Engine Seized

[Myumi-2]

Im restoring a 1972 CB750 after 20 years in storage.  The bike was largely complete and was relatively easy to get started.

Ive had the bike running for the last 6 months and run it regulary.  Ive not had the engine out of the bike.  On a run two weeks ago I noticed that it got sluggish when pushed hard i.e. up a steep mountain.  Last week I went for another run and only traveled about 1 km when the bike lost power.  It still ran smoothly but there was no power.  Ive recently sand blasted the tank and suspected that some grit had blocked one of the carbs.  Pulled the carbs off and cleaned them.

I took the bike for a run (5 minute) to warm the engine up before syncronising the carbs and had the same loss of power problem.  

I was just running the bike into the garage when the engine seized.  The engine did not seem particually  hot.  It has ample oil.  I could not kick the engine over.  I put the clutch in and pushed the bike freely.  I left it for 2 hours and came back and the engine cranked over easily.

So my problem appears to be cylinder seizure caused by over heating.  

My question is, what do I do now?  

Putting some penetrating fluid into the spark plug holes is not likely to help as it is not a permanent seizure.

I can only assume that oil is making it to the top of the head as there is a small oil leak out of the top of the engine and the oil in the resovoir on the side was hot.

Your views are most welcome.

Cheers
Tim

[Myumi-2]

Just though to add:
     I do have an inline filter on the fuel so it is unlikely that any grit made it to the carbs, but always possible that it made it past the airfilter.  
     The reason for identifying the Carbs as the problem was that Carb #3 was overflowing.

[Myumi-2]

More additional Info:

About two months ago I ran a complression test and essentially got 150 across all four cylinders.  (this was a source of another question to this forum.  Initially cylinder 3 measured 125.  With some oil poured into the spark plug hole it came up to 150)

I ran a test this morning and got 110 across all four cylinders.  It has traveled perhaps 80 miles in this time.

The consistency of this reduced compression suggests to me the cylinders are not the cause of the problem, as all four cylinders are unlikely to be effected identically.

I checked the spark plugs, they are a nice caramel colour, but could be a little lean.

One theory of mine is that it is running lean and overheating.  

[Rick V]

From what you are describing, it sounds to like you have gotten some sand through the fuel filter.If the carb was overflowing it most likely had something blocking the needle and seat. The loss of compression also sounds like the sand has caused wear in the cylinders or rings.I have sandblasted parts in the past and done what I thought was a very thorough cleaning only to find there was still sand present.(I am not an amature, I am licensed mechanic) I have switched to walnut shells for blasting anything that is around the engine.