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Author Topic: Revs hang at 3k  (Read 255 times)

Offline Cbb129

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Revs hang at 3k
« on: December 03, 2017, 05:47:19 PM »
When I rev my '83 650 Nighthawk, it hangs for a couple seconds at around 3k when the revs come down.  Did a full cleaning of the carbs with all new jets, and checked for leaks where they mount to the cylinder and there were none.  Bike starts and runs great, just has this annoying little hang when the revs come down.  Anyone else experience this, or have any input ? 

Offline Larry Fine

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Re: Revs hang at 3k
« Reply #1 on: December 03, 2017, 07:12:24 PM »
Lube the throttle cable, and make sure the carb linkages move freely, so they close instantly. If your bike has two cables, you should be able to close the throttles right away.

Offline Cbb129

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Re: Revs hang at 3k
« Reply #2 on: December 03, 2017, 07:56:35 PM »
Also checked that, throttle snaps back to closed perfectly.

Offline MrF

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Re: Revs hang at 3k
« Reply #3 on: December 03, 2017, 08:39:36 PM »
Did you change jet sizes with your new jets?

Your initial thought of a vacuum leak is a good start.  How did you check for leaks? 

If you completely rule out a vacuum leak, my next thought would be that your idle circuit is too rich and someone 'fixed' it by adjusting the idle down instead of setting the proper mixture with the fuel screws (or air screws.. I'm not that familiar with the carbs on your bike).  I'd check the idle mixture setting off of what your service manual recommends next. 

If that doesn't work, a plug chop might be your best bet to figure out where your mixture is out of whack, but that's a big pain to do.
1997 Nighthawk 750 (Sold) |  2016 V-Strom 650

Offline Cbb129

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Re: Revs hang at 3k
« Reply #4 on: December 04, 2017, 07:18:03 AM »
Did you change jet sizes with your new jets?

Your initial thought of a vacuum leak is a good start.  How did you check for leaks? 

If you completely rule out a vacuum leak, my next thought would be that your idle circuit is too rich and someone 'fixed' it by adjusting the idle down instead of setting the proper mixture with the fuel screws (or air screws.. I'm not that familiar with the carbs on your bike).  I'd check the idle mixture setting off of what your service manual recommends next. 

If that doesn't work, a plug chop might be your best bet to figure out where your mixture is out of whack, but that's a big pain to do.

Jet sizes are all stock.  Checked for leak using the method of spraying carb cleaner where they connect to the cylinder.  Fuel screws set at stock 2 5/8 turns out per manual.  It's really frustrating because bike starts and runs great, plugs are good color (not rich).  I just don't get why it hangs at 3k.   bh1

Offline MrF

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Re: Revs hang at 3k
« Reply #5 on: December 04, 2017, 06:44:07 PM »
Well, you’ve covered just about everything I would have.  I didn’t mean to be too pedantic... it’s hard to tell someone’s experience level over the internet.

Have you checked float height?  If they’re not adjustable, have you checked them for leaks?


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1997 Nighthawk 750 (Sold) |  2016 V-Strom 650

Offline Cbb129

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Re: Revs hang at 3k
« Reply #6 on: December 04, 2017, 07:13:23 PM »
Well, you’ve covered just about everything I would have.  I didn’t mean to be too pedantic... it’s hard to tell someone’s experience level over the internet.

Have you checked float height?  If they’re not adjustable, have you checked them for leaks?


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No worries man, I appreciate the input.  I checked the floats and everything internal when I tore the carbs down.  Only thing I can think of is that the rubber where they mount to the cylinder is old as they're original and there may be a very slight leak somewhere.  There is an available reproduction set you can get for $50. 

Offline MrF

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Re: Revs hang at 3k
« Reply #7 on: December 04, 2017, 07:59:40 PM »
Yeah, it's possible that the carb cleaner may have missed something on your leak test.  I've always used starter fluid since I know what it'll do to the engine.  Some brands of carb cleaner might not give you the RPM rise you expect.  You could always squirt a bit down the intake to see what it does.

You might try a carb sync if you haven't done one since you pulled them off last. 

Do you notice any change in the hangup as the bike warms up?
1997 Nighthawk 750 (Sold) |  2016 V-Strom 650

Offline Cbb129

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Re: Revs hang at 3k
« Reply #8 on: December 05, 2017, 07:41:42 AM »
Yeah, it's possible that the carb cleaner may have missed something on your leak test.  I've always used starter fluid since I know what it'll do to the engine.  Some brands of carb cleaner might not give you the RPM rise you expect.  You could always squirt a bit down the intake to see what it does.

You might try a carb sync if you haven't done one since you pulled them off last. 

Do you notice any change in the hangup as the bike warms up?

It actually gets worse the warmer the engine is.  Revs fine when cold, but once she warms up the hang at 3k shows up.  It's only for 2-3 seconds, then she settles down to idle. 

Offline mollusc

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Re: Revs hang at 3k
« Reply #9 on: December 05, 2017, 08:07:02 AM »
Are the slides hanging up inside?
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Offline MrF

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Re: Revs hang at 3k
« Reply #10 on: December 05, 2017, 08:27:29 PM »
Getting worse as it gets warm would indicate something is rich... if it's mixture related.

Are the slides hanging up inside?

^ Also a good thought.
1997 Nighthawk 750 (Sold) |  2016 V-Strom 650

Offline Cbb129

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Re: Revs hang at 3k
« Reply #11 on: December 11, 2017, 05:48:51 PM »
Slides were moving freely when I rebuilt / re-assembled them.  How can I tell if a slide is sticking other than taking them apart again (which I really don't wanna do). 

Offline Tellz50

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Re: Revs hang at 3k
« Reply #12 on: December 13, 2017, 02:31:38 PM »
With this trick you won't have to disassemble the carbs, but they will need to come off the bike.
1) Remove the carb rack and drain the float bowls
2) Hook a shop vac up to the engine side of each individual carb (While on) then open the throttle plate and observe the slide's upward movement.
3) Close the throttle plate and observe the slide's downward movement.
4) All slides should have pretty much the exact same upward and downward movement, so if any one of them exhibits something different (Ex. Very little or no movement upwards when throttle plate is open or very quick/slowwer downward movement when throttle plate is closed) you may have identified a problem with the slide, slide diaphragm or slide bore specific to that carb.

I would actually perform a carb sync first, as someone had mentioned already, then the above and if still not fixed perform all your carb cleaning, assembly and setup all over again (throttle cable routing and action checked 3 times over).

Hope this helps!
1985 CB700SC
2003 Kawasaki ZRX
2000 Harley FLHTPI

Offline ExTex

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Re: Revs hang at 3k
« Reply #13 on: December 13, 2017, 02:56:25 PM »
Good Idea, Tellz50

But less mess if draining the float bowls before removing the carbs.
I have a pieces of clear tubing that fits over the drain nipple, makes draining into a glass container easy.

Merry Christmas
Sold:1983 CB650sc & VT500c & VF700S

 

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