Hitchcocks Forum

Hi everyone, My 350cc clipper is blowing smoke. After starting the bike it does not smoke straight away it starts after about 40 seconds then as I increase the revs so the smoke increases it then continues to smoke, the amount depends on the revs. The exhaust pipe has blackened oil at the pipe entry, after running for five minutes I can see slight drops of "blackened' oil dripping where the pipe enters the head. The plug is also oiled up as is the exhaust port.
I have done the following: (with parts from H) replaced the valve stems and valves, (fitted by an engine reconditioner), these were very worn. The exhaust rockers were worn so that at higher revs, oil was being sprayed onto the exhaust valve so these have also been replaced. The compresion is good and the piston ring gaps, when inserted in bore, were well in tolerance, I did observe that the Top ring was a 'chrome' type, the 2nd ring and scraper were 'normal' and the 'top' mark on the 2nd ring was correct. I have just ordered a new set of rings from Hitchcocks just in case. I also removed the 'Breather' pipe and could sense air being expelled so I assume the engine is breathing. Prior to starting I drained the sump in case the smoking was due to 'wet sumping'. Any comments would be appreciated. Thanks.
Posted: 07:48am 30th March, 2018

I would deglaze the bore and fit the new rings and run it for a couple of hundred miles, don't be too gentle or the bore will glaze again.

While it's apart check piston to bore clearance, burning that much oil almost certainly won't be fixed by honing or new rings. And if you have access to the right tool (a bore dial gauge,) check the bore for truth, they often wear unevenly.

REgards, Mark

Also, the exhaust valve is usually designed to get extra oil, the purpose is cooling as well as lubrication.

REgards, Mark

Hi Guys-------- and Hi Clipper132, ...Here's a few questions for you, just for clarity's sake}----1) I've been looking at our hosts on-line parts book and the piston clearly shows 3 piston rings fitted ie... a plain top compression ring, a second plain compression ring and the third being the multi-piece oil control or "Scraper" ring ( As I've heard some folk call it )....Is that the situation you have also ??...........2) What do you mean when you say}--- "Valve stems" ???....I'm' guessing you mean the valve guides and that both Inlet and Exhaust were changed ?? ( Cast iron or Bronze ??).....3) The rockers... did you change both the arm AND the split bearing of each one as a matching pair ?? Remember that the two halves of the rocker bearing will also be showing signs of wear at this stage .... Finally I'd agree with Mark M about the cylinder bore.....ie}-- You need to know at what state of wear it's in.....Shown here are the two main ways of measuring it}------------------
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-------- sorry, call of Nature just then !!...When you measure the bore you need to measure only the area that the rings move in......Do 3 readings of "Top"----"Middle" and-----"Bottom" that are in-line with the frame of the bike, then 3 more again in the same places BUT now at 90 degrees to the previous ones.......So that's a total of 6 readings and enter them on a piece of paper so that you can visualise the whole situation......Remember that the Piston also will need to be measured at the same relevant places to give the total, overall picture................Some years ago I refurbished a pair of Honda PC50 mopeds ( 4-stroke, Cast Iron head and barrel ) simultaneously.....Upon starting them up one ran perfectly normal whilst the other one... after about 30 seconds ...started to smoke quite heavily...In fact it showed all the same symptoms that yours does....To cut a VERY long story short ( Gnashing of hair---pulling of teeth ) it was the new rings in the new re-bore that were at fault....The two top ones had been fitted UPSIDE DOWN on the piston, as supplied !!............It transpired that they have a very slight taper to aid swift bedding-in....As fitted they were acting like a form of pump and bringing oil onto the piston crown....Once changed over = instant cure !!!..........here's a photo of a spare that I still have....Imagine trying to change one ring at a time in order to eliminate each possible scenario......These were to be sold on so I had to get it right.........

Hi, I had a very similar problem on my 350 modelGdelux/Clipper and it turned out to be the piston rings. The piston seemed fine (an old "Heplex") & new. I could still see some honing marks on the bore but it smoked like a trooper! The piston ring gaps were way out though despite everything looking new; anyway I had a conversation with Hitchcocks and they suggested trying new rings - which they sent to me on approval, i.e fit them & check the gap and if ok keep them or otherwise send them back no problem. The new rings (top chrome) worked a treat and the bike no longer belches out smoke that is unless I leave the bike standing for a few weeks/month when it smokes for the odd minute or 2 on start up.. hope you get the problem sorted soon