Topic: CB350F: two failed top end rebuilds, stock bore and 393cc

[ekpent]

  Lookin' good,bet your anxious to get it all wrapped up and press that starter button for the big moment.

[markreimer]

It's killing me! This bike has sat since the spring, I just didn't have the time and money to deal with it. I'm really hoping this engine rebuild makes it as reliable as my 750. I don't see why it wouldn't.. I know the bottom end is solid, and now it has fresh pistons, rings, valve seats cut and so on. If it runs well for the rest of the season I'll start focusing on the many cosmetic improvements it could use 

[markreimer]

Oh and I forgot to note one thing for the build thread

I looked into the intake and exhaust ports to see if there was any indication of oil weeping down the guides, thinking that might have contributed to the heavy carbon fouling of the piston tops and valves, and the burning oil. I did have a valve job done right before this happened though, with seats cut and guide clearance checked. When I pulled the valves, one of them had a tiny nick from the keeper, and I was worried it might have scratched a guide on the way out.

I found evidence of some oil in the #4 exhaust port, which was the cylinder that was burning massive amounts of oil relative to the others. I removed the keeper and springs, and pulled the valve out to inspect the guide. The guide appeared to be perfect with no signs of scratching or damage. So I'm guessing that the bit of oil I saw in the port was blown out during combustion. The oil likely entered the cylinder by sneaking past the rings, and any unburned oil remaining was blown out the exhaust, leaving some wet spots in the port.


Does that rationale make sense? Again, the guide looked good and the clearance measured within spec. The exhaust guides do not have seals on this year.

[MRieck]

Cylinder is on! Had a couple of brain farts on the way, but it's together now.

I checked the ring gaps and installed the rings, verifying that they all had the 'top' markings facing up, and the order was correct. I had my first very close OOPS when I realized just before sliding the cylinder down that #2 piston had the two upper rings swapped! Glad that didn't happen. So I fixed that up, installed the pistons in the cylinder, then lowered the cylinders (with pistons inside) onto the rods, sliding the middle two wrist pins and circlips in first, then the outer two. This is my first time assembling a motor this way, and it was actually very easy to do by myself. The big bore totally eliminate the chamfer at the bottom of the cylinder, so it'd be super easy to break a ring without a compressor. This made it easy to ensure the rings made it in safely.

The machine shop should have cut a new chamfer. I'm only mentioning it as it may help somebody in the future. If you don't know the shop always ask.

[markreimer]

Well technically there was a chamfer, but it was tiiiiiiiiny. Boring it out to 393 has made the barrels super thin at the bottom. If the taper was any wider the cylinders would be knife edged on the bottom. Sure would have helped, although I gotta say installing the pistons in first made the whole process really easy.

[markreimer]

Motor is done!

More big progress today. I retorqued the cylinder studs, double checked the cam timing, lubed everything with engine oil and bolted the valve cover down.

Installed the carbs after much coarse language, and hooked up the control cables. I've always hated the throttle cable routing on this bike as one cable protrudes directly into the frame spine, producing a sharp bend that makes for a stiff throttle. Gonna have to work that issue out soon.

Anyway, I assembled the rest of the bike, put fresh oil in and went to start it up, only to realize I had no spark plugs and no gas!! Doh!! Gonna have to wait another day or two. But I did hit the starter a few times with no plugs in, and it spun up nicely, building oil pressure and shutting the oil light off.

Next chance I get I'll put plugs in, fresh gas, check the points gap and giver.


Forgot my phone though, no pics today


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[markreimer]

Just got back from my first ride on the cb393f!

A few concerns and comments.

First the good; fired up immediately and the engine sounds great. Much smoother and quieter than before. Settled into a pleasant idle.

The concern:
It was very smokey right off the bat. However, I did burn several liters of oil through that pipe, so I'm sure there is some stuff burning off in there.

I went for a quick test ride and it felt great. Pulled well and sounded good. But smokey. Yes, all the rings are in correctly, I double checked and had someone else confirm!

I got back and pulled the #4 plug, which is the cylinder that was mega fouled in spring. It looked perfect! Nice and tan. I parked it for an hour or two

Went for a second test ride, a bit longer this time. Again it fired up well and felt great. When I got back I decided to pull all four plugs to see. #1 was wet. I don't THINK it was wet with oil, but with gas. #2 also looked gassy, 3/4 were dark but dry. I did the sniff test, but it was inconclusive. I didn't really smell anything

What do you think based on the photos? I wiped the wet #1 plug wth my finger and didn't get any black marks on my skin. But the oil is brand new, so it wouldn't really be black yet I'd suspect?

