Honda Nighthawk/Printable version

Honda Nighthawk

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Guidelines for Contributors

The initiative for this guide is to consolidate useful Honda Nighthawk servicing information. All contributions are greatly appreciated but please:

1) Keep content as brief and to the point as possible.

2) Specific information pertaining to a model should be referenced as follows:


1984-1986/SB-700SC Low Oil pressure (identify year/model and topic)

Ref 2-12 (Refer to Section 2, page 12 of the Honda manual- the page where the information you are posting is applicable).

3) Please do not completely rewrite what has already been written. A comment on the discussion page regarding your concerns would be appreciated.


[1985 CB700SC/CB750 Carburetor]
[1985 CB700SC Cylinder jug]
[1985 CB700SC Commutator]

Carburetor Cleaning

1985 CB700/CB750SC Nighthawk S Carburetor

Rough idling, hard starting or other performance problems may be due to dirty carburetors. If the bike has sat for a few years or the gas tank shows signs of varnish and gum deposits the carb should be suspect.

When servicing carbs be aware that:

1) Parts are fragile and become more so with age.

2) Parts made from rubber or plastic can be damaged by carb cleaners.

3) Gasoline is highly flammable.

1984-1986 SB700&750

Recommend removing gas tank to do this job.

Ref 4-3 Rubber connecting tube between the air box and carb intake will be stiff due to age. Loosen both 10mm AF bolts at top rear of airbox and lever box backwards along the slots for 15mm until the bolts are fully back then tighten them there to give clearance for boot removal. Unclamp clips then push the rubber flange inward at the air box end. The connecting tube can now slide off the carb intake and out. WD40 applied to the joint may help.

Ref 4-3 The “Intake” pipe snootboot going to the cylinder head is extremely difficult to remove from the carb as the rubber is stiff.

Working the carb assembly up and down side to side may get results. Heating the rubber with a hair drier helps but rquires extreme caution, make sure no sources of fuel are present. Attempts to lever the carb off should be done with care to avoid damage to the linkage.
Using a Dremel tool to resize boot
Removing the eight allen hd cap screws holding the intake flange to the head can be done if all else fails. To attempt this you will need a 1/4" hex socket, a universal, with a selection of extensions. A ball end hex is also useful. Once off, resize the boot opening so next time you don't have to tear half the bike apart. A Dremel tool with a small drum works well.

Ref 4-4 Check each vacuum piston for movement (black cylinder) visible from the intake side of the carb. Place a finger below the piston and gently lift. Make note of any pistons that appear seized.

Ref 4-4 The vacuum chamber cover screws may be difficult to remove. Avoid stripping the heads by using a proper fitting impact driver.

Carefully lift the cover while separating the rubber diaphragm. Remove spring and lift out piston with attached diaphragm. The piston itself is probably not seized; chances are that the jet needle is stuck. Free it by spraying a small amount of carb cleaner around the end of the jet needle visible through the intake end of the carb. Allow to soak and gently work the needle buy rotating the piston from above. Remove the piston when free. Remember carb cleaner kills rubber; keep it well away from the diaphragm.

Ref 4-5 As you lift the bowl the gasket should stay in the groove on the bowl side. Carefully separate the gasket as the bowl is lifted.

Ref 4-5 to 4-6 Check to make sure the needle valves move freely before attempting to disassemble. Frozen needle valves can be lifted by a collar at the top of the valve where the plastic float is attached . Avoid applying force directly to the plastic float. Avoid using carb cleaner to loosen the valve as the pointed end is made from rubber.

Use a proper fitting screwdriver or where possible a socket to remove the jets.

Ref 4-7 Choke (bystarter) valve removal. Remember orientation of the spring on the choke shaft (bot of 4-7)

Ref 4-8 after loosening the choke shaft locking screws, check the shaft to make sure it slides with finger pressure. If still frozen, slightly wedge open each of the 4 arm clamps and apply some WD40. The dark colored nut on top of each choke valve is made from plastic and will shear off under excessive force. Do not attempt to remove the choke shaft by hammering the end.

Ref 4-13 Pilot screw must be removed for cleaning the low speed circuit. Do not over tighten when counting the turns in. The factory setting is two turns out.

Ref 4-10 Carb Cleaning –“Clean all carburetor passages with compressed air” who ever wrote this manual must have been dreaming.

1) Scrape varnish off as much of the float bowl as possible. Spray with carb cleaner and let soak.

