[kvinmn]

I am wondering if anyone has experience in reading the burn pattern on spark plugs. I always thought that it should be a light brown or tan color when everything is ok.

What would very dark and smelling of gas indicate?

How about different burns in different cylinders?

My #1 and #5 plugs looked light brown, #2, #3, #4 were wet and almost black. They did not appear to be oily. #6,#7, #8 were similar but not as bad.

I have a 68 XR7 - 390 4bbl with Edelbrock Performer manifold, MSD Ignition, Demon carb (that is ANOTHER story), stock exhaust.

Ignition timing is set to 8 deg btc.

I'll get this cat sorted out yet!!!

Ken (68 Kat)

 

[cougar1]

They should be a light tan color.

Black is a sign the mixture is too rich, especially if they smell like gas. In your case, since only some are black and gas smelling, it makes me think you are not getting enough spark (bad wires or bad dist cap possibly).

Also check that your vacuum is stable, if not it could indicate sticking valves which could also cause the plug differences.

Is compression even in each cylinder?

Throwing out some ideas!!!

Bert

 

[calcat]

In addition to what Cougar1 said, you may want to do a leakdown test to compare what is happening with the cylinders. You may also want to consider using a vaccum guage to set your initial, my guess is that it is a bit retarded right now to be optimal. Use the same vacuum gauge to set you idle screws. Then take it out, with new plugs, and do a plug chop to get a reading on what it is doing.
If it is still rich, start looking at the size of your mains, verify you have right size PV and that it is not blown.

 

[kvinmn]

I have not done a leakdown test or a compression test. These are next on my list.

I have thought all along that my main problem with the engine was that the PO didn't have a clue about what parts to put into the rebuild. I'm just surprised the speed shop that did the work for him didn't steer him a little bit better.

I still think the 750 carb is a problem and it's probably jetted too rich. I am planning on replacing it with a 600 or 650 cfm with vac secondaries.

It currently is pulling about 11 in of vacuum and it is steady. I set the timing using the vacuum gauge originally - but it pulled the most with the timing set to 22 deg btc so I set it back to 8 deg.

It has new ACCEL wires and a new MSD distributor and coil. As well as the MSD ignition unit. I did check each cylinder for spark.

I hope to test everything else this weekend and replace the plugs. The current ones are not factory spec. They are Autolite 35s and might be another problem...I think they are a colder plug than stock.

 

[calcat]

Just a guess, but with 11" of vaccuum it would'nt be uncommon to have a pv rated too high (like the ubiquitious 6.5) and have it opening too soon causing your rich condition.
If the 750 is a vacuum secondary it should not be a problem as long as it is jetted properly.

 

[modifiedmark]

Just pulling a plug on a engine that has seen Idle time will not be the best way to determine the mixture. That rich looking plug may still be lean at wide open throttle. So to check for that you need a clean set of plugs and a hard run on them. You need to cut the power to the engine before idleing it and then check the plugs again.

Calcat is right about the power valve maybe the trouble also. Very few actually understand the power valves function although I assume the Demon carb has built in PV protection a blown PV will cause trouble. mm

 

[kvinmn]

I forgot to mention the gap on the plugs when I checked them was about 2 times the factory spec (.0032 - .0036). I re-gapped them to .0035 when I replaced them.

I will try reading the new plugs after a WOT run - and shutting it down before idle to check them again.

As far as I know, the Demon carb does have PV protection but I also am not sure which valve is installed. It is a mechanical secondary carb.

It still bogs or pops back through the carb on hard acceleration.

I did order a Road Demon 625 cfm with vacuum secondaries yesterday...