So I'm thinking I have a fuel or maybe ignition issue. I have cruisin image jets in there now, I believe 35. But these plugs look very rich. Stock needle I believe.

I DO know that my petcock has a very slow leak. And the two wet plugs are the lower side of the engine when it's on the sidestand, which is how I had it parked. So maybe I have a carb issue that caused the plugs to get wet while parked for two hours. But wouldn't that have burned off after a 5 minute ride?

Is there a way to confirm this is fuel and not oil? I held a match to the wet plug and it didn't catch fire...



1-4 after I wiped the plug with my finger.






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[seanbarney41]

If your baffle got oil soaked, it does take a surprisingly long time to burn it out.

[markreimer]

Yeah I'm thinking so, and not getting too worried just yet.

I think I'll start by pulling the carbs and replacing the slow jets. I found a set of Keihin 35's, still in the package. I forgot I had them! So I can at least verify that I've got the right slow jets, and check the float levels while I'm in there. I'll order Keihin mains as well today so the carb is all OEM.

I know my petcock has a slow leak as well, pretty sure it's leaking past the two fibre washers under the screws that bolt it to the tank. I'll fix that up as well, as I'm thinking it's possible that the wetness might have been from fuel leaking into the chamber while it was parked, wetting the plug, before I pulled them. May as well try these easy things first.

Once those two are done, I'll take it for ride for a few hours and see if I can't burn out that oily pipe. it doesn't smell like burning oil, not like it did before. And it's not a huge plume of blue smoke so far as I can tell. It was night time, so I'll have to check again in the daylight. It smelled gassy to me, not oily.

Another possible cause of gas fouled plugs could be ignition. I have the original set of points on there. I've only heard bad things about the Diacchi replacements, but OEM is hard to find and very $$$. I could almost just put a Pamco in there for the same price. Anyone have a line on reasonably priced OEM points and condensors? I'll trip back the spark plug wires and replace the caps too, they are originals.

[flybox1]

If you replaced the float valves with aftermarket, your float level will need to be near 24mm (not 21mm as mentioned in the shop manual and clymer)
If so, this (high fuel level) may account for rich plugs.  Clear tube test will really show where you need to go.

[markreimer]

Thanks, I did infact replace the float valves. The originals were damaged during disassembly, it had been sitting outside for... 41 years lol. I'll check with the clear tube method!

[Ridem32]

I would take it out and ride it some. Let the motor get some miles on it and get broke in.  You said it runs good. Ride ride ride it. Then if plugs still look like rich fuel check out the carbs. I think your a little gun shy from before


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[markreimer]

Bad news.

Just got in from a 70 mile ride, all highway. The bike smoked like a chimney the entire time. Someone asked if I was on fire haha. It's burning huge amount of oil. Oil was spitting out at the header/pipe connection.

It smokes at all times. At idle, then a thick stream while accelerating, and even more after rolling on the throttle after engine braking.

After 60 miles I checked the oil. The dipstick was dry!! I had to add 1.5L to get t back to full.
 
What's strange is the engine absolutely hauls compared to last year. It pulls steady and hard all the way to 100mph in 5th without tucking. It never did that before. It feels strong and smooth but just so smokey.

I'm not sure where to start here. I know the rings are in right side up. I know the piston to bore gap was done within spec. The valve clearances were in spec and the faces cut. What could it be? What could cause such massive oil consumption?

Please help, I'm so bummed






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[innovativems]

Did you have your valve guides checked?


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[ekpent]

 Sorry about your situation Mark,sure it is disappointing. Classic sounding ring problem like I had once on a 836 rebuild but you sound quite confident that they are all orientated correctly. Hope you can get her sorted out after all of your effort.

[markreimer]

Yes, very disappointing indeed. But it is what it is. Time to focus on getting it sorted.

I agree that it sounds like a ring problem. Innovativems - I checked the valve clearance last year when I had the valve seats recut. It checked out, but come to think of it, the bike smoked like mad after I rebuilt it with stock-bore pistons. New rings, a fresh hone, valve seats cut - smoked like crazy. Now I do a big bore, smokes like crazy. Though it's definitely smoking way more now than it did before.

