Known Problems with the Nighthawk 1984-1986 CB700/750SEdit
Contributed by Greg Shell
There are a few problems which seem to be fairly widespread with this bike. The following notes come from my own experience combined with info from other sources, namely mechanics, parts counter people, magazines, and other owners.
Alternator shaft chain breaksEdit
The CB700SC was one of Honda's first inline-fours with the alternator (and starter) moved back onto a discrete shaft, decreasing the width of the engine. Unfortunately, the original chain driving this shaft (from the crankshaft) and/or the tensioner has a tendency to break. I understand there was an update to the chain to fix this problem, but I don't know if it made it into any production bikes. The symptoms of this problem are no charging, no useful starter (whirrrr!), and poor idle/running as the battery doesn't seem quite powerful enough when running with a "total loss" (no alternator/generator) electrical system.
This happened to my '85 bike. It took me quite a while to realize what the problem was and decide to fix it, five years in fact. At last I decided to return to riding, pulled the engine out (ouch!), and had a mechanic replace the chain and tensioner for about $650. I put that sucker back in (double ouch!) and now it lives again, although with more noise and vibration and oil consumption than I remember. Next time I'll follow Honda's long-term storage recommendations.
A lot of people have had starter problems. My bike has the following symptoms:
Oil in the starter motor, which tends to leak out and make a mess. I imagine this is caused by high compression in the crankcase resulting from leaky rings, possibly combined with a starter commutator shaft that's not quite in balance. I've been told there's a seal that can be replaced for this, but it's not obvious from the shop manual. And new seals for the starter motor case do a good job of keeping the oil from getting out.
Prematurely worn brushes. I've heard it said that the commutator shaft can get out of balance, wearing the brushes and commutator and probably allowing the oil to leak in. The brushes and "brush holder" are not terribly expensive from Honda, these fix things right up for a while. What's more, I've found I can simply stuff the wires behind the brushes with good effect, so I carry the required 8mm wrench and screwdriver when I tour.
After suffering through increasing starter problems for about a year, with ever less starting power and equally less effect from my dubious repairs, I finally went out and spent $150 on a rebuilt starter. It works great, I should have done it six months previously. Not only is it now reliable and leak-free (so far as I know, might be some internal leakage), but it also now starts the bike instantly. Just like a starter should.
I had the same issues with a leaky weak starter. Found a local electric motor mechanic who completely rebuilt my starer for $60. Works great.
Indeed a weak point in the design. The oil leaks are more likely caused by a faulty oil seal on the main bearing. Replacing the bearing fixed the issue on my bike. Wear: I had the starter rebuilt three times before giving up and replacing with a new unit. The commutator always wore out really quickly. I suspect the rear bearing wear was causing an alignment issue resulting in premature failure. Template:Kyrie
There can be a problem with the transmission, wherein the bike pops out of gear, starting with 2nd. Apparently the problem grows to include 3rd gear, etc. I've been told that the cause is a too-small part in or actually near the transmission that wears out. That's all I know. My bikes jumps out of 2nd occasionally, but only right after shifting from 1st, so I think my experience is more of a missed shift. And the jump out of 2nd can be avoided by I really firm shift from 1st to 2nd.
Instrument needles breakEdit
The indicator needles on the tachometer and speedometer often break off. On mine this happened with the tach, but it's still easy enough to read. Easier to read than the intact speedometer, I find.
I have had the same happen. It is due to sunlight degradation of the plastic needles. Replaced with homemade PET plastic needles because I could not source original speedo replacements. The bezel surface also darkens with age because of sunlight exposure. This can be sanded off and replaced with a clear lacquer.