16k mile barn find CB350 black oil changes?

k2-350

k2-350

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I recently got a 16k-mile 1972 CB350G. It was definitely owned by someone who was old, so I don't think the engine was revved very much, so there is probably a fair amount of carbon in it. My (mild) concern is that I did one oil change on it, ran it for a few minutes, and went to drain it and check it out. It ran for less than a minute and came out pure black, not even a burnt gold. It was not sludgy, it was a good consistency, just very dark. I should disclose it has only been running on one cylinder, and while gas was entering both cylinders, I only got spark on one (it is fixed now). I figure it was a dirty engine, and run the engine about for longer, a heat cycle, then drain it. It was still black. I did this again, and it came out a bit lighter, yet still mirror-black in a pan. I've put another change of oil in it and have not changed it yet. I plan on changing it after 3-5 short rides, easing into more spirited riding to restore some horses and get rid of some carbon. But I'm curious if anyone can tell me what's going on inside the engine? Could the fuel make it black? Is it just very dirty? I've attatched a picture of the 3rd oil change.Screenshot_20260513_164402_Gallery.jpg
 
What engine oil are you running?

Wet clutches also make oil black pretty fast, especially with a non-running cylinder and some carbon buildup over time.
 
The oil on an air cooled engine will “dirty” very quickly, not to mention there’s probably a decent amount of sludge in the lower case.

Did you clean the slinger and sump screen out?
 
EzPete said:
The oil on an air cooled engine will “dirty” very quickly, not to mention there’s probably a decent amount of sludge in the lower case.

Did you clean the slinger and sump screen out?
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Yes! I didnt know there was a screen, but it did have a centrifugal oil filter which i removed and did clean. It wasn't too dirty. I cleaned out the entire crankcase though, minus that screen. I will clean that next oil change since I'll be doing than sooner than usual anyway.
 
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Moved to 350 Engine and Transmission section.
k3350 said:
Yes! I didnt know there was a screen, but it did have a centrifugal oil filter which i removed and did clean. It wasn't too dirty. I cleaned out the entire crankcase though, minus that sludge filter. I will clean that next oil change since I'll be doing than sooner than usual anyway.
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The screen is on the bottom of the oil pump where the pickup is, so the clutch and oil pump usually have to be removed to get the screen off without doing damage.
 
k3350 said:
Yes! I didnt know there was a screen, but it did have a centrifugal oil filter which i removed and did clean. It wasn't too dirty. I cleaned out the entire crankcase though, minus that screen. I will clean that next oil change since I'll be doing than sooner than usual anyway.
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When you say you "cleaned the entire crankcase" how did you do that? Did you flush the crankcase with some solvent or something?
 
RobMan said:
When you say you "cleaned the entire crankcase" how did you do that? Did you flush the crankcase with some solvent or something?
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I took off the right side crankcase cover, and then I drained the oil, in that order. Then I dried the gears and general area with a cloth to wipe off any debris after draining the oil. there was a small amount of black goop in some areas. I opened the centrifugal filter, cleaned that, and put it back together. I also scraped the old gasket off, and put on a new one and used axle grease as a dressing.
 
Got you, I asked because a lot of sludge will collect in the bottom of the sump and if you tried flushing it would probably cause the oil to darken up quickly for several oil changes.
 
k3350 said:
I took off the right side crankcase cover, and then I drained the oil, in that order. Then I dried the gears and general area with a cloth to wipe off any debris after draining the oil. there was a small amount of black goop in some areas. I opened the centrifugal filter, cleaned that, and put it back together. I also scraped the old gasket off, and put on a new one and used axle grease as a dressing.
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There’s going to be a good amount of gunk behind the oil filter inside the case, that’s likely part of what’s turning the oil dark.

There’s a few ways to deal with it.

1. Completely strip the engine down, split the cases, clean and flush and inspect everything - including flushing the crank sludge traps.

2. Keep running it with frequent changes and oil pressure checks and cleaning the slinger often until it erodes out due to heat cycles. Possibility of clogged crack oil passages or other oil ways exists here.

3. Remove the clutch plates, replace side cover and fill with diesel or kerosene, drain flush rinse repeat until its starts getting a little cleaner then go with Option 2.
 
EzPete said:
There’s going to be a good amount of gunk behind the oil filter inside the case, that’s likely part of what’s turning the oil dark.