If I only had one ring upside down, I'm assuming that only that cylinder would burn oil, right? All four of my plugs look very fouled, and I'm pretty sure they're all burning. Hard to tell with a 4-1 pipe. But oil was literally spitting out the pipe while riding. Would bad valves even be capable of spitting that much oil out? Keeping in mind they only have 10,000 miles on them, and the valves felt very precise in the guides. I couldn't actually feel any play in them at all by hand.

Clearly the engine has to come apart again so I can check these things again. It's only the price of a few gaskets I guess.

If the rings ARE all in the correct orientation, then what should I look at? all the rings were gapped to the tighter end of the range in the manual. In fact I only filed two of the 12 rings, and only a a few very light passes with the file.

I'm just so baffled that this is happening! 

[markreimer]

I'm trying to deduce a few factors based on what happened, can someone verify if this logic is sound?


- engine pulls much harder than it used to. Smooth and quiet too. I don't run out of power, I run out of gears (need a 6th!): Deduction - compression should be good, piston-to-bore should be good too or else I'd be down on power or have a rattly engine, right?

- burning oil - Would an upside down ring affect the engine's ability to make power? Or does it only affect oil consumption? Lets imagine I've got all four oil control rings in upside down. Could I still have a powerful motor but just have loads of blowby?

- burning oil - lets say my valve guides are actually out of spec. Is it reasonable to expect that valve guides could cause this much oil consumption? Seems like a lot of oil to get sucked through a tiny hole...?

- burning oil - if the pistons and rings are good, and if the valve guides measure in spec and the seals are in good shape (my head only has intake seals by the way), is there any other way oil could be getting burned at this rate?

[bryanj]

I have no experience of the 350 as it did not come to the UK but the 750F2 was notorious for bad exhaust guides and when guides worn would use a UK pint in 350 miles then when 1 stem oil seal split a pint in 150 miles

[markreimer]

Huh, interesting. Well I'll pull out the dial indicator and measure the play in the guides, and also inspect the guides for any scratches or other possible sources of oil leakage.

I was just looking at some photos I took of the pistons and rings during assembly. They do appear to have been placed right-side-up. All three rings had very clear "TOP" marks so I don't think this is a problem with an upside down ring.. Kinda disappointing to be honest as that would be an easy fix. If it was valve guides, I'd assume I could see all kinds of oil in the intake and exhaust ports though, right? When I took the engine apart last month, 7 of the 8 ports looked bone dry. The 8th had a speck or two of wetness, but very very minimal. And that was after a few hundred miles on the first failed rebuild. Everything feels like it's pointing to rings to me. maybe the rings are just crappy quality? I hope not, nobody else makes 393cc rings so far as I know...

[innovativems]

Bad valve guides will cause oil consumption but I can't speak to how much.  If the condition existed with all three builds I'm putting money on the head


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[markreimer]

I see your point, I'll report back what the guide condition and clearance measures up as. Probably tear it open next week.

I guess I'll pull the block too and double check the rings are all right side up. It's just another base gasket anyway. Third times the charm!


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[innovativems]

You will get this figured out! 


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[markreimer]

Started ordering gaskets and seals for top end job 3.0. Looks like Cruizin Image doesn't have any 393cc head gaskets on his eBay page right now. I didn't think of that when I bought it... guess I've got to wait for his next batch t o ome through. I'll order more than one next time... Guess winter is coming anyway.

[Jerry Rxman Griffin aka MuthaF'er]

Cometic should be able to make you a head gasket. While you're waiting send the head to Mike Rieck  You won't be disappointed and you'll know it's right.

[markreimer]

Thanks for the Cometic tip, glad to know there are options! I've emailed C.I. to see if they're making a run of gaskets soon. If not, that's a good option.

I was thinking more about what Innovativems said, and I think it makes sense that my issue is with the head. When the motor was stock, it burned oil. When I re-ringed and honed it, it burned quite a bit more oil. Then when I bored it out, it burned way way more oil. The more work I do on the block, the worse the problem gets. Really seems like I'm looking in the wrong place.

Come Thursday I'll pull the head and report my findings. I found a supplier for new intake/exhaust guides if they measure out of spec.