CB350 spark plug analysis help!

[iContango]

Below are my CB350's newly installed b8es plugs after 5 miles of running around town going ~40-50mph MAX:

Left cylinder = sooty tip (I assume this is rich?)
Right cylinder = white tip (I assume this is lean?)

Setup:

Stock CB350 engine w/ cheap pods, cl pipes, shorty slash cut mufflers, pamco ignition, Mikes xs650 coils, 87 octane, 10w40 moto oil

Thoughts... If one cylinder is running lean, is it common practice to jet up slightly, or should 2 cylinders never be jetted differently? I may try swapping the pods for UNI foam filters as well, not really sure where to start. The pamco's timing is set with a strobe, and appears to be fine.

Where should I start?

 

[Sonreir]

Compression numbers on each cylinder?

 

[splineman]

Intake air leak?

Sent from my ST25i using Tapatalk 2

 

[carnivorous chicken]

You didn't mention it, but did you play around with the mixture screw?

Dunno if anyone will say otherwise, but no, you shouldn't have R and L carbs jetted differently.

 

[teazer]

Check the timing with a strobe light. The lower picture appears to have run much hotter just looking at the ground strap. A plug chop has to be done at WOT under load to tell much about mixture and the place to look is deep down inside the plug.

 

[iContango]

You didn't mention it, but did you play around with the mixture screw?

Dunno if anyone will say otherwise, but no, you shouldn't have R and L carbs jetted differently.

I have not yet - I am still at 1 1/8 turns out on both carbs. I may try to richen up the side that is running lean a bit and see what happens.

 

[iContango]

Check the timing with a strobe light. The lower picture appears to have run much hotter just looking at the ground strap. A plug chop has to be done at WOT under load to tell much about mixture and the place to look is deep down inside the plug.

I have already checked the timing with a strobe and it appears to be fine on both sides. I will check again tonight just to be sure though.

I never got a chance to go WOT at all driving around in the city, so the lean condition is from idle/low/mid range for sure. Are you saying that unless you do WOT under load that any plug readings will be invalid?

 

[iContango]

Compression numbers on each cylinder?

Have not tested compression yet - this is somewhat of an interim motor until I finish the rebuild on my other one. I was really just hoping this one makes it through the summer, but am not sure if it can handle a lean condition as the plug is indicating...

 

[carnivorous chicken]

I have not yet - I am still at 1 1/8 turns out on both carbs. I may try to richen up the side that is running lean a bit and see what happens.

In my experience, mixture screws can have a little difference in how far in/out they are seated, and that might be all you need.

 

[troybilt]

I think Teazer is spot on.

If I had to guess I'd say part of you problem is timing.
Plug one heat range is cold with the mark moving toward the end of the ground strap tip. Its losing heat too quickly.
Plug two the heat range is too hot with the mark toward the base on the ground strap.

Heat Range = Ground Strap, the ground strap indicates the heat-range of the spark plug. If the "color" of the ground strap "changes" too close to the ground strap's end, (which is above the center electrode), then the heat-range is "too cold" , meaning that the strap is loosing heat too quickly to the base ring, and is not able to burn off deposits until near its end. If the "color" of the strap changes near where it is welded/attached to the base ring (last thread ring), then it means that the plug heat-range is "too hot", because heat is not being transferred/cooled from the strap to the base ring quickly enough !!!! The strap might begin to act like a "glow-plug", eventually causing pre-ignition and/or detonation later on. Proper heat-range is when the "color" is at the half-way point on the strap, neither too cold or too hot.

I would check timing first and foremost. Make sure it is dead on. Then go from there. Depending on what Pamco unit you are running (new retrofitted or old)some of the old units can vary by 6-10*.

 

[iContango]

If I had to guess I'd say part of you problem is timing.
Plug one heat range is cold with the mark moving toward the end of the ground strap tip. Its losing heat too quickly.
Plug two the heat range is too hot with the mark toward the base on the ground strap.

