Cause of high idle that ebbs down?? - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 12 (permalink) Old 04-21-2008, 07:35 AM Thread Starter
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Question Cause of high idle that ebbs down??

Hi, All - This is a "ping" for a thread that storm16 started a couple of weeks ago. When you crack open the throttle on his bike, the engine revs up nicely....release the throttle and the engine is still revving high. The revolutions only come down in their own sweet time, leaving the rider a little, um, vulnerable to trying to make a turn with the bike zipping along at 4000 rpm in first gear.

Here's what I do know about his baby:
1. We cleaned the carbs and got the choke connecting rod set properly, so his choke works just fine now. Plungers move up and down, and the choke cable has plenty of range.
2. We double-checked that the carbs boots were snugly in place, so there's no vacuum leak from there.
3. The throttle is not sticking - when you release the throttle, the valves snap shut smartly. Even so, the revs will stay ridiculously high.

Some possible culprits, although, Brad, correct me if I'm wrong on these:
1. Carbs still need to be sync'd. We ran outta time for that.
2. Air filters need to be replaced.
3. Cam balancer dampers sound like they need to be replaced.
4. Looks like there may be an oil leak around the rear cylinder head gasket - not major, but there's some drippage there.

My thinking was that the combustion chamber is getting too rich a fuel mixture when the throttle's open - it's almost like once all that stuff's burned off, the idle can come back down to a normal range. But I'm stumped. Anybody got other thoughts on this? Elemental, your certs would come in handy right about now!!

C
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post #2 of 12 (permalink) Old 04-21-2008, 05:01 PM
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Lets bring it to basics and go from there. (Opps, I jsut saw you called for me. )


What makes your engine rev is the introduction of air. You twist the throttle, and more air is let in. You hit the skinny pedal on your car, more air is let in. (I HATE when people call it the gas pedal...It isnt!)

So, one of 2 things is happening.

1)The air inlet on the carb is stuck open for some reason....Dragging Throttle,something in the carb sticking,linkage...something.

Frankly, this is what I am thinking based on the symptoms you are describing.

2) A air leak allowing uncontrolled air into the engine. Like a intake boot leaking..Something letting air in from between the carb and the engine.

Again, the bottom line is a high idle concern is because of too much air coming in. This is true on your car,motorcycle, any 4 Cycle Otto Gasoline engine....

1990 Vulcan
------------------------------
Saturn Plum/55 Tbird red.
Accel Coils.Mangacore wires.Corbin Seat.
Cobra exhaust.Jetted to 1.1 lamda.
Wide levers,cushy grips, All steel lines....
Actually, I've changed more stuff then I can remember.
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post #3 of 12 (permalink) Old 04-21-2008, 09:01 PM Thread Starter
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Really? Too much air, not fuel? Shoot, the solution there is easier than I was thinking. Hey, Storm!!! Where are you??

Thanks, E!
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post #4 of 12 (permalink) Old 04-21-2008, 09:18 PM
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AH shucks! Nothing to it. Can I belong now?

1990 Vulcan
------------------------------
Saturn Plum/55 Tbird red.
Accel Coils.Mangacore wires.Corbin Seat.
Cobra exhaust.Jetted to 1.1 lamda.
Wide levers,cushy grips, All steel lines....
Actually, I've changed more stuff then I can remember.
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post #5 of 12 (permalink) Old 04-22-2008, 07:08 AM Thread Starter
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Quote:
Originally Posted by Elemental View Post
AH shucks! Nothing to it. Can I belong now?
I haven't checked, but did ya post a joke? Of course - we really aren't a clique.

C
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post #6 of 12 (permalink) Old 04-22-2008, 10:12 AM
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Oh, I'm here. I missed the thread initially, but I'm here now. In fact, let me throw another piece into the equation for Elemental...

I took the bike out about a month ago, the week after Cindy and I pulled the carbs. The main thing I noticed (and I complained about this before, but it seems worse now) is that the bike seems underpowered. I was riding on US 211, which is rolling hills. On the levels in 4th or 5th gear, I would be doing 55 with the throttle wide open (this in and of itself is a problem), and I would hit one of the hills...By the time I crested the hill, I would be down to 40 or 45...Again with the throttle cracked wide open.

I'm out of ideas, but am a little leery about getting out in traffic until I can figure out how to get some of my oomph back.

That said, I'm going to go home and check my boots again. I checked them before, even using the carb cleaner approach, and didn't find anything.

Storm16
Culpeper, VA
1985 VN700A1 "WindRider"
21,500 miles (9/07)
R/R relocate (9/07)
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post #7 of 12 (permalink) Old 04-22-2008, 11:08 AM
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No more part darts! Lets work the problem, so to speak.

EVERY drivabilty problem in the history of Modern Engines has come to one or more of the following not being in spec.

1)Air
2)Fuel
3)Spark
4)Timing
5)Compression.

If your engine runs bad, one or more is out of spec. Testing will help you hunt down with one.

Let me refer you to this thread as a primer. See my post at the end.

https://www.vn750.com/forum/showthread.php?t=6559

Then, lets go from there.

1990 Vulcan
------------------------------
Saturn Plum/55 Tbird red.
Accel Coils.Mangacore wires.Corbin Seat.
Cobra exhaust.Jetted to 1.1 lamda.
Wide levers,cushy grips, All steel lines....
Actually, I've changed more stuff then I can remember.
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post #8 of 12 (permalink) Old 04-22-2008, 01:02 PM
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After a carb rebuild this happened to me...

Quote:
The problem went like this. Twist the throttle and you begin to move out, then as you continue to twist the throttle it has less and less effect. As if it is starving for fuel or the throttle is not working.

Well I had a hunch I knew what it was. When rebuilding the carbs I had a problem with the Vacuum piston / diaphragm assembly. The diaphragm did not want to seal properly but I thought I had them in right. Nothing to do but pull the carbs yet again! Well that was part of the problem. Another thing was the spring seat for the needle jet. You need to pay attention to the hole in the bottom of the Vacuum piston / diaphragm assembly, there are two. One is for the needle and the other hole is to allow for air pressure differential, which is what moves the diaphragm up and down. I am not sure I checked this before reinstalling them the first time.

1996 VN 750 (Spark)
Extended Sissybar
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post #9 of 12 (permalink) Old 04-22-2008, 01:43 PM
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Excellent point. Pulling carbs suck so bad that I am confirming the other 4 things before I jump.
(Maybe not timing on a Vulcan..Messy and nothing I can do about it if its off, and it idles and revs good. Likely Ok) but Compression,Spark and air are easy to confirm before pulling those carbs.
God I hate taken carbs out of these bikes.

1990 Vulcan
------------------------------
Saturn Plum/55 Tbird red.
Accel Coils.Mangacore wires.Corbin Seat.
Cobra exhaust.Jetted to 1.1 lamda.
Wide levers,cushy grips, All steel lines....
Actually, I've changed more stuff then I can remember.
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post #10 of 12 (permalink) Old 04-22-2008, 02:28 PM
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Quote:
Originally Posted by Crobins365 View Post
I haven't checked, but did ya post a joke?
Couldnt think of a good one, so I just became a Supporter.

If you cant be a athlete, be a athletic supporter!

1990 Vulcan
------------------------------
Saturn Plum/55 Tbird red.
Accel Coils.Mangacore wires.Corbin Seat.
Cobra exhaust.Jetted to 1.1 lamda.
Wide levers,cushy grips, All steel lines....
Actually, I've changed more stuff then I can remember.
Elemental is offline  
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