Author Topic: CB350F: two failed top end rebuilds, stock bore and 393cc (Read 28698 times)

markreimer · « **on:** May 13, 2017, 03:24:01 pm »

So I rebuilt my 74 350F top end in winter. New rings, hone, had the pistons and bore measured by my machinist to confirm it was within honda's spec (it was, just shy of max). Also has new valve seats cut, all new oem seals and gaskets.

I rode it 30-40km and then did an oil change. Everything was running well.

Today I went to ride to a town 100km away. I varied the throttle as much as I could (its flat, mile roads here...), and kept it away from the redline. Mostly 6-8k rpm. Stopped halfway there and the bike was idling nicely, running well

Pulled into town and stopped. I was immediately surrounded in blue thick smoke that stank like burning oil. Sure enough it was me. Now it's smoking like crazy. Oil dropped 1/4 inch on the stick.

What happened! What did I do? Did a screw up the break in? I've done the same thing on two 750 rebuilds with no issues.

Any suggestions?

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Ridem32 · « **Reply #1 on:** May 13, 2017, 03:28:37 pm »

Sounds like a ring broke!

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Bodi · « **Reply #2 on:** May 13, 2017, 03:36:05 pm »

Check the plugs, an oily inner (2 or 3) one suggests a broken ring. If an end one (or both) are oily, possibly the dreaded orifice valve seal leak has gone into the cylinder rather than out to the fins?
Oil is getting into the combustion chamber(s) anyway: either past the rings, down the valveguides, or through the head gasket. Now figure out which. Rings or gasket are most likely... that much oil couldn't get down the guides, IMO... some smoking especially on decel... but not a smokescreen.

markreimer · « **Reply #3 on:** May 13, 2017, 03:51:42 pm »

Good suggestions. I'll go out and pull all four and report back. Crap!

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markreimer · « **Reply #4 on:** May 13, 2017, 04:46:54 pm »

Well nothing super obvious here. I pulled all four and they're all real black but only one had a tiny spot of shiny black. Hmm. When I ride home tomorrow I'll be able to do a compression test; which should help.

Photos are in order 1-2-3-4

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drumstyx · « **Reply #5 on:** May 13, 2017, 05:00:00 pm »

Compression/leakdown will give you a definitive answer, especially considering it's a fresh rebuild, so they should all be very high compression/very low leakage.

markreimer · « **Reply #6 on:** May 13, 2017, 05:57:56 pm »

Checked the oil after the plugs. I had to add 900ml!!!! After 100km. Geez. I might not be able to ride home tomorrow like that haha

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Ridem32 · « **Reply #7 on:** May 13, 2017, 06:08:32 pm »

When you installed the rings you did not line them all up did ya. I'm sure you know to rotate each ring so they not lined up. Seems to me like a oil ring has broken. Wish you luck.

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markreimer · « **Reply #8 on:** May 13, 2017, 06:28:04 pm »

Quote from: Ridem32 on May 13, 2017, 06:08:32 pm

When you installed the rings you did not line them all up did ya. I'm sure you know to rotate each ring so they not lined up. Seems to me like a oil ring has broken. Wish you luck.

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I had them all staggered 120 degrees from each other, and they were all put in with the labels facing the top/up.

It's strange that it only started part way through my third ride. What could cause an oil ring to break then? Riding too aggressively too early? Should I have kept the rpm down longer?

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scottly · « **Reply #9 on:** May 13, 2017, 07:10:02 pm »

Is the crankcase breather clear? Any oil in the airbox? It's unlikely that all four oil rings would suddenly break after 90 km?? BTW, are you aware that the wet-sump dipsticks are checked without being screwed in?

markreimer · « **Reply #10 on:** May 13, 2017, 07:37:05 pm »

I'll check the breather!

Air box is clean and dry.

And yes, I know you don't screw the stick in!

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Ridem32 · « **Reply #11 on:** May 13, 2017, 07:52:03 pm »

I don't think it's anything you did wrong. If it's a broke ring and was installed correctly nothing you could of done to make it break. Just one of those bad mishaps. I think it's just one cylinder not all 4. One broken oil ring and it will smoke like a train.

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scottly · « **Reply #12 on:** May 13, 2017, 08:02:48 pm »

One broken oil ring would only affect one cylinder.

Ridem32 · « **Reply #13 on:** May 13, 2017, 08:53:27 pm »

Quote from: scottly on May 13, 2017, 08:02:48 pm

One broken oil ring would only affect one cylinder.

Yes only one cylinder. His plugs are not oil fouled just running rich on all cylinders

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Ridem32 · « **Reply #14 on:** May 13, 2017, 08:54:39 pm »

#4 looks like oil

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calj737 · « **Reply #15 on:** May 14, 2017, 04:42:50 am »

I would find it extremely unusual for a ring to suddenly break, especially after 100kms. Inspect the motor cases carefully to make sure you are not leaking the oil. Some fresh plugs, let the bike idle for a few minutes and pull the 1 & 4 plug. Are they rich or are they oil fouled?