Heat Range = Ground Strap, the ground strap indicates the heat-range of the spark plug. If the "color" of the ground strap "changes" too close to the ground strap's end, (which is above the center electrode), then the heat-range is "too cold" , meaning that the strap is loosing heat too quickly to the base ring, and is not able to burn off deposits until near its end. If the "color" of the strap changes near where it is welded/attached to the base ring (last thread ring), then it means that the plug heat-range is "too hot", because heat is not being transferred/cooled from the strap to the base ring quickly enough !!!! The strap might begin to act like a "glow-plug", eventually causing pre-ignition and/or detonation later on. Proper heat-range is when the "color" is at the half-way point on the strap, neither too cold or too hot.

I would check timing first and foremost. Make sure it is dead on. Then go from there. Depending on what Pamco unit you are running (new retrofitted or old)some of the old units can vary by 6-10*.

Cool thanks for the info - I will check the timing with my strobe again as soon as I am able. I have the newer Pamco unit (I think) with the updated rotor design. Can't wait till Pamco Pete releases his fully electronic version (no more mechanical advancer plate).

The more I think about it, the more I am leaning towards timing as well. When I got this motor (used off of CL), I pulled the plugs and both of them were sooty with little variation. I didn't touch the motor beyond tightening up bolts and installing the pamco unit, so it seems as if the variable is isolated - I hope.

Will report back after timing check.

 

[teazer]

What we read on a plug is basically how hot it got and how much of the soot and other deposits were burned off. Plug tip temperatures change dramatically from say 200c to 800c (i'd have to check for the actual numbers) over the operating range of a motor. At idle and low load, temps are low and at high load temps are way higher.

What that means is that with perfect jetting all across the board, the plugs will look different at different speeds.

Next complication is that unleaded fuels tend to make more carbon that old unleaded fuels, so the plugs look richer even when the mixture is perfect. And what exactly does "perfect" mean? At Idle we might want a mixture of say 10:1 and at cruise we might want 15:1 for economy. For flat out we want it around 12.5.

So we have different air: fuel ratios across different operating conditions and coupled to a variable plug temperature.

That's why pros only use plug reading for WOT high load high RPM running. It's the only time when we can make an accurate assessment of conditions.

Plugs do tell us something in relative terms rather than absolutes. For example we would expect to see a change in color as we add fuel, but it doesn't tell us if the motor wants more or less fuel. Reading teh plugs at anything other than WOT does not tell us that.

That's why I tell people not to use the plugs to jet a bike - except wide open. What you can read though is a lean mixture at low revs because it should not be looking lean. Plugs also show us if one is whiter and hotter than teh other and that is useful information. Not so much about jetting but it tells us something.

In your case, one side electrode appears in the photo to be normal with just a short clean area close to the tip. The other one appears to be blasted white all the way down that side electrode. If that's right, that side is running hotter which is why I suggested checking the timing with a strobe to see if one side had a little more advance than the other.

It is not unusual to have one plug looking cleaner than the other, but your looked a little too different.

Going back a couple of steps. At cruise ie almost no throttle, the bike is running very lean and at idle it tends to be set up a little rich, so e're not sure what conditions we're looking at on those plugs. If it's low throttle riding they may be fine. If it was mainly at idle it's probably leaner than it should be.

Try adjusting the idle mixture screws aka Air screws to see where it runs best. On those carbs, I think that IN is lean and OUT is richer, but check the manual to be sure. A little too rich is safer than too lean.

Cheap pods and open pipes tend to screw up jetting on CB350s, so you may want to ask Crazy PJ what he can do to set up your carbs properly.

 

[troybilt]

Great info Teazer.

iCantango: The newer rotor does offer a tighter spec but still can have a 2-3* differential (from my own experience).
Anyhow, like Teazer said the best plug chop is at WOT. You want full load acceleration. Cut the ignition, pull the clutch and coast to a stop. Pull the plugs
Check heat range on the ground strap. Jetting at the plugs base ring. The porcelain for pre-ignition/detonation.