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Discussion Starter · #1 ·
Hello!,
I have an 79 Cb400A, that my grandpa gave me. This bike had supposedly an engine overhaul (+0.5 mm rectification, new pistons and rings, new valves, etc) on the year 2002. After that my grandpa only used it once a week until 2009, with a total of 250 km after the overhaul. From 2009 to 2019 it was not run and he gave it tom me on 2019. I tried to start the engine and in order to do that I cleaned the carbs and changed the sparks an sparks cables and sockets, oil and oil filter. The engine did start but after warming up it will start to smoke pretty heavy.
That is why I decided to open the engine, suspecting that the piston rings could be the problem. This is what I found:
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The ring had written 0.5 on them, so that is why I think the engine is in +0.5 mm.
I noticed that the exhaust valve has not carbon, Why?
The rings where not stuck, but I don't know if the oil ring is ok because is not s open as the other two. what do you think?

I was thinking about cleaning the cylinder head, valves, piston, changing the rings and build the engine again. Any thoughts??

Thank you in advance for any input.

Tomás.
 

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Hi Tomas,

Automatic....

You know of CMSNL for the exploded views which will be helpfull.

The 4 inner cylinder head bolts should have copper washers.

The cylinder head bolts are different lengths so be really carefull when assembling the engine.

The cylinder head bolt holes must be clean and sealer free all the way down to the bottom. 10 X 1.25.

Is the cam sprocket bolt in your picture the correct bolt ???

I would take one piston off, clean and inspect, keeping the wrist pin the correct way round. Only remove the outer wrist pin circlip but check both.

Rings fitted up side down can cause smoky engine.

You have a manual ?

Attend to one piston, take pictures of it and it's bore and Conrod and report back for comments, then repeat for the other piston.

Take a picture of all valve springs...

Take your time...

You have a manual ?

You have means of measuring things ?
 

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Well you are right you need to overhaul the motor. In addition to day dreamer comments somewhere along the way you have 400 T pistons in the 400 A motor. I also dont see a lot of oil where there should be. At least one o-rung missing. I suggest you get a 400T manual with the 400 A supplement. Take the motor clear down, check the rod and crank bearings, oil o-rings installed correctly, and oil screen cleaned... Take cylinder to a machine shop and have it checked. Maybe go to next oversize pistons. Buy 400 A pistons and rings. Clean up the cylinder head and do a leak test on the valves you may not have to do anything to the cylinder head. Clean and inspect your cam chain slippers. Inspect oil pump and drive. Just follow the manual when you put it back together. It is a very good manual and explains things as you go.

Bill H
 

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Discussion Starter · #4 ·
Hello and thank you Daydreamer and Bill,
Here is the update.
Removed and cleaned the pistons, and one of them is like this:
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- cleaned and checked cylinder head for leaks, and valves are ok.
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  • I have the shop manual for the cb400t and the 400a supplement.
  • couldn't get Cb400A pistons here in Argentina, instead I could get a Fresh +1 Cb400N cylinder with new pistons and rings. I noticed that the pistons had like a sort of bump in the center, that the ones installed in the engine had not, there were plain in the upper side.
  • cleaned all the sealant on the cylinder head bolts, they had a lot of sealant.
  • Mounted new pistons,rings, gaskets and cylinder according to the shop manual.
  • Torqued the cylinder head bolts according to the manual.

Pending:
  • tuning valves gap.
  • change cylinder head cover bolts
 

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The 400 A pistons are flat on top . If you can turn the motor over by hand and the pistons dont hit the valves and or the spark plugs. it must be ok. That was the only concern I had .Cylinder head looks good, .
 
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