Are you detecting any "performance" change in the motor? Is it struggling suddenly, misfiring, struggling at idle? A leak down test is in order when you get home.

ekpent · « **Reply #16 on:** May 14, 2017, 04:43:24 am »

All those plugs are new with low mileage ? Pretty dirty looking. I put some oil scrapper rings upside down in an RC 836 once and it was a major smoker till I went back in and flipped them over.

markreimer · « **Reply #17 on:** May 14, 2017, 10:04:03 am »

Carb jetting is set to factory settings. I have a 400f header and open pipe on it but that shouldn't matter much.

Bike idles dead smooth and pulls ok. It'll pull up to 5th gear till 8.5k, but can't pull right to redline. That was into some wind.

I'm sure I didn't break an oil ring during installation. But you're saying it may have broken while riding? I can't remember that the oil rings had a top or bottom, but I installed them five months back in winter... I know the top rings definitely had a top (they were labeled). The second scraper would have as well. Oil rings were one piece rings. I'll see if I can find any photos. I'll go pickup some plugs and check it after a few min as suggested

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markreimer · « **Reply #18 on:** May 14, 2017, 01:18:09 pm »

Alright, drum roll please......

New plugs, idled for 3-4 minutes. Photos are 1-2-3-4

There ya have it. #4 is soaked in oil. #3 is a touch darker than 1/2, I wonder if oil was somehow making its way in there too? But #4 is clearly the problem. I bet it's that damn seal.

I used new oem seals, copper coat on the head gasket and hondabond around the seal. I figured it'd be bullet proof. Any suggestions on what I should do differently?

I guess it COULD be a broken ring as well. I won't know till it's torn down. Guess I'll get to it and report back once I know.

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Bodi · « **Reply #19 on:** May 14, 2017, 01:22:20 pm »

maybe a broken ring, maybe a scraper ring upside down... even the best engine builders can make a mistake on that. And the less than best... I have.
At least the head and barrels will come off in the frame! You feel a bit stupider when it's a 750 and the whole damned motor has to come out again.

markreimer · « **Reply #20 on:** May 14, 2017, 03:05:59 pm »

Just got home. I stopped periodically to check the oil level. After 40 miles it hadn't dropped. The final 20 miles it dropped about 1/8 inch after riding harder.

Does this help in diagnosing? After engine braking, id roll on the throttle and a huge blue cloud would get spit out. My wife was in the car behind and said "well that smelled bad" haha

I guess no matter what I've gotta taken the top end apart, and at minimum order new gaskets.

If it's a broken ring, any reason I can't replace just the one set of rings? I wouldn't need to do al four right? And if it's an upside down scraper, just flip and reassemble?

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calj737 · « **Reply #21 on:** May 14, 2017, 06:52:33 pm »

Don't tear into it yet; perform a leakdown test instead. That will give you a better indication of head or jugs for the leak.

I suspect a valve stem seal given the description of when it smokes.

Phoenix · « **Reply #22 on:** May 14, 2017, 08:16:55 pm »

Risking the blind leading the blind.... I was stupid enough to gap ALL three parts of the three-part oil rings resulting in much oil and smoke. Don't know how you would ta gap a one piece oil ring. I took mine apart three times before we caught it.

markreimer · « **Reply #23 on:** May 14, 2017, 08:26:53 pm »

Cal - the engine smokes all the time, even at idle. It just smokes the worst after engine braking and then rolling on. But there is definitely a steady stream of blue smoke coming out the pipe. I did replace the intake valve guide seals when I rebuilt the engine, everything was OEM/Honda.

If there is a break in the oil orifice seal, would that show up in a compression or leak down test? I'd guess if it's the oil seal, I should have even compression across the board. If I have low compression on #4, I'd suspect a ring issue. Does that logic hold true? Or would the leaky seal result in lost compression as well?

I've never actually done a leak down test before, only compression. So the idea is you set the cylinder to TDC, apply pressure via the spark plug hole with an air compressor, and use a mechanics stethoscope to identify where the air is leaking out, correct?

So if I could hear air leaking into the crank case, that's a ring issue?

And if I hear air escaping into the head, is that a valve seat issue, or a guide issue? I had new seats cut, so how would I identify a valve guide issue?

Ridem32 · « **Reply #24 on:** May 14, 2017, 08:41:33 pm »

They sell a leak down tester cheap and they work great. Are you sure your oil ring is just one ring and not 3 ?

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Author Topic: CB350F: two failed top end rebuilds, stock bore and 393cc (Read 28698 times)