Topic: Cb350f fresh top end smoking like nuts!

[markreimer]

I think it depends which year 350F. I had a '74 and mine had valve stem seals on all 8 valve guides. But I have seen some heads where the exhaust guides had no seals. CB750's were the same I believe, maybe the F series, and 69, I can't remember exactly, but early models had no exhaust seals. That could cause some smoke at startup, but nothing like most of us 350F owners experienced. This was no puff of smoke, mine was more like a tire fire in the pipe.

I would say as long as your exhaust guides are the type that can accept seals, use em! No reason not to.

I hope those Honda rings fixes your issue, I'm really curious to hear about that, so please follow up with a report!

[SquierA]

Your input is consistent with what I read yesterday about some years using them on the exhaust side and some years not. I'm only doing the valve stem seals at this time and will let you know if that cures the smoking issue the bike has now. And yes it smokes really bad however not at all at start up. Once it warms up it is literally embarrassing to ride down the street. I will update as soon I have the valve stem seals installed.

[grcamna2]

Your input is consistent with what I read yesterday about some years using them on the exhaust side and some years not. I'm only doing the valve stem seals at this time and will let you know if that cures the smoking issue the bike has now. And yes it smokes really bad however not at all at start up. Once it warms up it is literally embarrassing to ride down the street. I will update as soon I have the valve stem seals installed.

What condition is your crankcase vent in ? I hope there is nothing obstructing that 'air way' so as to create pressure build-up inside your cases..

[MRieck]

Just joined this forum and have the exact same problem as jag767 had in 2016 with his 350F. Did anyone ever find the solution that could be shared here?

The CI rings are garbage.

I have similar stories about the CI rings (CBX kits).....poor tension

[markreimer]

Just joined this forum and have the exact same problem as jag767 had in 2016 with his 350F. Did anyone ever find the solution that could be shared here?

The CI rings are garbage.

I have similar stories about the CI rings (CBX kits).....poor tension

I contacted CI about this when it first happened to ask if they had any advice or similar experiences. They responded by blocking me on eBay.

[MRieck]

Just joined this forum and have the exact same problem as jag767 had in 2016 with his 350F. Did anyone ever find the solution that could be shared here?

The CI rings are garbage.

I have similar stories about the CI rings (CBX kits).....poor tension

I contacted CI about this when it first happened to ask if they had any advice or similar experiences. They responded by blocking me on eBay.

Seems to be their standard operating procedure.

[SquierA]

Ok update here. I took the valve cover off last night and all 8 valves do have valve stem seals installed on them. SO much for remembering the machine shop calling and stating that head doesn't use valve stem seals. So now it's a full top end tear down to install the 4 sets of Honda piston rings I've purchased. I sure hope this solves the problem because I'm out of ideas after this. I will update everyone once I get this completed. In the meantime if anyone has had this same issue and found a different solution please chime in. 

[markreimer]

Man, that sucks. I feel your pain... I never solved my 350f oil burning problem. Ended up selling it as-is. I tried OEM rings on the stock pistons the first time I rebuilt the engine but it still burned oil. Not as bad as the CI big bore kit, but still too much. Maybe OEM rings on an over-bore would have been the ticket.

There is such a disproportional number of oil-burning cb350f rebuild failure threads here. it seems to me that if I ever had a 350F again, and it started burning oil, I'd fix it by looking for a complete 400F engine to drop in.

Good luck and be sure to report back!

[SquierA]

Quick update - Got the head off and there is about a teaspoon of oil on top of each piston. Talked to the machine shop and he agrees it sounds like the piston rings aren't sealing and am going to take the CI pistons to him with the Honda OE rings to make sure they will fit the pistons properly. Also gonna put a fresh crosshatch on the cylinders to help the new rings bed in. Fingers and toes crossed that this solves the problem.

[markreimer]

That's wild, exactly the same result as I had with my Cruzin Image rings. A fresh hone and some quality rings should do it...

[chrisbc55]

Shiny side up bro, in all things motorcycle  (including these rings- hard chrome edge faces the fire and is top ring).

[HondaMan]

While it is apart...measure the ACTUAL piston-bore clearance. You can do this by sliding a feeler gage alongside the piston skirt (front or back, not sides) and slide the piston into the bore (sans rings helps). It must not be more than 0.0008" on the 350F bore. I usually set those at 0.0004" (no, that is not a typo...).

