69 CT90 oil leaking from crankcase vent

m8d8y

New Member
I’m breaking in my engine after full disassembly, cleaning, gaskets, seals, top end rebuild with honed cylinder, new piston, rings, and valves. A slow but consistent amount of oil has been leaking out the crankcase vent near the front drive sprocket during the first couple short rides, so far about 10 miles total. Oil level is good. I’m not confident all the gaskets were exactly the same as stock since my kit didn’t include the hi/low cover and had wrong oil pump cover gasket. The old gaskets came off in pieces or with a razor blade, so I couldn't compare them to new.

Could there be a blocked oil passage from crud or wrong gasket forcing oil out the vent?

If so, where in the engine would a blockage cause this symptom?
 

OLD CT

Well-Known Member
Sounds like the crankcase has just a little too much oil, or over filled. After ten break in miles, it's time to change the oil. Re fill with the exact amount specified on the clutch cover. Do not forget to lean the bike to the right slowly, several times while draining the oil. That helps gets a little more of the old oil out. Not big on CT90's but if you check the oil on a CT70 you ''do not'' screw it in to check the level.
 
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m8d8y

New Member
I haven’t changed the oil yet, but I drove another 10ish miles while carb tuning and it’s still dripping. On the dipstick it’s still right on, on the tip just before the Xs when set back in without screwing it. I’ll probably probably get to an oil change this week and drive a bit more next weekend to look for drips.
 

-Nate

Active Member
I hope you put a MAGNET on the drain plug ? .

Proper engine breakin is correctly done in the first 60 seconds of operation .
 

m8d8y

New Member
No magnet, smoked for few seconds on first start but clear ever since. I hadn’t seen your 60s breakin, and wouldn’t have been able to do it anyway since the carb wasn’t working good enough. I just drove around my neighborhood which included hills after setting mixture and idle. With carb re-jetted, it seems to pull pretty good, so I doubt I’m down on power.

I drained the oil, and pulled the left crankcase cover. The oil was never leaking from the crankcase vent, but some amount onto the chain that splattered off looking like it was coming from the vent. I think it’s from the cover gasket or outer sprocket seal, but it’s impossible to see where exactly. I’m considering removing the swing arm and back wheel to see which is leaking, or just replace both.

I’ll have to buy a new seal, and I have another gasket set. I’m also going to sand/file the edges of the splines on the sprocket to prevent it damaging the seal if thats what happened.
 

-Nate

Active Member
What main jet are you running ? .

I don't think smoothing the splines is a good idea .

Just wrap some tape around them before you use a deep socket to push it in .

*DO* use only HONDA seals ! . they're still made in Japan to very close tolerances .
 

m8d8y

New Member
I have a t90ka four bolt carb that came with 75, main, 38 idle jets. I replaced with 72, 35, and fixe bogging issue when giving slight to mid range throttle. I’m running at 6k ft now with altitude knob pushed in and will be operating if on off-road trails up to ~12k ft with knob out if needed.

I was only considering smoothing the end on the shaft that doesnt engage the gear not along the length. The feel a little sharp edge/burr on the end. I rebuilt at first with honda seals, but I think only used tape on the shifter spline. I’ll try another seal.
 

m8d8y

New Member
Took off the swing arm and wheel and saw the seals were fine and the oil leak was from the side cover gasket under the sprocket. 4into1 kit didnt have the complete side cover gasket missing that bottom corner, Vesrah kit was good. I should have suspected that first after noticing the wrong oil pump and missing hi/low cover gaskets in the 4into1.

Any reason to go 70 vs 72? Its running good with 72 now, but I haven't done a plug chop yet.
 

-Nate

Active Member
Well ;

Run it hard and put it away, look at the plug in the morning .

It'll wither have a white center insulator or not .

