Left cylinder STILL fouling

RDDave · May 08, 2026, 08:07:55 PM

I have done EVERYTHING I can possible think of to stop this left cylinder from fouling. I have changed the carb, swapped all the ignition components, jumpered around the bike's ignition, changed coils, put the coil power on a relay, it has 100psi on both cylinders, it has ebag cylinders on now, etc.

I have an Iridium plug on that cylinder because the Iridiums are slower to foul and I can clean them several times. The coppers are usually dead after the first fouling, so using an Iridium in the left cylinder makes sense at this point.

It looks to me like it is oil fouled. Anyone else share that opinion?

Is it possible that the case seal is leaking?

If I run the crap out of it, the left plug's center looks about like the right except that the outer ring is wet.

How does that right plug look to you? I fattened it up one more step thinking it looked a little too white before. I will get the right jets later, but it had 155s which are the biggest jets I have. They measured (with a drill bit) a loose .042. The next bit I had was .046, so that is what they are now.

This pic is after running around town for a half hour or so. The cylinder was still running.

m in sc · May 08, 2026, 10:10:06 PM

if its,still on the pump, swap the oil lines. otherwise... not sure

1976RD400C · May 09, 2026, 06:50:24 AM

Could the exhaust pipe be plugged on the left side?

m in sc · May 09, 2026, 08:10:57 AM

so,

thinking about this. have you changed the wire from the points to the coil in the harness? my 72 t500 had this issue it drove me insane about 7 years ago trying to get it figured out. would just run a bit weaker on one cyl, turns out thats what it was.

automan · May 09, 2026, 09:03:37 AM

2 different plugs may do it

RDDave · May 09, 2026, 09:44:03 AM

I have done all that except the oil line swap.

I have bypassed the bike's wiring and jumpered the points directly to the coils and changed every single piece of the ignition. Repeatedly. It had the stock pipes on when I got it; same problem. The Bassanis are clear and I have ran them with and without the baffles. I have had well over a dozen different plugs in it. Right now I have the hotter 7 Iridium in that left cylinder.

I have repeatedly changed EVERYTHING I can think of related to that left cylinder, including the carb, every ignition component, and even the piston, rings, and cylinder. The right cylinder always looks great. The left just sucks. Nothing I have done has had any affect on it, except for the Iridium plug which just doesn't foul as fast and can be cleaned and reused several times unlike the coppers. The right doesn't have the issue and the BR8HS does just fine. Caps are non resistor with new coils that were recommended here.

Does the plug look oil fouled to you? I guess I can block the injector and go to premix as a test. I just haven't done that because I didn't understand how the left could be getting too much oil without starving the right. But, if you think that is a worthy test, I would do it.

I have never had an issue like this that I couldn't resolve. But here I am. And this fouling issue is the reason that the previous owner sold the bike. After I got the bike home and had a chance to go through the box of extra items that came with it, I found a couple of dozen plugs in it. Some new, but many fouled.

The bike shows 5,000 miles and the overall condition reflects that. It doesn't appear to me that the cases have been apart, but I can't know for sure.

I have been working on this off and on for over a year. I would throw in the towel and let someone else look at it, but there just isn't anyone close to turn to. So, this board is it.

Thank you for the input. And don't stop. I really need to resolve this. For my own sanity.

1976RD400C · May 09, 2026, 10:11:40 AM

The plug could look like that just from running rich, not necessarily from oil. When you changed the carb was it completely a different one and not swap any pieces from the old one? Leak down test next??

m in sc · May 09, 2026, 10:16:16 AM

it does look oil fouled.

as unlikely as the case leaking into the crankcase, its not impossible. id swap the oil lines. that will just rule out an oil pump issue . if thats not it, tske the motor out and split the cases. good opportunity to clean stuff up and new oil seals. sucks. but i get it. was there last week with one of mine

RDDave · May 09, 2026, 10:40:14 AM

I'll swap those lines.

IF the left was sucking gear case oil, wouldn't it also be pushing into the gear case? Should the gear case eventually smell like gas? And how remote is the possibility that there is a crack or some other deformity in the cases themselves rather than a leak in the seal between the cases?

I know I may be getting into the weeds here, but it shows how desperate I am.

RDDave · May 09, 2026, 10:54:36 AM

Quote from: 1976RD400C on May 09, 2026, 10:11:40 AMThe plug could look like that just from running rich, not necessarily from oil. When you changed the carb was it completely a different one and not swap any pieces from the old one? Leak down test next??

I have changed the pistons, cylinders, and rings. I doubt that 2 completely different ones would exhibit the very same issue. But no, I haven't done a leak down, only the compression test.

And yes, it was a completely different carb, actually a right side carb. And that was after more than one complete disassembly and changing all the individual pieces from the right to the left. I have changed every single piece of the ignition and carburetor with ones from the right side and with others in my stash. Multiple times. And I check the condition of the points with my ignition tester and set and check the points with a dwell meter. Both sets are at 44* which is real close to .013. And I check the timing with the timing light after setting them with the dial indicator and buzz box.

I haven't experimented widely with the ignition settings because I have followed the experienced advice here and because the right runs so well at these settings.

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m in sc · May 09, 2026, 11:38:01 AM

my RH plug was doing the same thing on my 70 r5.

crank and seal were new last year, and i went thru basically the same thing. mind you, i have a cdi so that was likely not the issue, but i did change wires, swap carb, etc.

turns out, it was the primary gear sealing surface. 28k on the gear did it in, the seal was actually good, and this absolutely fixed it.

1976RD400C · May 09, 2026, 01:10:47 PM

I took a look at a set of cases and there is just a little area where the trans gear oil can get to the left side crankcase. The metal is real thick in the crank area also.

RDDave · May 09, 2026, 08:26:50 PM

I swapped the oil lines at the pump today and road it 25 miles or so. Center looked okay, but the outer ring was wet.....as usual. More miles and the whole thing would have been wet.

I am out of ideas. I don't know what I am missing.

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teazer · May 10, 2026, 12:16:11 AM

Left side usually runs lean when the oil seal behind the ignition/alternator starts to leak. Clearly that is not what is happening in this case. A couple of thoughts: Did you do a leakdown test yet? On a Yamaha twin with lab seal it's harder to locate the leak than some other designs, but it's a good place to start.

I wonder if the oil feed nipple or hole in the left carb is somehow larger than the right and allowing surplus oil into the left side. On Suzukis that isn't possible with the pump design, but it's been too long since I messed with an RD pump to know for sure.

RDDave · May 10, 2026, 12:04:19 PM

I guess I don't understand what a leak down test would show me at this point. If the compression is the same on two different sets of cylinders, I don't see how the problem could be there. And how would a problem between the two cylinders in the crankcase cause the left to foul? I don't understand how that could happen and would appreciate an explanation.

And as I have said before; I have changed the left carb, so I doubt two different carbs would have the same restriction.

I haven't disconnected the oil lines and gone to premix. But, as suggested, did swap the lines at the pump. That had no change in the symptoms. So, the only common denominator left are the lines themselves. But could the left get so much extra oil that it fouls without running the right cylinder lean on oil? I didn't think so, but if someone has differing information, I would welcome it.

Oh, and I have changed the left crank seal knowing that it only went to atmosphere, not to any oil. I have even ran different heads. I think the ONLY items that I have not changed on that left cylinder are the oil lines to the carbs and ...................well, I can't think of anything else that I haven't either changed or bypassed. Which is why I was thinking of the crankcase as the possible source of the fouling. I have ran out of other possibilities as had my predecessors on this bike.

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Left cylinder STILL fouling