Reading Spark Plugs - Long Post

R

Ross6860

Senior Member
After getting fuel back to the carb I took the '36 on a little highway run. Maybe 15 miles at 60-70 mph.

Had to stop at a couple red lights, and drove the 1/2 mile home through a residential neighborhood before shutting down, so it's not going to be the best indication.

Used to run the bikes at speed and then shut down and coast to the side of the road and pull the plugs. I may try that later.

In any case, the electrodes were clean bare metal. The top of the insulator around the center electrode was white, but only down about 3/8". It was black from there down and the ID of the body was black and sooty.

The insulator was also pulled away from the center electrode on a couple of the plugs.

I thought if the insulator was pulling away from the center electrode it was an indication they were running too hot, as is the white color.

All the black has me baffled a bit. I'm used to seeing a much more consistent color over the whole insulator.

Maybe because the plugs were not brand new and had some carbon on them from idling?

Engine is stock as far as I know except for the ignition and headers. Red's headers and a ProComp Ignition (distributor, high output coil, and a 6AL spark box).

Plugs are Autolite 386

The ProComp has me concerned. I thought it was a Mallory. No doubt the color and design mimics Mallory and MSD. The Mallory has a 20 degree max advance and a curve designed for a flathead. Not sure I trust ProCOmp, and their literature says nothing about the advance curve or maximum advance.

Also don't really trust the builder and prior owner. I have found too many loose bolts, poor splices, stripped threads, etc.

More winter projects...


Thanks, all
 
Interesting, I'm on a fresh rebuild, stock engine except for a Rochester carb and MSD dizzy. I have Autolite 216's. I see similar coloration and mysterious darker, oily stuff on the end of the threaded shell and deeper in along the ceramic. I thought it was pre-lube liberally applied to the valve stems (still could be), but it's not burning off as I accumulate miles. I'm worried about running too lean.

Do you have PCV?

It seems like
 
No PCV - Crankcase ventilation "as built in '36";)

Sure wish they put a timing pointer on these things. Probably going to have to spend the time and make one, just to be sure what's going on.

Hate to do too much to this engine since it still bleeds oil out the rear like a stuck pig.
 
Reading plugs is a lost art, especially with the new gas. I've converted over to the Air Fuel meters. These can tell you allot including what the Power Valve is doing. You can also get timing information from it. It is possible to run a very lean mixture in the flathead, if, big IF, these work together. Another factor is the Piston to head clearance, The tighter the better. WE keep experimenting. At present we're running over 16:1 lean in cruise with 28 degs advance, 8.5CR.
I have a set of heads that will give 9:1 Cr with a relocated spark plug that I think will do even better. but we won't know till we try it.
 
No I use an NGK, however it's Limited to 16:1. Richard has one that gped to 22:1. I can't find any data on the one you show here.
 
thanks Ron, I would like to know more about these air fuel meters, but would still "read" the plugs as a final assurance that this electronic wizardry is correct.
 
Ol' Ron said:
No I use an NGK, however it's Limited to 16:1. Richard has one that goes to 22:1. I can't find any data on the one you show here.
Click to expand...
This is the one that's part of my Road Rig. I'm very pleased with it.
 

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Ron is right it's very hard to read plugs these days. I have found ( here in oz) it's hard to get much colour on the insulator nose. I've done hundreds and hundreds of miles with size 50 jets and I'm just getting colour. The main thing to look for is detonation , little black / silver dots on the nose. I use a jewellers magnifying glass .
 
Ron what you described would tell me (guessing only)
that its too rich but the HOT spark from the MSD has it disguised with the electrode & area running so hot
but like Ron said this gas is very difficult to read on the plugs
Good luck
 
Other way round, Bill, 16 parts of air to one of fuel. That's by mass (weight). The term stoichiometric is used to describe the ratio where there is exactly enough air to burn all the fuel, and that is normally understood to be around 14.7:1. Therefore Ron's running at 16:1 is a "lean" mixture.

Mart.
 
Thanks Mart:

I knew what I ment & I looked at it 3 times some thing did not look correct but the Darn Hands have a mind of there own.

Does all gas weigh the same or does different additives change the weight??
I understand higher elevations air is thinner so should weigh less.
changing how the engine's run.
 
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Lets take a ride in Richards car. While visiting Richard last year I had the opportunity to observe the the engine i operation from the passenger seat.
Sitting at a stop light waiting for the green the engine was idling at around 14.?? the minute the light changed the car began to accelorate and the AF dropped to around 13.?? as we reached cruise speed the AF began to climb to 16.?? fluxuating between 15.8 to 16.2. Pushing on the loud pedal dropped the AF to 13.??
All this time the timing was going from 20 to 28 degrees.
Having an ignition system capable of providing 40KV is a waste of money because the amt of voltage necessary depends on the resistance between the electrodes of the spark plugs In a Flathead with 8.5CR and plugs gapped at .025 it's doubtful the plug ever sees 10KV.
 
Ross what carb you running? I'm just getting to fine tuning my 94 so am going through the same issues. My own opinion is if in doubt run a little rich. Running on the edge of being lean can bring problems. Being a little rich yr in a safe zone. At what cost, a bit of extra gas. Does nt cost a lot in fuel but it can in parts if yr lean.

On the '36 if you'll be cruising most of the time, maybe hovering between being on mains and mains plus PV. That's where the AF metre is very useful.
Put mine on this afternoon and its gone pop, so will be getting a new one.

Cheers, Tom.
 
Ron (and any others) -- I'm looking at adding bungs for an A/F meter. I have Red's 3-into-1 headers with extensions (about 2 ft long). I'm thinking the A/F probes like hot gas, and therefore putting the bungs in the headers between the last two ports (so it's catching the forward 3 cylinders, but not the 4th). Is that a good location, or is further back better?
 
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