SOHC.co.uk Forums > CB350/400
Smokey rebuilt 350
Trigger:
0.0005" differences on pistons is standard, that is unless you are buying top quality race pistons. Same as weight differences ;)
royhall:
Another potential culprit in the smoke problem is the exhaust valve guides....... The valves measure up bang on the money but they feel loose in the guides........ When I originally built the engine I checked all this, even turned some bar in 0.001" steps to check the wear from both sides. Some wear was found (3.5 thou) but it was well inside Honda's specified service limits which are stated at over 11 thou........ That can't be right, thats a back alley.......... The exhaust ports were spotless 280 miles ago now they are caked in thick black sticky carbon, that's got to be oil passing the guides. The cylinder head domes and piston tops are spotless, though very oily........ The new guides and o-rings are ordered and will be fitted by the same engineer that does the re-bores......... Also found a tiny nick (looks like a screwdriver scratch, not me) in the head face ironically between the oil feed and cylinder 4 (the really smokey cylinder). Can that be a coincidence? Going to have the head skimmed a few thou to get rid of it anyway...........I guess it's time to stop using Honda's specified wear limits as they are way too lenient, an example of which is maximum piston ring gap at 24 thou. I don't think so. No stone will be left unturned this time around, all limits will be returned to production line spec..........Whilst I'm on about sticky carbon deposits, what's the best way to remove it without damaging the paint?
Seabeowner:
Lots of silly mistakes in Honda manuals.
My Haynes book states max stem to guide clearance for CB350 as .003 inlet, .004 exhaust. Same as 500. And I like to stay a way inside that.
royhall:
Been undecided what to do about the exhaust valve guides.
Some engines (mine included) had stem seals on the exhaust side and some had unsealed guides that are machined to a point to shed the excess oil. According to some on the internet the excessive wear is due to the stem seals on this engine and the lack of oil on the valve stems, and they should have the other type. Many others disagree and say to fit the 400 type with the seal (which as we know should be designed to leak slightly).
I even checked the ones on Yamiya's site for the 400 and if you buy the full set they all have seals, if you buy the exhaust guides separately they don't have seals? All the guides on CMS for the 350 are without seals except for the French bikes. This is all a bit weird.
I rang CMS with my engine number and they say the non sealed version so that is what I have gone with (even though seals were fitted in my engine). On top of that there are 4 different versions of unsealed guides so I went with the latest version. Just hope it's the right decision as I have fitted them now.
This is not usual Honda and is very confusing.
royhall:
The mystery continues.
The 350F is now rebuilt again with all genuine Honda parts except for the +1mm Cruzinimage pistons. The pistons were supplied with exactly the same rings as my Genuine Honda set.
The barrels were rebored to exactly the right size with each bore individually cut to the relevant piston. The exhaust valve guides were replaced with the correct non sealed versions and reamed to suit. This time around everything was measured and double checked. The rings are all fitted the correct way up and verified twice.
Started the engine this afternoon for the first time, it started straight away on the button with no messing around. Guess what, No.4 is still smoking badly.
That just leaves the carbs that have already been rebuilt twice, once by Gerben and once by Matt Harper. I am beginning to see why the guy on the USA site gave up and sold it on.
Does anybody have a good set of 350F carbs that I could borrow for a short while to eliminate the problem. I'm thinking there may be a small crack somewhere in the No.4 carbs casting causing it to run very rich? Or it may be the Cruzinimage carb kits that were used are duff.
Cheers.
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