Motorcycle Repair: pipe rear smokin, head gasket leak, vacuum conditions

Question
QUESTION: hello-Ihope you can hel me is a honda shadow vt750c -#2 carburetor-compression test 175psi rear cilinder and  175psi front cilinder the rear cylinder smoke on pipe Im belive is blue smoke the end of rear pipe look a barnis on end of pipe=ok smoke at start up-5 to 15 minutes and clean up-after shut the engine down-engine hot make a compression test and is  175psi front and 175psi rear the spark plus is clear but oily not to much-on hot engine no smoke only on cool smoke-Im think its not noticeable while the is went runining because IM make a compression test take out the spark plus and look oily=the guestion is valve-o-valve steam seal or piston ring-Im check the oil comsummeate  dipstick only 1/8 down on up mark[Im riding the bike 50 mile go and return] excuse mi english thanks for your time and atention=gamalier[the shadow is 1983 vt750c]

ANSWER: Hi Gamalier,

There are a few things that cause smoke.

1. White smoke = steam or water vapor
  -Head gasket leak, milky oil.
  -Condensation gathering in the exhaust pipe.
  -Smoke when your engine is cool is likely water vapour in pipe
   or valve seal oil leak from previous running.
   It clears up after running awhile.

2. Blue smoke = oil
  -Valve seals faulty or worn oil rings.
   Smoke when shutting throttle off on downhill
   at high rpm = poor valve seals. (high vacuum conditions)

3. Black smoke = Unburnt fuel (too rich).
  -Carb flooding, choke, air filter problems, engine overheating (fan, timing, lean fuel mixtures)

Make sure your carb intake rubber is not leaking air causing a lean fuel mixture
as this can overheat the cylinder.

Your compression is very good but it may be
excess carbon build up, it may cause overheating which thins the oil
making it smoke.

You could try some "Seafoam" fuel addictive and 20W50 oil.
Do not overfill the oil, it might also cause smoke.

Luck!
Wayne S.
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QUESTION: Iwhat to thanks you very much for your explanation-Ican make a leakdow test Ido not have a leakdow tester can I do it with a compresol I think to put the compresol on[20-or-30psi] and make the test on spark hole only Im take oof valve oof on the compresion tester and use the hose on the compresol and make the test-dos the test tell me if I have a bad oil ring or rings o valve the leakdow test made engine cool o engine hot--ok Im maked a compresion test with the compresortester with the engine cool and the test is the seme compression-on cool 175psi front-175 rear-on hot 175 front-175psi rear-is the number ok with the test made the test with the open thorttle and choke open- thanks for your atention and time-gamalier

Answer
Hi Gamalier,

If you apply pressure in the spark plug hole it should
be done with that cylinder on the compression stroke (valves closed)
and the engine a bit warm.

If you hear air escaping in the carb the intake valve is leaking.
If you hear air escaping in the exhaust pipe the exhaust valve is leaking.

If you hear air escaping from the oil filler or breather hose
then the rings are leaking.

Air escaping from a different spark plug hole might be a bad head gasket.

You can also move the piston slowly downward and see if
the air leaks when the piston is further down the cylinder.
This indicates piston rings are not sealing at that point
in the cylinder.

A 10 per cent leak down is normal. You can use 70 psi or so.


Here is a leak down tester link:
http://www.motorcycleproject.com/motorcycle/text/leakdown.html


Wayne S.
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