Got my CL350

tmross said:
Yes and yes. Also can see lots of oil splashing around if I take off a tappet cover.

I'm pretty sure this isn't condensation, strong fumey smell.
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Cool. I feel like you've ridden enough on it that it shouldn't be condensation, but just wanted to mention that possibility since I had seen it on mine.

Kinda sounds like your motor is running hot. Have you inspected the exhaust to make sure the header pipes are clear? A restricted exhaust might cause overheating.
 
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RobMan said:
Typically excess crankcase blow-by would be caused by cylinder/ring wear (which doesn't sound like the case here), piston ring gaps lined up on the piston or the rings turned upside down. Any chance you got a ring upside down?
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I did space the ring gaps out correctly, I am almost entirely sure I put them right side up, thats the kind of thing I would be diligent about, but at the end of the day I am an amateur rebuilding engines in my living room so its not entirely impossible. Will have to confirm when I reopen the top end, which will probably be sooner rather than later at this point.

I'm still hung up on the rings not seating theory, seems like that would mimic the things you listed. I just don't love that compression drops at full temp.

Thanks everyone for sticking with me here, I really want to get this bike running right.
 
tmross said:
I did space the ring gaps out correctly, I am almost entirely sure I put them right side up, thats the kind of thing I would be diligent about, but at the end of the day I am an amateur rebuilding engines in my living room so its not entirely impossible. Will have to confirm when I reopen the top end, which will probably be sooner rather than later at this point.

I'm still hung up on the rings not seating theory, seems like that would mimic the things you listed. I just don't love that compression drops at full temp.

Thanks everyone for sticking with me here, I really want to get this bike running right.
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Sounds like you did due diligence but worth a look when it comes apart. I know you said your machinist checked the bore and pistons but I would double check myself just for piece of mind. I looked on David Silver Spares and they have genuine Honda piston rings available from standard to 1.00 mm over. Not saying to buy from them, just an option, but I would get Honda rings. I am like you, losing compression as it warms up is weird.
 
Guys, im starting to think there's something to this overheating theory, and wonder if it could explain everything I'm observing. This occurred to me when I was redoing compression tests today with a new Mityvac compression gauge I got - I didn't trust the cheap ones I was using. First I did a cold test and got L 190, R 200, so that doesn't really make any sense at all, but not important to this story. After that, for good measure, I increased the exhaust valve clearance by .002", total of .006". Rode around for 30 mins, idle started drooping and stumbling as usual. Then I did a hot compression test. In the middle of the test I heard a "pfff" noise and realized the o-ring that seals the adapter at the spark plug hole had blown out.
1775175722887.png

The reading got to 172 and wouldn't go higher. Ive observed that pfff noise with the other cheap gauges I used. So now I'm thinking the reason why the hot comp tests are lower than the cold tests is because the heat is softening/expanding the o-ring and it blows out, giving me inaccurate low readings. I wonder if excessive heat is causing this, and could also explain how the hose itself melted/blew off the adapter on the cheap gauge i was using yesterday.

Just looking at the symptoms - idle struggling, smoke out the breather tube, gas bubbling in the lines and tank, this could all just be simply overheating huh? The question is why is it overheating... its certainly not tight valves, they are currently both .002" over spec and click clacking so loud its comical. Checked oil and its right between the 2 lines. The plugs look on the lean side to me, so maybe bigger mains could be worth a shot. I think the 4into1 rebuild kit I got comes with one size up of each.

So anyway thats a thought. Its probably a simpler explanation than the rings not seating. I'll get an IR thermometer and verify temps.

Also I checked the pipes for blockage, used a snake camera, all clear.
 
The back pressure is reduced on the left cylinder only when the idle speed starts to droop, correct?

What are the float heights currently? 722A carbs?
 
Upjetted mains to 70/110, up from the stock 68/105. Problem unchanged, and performance throughout the rpm range noticeably worse. Damn, thought I was on to something with the lean/overheating theory.

Other thing effecting temp would be timing, dynamic is spot on, will check static timing next.

After that I've run out of things I can do.
 
IR thermometer arrives next week, ordered online. Didn't check the breather tube after this last ride. Even if its running cooler, it runs like trash with richer jetting.
 
I had my CB360G out for a forty mile round trip today on mostly suburban surface streets, twenty miles out and then it sat a few hours before I came back home. Each leg took about 45 minutes of riding. When I got home, I measured temperatures on the head with an IR gun, getting 250-260 °F near the headers and about 220 °F near the intake.

It's at least some frame of reference for your measurements, even though it's a different motor.
 
Had the carbs off over the weekend, took note of the throttle plate positions:
1775479261398.png

L plate open enough to see that idle passage hole, R only barely peaking through. This is the setting required for L and R exhaust pressures to be equal at temp, it represents about an extra half turn in of the idle screw on the L carb. When cold/warming up, the L exhaust pressure is higher than R, reflecting the larger L throttle plate opening, the pressures slowly equalize with higher temps.

