Trouble idling after WOT?

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Black_Moons

New Member
Oct 25, 2010
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Hi, I recently made a new foam air filter for my CNS carb and extended the intake manifold from the engine to the carb with 3/4" ID fuel hose to soften the bends and bend off to the side.

Problem is now after going WOT for awhile, when I release the throttle and put in the clutch to stop, it idles at like 4000~6000rpms for 5~10 seconds before returning back to normal idle. It never used to do this...

I guess the carb could be vibrating more now that not hard mounted to the frame. I don't think its the throttle cable.
 
2door

2door

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Sep 15, 2008
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Start looking for air leaks in your intake. This is the most common cause of a fast idle. To confirm it try closing the choke while the idle is high. If it slows immediately then you're sucking air somewhere. Spray some carburetor cleaner on the intake. A change in idle speed will tell you if something is leaking.
Tom
 
Goat Herder

Goat Herder

Gutter Rider
Apr 28, 2008
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Might check around your crank seals too if you can't find it. I would start around the carby first though. Like 2 door said..
 
2door

2door

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Staff member
Sep 15, 2008
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Goat Herder said:
Might check around your crank seals too if you can't find it. I would start around the carby first though. Like 2 door said..
Click to expand...
Goat Herder is right, but I didn't want to jinx or scare you. The crankshaft seals can and do go bad occasionally; usually it is within the first few miles and due to the seals not being seated correctly in the bores. If you find no apparent air leak in the intake circuit try removing the magneto cover, left side, and checking for gasoline/oil residue inside the cover. Same for the right side, primary gear cover. New seals are available from several of the sponsors here. Good luck, let us know what you find.
Tom
 
corgi1

corgi1

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Aug 13, 2009
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Actually being on a rubber extended tube it is probily not viabrating as much as when it is mounted solid ,make sure the carb slide operates totaly free of any binding including the cable ,and is secured to something solid
 
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Black_Moons

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Oct 25, 2010
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Hmm, the odd part is it quites down after a few seconds of high idle back to normal.

I did however happen to be running with the magneto cover off (I know, BAD!) this time, because one of my pull starters springs exploded.

Other thing I can think of is just the new air filter makes the whole thing run too lean due to not enough vacuum... Damn jets I ordered, Hurry up and arrive! (needle is allready all the way up)
 
GearNut

GearNut

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Aug 19, 2009
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Black_Moons said:
Hmm, the odd part is it quites down after a few seconds of high idle back to normal.

I did however happen to be running with the magneto cover off (I know, BAD!) this time, because one of my pull starters springs exploded.

Other thing I can think of is just the new air filter makes the whole thing run too lean due to not enough vacuum... Damn jets I ordered, Hurry up and arrive! (needle is allready all the way up)
Click to expand...
You do in fact have the classic signs of an air leak.

A free flowing air filter will not cause a loss of vacuum, in fact it can increase the vacuum signal inside the carburetor. The engine does not suck fuel out of the carburetor like a person would suck a drink through a straw. It siphons it.
A carburetor works on the Bernoulli principal. The throat of the carburetor narrows in the center (called a venturi) causing the air flowing through it to increase in speed and decrease in pressure. This pressure decrease is what allows the fuel to flow through the fuel jet, needle jet, into the carburetor throat and mix with the incoming air.
The pressure on the fuel that pushes it up through the jets is atmospheric pressure (14.7 psi at sea level). This is why it is important to have a properly vented carburetor float bowl. Without the vent to atmosphere you will not have atmospheric pressure inside the float bowl to push the fuel out.
Also, if the carburetor is not sized properly to the engine it will not have the ability to create the proper pressure drop inside the throat of the carburetor.
Too small and it will restrict air flow, too large and it will restrict pressure drop.

Bernoulli's principle - Wikipedia, the free encyclopedia
Carburetor | Ask.com Encyclopedia
 
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Black_Moons

New Member
Oct 25, 2010
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Canada, Bc
Ahhh k, I do have an air leak... I guess just stuffing the hose into the carb wasent nearly good enough, I assumed the pinhole sized hole at the top of the clamp was not a big deal.. then I realised that gasket inside seals perfectly to the stock intake and not to mine.. Off to make a small adapator I guess. Did'nt think such a tiny leak would matter. Now I see its gotta be 100% sealed.
 
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Black_Moons

New Member
Oct 25, 2010
205
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Canada, Bc
Wow, my new intake is even more sealed then my old one, Judgeing by the fact I had to raise the E clip to keep the thing from smoking like crazy and driping oil out the exhaust. Allways had problems getting it to run rich enough, Now its too rich and I finaly know why :)
 
GearNut

GearNut

Active Member
Aug 19, 2009
5,104
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San Diego, Kaliforgnia
Congratulations on repairing the leak! I hope that it stays sealed up for a good long time for you. As for the too rich condition, it seems like you have a handle on it. May you have good luck with the fine tuning!
 
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