Plug Chop Test is best but as long as no blubbering it's likely okay .
 

m8d8y

New Member
I haven't used my 90 much in the last year, but a few runs in the neighborhood this summer didn't go well. Going flat, up and down hill for short runs seems ok, but twice it happened sustained full throttle uphill, power loss for a few seconds, bike dies, back tire locks up skidding to a stop. Kick start cranks, but no start, open oil dipstick, have enough oil, but smoke from the crankcase. Motor is HOT! Looking around forums posts, it seems skidding to stop is probably engine seized from overheat.....I was running 70 main jet in the carb and the throttle was snappy with good power, but I hadn't done a plug chop. I tried the bike a few weeks later with a 72 jet and same. Today I tried the original 75 jet, and it didn't stop, or overheat, but removing the dipstick after the run uphill was smoking just a little, so oil is maybe still hot or maybe it's blow-by combustion gasses?

During that 1-1.5mi run uphill full throttle 3rd gear, low range, I did a plug chop finally with the 75 jet in. No soot and no color on the insulator, so I'm still running lean. I didn't cut the threads off the spark plug, but looking down in behind I don't see any tan coloring. Does that make any sense with a 75 jet running at 6,000ft elevation? I don't have bigger jets to test with now, but I'm confused why I would need to go bigger, or what else could be a problem. Any suggestions are appreciated.
 

-Nate

Active Member
When the engine gets too hot it can "bind up" enough to come to a halt without actually siezing up .

For now I'd drain the oil and remove the clutch cover and check the oil intake screen, it may have filled / blocked with clutch fibers .

I'd also open the center plate in the clutch and clean it out .

Re assemble and fill with fully synthetic oil and check and adjust the valves then the ignition timing .
 

m8d8y

New Member
I forgot to mention to keep the post shorter, after the first time it bound up and died, I changed the oil, checked the timing and valve lash. Timing was a little advanced and the intake valve was the slightest bit loose, so I set timing to F and tightened the valve. I didn't run it with the exhaust valve cover off to confirm if there was sufficient oil supply, but when I took the cover off today, it was filled with oil, so there is some flow. I'll check all that stuff again and see how it goes with the 75 jet for now.
 

-Nate

Active Member
Okay, sounds like you're going in the right direction .

That being said anytime an engine overheats the valves should be checked as they'll normally be tight again .

What about the litle square black oil intake screen ? .

Ditto the oil centrifuge in the clutch basket ~ these are serious pinch points for oil flow when blocked by crud .
 

__o7

New Member
Made my account to chime in and ask the same question because I had literally the SAME EXACT symptoms after doing a complete engine rebuild. Would run great on idle and putting around, but a sustained full throttle run overheated the bike. I soft siezed it a few times like you described and will be replacing the piston/rings preventatively (it still feels like it has good compression but the engine is noisier after these events so I wont sleep well without replacing them and checking the cylinder)

Opened up my clutch cover this evening to take the clutch out for a rebuild while I determined what to do about the overheating.

And this is what I found:
IMG_0714.jpg

IMG_0715.jpg

IMG_0718.jpg

Im 99% sure this is what caused the overheating at high RPM. At low RPM, with much less heat being generated and lower flow requirements the pump probably got by but in higher RPM it almost certainly would restrict flow so much that oil wouldn't circulate fast enough through the head and rest of the engine, preventing it from pulling heat away from the cylinder.
 

__o7

New Member
It was very small lint fibers presumably from the blue shop paper towels I use, mixed with very fine metal particles from break in. Not sure where they all came from but maybe a piece was stick in the corner somewhere or over the 30 miles or so of breaking it in tons of individual lint particles built up and clogged it.

This is a clean filter:
IMG_0720.jpg
 
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-Nate

Active Member
And, there you go .

I often find more than a little bit of sludge hiding way back inside the crankcase, why I'm so adamant about always cleaning until it's all gone .

Old oil tends to sludge up and stick where you can't see it, fresh oil is detergent an breaks the oil oil and crud free .

Wieh the clutch cover off one can reach deper inside with coat hanger or simple a snorkel to blast the crud loose with your chosen cleaner .
 
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