Got a chance to fettle a somewhat preserved (considering its age) 1973 CB750. A beloved, one-owner bike, ridden both overseas and in the US. But for some little while not running so good.

A visual inspection puts that into focus. The bike has had a pretty hard life. There are a number of signs of its age and use, and most of it really unnecessary.. Both steering stops are severely bent, one of them is welded.

Apparently some frame damage too, as evidenced by the right side mufflers fouling the frame mount and having been spaced out with washers. This is how it came to me.

The stud that does double duty as engine mount and footpeg mount is bent on both ends. I replaced it.

In addition the frame downtubes are distorted, requiring a scissors jack to remove and reinstall the engine.

Odd hardware evident all over the bike. Functional, but none too pretty. I wouldn't want my bike looking like that.

And more of that. How could someone accept the look of this?

The usual misinstallation of the front axle clamps that one finds when a person unfamiliar with Hondas works on the machine. Very common.

This is actually dangerous! Someone has hammered on the throttle linkage, breaking the locating pin and resulting in the linkage jamming fully open whenever it is actuated. These carbs were tossed out.

Almost as life-threatening, plastic cable ties have been used on the fuel hoses instead of clamps. This is hack.

The hammering on the linkage left marks, seen in this photo. Both ends are in fact mushroomed.

The carbs' slides are mangled also, looks like rods or wires have been stuck beneath them, a deplorable practice.

One of the four float valve seats is the later filter screen type yet is lacking its screen.

The carbs were rejetted for who knows what reason (the bike came in with the stock intake and exhaust configuration), resulting not only in a wrong mixture, but on top of that the wrong length idle jets were used.

So rich was the jetting that someone thought it necessary to try to mitigate it by lowering the slide needle as far as it could be lowered. And the bike was still filthy rich.

The poor bike has "been through the war". There is hardly a part of the engine that isn't scraped, cracked or broken.

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And inside the engine as well there are signs of very rough handling.

A surprise here. In all the years this bike has existed, no one has removed the chain oiler. Incredible. And a sign that no one with any measurable vintage Honda experience has been in this area of the machine.

The chain oiler was a failed experiment on the factory's part. I replaced it with a special bolt.

Looks like the drive chain was run pretty loose at some point. See the grooves?

You're looking at a crack in the crankcase. The wrong size engine mount bolt fastening nut was used and when tightened it wedged the aluminum and cracked it. Not the best thing, but it won't affect the engine.

Every single gasket had goop on it, that is every one except the one that should have had sealer and didn't. What a mess. And uber countereffective.

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Of all all the engine's gaskets, this one, the cylinder base gasket, is the last one, the worst one, to put sealer on. It practically fell off when the engine was disassembled. Experienced mechanics know better.

And of course there is silicone sealer, the unmistakable trademark of the hack.

One of the objects of this bike's getting attention this time around was this oil leak.

The proper way to diagnose a leak is to first clean off all the oil. A degreaser is applied and the engine warmed up, then completely hosed down and dried off.

Then a white powder aerosol is sprayed all over the engine. It goes on like and looks like flat white paint. But it washes off easily after diagnosis has been made.

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The most significant area of oil leakage turned out to be the head gasket.

Note how streams of oil have been pouring out the front and back of the gasket surface.

The gasket itself tells the tale also. Note that it has no sealer applied to it. That and missing o-rings are the reason for the oil leak.

And the cylinder head evidences the fault also. See the oil tracks on the freshly-removed head?

The poor running, in addition to very worn and mis-maintained carburetors, was the result of a bodged-up ignition system. The wrong spark plugs (one of which was cross-threaded), and a butchered up ignition trigger system were at fault. Note that this Dyna pulser module is turned fully counterclockwise. Not normal.

The Dyna S rotor and Honda advancer. Problem is, it's not a 750 advancer but one from a 350 four. Not only does each SOHC four advancer have its own advance curve...

...but also its own TDC indexing. The TDC and F marks are not in the same places on the two advancers. That is, each advancer's locating pin (where it joins the crankshaft) is in a different place on each advancer.

See that? The crankshaft is timed to the F mark on the 350 advancer and then when the correct, 750 advancer installed, the ignition timing is in fact retarded, that is, late. This did not help the engine run its best, but was a major part of its running problem.

While the engine was getting new gaskets and seals, I checked the cylinder bores. Pretty good! Round, straight. Good machine work!

A measurement of the pistons for comparison however shows the bores are pretty loose, they were fitted a bit looser than they should have been. No real problem though.

That and the years and miles have resulted in the signs of extreme blowby visible in this picture. But the engine is running good and strong now.

The cylinder flange sealing o-rings were loose. This was corrected.

And all the valve springs were installed upside-down. Curious, this. It was corrected also.

One of the many new oil seals I put onto the engine, this one the countershaft oil seal.

A new cam chain went in also, as the old one showed significant wear resulting in an unwanted retarding effect on the camshaft. The cam was degreed-in also for best result.

And the engine after repairing almost every screw thread and replacing all the fasteners, including many that were the incorrect length, went back into the frame. The bike still could use a lot of love, but the engine's top end at least is the way it should be.