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AP timing screw and idle

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I have a small slight hanging idle on 09 CRF 250 R. It's not all the time, and not consistent, but sometimes when you chop the throttle it doesn't quite die back down to the set idle. I try playing with fuel screw and idle screw combinations throughout the day, and sometimes have it right - ride for a little more and it will start doing it again. :bonk:

My question is: I have heard that adjusting the AP timing screw a half turn can make a difference to this as it reduces the volume of fuel in AP chamber? I presume the theory is that on small throttle openings the AP is dumping a little too much fuel in and the hanging idle is excess fuel being burnt off?

Correct or not?

Bike is standard,valves in spec etc. Has enduro Leo Vince silencer and Leo Vince lowboy torque header. Was dyno'd and jetted, running half clip leaner needle on third clip, 175 main, 42.5 pilot 55 leak and fs at 1 3/4

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AP is for acceleration only. Will do nothing for a hanging idle.

Assuming all it good with your carb, sometimes opening the fuel screw slightly and/or dropping the idle just a tad is all that is needed. In some cases, a larger pilot jet is called for (or a least a clean/fresh one).

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AP is for acceleration only. Will do nothing for a hanging idle.

Assuming all it good with your carb, sometimes opening the fuel screw slightly and/or dropping the idle just a tad is all that is needed. In some cases, a larger pilot jet is called for (or a least a clean/fresh one).

Thanks for your reply.

Would going up on the pilot jet not go against the recommendations of the dyno? I watched the air/fuel ratio on it when it had a 45 in there and it was reading too rich - which is why a 42.5 was installed. I have tried lots of combinations of increase/decrease idle and/or opening/closing fuel screw and it provides little help for long! Do you think the TPS could be playing up?

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Correct. You may of been better off witht eh 42 and having the fuel screw opened further. Theroretically, the 42 at 2.5 turns on the fuel screw is supposed to be the same as a 45 at about 1/5 turn. This assumes both jets are brand new. Over time, a jet can accumulate a film of varnish on it, does not take much. This can affect things. You might want to buy a new 42 and try it.

TPS adjustment should be checked and assuming your neutral sensor is still connected, should assist is lowering the idle.

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thanks. Have already done the pilot circuit setting test and can confirm that the 42.5 is the correct one according to this test. Should I try a new 42.5 or try another 45?

I forgot to add that in terms of starting the bike is usually good - between 3 and 5 kicks. However, very occasionally it will be stubborn for not apparent reason and take 567307 kicks! For example, Sunday. 8 or 9 kicks from cold (choke on and a twist of throttle to prime) temperature was cold. Day was riding on and off for 20 mins at a time until end of day. At about 3rd or 4th check after about a 15 minute wait for my time, I go to start the thing and it decides not to start. Had to be at least 50 kicks and several frustrated minutes! Rest of day though it was between 3 and 8 kicks I guess to start.

Does this suggest anything? Valves are all good....

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Sorry yes I meant a 42.

I have tried turning the idle down but then it makes the idle too low and prone to stalling.

The idle it is set at is perfect on the 1600-1800 mark, it's just that hangs sometimes/oftentimes before it gets there.

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Did try removing the close cable but made no difference. Did reduce the amount of drag though - quite surprising!

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checked the tps today - it within spec. The handbook I have is a clymer for 04-06 bikes (mine is 09) and it said resistance should be 4000-6000 - mine read 4800.

Cleaned the pilot jet and it's mounting position thoroughly with carb cleaner and compressed air and re-installed. Started bike and still doing it. Tried playing with fuel screw and hanging idle did seem to get less the further I opened it - I got as far as 3 turns and then installed a clean 45 and restarted at 1.5 turns on fuel screw. Got as far as 2 turns and it certainly seemed the best it's been so far.

I am now wondering if the drop in the temperature could be causing this? When bike was dyno'd and set-up temperature was 19 - 20 °C, but average temperatures have now dropped to around 12 - 13 °C as winter approaches. I am wondering whether to reinstall the 42 and change needle out for a 1/2clip richer one?

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Do not wonder. Do the test Eddie refered to in the link he provided. Here, this is the same thing in a more verbose presentation:

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 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 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 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 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 1,800 rpm, but go by the spec in your manual.

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Ok I give up - someone send me the nail to stick in this coffin!

Tried all of the above and went off riding today. Nothing solved it and to top it off the bike is getting worse and worse to start hot/cold (mainly hot though) and will quite often cut out at lower speeds. I tried every combination of needles/needle position/pilots/fuel screw/idle screw combinations and just when I think I;m getting somewhere - nope it does it again and continues to refuse to start! Is this even a jetting issue? Am I barking up the wrong tree!?

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Yes, you are changing and messing with too many things. Slow down

Your valves are good, right?

You hot start moves freely and has cable freeplay, right?

Your chock is in good condition, right?

If you had or have AIS, there are no residual vacuum leaks, right?

Your spark plug is new, right?

First, follow the procedure I mentioned above for selecting the correct pilot jet. If the jet is not in like new condition, get a new one. Once you have the correct pilot jet and the fuel screw set. LEAVE THEM ALONE.

Next, select a main jet that works, then LEAVE IT ALONE.

Next, work with the needle. Many have had good luck with the NCYS needle though if you have the JD kit, use his needle then LEAVE IT ALONE.

Now and only now, mess with the AP. I assume you have confirmed you are gettinjg a squirt. You oring'd or installed a stiffer AP spring. You installed a #40~#60 leak jet. You slowly tightened the AP timing screw, turning 1/2 turn and going for a ride.

When starting a bike, a cold start uses the pilot, fuel screw and choke. A hot start does not use the choke and may if the engine is really hot, need the hot start. No throttle whatsoever.

When explaining issues, be very clear. This post started out about an idle problem and setting the AP. You have migrated it to a starting problem.

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Hard starting is some combo quite a ways off from correct of;

weak spark ( very doubtful with modern ignition)

Insuficent fuel (I had one recently that had an obstruction in the idle circut,we kicked our brains out)

Too much fuel (when it does finally start it will be sluggish for a second)

Down on compression! It has to have pretty reasonable compression to have sufficent vacuum to draw in fuel. So how much compression does it have?

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Quick update - I've taken bike to shop to get sorted - I havn't the time for this!

The suggestions made have been noted and I shall update on what outcome is.

thanks (and sorry for going off topic)

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