New user maybe new bike - tips for buying '78 CB750 Four

[djolecar]

Hi guys,

 I've ridden bikes in past (start small 50cc and moved on the bigger motorcycles later). My question is, I was looking to buy a old 1978 CB750 Four (as I understand it is the same thing as CB750 K8). American model the last ones built with SOHC.

Now I don't need long to pick up on the knowledge of mechanical parts, but I do not have any experience with these motorcycles. The guy is selling it for pretty cheap $450 (figure I can get it down to at least $400). Cosmetics way it looks bad but I don't care about that (at least not for now). I want it to purr and ride well. Any suggestions on what I should look at when when buying? The bike runs but the seller says it needs a "tune up". Wondering what could be wrong, making sure I don't go over my head when buying and ended up paying $2000 for repair (maybe one day to polish it). Or just not bother and buy a nice one for 1500~2000.

Thanks for your time!

[LukeM]

I got my 79 CB750L for $500. It had some issues, but ran well until it ate itself up after breaking the long cam chain.  My '84 Shadow cost me $650. Both didn't run well when I got them, but after some sweat and chemicals and some cash, they got better, thanks to the helping people on this forum.

You should feel little puffs from the exhaust pipe when the engine is cranked.  That says you've got good compression, and means the rings and valvetrain are probably OK.

If you can get it to start at all, even a few pops and wheezes, that's a plus.  That says your ignition and timing are probably close, and there's some gas getting to the cylinders.

A "tune up" in my mind includes:
New or cleaned spark plugs
New points (if applicable) or cleaned points if they're fairly new
Carburetor adjustment if the engine runs at all
Timing check (static and dynamic-Involves timing light and dwell meter)
Cam chain(s) adjustment
Oil change if due
Battery water level check

A more involved "tune up" in my mind should include:
Adjustment of cables for throttle, clutch, and brake and check of brake cylinders
Check of screens in tank/petcock or external fuel filter
Electrical system check for charging and battery health
Valve lash check/adjustment

I'm sure if it's been sitting a while, there's a fair amount of gunk in the carbs, and will need a good disassembly and cleaning.  There are MANY posts regarding this.  Get the Factory Service Manual on our home page, and use the SEARCH bar to look for other postings.

I hope this helps. Keep us in the loop, and we'll try to help more.
Luke M

[djolecar]

Thanks a lot for the tips. I'll keep you posted on how things work out. First, when I see the bike!

[Spyder]

Check the intake boots, they cost alot, and if there all rotten , the bike was stored outside for years.

[shinyribs]

Cleaning carbs is cheap.Almost free,all it cost is solvent and time. No biggie there.

Check the chain and sprockets to see if they are wore,rusty or of the chain is kinked. Also check the age/condition of the tires. If all those things need to be replaced it can get pricey.

BUT, if you can get a running 78K for $400 it's is a good deal IMO. If the motor sound good and it CHARGES then you'll be ahead of the game already. Easy way to check for charging is just to see if the headlight brightens when you rev it off idle. It wont tell you if it is charging strongly,but it will tell you if it is charging at all. I'd go ahead and check out the wiring harness and see if it has been butchered.

It wouldn't hurt to take a peek into the tank and see what it looks like in there. Check for rust and the smell of rotten gas,as always.

These bikes are known for worn steering bearings. A good way to check for that is to get it up on the center stand and rock the bike back to get the front wheel off the ground. If you can feel a notchy spot in the center position ( front tire pointed straight ahead) then it needs new bearings. They are easy to replace and cost around $40.


Other things to check would be sticking breaks, dry rotted brake hoses,bent handlebars....stuff like that. Good luck!

[Thorby]

What has been posted makes a lot of good sense.  I'll add that don't freak out if it backfires (loud bangs coming out of the tail pipes) at first.  When I first got my bike it would do this, but it was from lack of use.  The backfiring while cold dissapeared with use.