Constantly fouling spark plug on #3

Hello XJ community,

It's been a while since I've posted here. I have a 1982 XJ650 (with a non-YICS 1980 engine from a PO's swap) that I've been working on rebuilding the last few years. In 2018 I tore into the motor and installed new pistons, rings, had the head serviced, went through the carbs, and did some overall maintenance on the bike. Since then, I've been troubleshooting poor fuel economy and a spark plug that keeps fouling. I have worked through numerous issues with the bike:

- Initially, poor compression, thus the rebuild - all cylinders are 145-150 psi cold now
- Swapping a carb body from a spare set because PO had done a float post repair which had started to fail
- Setting fuel levels in the carbs a couple of times, including after swapping in new float needles/seats
- Replaced all main and pilot jets, thinking that PO had damaged/enlarged these
- Replaced the needles (new) and emulsion tubes (used in great shape) thanks to Len/chacal
- Upgraded the ignition system to Dyna coils
- Most recently, replacing several valve stem seals that the machine shop that did my head work seemingly damaged during the install? Backside of multiple valves had oil on them.

Now the bike is running good, nice stable idle, returns to idle after a throttle blip, has remarkably more power, but is still only getting 28MPG. Rides are good, until ~30 miles or so in and plug #3 gets fouled, starts bogging and rough idle. I can clean the plug (or replace) and bike runs perfectly for another 25-30 miles. It happens consistently no matter how I'm riding that day - in traffic, or on clear roads. Lower speeds in town, or 70MPH on the highway.

In reading the plug, it sure looks oil fouled to me, but I replaced both valve stem seals on this cylinder and it is still fouling. Maybe I'm reading the plug wrong... Thoughts?

R is cyl 3

1 on L, 4 on R. I know 4 is rich but I fixed that...
Exhaust valve on cyl 3 - looks like oil and carbon

A few days ago I popped the head and cylinders off, thinking that the only thing left is the oil control rings (or the valve guide). I was expecting to find a broken or failed oil ring. They were in perfect condition! So I'm at a loss for where the oil was coming from.

Today I talked to a local motorcycle shop over the phone, and the owner there convinced me to take a step back and look at the carb again. After an hour or so of poring through the carbs, I found that the emulsion tube on #3 was seriously damaged. I think when I swapped the carb body, I didn't reinstall the newer emulsion tube and mistakenly used the one from the spare - this was the smoking gun I've been looking for. This would create a large gap around the needle, and so the engine would be sucking up way more fuel anytime the throttle is opened. I can't believe I missed this!!! But that would make the plug fuel-fouled, not oil-fouled.

Looking at piston top on cyl 3 - looks like no carbon and a thin film of oil?

Borescope shot of the piston crown - super clean, can still see casting marks

Emulsion tubes - R is very damaged

What do you all think? Have I been reading the plugs wrong the whole time and chasing the wrong problem?

Thanks for any input!
Aaron

 

white tips on plugs indicates lean paperbag brown is good. a lean plug will have misfires in the cylinder due to air fuel mix ratio and will burn and leave soot .

did you running sync carbs?

what plugs are you using?
what air filter?

are you using a nonyics tci that would be black or green label

have you ohmed out the spark plug caps?

are the header pipes all getting hpt on startup?

whats your running voltage?

 

Hey XJ550H, appreciate the help, but... Did you read the whole post and what I was asking about? I'm mainly trying to figure out if cyl #3 is fouled from oil or fuel based on my pictures. Having said that, here are the answers to your questions:

I have done a running sync and colortune and was able to get all looking great at idle except #1, PO stripped the spark plug hole and heli-coiled it so I can't get a good bite with the shallow Colortune plug. But I've got it almost there using matching the other cylinders, reading the plug color at idle, and plug chops.
Plugs are NGK BP7ES. Air filter is stock in the OE airbox, no holes drilled, etc.
TCI is correct (you've helped me verify this in the past)
Spark plug caps ohm out correctly on their own as do primary and secondary circuits on the coils and whole system resistance, cap to cap is in spec.
Header pipes are getting hot across all four cylinders.
Running voltage at idle is ~12.8 but jumps up to 13-14 off-idle. Never had charging system issues and I don't think that's the issue.

Like I mentioned the bike starts and runs fine at idle but it keeps fouling the plug on #3 (which will eventually cause a rough idle). Swap out the plug for a clean one and it runs great.

 

it looks fuel fouled

the needle for #3 may be damaged

 

Well, I have verified that all the oil control rings are good and offset correctly. I have a gasket set on the way (Thanks Len!) and am planning to reassemble when that shows up. While the bike is down, I did install new clutch discs and the EBC clutch springs. I've loved the improved clutch feel on my other bikes. Now just need to readjust the clutch free play.

I did want to share more info on what I found in the carbs... Maybe this will help somebody else.

After finding that damaged emulsion tube, I am pretty sure that it is fuel fouling and not oil (or possibly both, I do think there was a bad valve stem seal on that cylinder and it may have been burning through residual oil). Here's a better picture of the damaged tube:


And I used a tapered pick to gauge the degree of wear on the bad tube vs. a good one:

You can see that the damaged tube is much larger diameter based on the taper of the pick (which is similar to the needle). This would definitely let in way more fuel than it needs at any given throttle opening, which is why I could tune the carbs at idle and they'd be great, but the plug would foul as soon as I took it out for a road test.

Aaron