[Trevor from Warragul]

A fresh crosshatch could possibly make the piston to bore clearance too great. Listen to HondaMan, the clearance is critical on these tiny cylinders!

[grcamna2]

A fresh crosshatch could possibly make the piston to bore clearance too great. Listen to HondaMan, the clearance is critical on these tiny cylinders!

++1  Make sure they're all 100% round,rather than oval-shaped.

[markreimer]

While it is apart...measure the ACTUAL piston-bore clearance. You can do this by sliding a feeler gage alongside the piston skirt (front or back, not sides) and slide the piston into the bore (sans rings helps). It must not be more than 0.0008" on the 350F bore. I usually set those at 0.0004" (no, that is not a typo...).

I had mine done at .0004” and had the same issues. Rebuilt it again and had a second machine shop verify the piston to bore clearances as well as check for cylinder hour-glassing. It was all in spec. The cruzin image rings are garbage!


Sent from my iPhone using Tapatalk

[Erwin83]

Just throwing my experience in here:

I also had a smoking engine (400f) after rebuilding the top-end, using a Forseti kit from Australia (should be same or similar to CI). Also oil on the pistons (2-3) and subsequently in the exhaust which takes a lot of time to burn off.

Turned out to be the head-gasket. Apparently, the headgasket with this kit shrunk quite a lot.
Replaced the head gasket, torqued it, left sitting 24 hrs. Torqued again. Rode the bike gently for a couple dozen kilometers, and retorqued again. Seems OK now but I don't ride the bike that often.

[caluser2000]

Did you fit the four large skinny O'rings between the crank case and barrels? A randdom question I know.

Only thing I can think of is the rings gaps aren't set 120 degrees apart.

[bryanj]

Wouldnt cause that with the 3 piece oil rings

[caluser2000]

Wouldnt cause that with the 3 piece oil rings

I the gaps on those three rings are aligned together?

[HondaMan]

Here is an off-the-wall question, it might seem:
what torque wrench are you using for the head nuts?
Reason I ask: I had access to a brand-new set of 'click' torque wrenches a few years ago (2010, I think it was). Nice ones, it seemed, with calibration certificates in their boxes. Like all things lately, they were from China (I thought they were Harbor Freight, turns out not). I torqued a 750 head down with them to the 210 in-lbs I normally use. During the engine startup, about 5 minutes into it the shop filled with smoke(!). After cooldown (next day) I checked them again, retorqued with the 1/2 turn off, then back to torque, started up again, and...the shop filled with smoke! Third day, I brought in my trusty old beam torque wrench (300 in-lb max), now 50 years old. Torqued it to 210 in-lbs, but this took more than a WHOLE TURN MORE on every nut to reach that torque. Started up, ran for 20 minutes, no smoke. 

I don't use click wrenches anymore.

[markreimer]

Interesting idea there. Still, the fact that there are now maybe ten threads on SOHC4 with 350F smokey rebuilds, all using Cruzin Image kits, my bet is still that the common denominator is the culprit.

I’m not sure I’ve read any rebuilds where OEM oversize kits were used. Probably because they’re impossible to find. But it would be interesting to compare success rates. I think everyone knows how tight these little fours need to be, but still, it’s getting crazy how many guys are ending up with bad engines now.


Sent from my iPhone using Tapatalk

[Trevor from Warragul]

I rebuilt my 350F using OEM Honda pistons & rings (2nd oversize).  Smoked like crazy & fouled the plugs, especially on cylinder number 2.  It was a piston to bore clearance problem.  In the end I bought a used standard size cylinder block with matching used pistons.  From memory, I sourced a set of standard OEM Honda rings, then went old school and cleaned up the bores with fine wet & dry sandpaper.  No smoking & dry plugs ever since...

[HondaMan]

This seems like a strange one to me. I've rebuilt 2 CB350F and 3 CB400F engines all with 0.5mm oversize CI piston sets. Not a lick of trouble with any of them. I also did a member's cylinders for a big-bore (393? CC kit) from CI, and that went fine, too.

[grcamna2]

A fresh crosshatch could possibly make the piston to bore clearance too great. Listen to HondaMan, the clearance is critical on these tiny cylinders!

++1  Make sure they're all 100% round,rather than oval-shaped.

?

[Trevor from Warragul]

I've rebuilt 2 CB350F and 3 CB400F engines all with 0.5mm oversize CI piston sets

Do you use a lathe to bore out the sleeves?  Seems to me this would be the way to most accurately do it...