The 72cc Install That Smokes On Cold Startup  By  mopojo (Read 2,952 times)

The fact it doesn't smoke right away, and the smoke disappears so quickly often indicates condensation being burned off, possibly from inside the exhaust. It takes a couple of seconds for the heat to start burning it off. Together with the amount of rust on your exhaust, I'm betting on this being the issue.

If the smoke starts right away, it's usually oil burning in the cylinder, often meaning valves need to be lapped or rings/jug issues. However, it still could be an oil issue, since it takes a couple of seconds to get the oil pumping up to the head, but the fact it goes away makes me think condensation. If you are experiencing a lack of power, I would recommend a leakdown test on the head and cylinder.

That smoke is exactly what was happening on my "100cc" BBK before I reinstalled it with a new kit. I noticed when I was installing the new kit that there was some gasket stuck to the engine case from the old 50cc gasket that I just didn't see the first time. I believe this was causing vaccuum issues that also caused oil to get into the combustion chamber. It would smoke after a few seconds of being on then go away after it warmed up. The smoke I had was we think too rich because of the oil getting in and not burning off till it warms up enough to or possibly buildup of some oil/gas in the muffler from it not burning off.

Not saying that is the problem but just giving my experience with the same issue.

That engine died not long (maybe a week) after posting this thread.... LOL Just lost all compression and would not start. Thought I had burned a hole in the piston or rings had become digested.
Finally got the time to tear it down... talk about procrastination.
That not lapping the valves is most likely the issue. Don't know yet but will install to a ready-for engine chassis tomorrow and find out.
Exhaust valve was not seating, toasted and a divot had burned (whatever) its way into one portion of the seat (lots of leakage coming from intake and exhaust. Tried lapping but still had the tinniest bit of light shining through the burned spot and the excessive hiss on compression stroke got on my nerves. Prolly would have run fine but OCD kicked in. Pulled a valve from an older head and lapped it: Takes some quite a bit of effort to turn the crank now and hiss is almost non-existent.
Lapped both intake and exhaust, BTW.
Moral of the story? Lap those valves, Ladies and Gents. Don't get too hasty to get that ride on the road. "A stitch in time saves nine"?
As always: Any and all input on this is/was greatly appreciated. Sorry I am slow to post and help out here: Lot going on and it ain't all good. Long as it ain't all bad; we good.

NO!. Machine those Valves and Valve Seats Properly, 1/2 Degree Difference on the Valve Angle with NO Lapp and she will Hammer Home upon start up to a Near Perfect Vacuum.
Pulling a Valve out of another head and Not Machining Either Surface is Butchery, Not to be Teaching others that this is Proper Practice. 
That Lapping is Not a way to Correct Valve Seats and Valve Face that need to be Machined.
You do NOT make it Better when you do after a proper machined valve job.
Lapping? this practice needs to be Stopped from being taught. Old timers passing this down for a hundred years.
Ever see Any OE in the World lapp Valves? Auto, Cycle, Marine, Diesel, anything with Valves?
No Pro racing or Aftermarket Hot Rod heads are lapped.
Peeps just don't understand what there doing to the two surface when ya Goop a Glob of Abrasive Paste in between. 
Lets say my Valve Seat are on the Valve face is .030", All My Spring Pressure is on the contact area and Equally Important that Contact area when Seated is How the Valve Dissipates its Heat.
Now take a Glob of crap and Lapp like Heck, You have Now Made the Valve Face Seat area Concave.
So Its Only Sealing on  the Very Top and Bottom of that .030" Seat Area and NONE in the Middle.
Now it Cant Dissipate Heat properly and will Not Seat as well. 

Per the 2009-2013 Piaggio Scooter Factory Service Station Manual published and distributed by Piaggio - after covering how to check if valves and heads are suitable for reuse they instruct:


"If no anomalies are found during the above
checks, the same valves can be reused. For better
sealing results, it is advisable to grind the valve
seats. Grind the valves gently with fine-grained
lapping compound. Upon grinding, keep the cylinder
head in horizontal position. This will prevent
the lapping compound residues from penetrating
between the valve stem and the guide (see figure).
CAUTION
TO AVOID SCORING THE FAYING SURFACE, DO NOT ROTATE THE VALVE WHEN NO LAPPING
COMPOUND IS LEFT. CAREFULLY WASH THE CYLINDER HEAD AND THE VALVES WITH A
SUITABLE PRODUCT FOR THE TYPE OF LAPPING COMPOUND BEING USED."


For checking how the valves are seating:

"Inspecting the valve sealings
- Fit the valves into the cylinder head.
- Alternatively test the intake and outlet valves.
- This test should be carried out by filling the manifold
with petrol and checking that the head does
not excessively ooze through the valves."

Bottom line per one of the most highly respected makers of motorcycle, scooter and air craft engines and not by my personal opinion you lap the valves on a scooter if you go by the book.

YA I knew someone would come up with a Chinese manual that say to lapp even tho the same OE Does Not at the Factory. Pure Bunk. the "Book" is simply passing down misinformation of Old school thought.
Currently the Big Five makers Do not advocate Lapping, were dealing with Allot of TI in Offroad and want to ruin a TI Valve fast lapp it.
Why would someone think there perfecting fine machined surfaces with Lapping compound. Unreal and have No Clue what There Doing.
I have work with every Factory MC Pro Show and None Ruin a Valve Job with Lapping compound.
Yup, I'm one of those who gets Paid to be at races every weekend.
You are pointing out that you simply have never done a proper valve job and don't know how or know what you are looking at.
Piston guy didn't move from TX to NC for the BBQ.  I have been involved with one of the Nations Highest End manufactures of Valves, Valve Seats, Valve Guides and retainers. Turn the TV on this weekend and you can watch a few thousand of that Valve Train Not Lapped.

Here are some tasty nice valves One Steel that rest TI
Some Honda and Yamaha, See any lapping here?
A couple of these I had made for HRC of Europe. yup Honda Racing Europe , and they Don't Lapp valves.