Topic: First Post: CB350F engine help

[DMMartin]

Hi everyone, this is my first post though I have been reading many of the posts here previously. I used to own a 72 CB500 and came here to find many solutions to my problems. Unfortunately this time I need a little bit more specific help with my newly acquired 1973 CB350F. This may get slightly long but I think it is all helpful information so here goes:

I bought the bike a few weeks back as a project, after taking a while to get it running when I went to look at it it did run pretty well. The PO told me that his mechanic had recently put a new timing chain and new rings in it because the timing chain was making noise. Everything sounded fine when I test rode it so I bought it at a decently low price. When I got the thing home I did all the standard maintenance ie: checked timing/points, adjusted valves etc. When I got it running pretty well and took it out for a long ride I discovered that it smoked REALLY badly once it got hot. Both out the exhaust and the crank case breather. I wasn't too discouraged though as I figured i'd do a top end rebuild and get rid of some of the small oil leaks it had all in one shot. So apart it came. None of the plugs showed any signs of oil, but when I pulled the exhaust the #1 and #3 exhaust valve guides looked as if they were leaking oil. I ordered a gasket kit from the company 4into1 and a set of rings off ebay then got ready to rebuild.

Here's where things got interesting, I found stampings of someone measuring the bore size on the cylinders and stampings in the pistons 1-4 but the pistons were in the cylinders backwards. The piston marked 1 was in cylinder 4 and the one marked 4 was in 1. They looked like they where professional stampings and given the fact that the PO said it had just been done I decided to put them back in their labeled holes. I don't have a dial bore gauge or Mic so I couldn't double check, but after building quite a few car engines the pistons seemed about as tight as any of the other cast piston engines I've assembled. I continued. In addition to that I found that cylinder #4 was missing a circlip for the wrist pin and the pin had scored the wall fairly badly. I gave each a light hone for maybe 20-30 seconds and this helped the situation but the score was still there. I made a pretty poor decision and decided to assemble it as is and hope for the best. New rings, lightly honed, and a new set of circlips and I threw it together.

As for the cylinder head, I pulled every single valve, each and every one seemed perfectly tight in the guide and it had valve seals on the intake and exhaust that were rock hard. I replaced all 8 seals and cleaned/lapped the valves. I also checked all the springs and they were well within service limits according to the clymer manual. So I cleaned all the gasket surfaces and even ran the cylinder head and cylinders over a piece of sandpaper on a glass table hoping to flatten it out if it had been warped at all.

I got it all together and fired it up, it ran great and the engine/valvetrain noise seemed much quieter than before. I kept it at around 1500 rpm varying it a little bit for a minute or 2 and then gave it a moderately hard break in procedure. Lots of full throttle without going over maybe 8k rpm. It smoked some but it was also a cold night and I attributed it to condensation. The next morning I start it and it smoked worse than before. I gave it a couple long rides hoping to break in the rings to no avail. In my troubleshooting I did a compression test. I found lots of oil on the number 2 and 4 plugs while 1-3 look perfect. All 4 compressions are the exact same. So I pulled the rocker cover off to re-torque the head and see if that helped.

Now for the worst part: after getting it together again I start it up and let it idle for a little bit to warm up. I note that it is still smoking. After it idles for a few minutes the cam chain starts making a racket and as I walk over to shut it off and investigate it makes more noise, dies and the crank wouldn't budge. To avoid further damage I didn't mess with it and started taking it apart to investigate. From what I could tell it looked as if the cam chain had lost its tension and binded on the crankshaft. After removing the guides and moving the chain around I got the engine to spin just fine and I couldn't see any marks on the chain. One thing to note is the first time I pulled the chain guides the bottom damper that goes on the rearmost guide came out with it, and when I pulled it out this time around it did not. Do they stick in the tensioner or has it fallen to the oil pan?

Now for what I need help with: I've spent enough time and money messing with this thing that I just want to do it one more time and get it right. Maybe bore it out to get rid of that #4 score, whatever it takes. But I would like to figure out the cause of the smoke. It is odd to me that the cylinders that showed signs of smoke moved with the pistons. When I swapped the piston order the cylinders that where smoking also changed. I have not removed the head or cylinders yet, anything to look for when I do? I should probably be safe and replace the cam chain, right? How about the cam chain tensioner and that little damper that is now missing? Can this be done without splitting the cases? I'm thinking I need to check for bent valves as well, correct? Any other thoughts for me? I know it's a lot but I really do appreciate anyone who tries to help me here

[HondaMan]

I'd bet you $10 the valve guides are the #1 cause of the smoke, and $5 that the damaged bore has more to do with it...

That said: if the engine is running unevenly, like from ignition timing not being equal on the 2 sets of points, or from a weak coil on its 2 cylinders, this will make the chains very noisy. Bear in mind the Hy-vo primary chain in this engine is only 2mm from the upper engine case, and when the engine is uneven in running and the chain has a little slack, it hits the case, right behind the cylinders, and often sounds like a cam chain.

The tensioner: those parts should be coming out intact. It might be broken? I'm not quite sure, from your description, of which parts you are removing, there?

