1982 Nighthawk 450 trouble

I bought a ’82 nighthawk about a month ago only 2000 miles, clean I was loving it. Till about a week ago when it left me stranded on the side of the road without warning. I was crusing at 70 when the bike just lost power, the bike didn’t actually die till I came to a stop. After checking some things and making some phone calls I tried to start it again, it started back up. About 100 yards down the highway it lost power again. That repeated one more time before I chalked one up to the bike for the day and waited for a friend with a truck. Since then I’ve checked spark plugs, fuel strainer, carbs (although not extensively) nothing seems to clear it up. Some days it won’t start at all some days it starts right up idles fine in the garage even revs at 4000 for 20-30 seconds fine, but ride it 100 yards or so and it looses power and dies. Please tell me you have the golden nugget of truth, this is my first bike and I am itching to get back out on the road. Thanks for your time!

Hi Tyoung

Bikes only need 3 basic things to run – fuel, compression, spark. If I was you I would tear through the carbs thoroughly. Make sure every passage is clear. Make sure the floats operate smoothly and are set to factory spec. Also verify fuel flow from your petcock.

If you have a compressions gauge you should do a quick check. With only 2000 miles the bike should not have any sort of compression issue, but you never know. Rust could have built up in the cylinder. A compression gauge can be bought at Harbor Freight for around $15 or so.

If the carbs and the compression both check out then it’s on to spark.

BEFORE ANYTHING: Make sure your battery is in good condition!!!! I see so many strange problems with bikes due to a worn battery. If your battery is old, or isn’t holding voltage – REPLACE IT!

Let me know how it goes.


Notes from a Reader:
TyoungYL: I have your golden ticket for this one! (But it’s a year late -_-)

EvanFell: you got it right on with your diagnosis – that’s what my mechanic buddy told me too.

Th OP has probably solved his problem by now but I thought I’d give away the answer anyway.

He blew an air cut-off valve diaphragm.

I experienced exactly the same problem last year on my ’84 Nighthawk 450 (32,000 km). My bike lost power in the above explained fashion as I downshifted to slow down for a minivan on the highway. When I tried to rev up nothing really happened.

It started twice more and then wouldn’t start the third time. I was lucky enough to only be about 8 blocks from home (and driving a light bike)

Regardless of the miles operated the ‘rubber’ (elastomer) components of a bike (or anything else for that matter) will degrade. After twenty years many seams, seals, and gaskets is will be weak and/or cracking.

You can get to one of your air-cut of diaphragms by taking the cover held down by two screws off of the side of your carb. You can’t really get at the other side so you might as well take the whole carb off, clean all the gunk out, lubricate it up and put in 2 new diaphragms (don’t just replace one because the other is liable to blow too). Be very careful not to strip any screws because replacements will be tough to get.

Go to Carb Kit  Capital dot com to buy the parts you need for about $25 bucks each….. or get the Honda parts for about $95 each. The important, rubber part of the CarbKitCapital diaphragm fit, by the metal actuator did not so I had to file it down to the same size as the original, which was tricky business.

If I had to, I’d buy the cheep ones instead because they seem to be working fine.

Good luck!


Evan Fell

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