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Won't idle

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I have an 08 crf150r. Bought it used last week and just to be safe i wanted to put a top end in it. Put the new top end in today and it wont hold an idle. If i increase the idle screw it idles way to high. As sooner as i back it down it stalls. While it was apart i also raises the pilot jet from a 38 to 42 based on wbat i read in the crf150r section. