650 Honda Nighthawk

[Canuckskie]

Hey guys,
Question about a buddy's bike - an 82 Honda Nighthawk 650 - works great, good RPMs up to 5th gear, then its impossible to get it up past 80-85 km/hr.
carbs have been done- cleaned, new jets, new seals, new plugs, etc.
the shop who did most of this work are kinda stumped. they're thinking its the ignition modules at about $800. cost to replace. ???
exactly what are these ignition modules ( sounds like there's 2 of them)
another shop we talked to says its the airbox that needs cleaning.
anybody have any experience with this bike or with this kind of problem?
any suggestions greatly appreciated.
thanks to all..

[Johnj]

Clean the airbox out. Clean and sync the carbs. Check the exhaust for stoppage or leaks.

[Canuckskie]

thanks bud,
carbs redone at bike shop, cleaned, new jets, gaskets, etc.
exhaust looks good, the only thing we don't know for sure is what was done to airbox besides putting in a new filter.
my buddy's going to get the bike from the repair shop and we'll have a look at the airbox, etc when he gets it back home.
thanks for your input

[Johnj]

Okay your carbs were done at a bike shop. Why did they replace the jets? Which jets did they replace? Are the new jets the same size as the old jets? Did they change the needle settings?

80 kph is about 50 mph, can you go over 80 kph in fourth gear?

Does the engine lose power at the same rpm in every gear?

[Canuckskie]

supposedly carbs were completely cleaned, gaskets replaced, jets replaced because it was cheaper than trying to clean gunk out of old ones. it revs out fine in every other gear except 5th. they're thinking its probably the ignition modules, but they're not absolutely sure until they try it, but at that kind of price he can't afford to keep experimenting as he was only trying to get the bike in good running condition so he can sell it, but now repairs are going to end up being more than what the bike is worth. wondering if this ignition module they're talking about is the CDI module?? but they're saying there's 2 of them?????
another shop my buddy spoke to said to check and clean the airbox.
BTW - thanks for your help

[Canuckskie]

ok, my buddy got his bike back from the shop, we checked the airbox, clean as a whistle. the bike runs like a charm in every gear but 5th.
had it wound out in 4th, no problems, goes like a raped-ape but shift into 5th and it seems like it doesn't have the power to keep it at that speed.
dealer recommended changing the coils (2) and ignition modules (2)
wouldn't a problem with these also be a problem in any gear??
thinking the next step should be to sync the carbs, get it tuned by someone who knows something about older hondas.

[BuzZz]

Don't sound like the ignition. Don't really sound like un-synced carbs. Sounds bloody wierd is all. Runs fine in every gear but 5th..... all that springs to mind is something creating drag(bearing, chain? and the bike has the snot to pull past the extra friction in all gears but top) or jetting is off(same thing, it runs well enough to pull every gear but top gear).

Ask the dealer if he replaces the coils, etc. that he recommends, and it don't work, will he take it off and put the old stuff back without charging you for the parts. I doubt he will, but that's a lot of coin to drop on his guess.

[Sev]

Is it possible that the shaft splines are worn down around the 5 gear gears... causing it to slip? Seems next to impossible, but who knows.

[flynrider]

I agree with Buzz. Definitely doesn't sound like an ignition problem. If the ignition is creating a good spark up through the rpm range in 4th gear, then it should do the same in 5th. The ignition system doesn't know what gear you are in.

One thing that could be related is fuel flow. The faster you go, the more fuel needs to flow through the carbs. If there is a restriction anywhere in the system, from the petcock, through the carbs, it should show up when the fuel demand is higher (i.e. when you go faster).

I've seen this same problem show up on other bikes. Once they'd get up to a certain speed, they'd just bog down. Last time it was a petcock that was gummed up from old fuel.

Edit : An additional clue to the problem might be that the old jets were gunked up. That could mean that there are clogs upstream of the carbs as well.

[BuzZz]

One thing that could be related is fuel flow. The faster you go, the more fuel needs to flow through the carbs. If there is a restriction anywhere in the system, from the petcock, through the carbs, it should show up when the fuel demand is higher (i.e. when you go faster).

I've seen this same problem show up on other bikes. Once they'd get up to a certain speed, they'd just bog down. Last time it was a petcock that was gummed up from old fuel.

Edit : An additional clue to the problem might be that the old jets were gunked up. That could mean that there are clogs upstream of the carbs as well.

There's your best avenue of investigation I can think of.