What to look for when buying a CB750

[Redox6022]

Hey all. I am looking at two cb750's for sale in my area. I am very new to motorcycles, but have been an amateur mechanic for 13 years, so turning a wrench is nothing new.

One is a 1980 cb750c with 13k miles, and the other is a 1982 cb750k with 29k miles. First off, what is the difference between the "c" and "k" types? The 750c has mag wheels and the 750k has spokes, that's all the difference I can see.

I am more interested in the 750c because it has less miles, but I've talked to the guy and he says the starter switch was bad and draining the battery. Is there any way to start the bike while bypassing this switch? I really don't want to buy a bike without knowing it can start.

I also don't know really what to look for (i.e. trouble areas) in a bike this old, other than the gaskets and any rubber that's on it. Any help would be appreciated. Thanks!

[TOOLS1]

Welcome to the forum. You are on the right track. Check the oil, rubber, breaks, look in the gas tank, for rust, and so on. I am not real familiar with the DOHC models. I am more into the earlier SOHC models. However I know the switch can be bypassed. Jump across the solenoid. It should crank. You can also hook jumper cables right to the starter wire, and the frame.
The C model is a cruiser style bike. Higher handle bars. The foot pegs are slightly forward, and the seat is lower. It also is geared a little lower then the K, and has a 16 inch rear tire. It also has duel front disc breaks. The K is the standard model. Not as fancy as the C. It is what would be referred to as "the universal Japanese motorcycle."
Let us know what, you decide.
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[sgtslag]

The C model is more for in-town riding, due to the lower gearing.  If you plan on touring on it, go for the K model.  Check the oil:  look at the color, sniff it to see if it smells burned (you probably know this stuff already, but I prefer to cover the bases, than to miss something...); check the exhaust color upon start-up, to see if it is blue (burning oil), or maybe a little white (normal); check the exhaust pipes carefully, looking for rust (can be replaced, but 4-into-4 is a bit of a challenge to keep); check around the header pipes where they bolt onto the engine -- feel (when it's cold!) for exhaust blowing past the seals; check the condition of the chain (630 on the K, 530 on the C; 630's are more $, and more challenging to locate, but they're heavier, too), to see if it is rusted badly; look at the sprockets, as well, since the book recommends changing all three at the same time (NOT necessary...); check the condition of the spokes on the K; check the condition of the rubber (should be replaced unless they're less than three years old, and they have plenty of tread left; check the front fork tubes for leakage -- oil smeared up/down the sliders, indicates the seals need to be replaced along with the ATF fluid within the tubes.

Some things to realize no matter what used bike you buy:

***Download the Factory Service Manual (FSM) from this site's Home Page!!!***

1)  Plan on changing all fluids (oil and ATF in forks, only), no matter what!

2) Plan on performing a full tune-up:  plugs, adjusting the timing chain tension, possibly adjusting the valves, cleaning and balancing the carbs.

3) Replace all of the light bulbs, including the instrument bulbs (avoids losing them at the most inopportune time...).

4) Possibly needing a new chain, and/or sprockets.

5) New tires -- you only have two!  Your life, literally, depends upon them!  The C will run tubeless; the K will need tubes with the spoked rims -- you can use tubeless tires, no problems, whatsoever, running tubes within them.

6)  Changing the oil and filter; changing the ATF fluid in the forks (check the seals for leaks, replace the seals as needed).

7) Drain the blow-by box, beneath the battery:  catches water, and other gunk, which come out of the engine; can cause engine to run rough, if the bottle fills up, easy to drain, easy to check.

8) Check the health/fluid level of the battery, replace it if necessary.

If you want recommendations on tires, plugs, and other parts, give us a shout.  Cheers!

[Redox6022]

Wow guys thanks for the info. I am going to look a the "c" this Wednesday, the one with the bad switch. I found another 750c that I'm going to look at too. I talked to the guy who is selling the 750k, and I think I am going to pass on it, he said the fork seals were leaking, it had aftermarket jets, and "didn't run to well on the 10% ethanol fuel so use an additive each tank"......To me that's a can of worms I don't want to open.

I plan on replacing all fluids plus the tires when I finally get a bike. What is the difference between a 630 and a 530 chain? Are they interchangable? Never heard of a blow-by box, thanks for pointing it out. What exactly does it look like? Is it possible to bleed the brakes with one person? I don't know if the rear drum is hydraulic or not.

Is it really ATF that goes in the forks? What type, i.e. Dextron, type F, etc...? How hard is it to replace seals, do you have to separate the lower half of the fork from the part connected to the steering head?

I would love to have some recommendations on tire type, plugs, any aftermarket parts that make it ride better and safer, and so on. One thing that is guiding my choice of bike is how easily parts are available.

Is the gearing of the transmission of the "c" that it's too buzzy at interstate speeds? I don't really plan on riding it like a crotch rocket, just a commute to work.  


Sorry for all the questions, I just want to know as much as I can and I don't know many people who are riders.