Its a clue, just not sure what it points to. I'm considering finding another L carb, to see if its a problem with the carb body. I'm starting to feel this project in the wallet though.
 
tmross said:
Its a clue, just not sure what it points to. I'm considering finding another L carb, to see if its a problem with the carb body. I'm starting to feel this project in the wallet though.
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Have you verified that the larger idle passage is open on the left? I will typically spray carb cleaner or compressed air through to verify when reviving/cleaning an old set of carbs. You've probably already checked, but an old o-ring can sometimes get jammed in the mixture screw hole.

At the same time, it's not unreasonable for the throttle plates to be in slightly different positions — even on a carb rack the throttle plates are adjusted independently when balancing vacuum.

I get what you're saying about the cost. It doesn't feel good throwing parts and money at something on a whim, so hopefully the right steps can be identified and followed to avoid wasting money.

You could swap the slow jets to make sure it's not a problem with the jet. Have you seen daylight through them or run a thin wire (guitar string ~0.009") through them? Based on how clean the venturis are, I'm guessing you've already done this.
 
Yes I've shot carb cleaner through the idle passages and it blasts out the little holes under the throttle plate in a strong healthy stream. Also ran guitar string through pilot jet, can see daylight through it.

stl360+450 said:
At the same time, it's not unreasonable for the throttle plates to be in slightly different positions — even on a carb rack the throttle plates are adjusted independently when balancing vacuum.
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True, but in this case, since the L cylinder power seems to be dropping with temp, it seems like the more open L throttle plate is masking/compensating for some other abnormal condition/process.

i.e. if the left cylinder idle were just weaker than R at startup and at full temp, I'd just bump up the idle on the L and call it a day. Its the inconsistency thats complicating matters.
 
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When you replaced the felt rings on the throttle shaft, were you careful about the orientation and centering of the throttle plates upon reinstallation? The orientation is important due to the bevel on the upper/lower edges.

Did I read correctly that you have new insulator boots? So, no air leaks at this point, whether motor is warm or cold?
 
I'm not much of a carb guy, but when I see throttle plates oriented like that it makes me wonder if the way the engine runs might just have something to do with the sync of the carbs prior to any adjustments being made. If the one side's idle passage is partially visible but the other is not, I wonder if going back to the beginning and doing the setup/sync like Bill Lane did it might arrive at a point where both plates are closer to being the same, then mixture adjustments and idle speed adjustments might be more equivalent and responsive. But don't mind me, I haven't touched a set of stock carbs since most were new enough that they didn't need rebuilds. :rolleyes:
 
I'm still curious about the engine temps.

The thing that bewilders me the most is the fact that if you adjusted the idle up on the left when the motor was warm, then it would be too high when starting the motor cold and would still droop when the motor was warmed up again. I could live with the higher idle during warmup, as long as it stabilized at temperature.

tmross said:
I still think somethings up with the left cylinder, during warmup the left and right exhaust pressures are equal but at full temp when the idle starts sagging, the left exhaust pressure is noticeably weaker than the right. If I try to boost the idle speed on the left cylinder to compensate, the next time I start the bike up the left exhaust pressure will be higher than the right, and the idle hits like 1600-1700 during warmup before it starts slowly sagging down to below 1k at full temp.
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Just found an interesting explanation for compression falling when hot, from a moto guzzi forum:

1775591039452.png

OP had similar symptoms as me, too small ring gap did end up being the problem after he reopened the top end and remeasured.

I mentioned before my ring gaps measured .016" new out of the box, I really want to remeasure that now.
 
Interesting, not something I'd considered since most rebuilds are with OEM oversizes and proper work by the machinist who does the boring and clearance so it shouldn't happen. Now if someone used the next oversize rings (.010" bigger) and hand-filed them, then it could easily be a problem if left with too tight an end gap.
 
This could potentially tie into the upside-down ring theory as well, which could interfere with the oil film protecting the cylinder wall and contribute to overheating. Too much heat might also cause the ring gap to close.
 
Got some more data today. Thermometer arrived + a new Lisle compression gauge. Out of all the gauges I've used the Lisle seems to be giving the most believable numbers. The Autozone rental read low, the Mityvac read high, the Lisle reads in the middle.

Warm comp tests L/R:
Autozone - 147/153
Mityvac - 182/188
Lisle - 174/180

So I'm hanging on to the Lisle, reviews seem to indicate its an accurate gauge. I did cold comp tests with the Lisle as well and got 180/187, so the compression drop from cold to hot is consistent with every gauge I've used.

Also I did a leak down test (warm), L ~ 5%, R ~ 2%.

