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Old 29th January 2015, 11:29   #1
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Default Honda Stunner: High rpm on throttle input, engine not settling back to idle

To the Mods: Searched similar threads but could not find one. In case one exists, please feel free to move the post. -Thanks

Hello BHPians,

My Honda Stunner has run into a strange problem. All solutions to the classic indications have been tried as a DIY but to no avail. The bike in question is a 2010 Honda Stunner closing 26,000 clicks on the odometer. Apart from regular service, the bike has never needed anything other than fuel to run and is completely stock, except for Fr-tire size which of course does not relate to this problem.

The problem is that on cold start, the RPM runs as high as 2.5K-3K RPM. If I use the idle speed control screw to bring it to about 1.1K, the bike will hold the RPM for a while and very slowly it will start dropping and finally the engine will die. Also, if I give some throttle inputs, either at a standstill or while in motion, the RPM will again rise to 2K+ and if left to idle, will slowly fall to idle and finally stall.

What I have checked and found no problems:

1. Any vacuum leak through rubber hoses.
2. Choke Cable getting stuck.
3. There is a TPS - Throttle position Sensor, though the bike is carbureted. I could not check the sensor but taking our the wire coupler does not have any effect on the engine at any RPM range and the behaviour is same. - Can it be at fault?
4. The carb was cleaned some 2-3K kms ago and I do not expect it to be dirty at all.
5. Spark plug is almost new.
6. The performance is as good as ever, of course, for a 125CC bike.

I am planning to take the bike to HASS and let them do their thing once. Also requesting members to share their diagnostic views on above. Also, it will be worth noting that this behavior did occur in past once or twice but resolved itself within one ride.

Thanks.
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Old 29th January 2015, 11:39   #2
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Default re: Honda Stunner: High rpm on throttle input, engine not settling back to idle

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Originally Posted by saket77 View Post
The problem is that on cold start, the RPM runs as high as 2.5K-3K RPM. If I use the idle speed control screw to bring it to about 1.1K, the bike will hold the RPM for a while and very slowly it will start dropping and finally the engine will die. Also, if I give some throttle inputs, either at a standstill or while in motion, the RPM will again rise to 2K+ and if left to idle, will slowly fall to idle and finally stall.
I ran into a similar issue on my RX and it turned out to be the carb. Not sure how the sensors come into play in your Honda, but another look at the carb may be worth while. If I recall correctly, the issue had to do something with the float on my bike.

Last edited by GTO : 29th January 2015 at 15:13. Reason: Please quote ONLY the relevant bits of a post. Quoting a full, long post inconveniences our mobile readers.
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Old 29th January 2015, 12:10   #3
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Default re: Honda Stunner: High rpm on throttle input, engine not settling back to idle

Points to check:
1. Carburettor slide.
2. Diaphragm.
3. Throttle body.
4. AFR.
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Old 29th January 2015, 12:27   #4
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Default re: Honda Stunner: High rpm on throttle input, engine not settling back to idle

Quote:
Originally Posted by saket77 View Post
To the Mods: Searched similar threads but could not find one. In case one exists, please feel free to move the post. -Thanks

Hello BHPians,

I am planning to take the bike to HASS and let them do their thing once. Also requesting members to share their diagnostic views on above. Also, it will be worth noting that this behavior did occur in past once or twice but resolved itself within one ride.

Thanks.
Hi saket77, check if your plunger/ choke assembly is seating properly. A worn out plunger tip allows more fuel into the carburetor causing the engine to idle high. If the plunger is cable operated, check for stuck / tight cable. And if it does, as a next step check your float height using the clear tube method and see that it is up to the factory specification. Oh and another thing to be checked is that your carburetor's vacuum hoses are not clogged / pinched, the fuel filter is not clogged.

Edit : Oops, I missed the part that the engine dips and dies. So you can exclude the plunger assembly. Does the bike get back to normal when the engine is hot? Then it is the classic lean pilot symptom. Just fiddle with the fuel screw and see if it gets any better.

Sorry for the constant edit /addons. I am multitasking from the Office under the nose of my 8055
regards adrian

Last edited by GTO : 29th January 2015 at 15:15. Reason: Typos
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Old 29th January 2015, 13:52   #5
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Default re: Honda Stunner: High rpm on throttle input, engine not settling back to idle

Thanks guys. I did try to make the mixture little richer by changing the AFR but apparently it did not help. Will give it one more shot before visiting the service center. Until, please let the advice flow.