2) Fill a dish with carb cleaner and drop in the brass jets and choke plungers.

3) Visually trace all of the fuel and air passages in the carb body.

The fuel passages are: a) High Speed (main jet) b) Low Speed (slow) c) Choke

Jet towers, float valve opening/seat and choke valve opening should be thoroughly cleaned by soaking in carb cleaner. Working the cleaner with a Qtip and a small bristle brush works well. Next spray carb cleaner into each of the fuel passages. Let sit for 10 minutes and use compressed air to blow each passage out out. If you haven’t figured out the circuit, you may get a face full of carb cleaner at this point. Repeat procedure until the carb cleaner is clear when blown into a white pape towel.

4) The needle valve requires special attention to the small spring loaded rod at the top of the valve. Gently push on the rod and see if it moves into the valve body. If it does not move freely, set the pin end of the valve in a shallow tray of carb cleaner with the rubber end up and out of the cleaner. Soak until the pin is free.

5) Small fuel tubes inter-connecting the carbs contain plastic and rubber components. Any carb cleaner leaking into these should be blown out promptly.

6) Carb cleaners may attack the plastic floats. Exercise caution. <is there any way to adjust float levels by bending the plastic?>

7) After a thorough cleaning, hold the jets up to the light for inspection.

8) Blow out all air passages.

Known Problems

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. 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.

Starter woesEdit

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 starter 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 a really firm shift from 1st to 2nd. (the problem with the shifting is due to one of the three shifter forks which will wear and snap off causing you not to find 2nd. You can find the central and right (I think) shifter forks on ebay but not the left because that is the one that had premature wear.

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.



Remember the old saying about "Estimating time and doubling it" to come up with something reasonable when it comes to fixing anything? A complete bike restoration can take several months to years to complete. It’s probably going to take more time for several reasons. Most likely you'd be fitting the work around other things happening in your life, expect lots of interruptions and delays. You tend to discover parts that need replacement as the project is proceeding. Sometimes parts need to be sent away for servicing or they might not be readily available and you have to find them. Then there is the money thing. What level of perfection do I want, and how much is it going to cost. It always takes longer and cost more than you think

Teardown. Having a bike broken down into five hundred pieces at the end of the day can give one a terrific sense of accomplishment. Don't do it!. Putting everything back the right way and in the right place is the hard part. Here are some suggestions that might help.

[Freezer bag with parts]

1) The time between the teardown stage and assembly will take a while. Knowing exactly where things belong at some point in the distant future can save you an enormous amount of time. Invest in a selection of different sized freezer bags that have the nifty seals and a place for writing a label. As a part comes off, clean it with its respective fasteners, washers, gaskets etc. Put the pieces into a bag and label what it is and where it goes. Apart from left, right, front and back some things like carburetors, pistons and valves require referencing to the cylinder numbers. The plastic bags also protect the parts from corrosion. Spending extra time staying organized at this stage will have a huge payback six months from now when assembly starts.

2) Set parts aside that require further repairs or refinishing before assembly. Sometimes it's nice to have a few jobs that can be completed in an hour or so.

3) Follow the shop manual as you take things apart. If your not sure where to start or how things work, the manual will tell you. If you have a digital camera take lots of pictures especially areas that look complicated.

4) Invest in good tools. Fortunately you don’t need a lot of tools to work on a bike. A must is a set of 6 point sockets(not 12 point)in ¼’and 3/8” drives and a good selection of extensions. Universals and wobble extensions are handy to have for tight spaces. A set of hex sockets for ¼” drive are also a must have. For assembly you need a torque wrench. The newer types that go “click” are slick. Combination, box and open end wrenches, use them where you have to, but make sure they are in good shape. Hopefully Philip head screws will be a thing of the past some day, but until they are an impact driver is the best way to get them out.

5) Avoid damaging or breaking parts. A broken bolt may use up your valuable time attempting to remove it, or worse, destroying the part your working on. Parts for older bikes are expensive and hard to find. You just have to walk around a salvage yard to see bikes that have been scrapped because the parts needed for repairs are too expensive or not available. Not sure how to remove a rusty bolt with the stripped head. Here is practical advice from someone with years of experience on all aspects of MC repair See Dan's advice

6) Bikes built since the early 1980’s have lots of plastic and rubber components. These tend to become more fragile with age. So be careful.

7) If you don't have the special service tools specified in the manual. Make sure what you do use as a substitute will work without causing damage.

