Spark plug reading

I only ever use plug readings for finding the correct main jet. So I run the motor on full throttle for a substantial distence. If I had one plug which looked as rich as that I would suspect either a leaking valve (low compression) or a carburetor which is affected by vibration (fuel frothing). I set both of my carbs to be identical. When I set the idle, and when I set the needle position, I do both in the same way - I induce the cough by leaning off, then enrich slightly. My main jets are usually slightly too rich, so when I give the bike a real blast, I do not burn valves or pistons.
If you read LAB's document on Amal carbs, it says most of our riding is done using less than one-third throttle. If you burned a valve while doing that, you motor would have been coughing and spluttering for miles.
 
Do the silencers match? It took me forever to find a rich condition on one cylinder, which would foul the plug with #230 mains. I eventually discovered one Atlas silencer with the end of the baffle cut out and the other not. Granted, my hearing isn't what it once was but they sounded the same. I found another already-cut-out silencer, exchanged it and the problem was solved.
 
concours said:
Oil control issue (minor).

If you are running unleaded pump gas, spark plug readings are impossible.

please, read that again.

Unless the fuel mixture is OFF THE CHART HOG RICH, the plug will not give any jetting information.

If you are hell bent on a plug chop, get some racing fuel.
Click to expand...
I have read this statement many times and my own experience bears this out. But what's a body to do? I have a 6-cylinder pickup with twin carbs that have adjustable main jets and dual exhausts. After nearly twenty years of twiddling to squeeze that last MPG out of it, last winter I was blindsided by a load of ethanol gas with a little extra, I think. This was California and Arizona, mind you, and it was hotter than usual. Gas in California was over $6/gl when I started home and the incentive to lean it out was strong at 10 MPG on a good day. Anyhow, it began using oil and by the time I got back to Washington State, I was up to a quart a day (400 mi. or so ) and blowing that out the breathers all over everything. When I pulled the head I found a burnt piston.

A friend gave me a pyrometer setup he used in experiments running straight alcohol in an air-cooled VW. There are two probes, a switch and the meter head. I plan to weld a bung on each down-pipe and set up the pyrometer as a permanent gauge. Does anyone have an idea as to what a reasonable maximum temperature might be? Hopefully, this will save me a similar fate on the new shortblock.

This is the sort of thing that gives me nightmares when tuning customers' bikes, to the point where I'm considering using .1065 needle jets rather than .106s. L.A.B tells me they are still available. This would be insurance for me should someone revert to ethanol gas. Ethanol-free is not universally available as you all know.
 
I have x 4 K type probes on one bank , running through a MGL avionics gauge , unfortunately my car has been off the road for a couple of years , the dyno operator took a photo at WOT and said they evened out , that said light criuse and full throttle show huge difference in exhaust temp , maybe weld in another 18x1.5 and get a AEM air fuel gauge they read fast and can read % etc you can look up tables for your fuel . Cheers

X-Series Wideband UEGO AFR Sensor Controller Gauge​

AFR VALUE RANGE BY FUEL TYPES:

  • Display Gasoline values from 8.0:1 to 20.0:1 AFR
  • Display Lambda values from .55 to 2.00
  • 0-5V Output Gasoline values from 8.5:1 to 18.0:1 AFR
  • CANbus Output Gasoline values from 8.0:1 to 20.0:1 AFR
  • CANbus Output Lambda values from .55 to 2.00
  • 0-5V Output Lambda values from .58 to 1.23
  • 0-5V Output E85 values from 5.7:1 to 12.0:1 AFR
  • 0-5V Output Ethanol values from 5.2:1 to 11.0:1 AFR
  • 0-5V Output Methanol values from 3.75:1 to 8.0:1 AFR
 
It can be very difficult to get plug color to balance with Amals, I switched to another brand.

Spark plug reading
 
Last edited:
I'm wrong carbs are 32mm But everything else is correct. Doesn't look like oil but I could be wrong. Storm coming this week end. Going to pull carb and check for possible air leak, raise the needle a notch. My concern was that possible un-synchronized carbs on the top end was the cause and some type of damage could result
 
Thomasa said:
I have read this statement many times and my own experience bears this out. But what's a body to do? I have a 6-cylinder pickup with twin carbs that have adjustable main jets and dual exhausts. After nearly twenty years of twiddling to squeeze that last MPG out of it, last winter I was blindsided by a load of ethanol gas with a little extra, I think. This was California and Arizona, mind you, and it was hotter than usual. Gas in California was over $6/gl when I started home and the incentive to lean it out was strong at 10 MPG on a good day. Anyhow, it began using oil and by the time I got back to Washington State, I was up to a quart a day (400 mi. or so ) and blowing that out the breathers all over everything. When I pulled the head I found a burnt piston.

A friend gave me a pyrometer setup he used in experiments running straight alcohol in an air-cooled VW. There are two probes, a switch and the meter head. I plan to weld a bung on each down-pipe and set up the pyrometer as a permanent gauge. Does anyone have an idea as to what a reasonable maximum temperature might be? Hopefully, this will save me a similar fate on the new shortblock.

This is the sort of thing that gives me nightmares when tuning customers' bikes, to the point where I'm considering using .1065 needle jets rather than .106s. L.A.B tells me they are still available. This would be insurance for me should someone revert to ethanol gas. Ethanol-free is not universally available as you all know.
Click to expand...
Running lean burning a piston is a very misunderstood phenomenon.
Was that pickup hauling a camper?
Racing, or working very hard is the only way the elevated combustion temperatures can overcome the pistons ability to transfer the heat away.
At Part throttle cruise, LOOOOONG before an engine "burns a piston", that engine will run VERY POORLY, and be way down on power output.

Ethanol mixed fuel doesn't burn pistons, it has less heat value per volume.
 
I think youd definately want BOTH TAPS open , doing ' plug chops ' .

Id do a flow test too . Maybe Bowls off , but through the fuel lines - anyway . for G P H . feed .
 
mean gene said:
Running non-ethanol and Aviation 100 octane mix
Click to expand...
Translated: non-ethanol unleaded, and Aviation 100 formerly called 100LL (low lead).
Not enough to color a plug.

This fuel is widely distributed, you will be able to see plug coloration.

vpracingfuels.com

Race Fuels, Lubricants, Coolants, & Additives - VP Racing Fuels

Your one-stop shop for race fuels, performance lubricants & motor oil, coolants, street additives, diesel care, small engine fuels, & detailing products.
vpracingfuels.com vpracingfuels.com

Official Fuel of NASCAR | Sunoco Race Fuels

www.sunocoracefuels.com www.sunocoracefuels.com
 
Last edited:
Matt Spencer said:
I think youd definately want BOTH TAPS open , doing ' plug chops ' .

Id do a flow test too . Maybe Bowls off , but through the fuel lines - anyway . for G P H . feed .
Click to expand...
O.P. is "plug chopping" at 30 mph, one should flow more than enough
 
mean gene said:
I'm wrong carbs are 32mm But everything else is correct. Doesn't look like oil but I could be wrong. Storm coming this week end. Going to pull carb and check for possible air leak, raise the needle a notch. My concern was that possible un-synchronized carbs on the top end was the cause and some type of damage could result
Click to expand...
Is your concern the richness of the dark plug or the leanness of the light plug?
 
concours said:
That's not unleaded pump gas.
Click to expand...
Yes it is unleaded pump gas. Thats all we have here in Calif. You just have to have a few miles on them for the color to show.

Spark plug reading


The "leaness of the light plug" with the speckles shown in the previous thread looks like it could do some damage.
 
You must log in or register to reply here.
Back
Top