As for temp tests, took temps after a 30 min ride. It was hard to be scientific about it cause the reading would change like 30 deg if I moved a couple inches closer to the source. But generally I was getting in the 280-300 range around the exhaust ports, 280 around the cylinders, low 200s around the cam box. There didn't seem to be much difference between the L/R sides of the engine.

Lastly, I replaced the L spark plug before riding today (damaged the old one), took a look at it after the ride and the tip was bone white. I then observed the spark itself on both plugs, L spark was bluish/violet and R seemed more whiteish. While observing the sparks I noticed the bike will idle on just the R cylinder, but not on the just the L cylinder, needed a little throttle. Consistent with the lower exhaust pressure finding on L exhaust. Somethings definitely up with that L cylinder/carb.

P.S. I ordered another set of carbs today, to rule out carb.
 
The points gap on the left is within spec? You could try swapping the coil leads also — if the caps reach the opposite side plugs you would just need to swap the points leads.

Your motor is running a bit hotter than my 360G, but it doesn't seem like a huge difference.
 
stl360+450 said:
The points gap on the left is within spec?
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yeah i've redone points and static timing like 5 times now.

Does seem like its running hot, not surprising given the spotless white spark plug tip. seems really lean. But upjetting gave it that undeniable too-rich boggy flubbery feeling. Need to check dynamic timing.
 
I was looking back at old posts. In post #60 you mention smoking (figuratively) the left coil a little bit by leaving the key on. It's still providing spark and the plugs are clean, so it seems an unlikely culprit, but it is on the troublesome side. Could it be cutting out or failing to combust the mixture at idle when hot? Does the left mixture screw have any effect during the droopy idle scenario?

Did you happen to check the breather tube when you got the temp readings? Incidentally, I was getting pretty similar temps to yours on my CB350 K3 head after a spirited tour of four-way stops on my way home from work this afternoon.
 
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I replaced the coils altogether a few weeks ago, I didn't trust the original coil after cooking it. Condensers and caps too. The only ignition component I havent replaced is points, I have a reproduction set from 4into1 and might swap those in. I did actually swap the coil leads across cylinders today too, no change.

Breather tube was smoking, its pretty much always smoking at temp.
 
Modern reproduction points for the 350 are sometimes difficult to adjust without modifying the holes that limit the adjustment. I'd hold off on putting those in unless you see something wrong with the originals.

That breather tube smoke bothers me a little. Might fit in with your ring end collision theory.
 
stl360+450 said:
Does the left mixture screw have any effect during the droopy idle scenario?
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Yes, yesterday I tried adjusting it a little richer (new spark plug is still bone white after 2 30 min rides, freaks me out). 1/4 turn out drops rpms, 1/2 turn out kills the engine. It seems a little less responsive turning clockwise but 1-1/4 is highest idle, for both carbs, thats just where it wants to be. Its weird cause it seems like its starving for fuel, but any attempt to enrich pilot circuit or upjet mains and the bike just hates it.
 
It is interesting that the exhaust pressure is weaker, yet changing the mixture cannot effect any gain. Were the followers in pretty good shape when you rebuilt the top end?

You had mentioned it running on just the right cylinder, but not on the left only. Yet turning the mixture screw out a half turn on the left kills the motor. Goofy.

Are you using NGK BR8ES plugs with non-resistor caps? Mostly curious about the heat range based on the white insulators.
 
Well guys i got some good news and some very very very bad news

Good news is advancer is functioning properly verified via dynamic timing test.

Bad news is the overheating is indeed due to top end oil starvation. I rechecked the oil check bolt at the top of the head today, bone dry. I had checked it last month and oil was flowing. I should've rechecked it when I first started noticing the smoke out the breather tube, but my mind fixated on theories of unseated rings and blowby. I drained the oil and saw more rubber chunks - further disintegration of the tensioner roller. I would bet thats whats clogging up the oil passages too.

So I'll be pulling the engine this weekend. Might be in the market for new cam/bearings.
 
I'm sorry to hear it. I hope the cam bearings will be okay. My CB350 project involved two motors and all four cam bearings were unusable. I wonder what percentage of the originals are still in service.

You might have a look at this thread by member @GaryJames on a ball bearing modification for the cam bearings. I believe others have also done this and it shouldn't be too difficult for a machine shop to do.

And here's @Boomer343's thread.
 
I plan to pull the engine tonight. thinking back, I'm pretty sure the idle dropping issue was already there before I checked the top end oil bolt the first time (and verified oil flow). The idle issue seems to have always been there in fact. So I hesitate to attribute that to the oil starvation/overheating. Certainly wasn't helping matters. But I still suspect theres something else at play here, particularly with the L cylinder.

I took a look at the oil filter this morning, pick up screen was clean, filter really wasnt that dirty. "oil guide" thing on the oil filter cover is free and springy.