Thanks.
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Old 29th January 2015, 20:45   #6
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Default Re: Honda Stunner: High rpm on throttle input, engine not settling back to idle

Hi saket77, I though I would share some of my experience with the carburetor in no particular order, which might be helpful to you.
1. A clog in the atmospheric vent of the carburetor results in improper feed of fuel to the jets mimicking a lean condition.
2. Cleaning the carburetor passages and jets with carburetor cleaner alone won't suffice. Following it up with a burst of compressed air will ensure that stubborn clogs are expelled.
3. Before seating the carburetor in the rubber boot, always use silicone lubricant spray and seat the carburetor before the lube dries up. This ensure that an air tight seal is acquired.
4. Minor cracks in the rubber boots should never be overlooked.
5. A high float level causes a low fuel level in the float bowl causing a lean condition. A low float level causes a high fuel level in the float bowl mimicking a rich condition and will affect the proper atomization of the fuel. In the latter condition you get fuel in the resonance box of the air filter and in worse condition, fuel reaches the engine diluting the oil.
6. Improperly tightened plunger assembly will drive you crazy when you try to tune the pilot circuit of the carburetor.
7. The same applies if the metallic washer and rubber o-ring of the fuel screw is interchanged in position. The correct order being -spring -> metallic washer -> o-ring.
8. I don't know about the Hondas, but in the Royal Enfields, there is an o-ring in between the rubber boot and the intake manifold. Over time, the O-ring hardenes and looses the sealing property causing air to leak in through the rubber boot intake manifold joint sabotaging the air fuel ratio.
9. Before tuning the carburetor, the condition of the air filter and vacuum lines are to be inspected.
10. Air leaks from the exhaust manifold as well as bend pipe-muffler joint should be eliminated.
11. It is always good to experiment with the fuel screw adjustment in both direction. Some times the idle hangs due to a lean condition and some times the idle stays high due to a rich condition.
regards adrian

Last edited by adrian : 29th January 2015 at 20:51.
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Old 29th January 2015, 20:51   #7
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Default Re: Honda Stunner: High rpm on throttle input, engine not settling back to idle

From what I understand, in a cold start, you use choke to start a carb vehicle. RPM will shoot up of course till optimum temperature is reached.

Once idle temperature is reached, it will go back to its normal revs. IMO, you do not need to set it via idle screw.

Or am I not understanding your query?
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Old 29th January 2015, 23:01   #8
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Default Re: Honda Stunner: High rpm on throttle input, engine not settling back to idle

Adrian, thanks a lot for your detailed advice. I will check all the points that you mentioned in your post and report back. I am suspecting it to be a case of lean AFR. And to make matters worse, the fuel screw in stunner is really tough to reach out due to its location. But Really appreciate your time and the points you mentioned will go long way in my troubleshooting.

Sheel, at times I do use choke for a cold start. RPM obviously will be high at that time. The problem is that regardless of the use of choke, engine RPM is high during all cold starts. Leaving the bike for a while in the same condition slowly sees the RPM falling and ultimately the engine stalls as rpm falls to minimum. Any throttle input will again raise the rpm for a while and then it will start to fall very slowly again, just to stall after a while.

Thanks guys!
Saket.
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Old 29th January 2015, 23:48   #9
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Default Re: Honda Stunner: High rpm on throttle input, engine not settling back to idle

I had similar issue with my Suzuki Zeus ( which I have sold off ) and the problem was traced to a sticky piston slider assembly in the carb. My concerns started with symptoms which you are facing. Though my bike never had a comfortable idle, during summers it did perform reasonably. But in winters and rainy season, it was very irritating. Adjusting the idle wont give any result. As adrain has pointed out, check for o-ring problem also.
Honda service center might be having a spare carb with them. Ask them to bring it at your location when your bike engine is cold, swap the carb and see the result. This would be sufficient to pinpoint the problem and any further work with spark plug, air and fuel filter could be avoided.
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Old 30th January 2015, 00:19   #10
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Default Re: Honda Stunner: High rpm on throttle input, engine not settling back to idle

Check the accelerator cable too, since you are mentioning this happens during cold starts, the cable might have worn out.

Also it may have happened that the position of the cable has shifted. I am stating this because ridiculously many times people sit on the parked bike and fiddle with the throttle or levers thereby shifting the throttle assembly or at times loosening the cables.
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Old 2nd February 2015, 12:33   #11
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Default Re: Honda Stunner: High rpm on throttle input, engine not settling back to idle

Thanks a lot friends for the useful advice on the thread. The problem was finally solved yesterday.

Went to HASS in the morning but the way the ASC was crowded, I wouldn't have got my bike until next day. Hence, my next stop was a good FNG. Went to an FNG and he inspected the carb and told me to get a new choke cable from HASS. Went back to HASS and bought a choke cable OTC for Rs. 67/-. Got it fixed at the FNG for Rs. 40/- and within half an hour of this circus, the bike as fine as before; idling smoothly at 1000 RPM. So, the culprit was the choke cable. Cleaned the spark plug as due to bad choke cable, the bike was running rich and it was evident on the spark plug which had deposits of black soot.

Thanks to all for the detailed advice on this thread. It surely will be the place to look at for similar problems.

Regards,
Saket
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