8) If you are fortunate enough to be working with a bike that runs, test as many systems as possible beforehand to identify any problems and determine which parts need to be replaced or repaired.

9) If you’re uncertain about a procedure, the Nighthawk Groups in the links section are always very helpful.

10) Start looking for replacement parts as soon as possible, even if you're not buying right away-just to get an idea what is available. eBay and the various online groups are good sources, as well as MC salvage yards. Parts for items that wear out frequently tend to still be available from Honda or other aftermarket suppliers.

11) Don't get overwhelmed with the job. Break it down into smaller steps, each with a beginning and a finished end.

12) Gas tanks seem to get more easily damaged off the bike than on. Store them in a safe place, preferably far away from your work area.

13) You don't like wrenching on motorcycles but want a cheap bike, don't even start.

Stripped an internal thread here is how to fix it Using a Helicoil insert

Restoration ProjectsEdit

1985 750 Nighthawk S

If your complete restoration of the bike includes major engine work, I would suggest the following teardown order to make engine removal as painless as possible.. 1)Seat, 2)Gas tank, 3)Exhaust pipes, 4)Rear tire, 5)Air box with boots, 6)Engine.

[1985 CB750S Nighthawk S]

Engine Removal

Ref 5-2 Remove both ends of the hoses and look out for the O rings

Ref 5-4 Subframes (are these on 750SC’s also?)

Ref 5-4 Remove and examine gear shift position switch check for continuity. Ref 21-8

Ref 5-5 (Top) Engine Removal. The manual makes this job sound like a Sunday stroll.

If you’re planning to take the carbs off, this is a great time to do it and make a little extra room to squeeze the engine out of the right side. If you happen to be on the Olympic weightlifting team follow the manual. . If not, here is what works.

Save the frame paint, wrap it with some thin padding along the bottom right cradle tube where the oil pan has to clear. To have good control, you need to pull from the top, not push from the bottom as the manual suggests. To do this, locate the bike under a ceiling joist or roof rafter. Attach a light chain or rope hoist at a point above the engine to the joist. On the bike remove the two outside acorn nuts from each side of the head and make up two short straps from seatbelt material or a 1/16” X 1 1/4 steel strapping. Cut or drill two holes to suite the bolt spacing on the head in each strap. Attach the straps (use flat washers with the seat belt material). Next attach each end of a rope about 8 feet long to the straps in such a way as to make a bridle with the spine of the frame in the middle. Attach the bridle to the hoist and lift to take the full strain of the motor. Lift the motor just enough to make sure all of the engine mounts are free. Have a peek at the rubber boot at the driveshaft tube. It should just be starting to slide. Watch the boot for any pinching. Use the scissor jack under the oil pan to make tilting adjustments. The engine needs to go forward and as high as possible to clear the frame. To do this, lift up with the hoist and pull the bike back. The kickstand should slide on the floor. As you do, the spline connecting the output shaft from the motor to the universal joint should slide off. Look inside the rubber boot to make sure it has cleared before proceeding. Once the engine is in position, block it up and slide the bike sideways (left). When the strap on the left side of the head slides under spine of the frame, disconnect the rope and reattach to the strap, passing it under the right side of the spine. Pull the motor up a bit, continue sliding the bike to the left and the engine slips out. No strain, no pain and if your careful, not a scratch on the frame.



A General Guide to Trouble Shooting - Starting'

contributed by author: doclockwood...700S rider/revivifyer)

HI, it looks like there's a lot of Nighthawk starting issues (on the list) right now. No doubt, it's mostly from all the winterized bikes coming out of hibernation and from plain old gettin' old, the bikes, I mean. If we have some common tips for starting woes, we could all post 'em to the same page and maybe save someone a little time and work by looking for help (t)here. Many many thanks to all who contribute.

Anyway, I have had my share of starting trouble lately, (I sure have a lot to learn about starters) but if there's one thing I'd check first, it would be:

1) Clean the grounds to frame and engine. Shine 'em up, coat 'em w/ something or other(not biting on THAT one...dielectric grease) and then follow up by cleaning all the contacts in the starting/charging circuit. Test 'em all with an Ω meter (couldn't "resist") and a manual. This includes the kill switch and starter button.

2) Make absolutely sure there's a fully charged battery. It may be necessary to buy a new one every season but that would indicate a charging issue. Even if it's just recently charged, it may lose it's cranking ability after just a few seconds when you hit the start button due to the age/inefficiencies of other components. Take a jump from a car and see if it's appreciably stronger(faster spinning starter) than the bike alone. Cleaner contacts and a "known" strong battery work wonders.