One reason I never bothered to check the top end oil bolt again was cause the head gasket was still leaking oil healthily, so I knew oil was at least making it that far!
 
tmross said:
One reason I never bothered to check the top end oil bolt again was cause the head gasket was still leaking oil healthily, so I knew oil was at least making it that far!
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I wonder if the head gasket leak was exacerbated by the blockage that kept oil from getting to the top end.
 
tmross said:
But I still suspect theres something else at play here, particularly with the L cylinder.
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You can certainly investigate that end gap theory when you take it apart. It will be interesting to see if there is any scoring of the cylinder walls also. Hoping for the best.
 
Some pics, it wasn't totally bone dry up there. I don't see any clogs in any of the oil passageways. The roller was toast but the chunks fell to the bottom and the filter caught the rest. Wonder if it could be an oil pressure issue with the oil pump/filter itself.

L cam Bearing - some scoring. But Idk it was running fine like this.
Image_20260411_111017_476.jpeg

R cam bearing:

Image_20260411_111017_210.jpeg

Camshaft:
Image_20260411_111017_079.jpeg


Image_20260411_111016_893.jpeg


Image_20260411_111136_948.jpeg

They don't make rubber like they used to. Anyone used the 4into1 rollers?
 
I used the big roller from 4-into-1 and purchased the tensioner cam with small roller from 2fastmoto. I do not have a ton of miles on the motor yet, probably around 150.
 
Rechecked the ring gap on L piston rings, in spec, so tight ring gap theory is out. Weird thing is top ring doesn't have a letter stamped on it, middle ring does. Same on R piston rings. I remember there being stamps on the top rings, and the pic on 4into1's site shows stamps. Think the stamps must've worn off or something. Unfortunately I lost track of which way was facing up on the L top ring and since theres no stamp now Idk which way to put it back on. The top and bottom sides of the ring look identical, is there a way to know which way is up without a stamp?

EDIT - looking again at the 4into1 pic the top rings indeed do not have stamps so I guess the orientation didn't matter.
 
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Cylinders look fine, did leak test on valves they're sealing perfect.

Screen Shot 2026-04-11 at 12.52.24 PM.png
 
tmross said:
EDIT - looking again at the 4into1 pic the top rings indeed do not have stamps so I guess the orientation didn't matter.
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Maybe I'm looking at the wrong set, but this is a line from the description of the ring sets on 4-into-1:

4-into-1: Make sure the Letter stamped on the top two rings are facing up when installing them and stagger the ring gaps according to the manual.
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When I look at the picture of their ring set, there are two rings shown at each side, each with a capital R on them. The three-piece oil ring is laid out in the center triangular grouping.
 
I actually have two sets of the 4-into-1 rings that were not needed for my build. I *think* they are standard. The boxes show 13011-391-004. I'd be happy to mail them to you, if you'd like to start fresh rather than gamble on the ring orientation.
 
stl360+450 said:
Maybe I'm looking at the wrong set, but this is a line from the description of the ring sets on 4-into-1:



When I look at the picture of their ring set, there are two rings shown at each side, each with a capital R on them. The three-piece oil ring is laid out in the center triangular grouping.
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you're right, I was mistaking the oil rings for the top rings in the pic. Well thats weird, I swear the markings must've worn off then.

Thanks for that offer, I was thinking of finding some OEM rings this time, I'll let you know.
 
tmross said:
you're right, I was mistaking the oil rings for the top rings in the pic. Well thats weird, I swear the markings must've worn off then.

Thanks for that offer, I was thinking of finding some OEM rings this time, I'll let you know.
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I was just looking at some OEM rings for my CB360 and I can see how the second ring in that kit matches the profile shown in the image I posted earlier. The 4-into-1 second rings don't seem to have that same profile. I was checking for the marking on the OEM top ring and as best as I can tell it is a small 0 on the kit I have.

It looks like CMSNL has OEM ($$) and non-OEM ($) rings. Send a PM if you'd like the free 4-into-1 rings from my leftovers.
 
think I figured out which way is up judging by wear marks on the edges of the rings. theres a little bit of wear on the lower half of the edge on all the rings. makes sense from what I read the lower edge of the ring is pushed outward against the cylinder wall on the power stroke which creates the seal.
 
tmross said:
One reason I never bothered to check the top end oil bolt again was cause the head gasket was still leaking oil healthily, so I knew oil was at least making it that far!
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That is the erroneous thinking I had as well, until a buddy pointed out that with a blockage the oil will seek to squirt out wherever and can do so with myriad unexpected results

In my case it was the wrong hole not poked out in the cam bearing gasket, I caught it in time thankfully on the roller starter but my head gasket was spewing oil


ancientdad said:
I wonder if the head gasket leak was exacerbated by the blockage that kept oil from getting to the top end.
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Yes, and there was no head gasket leak after my fix, whew
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I'm curious to know your plans with those scored cam bearings and the cam ?
 
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