1974 Honda CB750 Four K4
Mechanical Service with some Cosmetics
Owner & Writer: Randy Creel, Jr., Owner & Restorer of Randy's Cycle Service & Restoration
Revitalize the Spirit & Beauty of your Classic Bike
Located in beautiful Central Virginia since 2001
8175 Black Walnut Run Road, Rhoadesville, VA 22542 USA
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This page was last updated on: August 6, 2013
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COMMENTS: This page is an ongoing write-up of Randy's mechanical & cosmetic service to the bike he'll personally be riding starting the spring/summer 2010. Write ups - some technical some personal - will be posted along with pictures & a video of the progress. Notifications of new entries will appear on Facebook, so please join us on Facebook by clicking on the icon above.
Posted May 17, 2010 - 5th entry
Before getting into the complexities of the 750 engine, I first want to send out a heartfelt thank you to everyone who stopped by on a rainy Sunday (May 23) at Butler’s Orchard in Germantown, MD to say hello and compliment the web work of Ursulina and my mechanical ramblings on this Honda. I am very grateful that so many folks are taking such an interest in my little project and I hope to keep bringing you more useful and entertaining information. There may be another project after this one documented in the same fashion. As long as you guys want to keep reading, I'll keep posting, so stay tuned!
As promised, I wanted to share the engine go-through. I hesitate to say rebuild only because I haven’t really rebuilt it, only freshened it up a bit and resealed all the gaskets and oil seals. The original plan was to not go through a total ground-up rebuild, but instead take a reasonably sound engine and use it as is, albeit fixing anything that could be a potential problem beforehand. The engine I began with came from a good friend and was not the engine that was included with the original bike when I acquired it back in ’99. The main reason I decided not to use the engine that was originally in this bike was that when I pulled the oil sump to have a look, it appeared as if the oil had never been changed in its entire 20,000 odd miles. Normally, I expect to see some clutch material, a small amount of fine metal particles, and general sludge produced by 30+ years of running, sitting, abuse, poor quality oils, etc, but what I found in the sump was just downright scary, and this engine was too big of a question mark for something I didn’t want to rebuild but wanted to trust far from home. So the decision was made to grab another engine from the reserve fleet and see what it looked like. This one was pretty close to new, literally.
Once the sump was removed, the bottom end was almost as shiny as new. There was a slight trace of some dark material in the pan, but that was easily washed out and the pump screen for the scavenge pump looked as if it had never seen anything but clean, new motor oil. I can’t say I’ve ever personally seen a CB750 K engine this clean inside. Either this engine had almost no mileage on it, or it was very well maintained. Coming from a 1975 bike according to the serial number, it has at least been around for 35 years, so even storage was kind to it, but the true mileage is unknown. No matter though, as what I found inside has me confident that it will prove to be a very reliable engine for years and miles to come.
I did also remove the oil pump at the same time I had the pan off, and disassembled it to inspect it, mainly to be sure that the check valve for the feed line was intact and clean so I won’t have any wet sumping problems later, but also to inspect the rotors and clean it thoroughly, then pack it with assembly lube. I did this for two reasons; one to insure we don’t have a dry pump to start and risk scoring the housing, but also to be sure the pump is “primed” and able to draw oil from the tank and pressurize the oiling system. A well oiled engine is a happy engine. Back in place now, and with a new sump gasket, and drain plug to replace the somewhat roughened up original, I headed for the top end.
Almost every CB750 engine I’ve had my hands on that has been put up for a length of time then put back into service will develop a substantial oil leak at the cylinder head. Usually, it’s misdiagnosed as the head gasket when in fact it’s the rubber sealing discs under the cam carriers. To avoid this potential scenario on an engine which you cannot remove the cylinder head after it is installed into the frame, it is a wise and preventative repair to go ahead and reseal this potential leak point right now, which I did. I am glad I chose to do so as the original discs were hardened and brittle. Of course going this far, and seeing all of the head bolts, a new head gasket was decided on. Fortunately, it wasn’t a leaker to start with, but when I did remove the head, the gasket didn’t have any real “stick” to it on either side. It came right off. But the good news was that the tops of the pistons had very little carbon on them and the cylinder bores were near perfect. The factory honing marks are still very visible, and there wasn’t much trace of carbon at the tops of the bores where the top piston ring stops either. Even the area under the valve cover and around the cam carriers was nice and clean just as the sump was. These are all very positive signs of a good choice of engine.
Next update: The cylinder head, clutch, engine covers, valve gear and other small nuances of the 750 power plant
Valve Guides
Piston chambers