Here's the "numbers" you need to know, as your Clymer/Chilton manual either does not have them, or it has them wrong:

Pistons: the piston-to-bore clearance is 0.0006"-0.0010" max, new pistons. This engine is considered to be seriously worn when the clearance is more than 0.0022" in this tiny bore size.

Valve stem clearances: the intake must not be more than 0.0022" of "wiggle" at the edge of the valve face when it is lifted 5mm from the seat. For the exhaust side this number is 0.0032" of "wiggle" back-and-forth, toward the front-back of the engine. These numbers figure out to stem-to-guide clearances of 0.0020" intake and 0.0028", respectively. If they are more than this, it is impossible for the valve seals to stop oil from sneaking past them, into the chambers. They must be "extremely tight" in the terms of car-engine builders.

You can get oversized pistons: even CI makes sets in 0.5mm and 1.0mm oversize, or there is a "big bore" kit of 390-ish cc size out there, too. Be SURE to get cast pistons, not forged types, or you will suffer many other trials that you probably don't want to see...also, check the O-rings in those carb hose castings where they bolt to the head. Most are hard like plastic now, and don't seal at all against the vacuum needed for the carbs. This makes the cylinders run alternately too rich and too lean, in alternate firing order, which will make you chase your tail trying to debug! Also, make sure the mainjets in the carbs are no larger than #78 size (most are #75), despite what the internet gurus are trying to tell you about this bike. It is, after all, a weed-whacker-size engine, so it takes almost nothing to over-fuel it! Even having too-deep float bowls in the carbs will make this one have black, sooty sparkplugs.

...and if that's not intimidating, remember than Honda also made the engine in 250cc size, and a smaller version of it in 125cc to boot (that latter one had 2 cams, though, so it doesn't really count...).

[DMMartin]

I'd bet you $10 the valve guides are the #1 cause of the smoke, and $5 that the damaged bore has more to do with it...

The tensioner: those parts should be coming out intact. It might be broken? I'm not quite sure, from your description, of which parts you are removing, there?

Here's the "numbers" you need to know, as your Clymer/Chilton manual either does not have them, or it has them wrong:

Pistons: the piston-to-bore clearance is 0.0006"-0.0010" max, new pistons. This engine is considered to be seriously worn when the clearance is more than 0.0022" in this tiny bore size.

I do have a dial indicator so I'll measure the valve wiggle when I get it apart. They did seem very tight like new to me but that may just be me getting used to the bigger engine clearances. Any reason you can think of as to why I saw no oil on the plugs before and I do now?

As for the cam chain parts I have been referring to this parts fiche:

https://www.cmsnl.com/honda-cb350f-four-1972-usa_model435/partslist/E++05.html#results

When removing the rearmost guide: #8 the lower damper: #4 did not come out with the guide the second time around. Is this cause for concern? I still haven't really found an explanation as to why my cam chain came loose and caused it to bind up. I had just assumed when the chain binds up it has the chance to mangle the chain and a new one is in order. Is the chain replaceable without splitting the cases?

[DMMartin]

Here's the "numbers" you need to know, as your Clymer/Chilton manual either does not have them, or it has them wrong:

Pistons: the piston-to-bore clearance is 0.0006"-0.0010" max, new pistons. This engine is considered to be seriously worn when the clearance is more than 0.0022" in this tiny bore size.

Valve stem clearances: the intake must not be more than 0.0022" of "wiggle" at the edge of the valve face when it is lifted 5mm from the seat. For the exhaust side this number is 0.0032" of "wiggle" back-and-forth, toward the front-back of the engine. These numbers figure out to stem-to-guide clearances of 0.0020" intake and 0.0028", respectively. If they are more than this, it is impossible for the valve seals to stop oil from sneaking past them, into the chambers. They must be "extremely tight" in the terms of car-engine builders.

Got it all apart and here's what I got:

I measured the piston - cylinder wall clearance at around .0018" with a variation of +- .0003" per cylinder. I planned on boring it anyway to get rid of the score so I'm not to concerned here

Measuring the of the cylinders that showed oil on the plugs I got about .0018" on the intake and .0028" on the exhaust. Tired, yes but doesn't really explain how much smoke I was seeing.

I had closer look at the pistons and it looks to me that I may have installed the 2nd ring upside down in the cylinders that where smoking. Now I had read a few posts here about the direction of the 2nd "scraper" ring and therefor I took a good long hard look at the rings when I got them. The top one is labeled with "top" so that side went up. But I could not find a single identifying mark on the 2nd ring. I also thought the ring looked pretty square on the sides so I had assumed they could go either direction. In the end I installed them the direction they came in the package. Maybe this is due to the fact that these are just cheap ebay rings? I'm not sure. But there was definitly a wear pattern that was different on the 2 cylinders that where smoking.

What would you guys do? Re-do the head with new guides/valves or leave it as is with a new bore job?

[calj737]

You might take an hour and read through this thread:
http://forums.sohc4.net/index.php/topic,165714.0.html