82 CB650SC Nighthawk Idle Mixture Screw Setting

[Flarrow13]

Hi all, I have an 82 CB650SC Nighthawk and am currently rebuilding the carbs. I was wondering if anyone knows how many turns out I should set my idle mixture screws at? Any help would be appreciated.

[mollusc]

I don't have info for your bike on the datasheet that I have (see the first post on https://nighthawkforums.com/index.php?topic=40.0) but it looks like stock for almost all Nighthawks is 2 to 2 5/8 turns out from lightly seated.

[Pete in PA]

Too late to count yourself where it's set at???

[Flarrow13]

Yes, but I think where the previous owner had them set was way too far off. When I bought the bike, it ran horribly, and where the screws were set at, the bike was running way too rich.

[Bob H]

...idle mixture screws

I am of the opinion that an idle mixture screw is NOT going to make a bike run horribly, and run way too rich.

We are talking about a very, very small passage right in front of the throttle plate that controls fuel mixture when the throttle is closed.

Yes, it contributes to the overall fuel mixture as the other jets start to contribute when the throttle is opened up, but comparatively it is negligible. Your bike was running bad for other reasons. Could be a bad spark plug lead, prior owner put wrong jets in, who knows.

In a perfect world, you take the used carburetor and gently screw the idle mixture closed, and see how many turns it takes. Did you do that? if so what was the result?

I did mine at 3 and believe it is too far, should have been more like 2 1/2 but the bike idles fine and runs fantastic (1993 NH 750). I know if I backed the idle mix WAY OUT it would be a rougher idle, but once rolling I don't think the bike would run horribly.

My idle screws were set at about 2 1/2 (maybe 2 5/8 - around there) before I rebuilt my carbs. I was going off advice of another mechanic to try 3. If they weren't so damn hard to reach i would do the idle drop test - but for now I am leaving them at 3.

[Pete in PA]

I think it affects more than you think.

For high altitude (6500 ft and higher) adjustment all you do is turn that screw in 3/4 turn.

You also spend a lot of time at under 1/8 throttle going down the hiway at 55-70 mph. There you're on the pilot circuit for a large percentage. Careful tuning will help mpg.

[Bob H]

Perhaps. Certainly the idle screw is going to affect idle big time.
Going down the highway at 55-70 mph on a Constant Velocity carb is going to involve a lot of other circuits with the diaphragm lifting the needle out of the other jets even if the throttle butterfly isn't open very far.

Although it is possible the prior owner opened the idle screw setting too far - the most common thing I see with old motorcycles is the idle circuit clogged up. It's one of the first fatalities when a bike get stored and fuel evaporates. The resulting residue may even persist despite attempts to clean the carb if people don't take the time to blow out every passage and confirm it is clear.

[Bob H]

Here's my idle screw after cutting off the tab that limits rotation



When I did my carbs last year, I saw an aftermarket idle screw with big thumb knob that said it would fit our carbs but it was like $50 each and I had already spent a ton on new vacuum diaphragms etc.

I just did a new search and not sure if this one fits but looks like they are coming down in price.
I thought about modify my stock screws (put something on the end, like an extension) but I was already juggling lot of stuf.


https://deadbeatcustoms.com/all-balls-racing-extended-fuel-mixture-screw-for-keihin-40mm-cv-carbs/

Same retailer has one says fits all keihen carbs for $37 each. The end of the screw looks very similar to the stock one - seems like as long as it bottoms out and you can back out the 2 or 3 turns needed it should work.
https://deadbeatcustoms.com/harddrive-pro-carb-rebuild-kit-mixture-screw-fits-keihin-cv-carbs/