There’s a few ways to deal with it.

1. Completely strip the engine down, split the cases, clean and flush and inspect everything - including flushing the crank sludge traps.

2. Keep running it with frequent changes and oil pressure checks and cleaning the slinger often until it erodes out due to heat cycles. Possibility of clogged crack oil passages or other oil ways exists here.

3. Remove the clutch plates, replace side cover and fill with diesel or kerosene, drain flush rinse repeat until its starts getting a little cleaner then go with Option 2.
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I think I am not wanting to split the cases lol, I will just settle for option 2! I've already had to mess with the previous owner's wiring as they did a horrible job relocating the indicators, change coil packs, tires, brake systems, spark plugs, strip the tank of rust and reline it, and do countless hours of troubleshooting. I still have to replace the rubber brake hoses, fix the speedometer, install a horn, and get the second mirror before I can safely hit the road with it.

mike in idaho said:
Apparently, the PO was a big fan of non-detergent motor oil. I would switch to some high detergent, diesel spec, motor oil.
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I gotchya, thanks.

I'm sure after a few good miles to wake her up it will clear up. After all, surely it cant be dirty forever.. right?
 
As long as the crud doesn’t pop loose like an embolism it will eventually fade a bit.
 
k3350 said:
Im running 10W-40 JASO
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Make sure whatever it is, its not synthetic. You want full dino juice for these older bikes. Makes a big difference in power transfer for the clutch. High mineral oil has done well for me thus far, but my first oil change I did with synthetic...it ran weird.

I run Bel-Ray 10w40 Mineral, it has been amazing.
 
cellomaster said:
Make sure whatever it is, its not synthetic. You want full dino juice for these older bikes. Makes a big difference in power transfer for the clutch. High mineral oil has done well for me thus far, but my first oil change I did with synthetic...it ran weird.

I run Bel-Ray 10w40 Mineral, it has been amazing.
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Thanks for the tip, I'll try that one out!
If you don't mind, what kind of fuel do you use? I heard that modern 10% ethanol will clog carbureters if left to sit for ~1 month or longer
 
If it sits up too long the gas will cause havoc inside, plugging jets and coat everything with varnish type gunk. I had one that sat up for only a year without draining the gas and had to do a complete carb overhaul.
As for oil, I use Delron T4 because it has the JASO MA/MA2 rating and works good with the wet clutch.
 
That sounds wonderful! I'm sorry you had to go through that, but I appreciate the info. It looks like I can grab ethanol-free/marine gas for $1 more than a normal gallon. Since I'm only planning on riding here and there, seems very worthwhile.
 
cellomaster said:
Make sure whatever it is, its not synthetic. You want full dino juice for these older bikes. Makes a big difference in power transfer for the clutch. High mineral oil has done well for me thus far, but my first oil change I did with synthetic...it ran weird.

I run Bel-Ray 10w40 Mineral, it has been amazing.
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That is your experience, and you mention it as it is the truth. I have a complete different opinion based on lot's of experiences, but I don't bring it as the truth, instead I wrote a comprehensive thread about the experience and let people decide for them selves, bases on facts.

There are many types of synthetic oils, with all kind of specs, and JASO MA/MA2 are the most important.

I can only think of one reason to use dino oil is and that is ZDDP (Zinc Dialkyldithiophosphate), but even in dino oils levels are lower every year, and ZDDP has nothing to do with clutch slip either.
 
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jensen said:
That is your experience, and you mention it as it is the truth. I have a complete different opinion based on lot's of experiences, but I don't bring it as the truth, instead I wrote a comprehensive thread about the experience and let people decide for them selves, bases on facts.
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Certainly didn't mean to offend anyone or make any blanket statements. I have talked to plenty of people in and outside the forum, and they all agree, so I thought it was the standard, especially based on the experience I have had. But, anyone can of course do whatever they want!

I have seen your thread, and all the immense data, super fascinating! I guess I never caught in there that you used synthetic. I guess the bikes like different things!
 
k2-350 said:
If you don't mind, what kind of fuel do you use? I heard that modern 10% ethanol will clog carbureters if left to sit for ~1 month or longer
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I always use non-ethanol. I have heard various people use 87 or 91, and have tried both, and my bike seems to run better on 87. Not sure why, but maybe you will have a different experience.
 
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