3) Install new plugs and wires. This makes an easier path for the battery plugs every oil change, even if the bike is running great? I each his/her own. But it eliminates a variable if it won't plugs can't hurt.

4) A rusty tank? Bad petcock? Got to be clean. And use fresh suspicion is that Sta-bil may inhibit restarting in the Spring for folx above the Mason-Dixon line...good stuff to keep the water out of the gas tank, but makes for hard starts initially. Another rookie mistake is to use Stabil in the tank but turn off the bike before it's had a chance to get all the way into the carbs and pistons...I'm certainly guilty of this one.

5) Carbs need to be clean, too. Just from the numerous posts on this topic, it's likely the carbs will gum up or clog sometime during your watch so it has to be considered a chronic problem for Nighthawks, esp. the older models. Lo-speed jets need attention here. Bad idle indicates clogged jets, but it may be electrical, too.

6) The starter should be in good condition. It seems like a no-brainer, but if all the above conditions are favorable, then a bad starter may be the problem. No duh, right?:) There are eBay starters out there and folk who refurbish them, so don't despair.

7) If the starter sounds good/performs well and the bike STILL doesn't start, it MAY be a bad coil, though it's a long shot. My bike was firing on cyls 2 and 4 only so I suspected the 1-4 coil as bad, but it was just a cruddy ground. A meter will confirm coil health.

Lots more tips are in the troubleshooting section in the NH 700S manuals in ignition/starting sections.

Fuel System


CB700/750SC Petcock. At the top is the brass fuel tube with the intank filter removed and used to draw fuel in the "ON" position. , Left side is the fuel supply to carb assembly, far right is vac hose to carb, at bottom is the fuel cup

The Petcock (fuel valve) seems to be an on going issue. This device serves a number of functions which may cause confusion when troubleshooting. They are:

1) Manual fuel ON valve setting for normal operation

2) Manual fuel RESERVE valve setting for reserve operation

3) Manual fuel OFF valve setting

4) Automatic fuel shutoff valve which overrides the manual settings. This valve is operated by engine vacuum via the small rubber hose. The valve opens when the engine is started and closes when the engine is stopped.

5) Primary and secondary fuel filtration. One protrudes into the tank, the other is in the small fuel cup at the bottom of the unit.

Before proceeding, make sure there is a fire extinguisher handy

If the bike has been recently running, turn the petcock to OFF and unscrew the small fuel cup cover on the petcock. Inspect and clean the small circular fuel strainer. Reinstall strainer and cup.

To determine if the petcock is working properly, turn the petcock to the OFF position. Disconnect the fuel line from the petcock and the smaller vacuum line from the carburetor. While holding an empty can under the petcock, turn the valve to the ON position. There should be no gas flow. Suck on the vacuum line or use a hand vacuum pump. Gas should start to flow into the can. Release the vacuum on the line and gas should stop flowing.The above test should also work when the petcock is in the RESERVE position.

If there is no gas flow . Turn the petcock to the OFF position and Ref 4-16 remove the gas tank The manual suggests servicing the petcock while still attached to the tank. It is better to remove the petcock completely and check the filters and the fuel valve diaphragm. If the tank is full, drain out most of the gas through the fill hole. Ref 4-17 in the manual(bottom of page) loosen the petcock lock nut and remove petcock. There should be a tube like fuel strainer attached. If not, check the tank for the strainer and remove it. Remove the O ring. Drain the remaining gas from the tank. Ref 4-16 (bottom of page) carefully remove the cover, spring, and both diaphragms. If the bike has sat for a while, chances are that the diaphragms are stuck in the closed position from gas residue. Exercise care in freeing them. Remove the fuel cup, and the small circular strainer.

Set the petcock valve to ON and inspect the tall fuel tube which sticks up into the tank for gum and varnish residue. Clean with carb cleaner and blow out with compressed air. Set the petcock valve to RESERVE inspect the opening at the base of the tall fuel tube and clean as above. During use, recommend using the RESERVE setting occasionally to prevent the buildup of gum in the passage.

Part of the petcock is assembled with rivets. Some internal components may be rubber and could be affected by prolonged soaking with carb cleaner.

Do not use carb cleaner on the rubber diaphragms. Inspect for tears and wear, Replace if required.