[Redox6022]

Also, what tools will I need to service the bike? I plan on doing almost all maintenance myself. I would like a toolkit that came with the bike, also a bike stand (or jack?), a carb synch tool, and timing light.

[TOOLS1]

Lets see if I can take this one question at a time.
A 630 chain is a lot heavier/stronger then a 530. the C model has a 630 stock.
yes they do take ATF in the forks. dexron/mercon. The forks do need to be taken apart to change the seals, but it is not hard. You do not even need to remove the tubes from the triple trees if, you can get the front high enough to remove the sliders. I have described the procedure here on several posts. I am doing a set tomorrow, for a friend. Maybe I will take pictures, and make a post of it.
Tires? What ever, you like, and can afford. Parts are readily available, for these bikes. We have several parts suppliers listed here. To make it ride better? Progressively wound springs, for the front, and shocks with progressively wound springs, for the rear.  I also recommend Iridium plugs.
The difference in gearing is in the chain, and sprockets. Not the transmission. You could just swap a sprocket to change gearing.
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[Redox6022]

I will search the posts for the fork seal rebuild thanks! And I didnt know about the sprockets, are they available and do i need to change both front (at trans) and the rear (at wheel)? Will go with iridium plugs.

[Redox6022]

In regards to changing a sprocket from a 750c to a 750k, is it the front or rear sprocket, and how many teeth does each have so if I want to change I know which part to get.

[Peterwh]

It's not the number of teeth on a sprocket that matters. It's the ratio between front and rear. Going up on the rear is the same as going down on the front and vice-versa but not tooth for tooth because the front is smaller than the rear.

[TOOLS1]

vintagecb750.com has a listing, for chains, and sprockets that explains quite a lot. You should check it out. If the chain, and sprockets are in good shape, and you want to change the ratio. Just change the front one. On the front more teeth will raise the ratio (faster) less teeth will lower the ratio (slower top speed).
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[Redox6022]

Well, I am going to look at the 1980 cb750c tomorrow. It has 13k miles and hes asking 1500 obo. Sound fair if everything is in running order? It needs a starter switch, but I have that covered.

[LukeM]

Is it possible to push start the bike?  That way you can assess its running condition (if you haven't done so already).  Also, is it the switch on the handgrip that's "bad" or the starter solenoid?  The switch is easy enough to diagnose: remove one of the leads of the motorcycle battery, connect a VOM to the switch, and it should go to zero ohms when you press it.  If it doesn't, there may be dirty contacts, or it's just broken.  If it does show zero ohms, then it's a wire problem back to the starter solenoid (should be under the seat, with a pair of small wires, and a couple of large ones going from battery and starter).  When the switch is working, the small wires energize a coil, which pulls the large current contacts closed, connecting the battery to the starter. The solenoid should click when everything is working right.  Again, dirty contacts could be the problem here.  You can find solenoids on EBay regularly.

If it were me, buying a used MC, I would look for the one that runs the best, or the one that needed the least repair to get running.  Appearance is secondary to me, as I'm a form follows function kind of guy.  Although I did lay a coat of wax on the tank, fenders and sidecovers yesterday.

Good luck with your shopping.  I'm sure we'll hear from you again. :-)
Luke M

[sgtslag]

The K model sounds like he might have been tweaking the carbs, possibly he put pod air filters on it -- it would be quite a bit of work to put it back to factory.  My '79 runs just fine on 10% Ethanol, so it sounds like you might have to do the carb work to save on the additives.

The blow-by box is a plastic tank, beneath the battery.  The FSM shows it clearly, and draining it means removing a plug from a hose, waiting for the crap to flow out (30 seconds?) into a catch container, then replacing the cap.

For tires, I am sold on the Pirelli Route MT66.  Search the Internet for reviews.  They are the tire that I put on, front and back, on all three of my bikes (one sold earlier this year, the Honda is in the shop having blown a tube on the rear tire, last Sunday -- not a failed tire).

The Iridium plugs are NGK:  lower resistance to spark, fire nearly perfectly, every single time; runs smoother; starts very easy with the weaker coils Honda used in these bikes; will last 50k+ miles, which is good since they cost around $8-$9 per plug.

Progressive brand suspension:  made my '79 ride like it was a 2009!  Replace both the front fork springs, and the rear shocks at the same time.  The progressively wound springs means you get two different springs, in one:  one heavier duty, one lighter duty, each functioning simultaneously, giving a very smooth ride, for not a lot of money, all things considered.  Only buy the heavy duty springs on the rear shocks, if you will be riding two-up most of the time -- otherwise they are really stiff, and nowhere near as good at tracking the road surface, nor as comfortable or smooth...

A K&N Air Filter is nice, but spendy:  oiled cotton; clean once every 50,000 miles, good for 1 million miles; passes more air than a conventional filter -- no need to re-jet, or re-tune.

SpeedBleeders will make bleeding your brakes/clutch super easy:  just open 1/4 turn, attach drain hose; drain old fluid from reservoir, fill with fresh; start pumping the brake/clutch lever until reservoir is empty, refill with fresh fluid; repeat pumping/filling, until clear, fresh fluid, passes out the drain tube, then close the SpeedBleeder, cap the reservoir, and done until next time.

