Loud intake (?) growl when hot

[waterskibum]

Hi all,

I have an 83 NH 650 that I've restored over the past couple years. It runs great. The only issue I've noticed is that I get a loud intake (I think?) sound, I'd describe as a growl, when it's hot, under load, between 3-4.5k rpm. If I let off the throttle, it goes away instantly. Best I can describe it is a louder version of the typical Nighthawk burbley growl (roar?) you get on acceleration. I've had another Nighthawk in the past, and I don't remember it making this noise so loudly. It does sound like it's coming from the intake, not exhaust, and it's not a ticking/knocking/metallic noise at all. Definitely not the typical cam chain noise. I'm going to try to record audio/video of it soon and add that to this thread. Anyone have any ideas? My first inclinations are carb sync or some sort of airbox or filter issue. The filter is clean and intact and carbs feel synced, although I've not verified that with a manometer.

Thanks for your time!

[Gene]

 Have you checked inside the airbox?

[Bob H]

If I let off the throttle, it goes away instantly. ...growl (roar?) you get on acceleration.

Get the rear wheel off the ground, slowly rotate the wheel (by hand) and watch the bottom run of chain carefully for any links that don't straighten out.

Get a rag and hold the chain in the middle of the run, and push it up and down, measure 1 1/4 to 1 1/2" movement. No more, no less.

Now pull on the chain, away from rear sprocket. If you can pull it back at all, more than 1/4 inch the chain is wore out and you are hearing it "climb" up the teeth of the sprocket.

New chain and sprocket will be fairly silent.

[waterskibum]

I have an 83 NH 650

Get a rag and hold the chain in the middle of the run, and push it up and down, measure 1 1/4 to 1 1/2" movement. No more, no less.

Now pull on the chain, away from rear sprocket. If you can pull it back at all, more than 1/4 inch the chain is wore out and you are hearing it "climb" up the teeth of the sprocket.

My chain is smooth, immobile, and oddly shaft-like, and the fluid inside tastes fine to me.  It is certainly an engine-related noise.

[Bob H]

OK. Not sure how you get an odd intake sound unless you modified the stock intake, or the air filter wasn't put back in.

What is the history - you have been restoring it - did you ride it previously or was this a barn find that needed restoration before you could just now ride it and hear this sound?

I mentioned the chain because a bad chain can sound like a "growl"

[mollusc]

The '83 650 doesn't have a chain -- it's a shaft drive.

[mollusc]

I wonder if it could be cavitation in the oil pump?  I've never heard of it happening on a Nighthawk but it does sometimes happen on other engines if the pump lobes or outer ring fracture.

[Gene]

I have a 85 650, and I notice no such noise. Try removing the air filter, and see if that makes a difference. Just a note, you probably have the same filter as I do, the original is NLA. I bought a aftermarket filter, and it's foam, which means I had to rejet it. The "carbjetkits" site was very helpful. Have you checked the boots between the carbs and airbox? One or more may be loose, and leaking.

[Bob H]

The '83 650 doesn't have a chain -- it's a shaft drive.

Oh... I get the dumbass of the week award. Jeeeze.

[mollusc]

I bought a aftermarket filter, and it's foam, which means I had to rejet it.

Do you oil the filter?  If you drop them in dry, they often mess up the mixture but oiled they are usually pretty close to the specs for the OEM paper.

[Gene]

 I oiled my filter. I went for a long ride today on my favorite country road, (that means fast) and when I open my 650 up it does exhibit some intake noise, kind of a moan. The rest of the time, all I can hear is the exhaust and the timing chain. (No problem, that's normal)

[draser]

Read the post and can't think of anything related to intake growling.. however, you'll get similar "growling" sounds from the speedo mechanism (dry grease) and front wheel bearings - both noises will come from front of bike and be rpm related (maybe). I'd disconnect speedo cable and go for a ride. For wheel bearings... I'd suspend bike and turn wheel and check for side movement. All easy checks.

[waterskibum]

Update!

I recorded the sound with my phone with some success. See YouTube link below. I'm convinced that it's coming from the engine itself at this point. It sounds like valves perhaps? I'll leave it to y'all to listen and tell me if it's normal or the sign of a problem.

https://youtu.be/bjyfyg_ktUU

[waterskibum]

I think I can hear the same noise as the guy accelerates in this video, right there from 3.5-5k. Video starts at the time stamp where I can hear it.

https://youtu.be/ESu8ug_jn5I?t=105

Maybe this is just a Nighthawk thing?

[waterskibum]

Anyone have any ideas? Rode it about 180 miles today and it made the noise the whole time (under the conditions described above) but didn't blow up. I'm tempted to check rod bearing by pressing on the piston w/ a screwdriver... and/or dropping the pan to check for metal. Did an oil change a few days ago and found very minimal tiny metal shavings in the filter housing. This was the first change following a new valve cover gasket, so I assume some of it may have been from getting the gasket off.

Does that audio sound like a bearing? Valves? Nothing? Thanks again for your time.