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hi all,

i have a 2007 Honda crf150rb that has an unsolvable hanging idle. bike has full fmf exhaust (powerbomb header and q4 muffler with spark arrestor taken out. you can turn the idle screw up and it will sit there and idle great but as soon as you touch the throttle the idle will hang very high and wont come back down unless i tap the kill switch a couple times.  I have replaced the vacuum plate seal and installed in correctly and it did absolutely nothing.  carb has been cleaned about 4 times before that. Jets are 135 main, 48 pilot and the others are stock. The throttle cables are not binding up i have checked this. I have tried the fuel screw turned really in really lean and turned out really rich. it made little difference except making it almost impossible to start. The screw is currently set at the factory specified 2 turns out. I have set and tuned the throttle position sensor to exactly what it should be. I've checked all around the engine for air leaks and have found none. Other than these issues the bike starts first kick and runs great. I feel like I've tried everything there is to try. Any help would be greatly appreciated.

 

Thanks 

Luke

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It can be a delicate balance of idle speed and fuel screw sometimes. A fully heatede bike, I tiny adjustment, a short ride and check for the bike returning to a proper idle. You cannot just leave it stand in the driveway as you fiddle with the fuel screw and idle speed. Stock pilot jet for you is supposed to a #40, I cannot imagine why you went to a #48, that is way too large unless you were riding on the polar ice cap.

Fuel Screw/Pilot Jet
Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.
Gently turn the fuel screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle knob to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly).  Once warmed, slow the idle speed (knob) to the lowest possible speed.
*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***
Turn the fuel screw in until the idle becomes rough or the bike stalls.
if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.
If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.
Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.
If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.
If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.
If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,850 rpm, but go by the spec in your manual.

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It can be a delicate balance of idle speed and fuel screw sometimes. A fully heatede bike, I tiny adjustment, a short ride and check for the bike returning to a proper idle. You cannot just leave it stand in the driveway as you fiddle with the fuel screw and idle speed. Stock pilot jet for you is supposed to a #40, I cannot imagine why you went to a #48, that is way too large unless you were riding on the polar ice cap.
Fuel Screw/Pilot Jet
Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.
Gently turn the fuel screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle knob to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly).  Once warmed, slow the idle speed (knob) to the lowest possible speed.
*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***
Turn the fuel screw in until the idle becomes rough or the bike stalls.
if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.
If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.
Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.
If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.
If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.
If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,850 rpm, but go by the spec in your manual.

Thank you I'll try it. I went to a 48 pilot because the bike had a terrible low end bog and had trouble getting off idle

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2 minutes ago, luke8500 said:


Thank you I'll try it. I went to a 48 pilot because the bike had a terrible low end bog and had trouble getting off idle

Bog is resolved by proper AP setup. AFTER the correct pilot is installed. You have a race engine and it will bog off of idle it you nail it. You must be 3,000 rpm or higher before you hit the gas hard.

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Bog is resolved by proper AP setup. AFTER the correct pilot is installed. You have a race engine and it will bog off of idle it you nail it. You must be 3,000 rpm or higher before you hit the gas hard.

It would die idling on the stand as soon as you touched the throttle. The 48 pilot solved that

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Pilot jet and fuel; screw are for idle ONLY. AP is for bogs. You also have to have ht eidle speed set to appx. 1,850 rpm, even though the engine can idle lower.

If you try to tune by adjusting carb circuits for what they are not intended for, you will never get it to run right.

You cannot use electrical conduit to run water though your house.

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William is right. I think you are attempting to tune past other issues with the pilot and screw. If you reduce your leak jet size and set your ap timing correctly, I think you may find yourself going down a couple sizes on that pilot. Of course we assume there are zero air leaks and your valve clearance is proper.

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I have this same exact problem and if I turn both idle and fuel screw down low I can get rid of the hanging idle but it idle at all. I’ve gone through the carb and bought a brand new carb and gone through that one. I have a 42 pilot and tried the 45 and 48 and still does the same thing. I’ve spent hours turning the screws in different ways 

Edited by Cameron Craig

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Perhaps check the rubber intake boot for cracks (air leak),

spray some carb cleaner around it while the engine is idling and see if the revs pick up.

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IMO, the 2012+ carb is a big improvement and is much less sensitive than previous models.  We had 2 races at 3000 ft on a rainy day and were tuned for 1000 ft.  Didn't have time to rejet because of work.  The 2012 bike ran fine but the 2008 would not idle once warmed up and DNF'd.  The only time we have ever fouled a plug.

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