I'm looking at buying a 92 Honda Nighthawk with 47k. The main issue is it is in need of carb cleaning. How hard is it for DIY? What should I expect if I took it to a shop?
I'm looking at buying a 92 Honda Nighthawk with 47k. The main issue is it is in need of carb cleaning. How hard is it for DIY? What should I expect if I took it to a shop?
When I ran a Yamaha service department back in the '80s we charged five hours labor for cleaning a four carb setup. One hour for R&R and tuning and an hour per carb.
I wouldn't recommend doing it as a first time I've used tools diy project but if you have some mechanical experience and are patient and able to be meticulous then you should be able to do it. It's not rocket surgery.
Getting the carbs of the bike can be a pain. I haven't touched a Nighthawk in decades so I don't remember any particular tips or tricks but the following in generic advice for a four cylinder universal Japanese motorcycle. Remove the fuel line(s) and disconnect the throttle and choke cables. You'll need a long screwdriver to loosen all the clamps on the manifold and airbox boots. I usually pull them all the way out. You'll have to straighten them as you pull them around the boots but they'll bend back in shape when you re-install them. If you're lucky you can loosen up whatever fasteners are holding the airbox in place and push it back far enough to wiggle the carbs out. They come out as one assembly. As old as that bike is you're likely to have to do some significant prying and wiggling to get them unstuck from the boots. If there's not enough room to slide them out you'll have to start removing whatever's in the way to get them out.
Don't separate the carbs and don't mess with the linkage adjustments between the carbs. That may have or have had plugs over the idle air mixture screws. If they're still there you 'll need to remove them. Drill them for a sheet metal screw. Run the screw in and either pry them out out pop them out with a slide hammer.
Lay out something you can write on and lay parts on like a sheet of paper or a paper towel for each carb. I like the blue shop disposable shop towels. As you pull parts out of each carb put them on the appropriate sheet. Write any notes on the same sheets.
Count the turns on each idle mixture screw as you turn it in till it lightly seats and then remove the screws. Write the number of turns for each carb down on the associated sheet. There's probably a spring, flat washer and o-ring in the bore of each mixture screw.
Disassemble the choke linkage (technically it's a cold start enrichment circuit and not a choke) and remove the plungers.
Remove the top covers and remove the springs and diaphragm/slide assemblies. Remove any air bleeds that are under the diaphragms. Keep track where they came from. They're is probably a pair and while the holes they go in can be the same size the orifices are going to be different sizes.
Remove the float bowls, float and needle and seat assemblies.
Remove all the jets except for the main. That's the large one that's more or less in the middle of the carb. Again make any required notes to keep track of where the jets came from.
Loosen but don't remove the main jet. The main jet threads into the emulsion tube which is inserted from the top of the carb. Tap on the face of the main jet with the handle of a screwdriver or a small brass punch to start to drive the emulsion tube up out of the carb. Once the main jet is seated against the carb body remove it and the washer under it and push the emulsion tube the rest of the way out of the carb with your tool of choice.
I soak everything except the carb bodies themselves in carb cleaner. If needed I'll setup some cut down oil bottles or other container, fill them with carb cleaner and set the carbs on top of them in such a way that the emulsion tube holders are submerged but generally I clean the exteriors in the parts washer and then clean the passages using the following procedure.
Pretty much every circuit in the carb goes to the float bowl for fuel, intake air either directly or from the area under the diaphragm and into the air horn somewhere to get the air/fuel mixture into the engine. I use wd40 and spray through each circuit making sure each passage is clear. If it's not I'll use some combination of carb cleaner, compressed air and wire to fix the situation. The vast majority of passages will clean with just the wd40. In fact most of the junk will be in and around the emulsion tubes which is also the part that most DIY carb rebuilders won't touch.
I follow up the wd40 with either brake clean or carb clean, whatever I have in stock to remove the wd40 and then blow the whole thing dry.
Reassembly is the reverse of disassembly. Put stuff back where it came from. Be careful to seat the diaphragms correctly. The top covers should sit flush and level and the slides should move smoothly with a slight resistance from the air moving through the large passage to the intake side. Gently blowing compressed air into the intake should open them.
Make sure that the throttle opens and closes smoothly. Turn the handle bars from lock to lock and confirm that there's a slight amount of free play in both the throttle and choke cables at both extremes.
Ideally the the carbs should be tuned with an exhaust gas analyzer and a set of carb sticks but if nobody messed with the adjustments trying to get dirty carbs to run right you should be pretty close just by having put the mixture screws where they were and not messing with the throttle adjustment screws.
If you have specific questions feel free to post here or IM me.
Yeah, so a full service is not within my wheelhouse. I wonder if a sea foam treatment, or any fuel additive, would be a reasonable stop gap if I can get it fired up and running. It will run on throttle, but won't idle. This is also a bike with 47K.
Find one of these blow guns, shoots out small orifices and such, ~$12. If like my Goldwing and hasn't run for 30+ years, rebuild kits are available.
In reply to Brian(formerly neon4891) :
It may, and I think Seafoam or carb cleaner may help. If they don't you probably need an ultrasonic cleaner or finding better condition carbs.
And someone else bought it. I'm not gonna lose sleep over it.
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