Topic: CB750 has not run since 1998.

[Squiddog1]

Hi All, back in November I picked up this 1974 K4 Cb750. It was originally a California bike and was last licensed in 1998. Speedo reads 75,000 miles or so, but the engine is not the original engine. In fact according to the serial number it has a late 1975 Cb750F0 engine in it. The guy I bought it from had no history on it as he picked it up from an estate sale other than it was a one owner bought new by a guy who was a Korean war vet and it was parked after he was too old to ride any more and just sat until it was sold to the I bought it from 5 years ago. The engine turns freely and seems to have compression although I did not do a cold compression test yet. It's hard to tell by the pict, but the whole engine and carbs all have a bad rattle can silver paint job. Very sloppy. This bike is going to be my retirement project and I'd like to do a nice restomod on it. I plan on doing most of the work myself, save machine work etc. My question is this, as I plan to do a complete rebuild on this bike including the engine, is there any reason to try restarting it now after it hasn't run for 20 plus years? One service shop here in the Phoenix area told me "no reason to tear into it if you have not started it yet and see how it runs". That did not seem like good info in light of what I've read on this forum about chain tensioners etc. It needs a lot of work, but I am excited to get started this summer. By the way, I rode motorcycles my whole life and my second bike back in 1975 was a K1 I had for 5 years before being tempted away by a clean KZ1000LTD. I have not had a bike however since 2001 or so. I want to take my time with this build, a year or two, and do it right. I am a bit worried about parts and know the whole issue of the F pistons, vs the K ones and that concerns me a bit, as does the availability of things like rod bolts etc. But I am ready to tackle each challenge as it comes up. I already Bought Hondamans book and it has been a wealth of info. Anyway, any feedback or wisdom will be greatly appreciated. Last pict is the K1 I had back in the day.

[fxef79]

You haven't said... do you intend riding it before you begin your restomod?

My 2 cents:  If you already intend to tear the engine down, and if the answer to question above is "no", why risk trying to run it now?

[scottly]

Squid, that shop was giving you good advice. The '75 F engine was not the same as the 77/78 F2/F3, which had different pistons and cylinder heads. There is a member here with over 160,000 miles on an early F. I vote you try to start that puppy up!!

[DesertKyorugi]

My K1 sat in a storage unit since 1981. When I bought it from the original owner a little over a month ago, it had rust all over with a nice layer of green moss on it. I checked that the motor is not seized by gently hand pushed the kickstarter.
Trailered it home, dropped the oil pan, cleaned out the 39 year old oil sludge, changed the oil and oil filter. Checked and adjusted the valves, adjust the chain tensioner, changed the spark plugs, cleaned out the gas tank and the carbs. Put fresh gas and new battery in, cranked it and it fired up on the 3rd try. It ran so good, all I did after was getting the carbs rebuilt by a pro named Rick West.
Now I ride it whenever I can, clean the rust and slowly restoring it piece by piece. It was my first CB750. I also have 2nd K1 that's a Japanese Domestic Model that I bought originally as a parts bike. It was stored since 1999 and it is in a partially assembled mode. I am going to work on it and probably try to get it started as well before deciding on rebuilding. These bikes are extremely well built. If there are no telltale signs of abuse or damage, I would try to start it first and see how it is. You will be pleasantly surprised     

[seanbarney41]

Exactly...these engines are extremely well built.  So well built, in fact, that many parts and amateur procedures result in an engine that is not as good as what came from the factory. 

[Bankerdanny]

Two of my 3 rider SOHC4's had been sitting for 20+ years when I bought them, including my currect 750F. As long as the engine turns over smoothly there is no reason to think that the 22 years had damaged the engine. On both my bikes I cleaned the carbs, replaced fuel lines and spark plugs, double checked the point gap, hooked up a battery and temporary fuel tank and started them up.

[Jerry Rxman Griffin aka MuthaF'er]

Fire that #$%* up!

[MauiK3]

When I purchased my K3 750 from the original owner a few years ago it was "running but a bit tired", mostly well maintained but in need of the entropy to be stopped. I cleaned it up some and rode it before starting the project. It was a good thing to do; helped me make a mental list of what I needed to pay attention to. It also afforded time to locate some of the big ticket items I knew I wanted such as new stock type repop exhaust pipes (Yamiya), K0-K1 type body work (Yamiya again, top grade stuff) and engine work (Hondaman for my head and cylinders).
After that period I took it all apart down to the frame and empty engine block. The rest is history.
Run it some.

[Airborne 82nd]

Like the others I say fire it up. My bike sat for years also. When I started it up it smoke for about the first 100 miles then everything cleared up so don't be to concerned if you see oil smoke for awhile.

[PeWe]

Lift the oil pan and look inside for sludge or dirt that can block the oil pump strainer which will not allow pump to deliver oil back into tank if blocked. No oil in tank will soon be visible as no oil pressure, no oil to crank and top end.

Terry in Australia had that adventure not long ago.

New oil filter and oil too, the old filter and filter cover can tell a story.
Make sure that there is a spring and washer pressing the filter to case. If not dirt can block the oil orifices in head, cam and other stuff up there will quickly go bad.

I should open clutch cover too and have a look inside. If clutch plates are ok, not cracked etc. 

Feel with finger inside for hard particles.

[low-side]

If you are absolutely certain that you are going to pull it down to constituent parts no matter what, no reason to run it first.  That said, you might drop the oil pan and have a look.  If it's not full of cam chain tensioner chunks and it's not moist around the cylinder head block joint, you are probably better off just running it.

[Squiddog1]

Thank you all for the collective wisdom, I guess it is worth a try for me to fire her up. I will take all the tips in ernest and prep her well. I do still worry about those tensioners being made of rubber and plastic after 45 years they can't be in good shape. I know i'd have to take the engine apart to replace them. Any of you still running with original tensioners with no trouble?

[scottly]

I had to replace some tensioner parts back in 1979 when I first got my bike, but that was due to the previous owner messing them up.

[carnivorous chicken]

Another vote for fire it up. You'd be surprised at how well preserved and how well put together originally these bikes were. It's too bad about the bad paint, but if it fires up and runs well it'd be a shame to pull it apart.

[seanbarney41]

Thank you all for the collective wisdom, I guess it is worth a try for me to fire her up. I will take all the tips in ernest and prep her well. I do still worry about those tensioners being made of rubber and plastic after 45 years they can't be in good shape. I know i'd have to take the engine apart to replace them. Any of you still running with original tensioners with no trouble?

yes, never have had any trouble with them.  Actually, have never replaced a single chain tensioner part...dozens of different bikes, hundreds of thousands of miles collectively.