the wire. Try changing 2 wires and see if the new wire fires the spark plug better.

[Terry Stapley]

Thanks all and great ideas!! Here is what I have done that I did not put in the first post because it was so long:

Checked New Dist cap and rotor. Both are ok with no carbon tracking or crack
Started engine in dark garage - no arcing apparent
Replaced offending cylinder spark plug wire with old one - no change
Replaced offending cylinder spark plug - no change
The wire separators are chromed plastic

I did notice that when I sprayed carb cleaner in the air bleeds that it flowed through the secondaries but when I sprayed it into the air bleeds on the primary side it blew back at me!! Could this be an issue??

[xlr8or]

Can we assume that you have double checked the plug wires location and firing order? I know its a stupid question but sometimes we don't see the obvious oops.

[Terry Stapley]

Yes I have checked the firing order and plug wire location multiple times!!

But I will be more than willing to check again!!

[xlr8or]

Another thought that crossed my mind is continuity in the cap from the inside terminal to the wire side of the terminal.

[Terry Stapley]

xlr8or,

Do you think that would cause an intermittent miss?? I will check that out also!!

I think I have a terminal case of small block - itis!!

[Three Peaks]

Maybe your rev limiter is set waaayyyyy too low?

Bob

[jhv48]

You've identified the misfiring cylinder.

The problem is ignition or carburetion. If the cylinders in question are getting spark, then it's a carburetor issue.

The smell seems to indicate the cylinder is getting gas, but no spark to ignite it.

Pull number one spark plug from the engine and then reattach it to the plug wire. Now crank the engine and see if the plug is firing outside of the engine.

My guess is you will need to replace the plugs again and fire it up again.

Check for moisture inside the dist cap too.

[xlr8or]

If it's the same cylinder/s mis firing as you indicated by temp differences there are only so many things are are unique to them. Plugs, wires, teriminals on cap, tabs on distibutor shaft that the pickup sees. Swap the plugs and wires from those cylinders to another pair and see if the problem follows them. If the problem remains on the same cylinders a closer inspection of the cap and distibutor internals is required. If the problem follows them you have your answer.

[Jerry Clayton]

Terry

Switch wires at the cap for #1 and # 8----it is common error to mix these two wires up as when you look at them they look correct when in many cases they are reversed.

Also make sure your wires are separated as cross firing between 7 & 8 was common .

Back in the day----people would line up the wires 1234 on the right side and 5678 on the left side whhen really the factory correct wire placement was 7568 on the left which put three wires on wrong---always happened with a tune up of an engine that was running fine before hand

[GlynMeek]

Jerry has a GREAT point. I had the wrong plug wiring JUST like this the first time I ever fired up my small block!

Glyn

[jhv48]

Seems to be a lot of nasty s--t going around about Comp Cams and the dizzy gear and the cam gear devouring each other.

Could this have happened to you too?

[cobred]

A compression test is free and a good idea to do before replacing more parts. Possible vacuum leak, valve spring, cam lobe, or other mechanical problem.