Seafoam:  add to the gas tank to dissolve varnish deposits in the fuel system -- 1 oz. per gallon of gasoline.  Add 1 oz. per quart of oil, to the crankcase, run the bike for five minutes on the center stand, shifting through the gears in idle, with the rear tire in the air; shut down (back in neutral gear), drain the oil, change the filter.  This will likely turn the oil black, as it dissolves the varnish, and sludge deposits, within the engine, and the transmission.  With regular oil changes, this won't need to be repeated for 20k to 30k+ miles.  Top off your gas tank, add 1 oz. per gallon of gasoline, prior to Winter storage, run it for a few minutes to circulate it throughout the fuel system, then shut it off for the season:  stabilizes the gasoline, dissolves any deposits over the Winter storage, helps it run great come Spring.

Battery Tender Jr. trickle charger:  smart trickle charger, maintains the battery without boiling off the electrolyte over the Winter months -- top off the battery's cells, plug it in, and forget it until Spring; install the plug connector to your battery, and plug it in after every ride:  tops off the battery's charge to full, helps prevent sulphation, which shorts the internal battery plates, destroying your lead-acid battery.

Optional safety/conspicuity devices to help motorists see you better:

http://www.amperite.com/assets/Documents/HEAD-ALERT-2%20Series%20Tech.pdf

HEAD-ALERT-2 Headlight modulator, with photo-switch (auto-magically turns it to on solid, when it gets dark out); legal in all 50 States, federal law, overrides all State laws; must be ordered directly from Amperite, at around $60 delivered.  No scientific evidence, but in my personal experience, riding three different bikes, all equipped with this, and the Brake flasher (STOP-ALERT), below, I've had fewer close calls with drivers not seeing me, not pulling out in front of me, as often.  YMMV.

http://www.amperite.com/assets/Documents/Stop-Alert%202%20&%202M.pdf

STOP-ALERT Brakelight flasher (motorcycle model):  flashes five times, in five seconds, then on steady, until you release the brake.  Really makes a difference in people noticing you in front of them, helps avoid rear end hits, tends to reduce tail-gaiting.  Can be ordered either from Amperite directly (order together with the HEAD-ALERT-2, to save on S/H), or you can get it from JC Whitney.

[Redox6022]

This is great info. I looked at the bike today, it tried to crank but the starter button worked spotty at best. It looked OK overall, there was a little rust, the brake and clutch cables need adjustment, and the foot pegs need new pins. There was pitting on the fork tubes, and the rear shocks definitely need replaced.

Is it normal not to be able to shift through all the gears when the bike is stationary? I could get it into 1st, neutral, 2nd, and 3rd, but nothing above that. Also, the throttle wouldn't return to its original position, but the starter switch was loose also, and they seem connected to each other.. Same with the left hand switch (signals, lights, horn). They are not supposed to rotate around the handlebar if you push them correct? The clutch handle had a loose screw, and the brake handle went all the way to the accelerator handle without too much resistance, but it stopped the tire. Pads look good.


It looks like a project bike to me, I may see it again this weekend with a friend, but if I can't get it to start then I'm looking elsewhere.

[TOOLS1]

The transmission not shifting when stationery is normal. I would take an SOS pad, and try to clean the fork tubs before buying. Fork tube are expensive. The starter button is only $8.00 at vintage, and easy to replace. However unless your mechanically inclined I would pass on this bike. Unless it is really cheap. Like $600.00-$800.00.
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[Redox6022]

Although I am pretty mechancially inclined, he's asking 1500 obo for the bike, and after I looked at it I would only pay 600-800 dollars like you said. Don't think I could bring him down. Maybe if he's still selling it a month from now I could get it, but the pitting on all four shocks, and the dodgy way the accelerator and switches felt, I think I just need to keep looking. It only had around 12.6k miles on it, but looked like it did over 40k.

[rusty vango]

arent our single cam bikes rated at 14 ampre hours?? wouldnt hooking a car battery up with a pair of jumpers melt some stuff?? im just curious

[TOOLS1]

12 volts is 12 volts. No matter how many amps it has. You can jump a bike off a car battery as long as the car is not running. I use a car battery around the shop to check out/work on bikes with dead battery's.
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[sgtslag]

All the loose bits, the brake lever going to the hand grip, and everything else, indicates this bike has suffered a lot of neglect over a long period of time.  There may be hidden failures, just waiting to reveal themselves to their new owner...

This bike will require a lot of TLC, and elbow grease, to get it in a safe running condition again.  I'd keep looking, unless you can get it for little money.  Cheers!

[LukeM]

I believe the term is "Rode hard and put away wet".

The seller may have to sell, but you don't really have to buy.  Me personally, I'd hold out for a better bike, even if you have to spend a bit more.  It could be this guy is wanting to cash in on the CB750 craze.  Your mechanical ability is something to factor in as well: if you're not gifted in that field, perhaps getting a bike that's in better shape that only needs basic maintenance.

Luke M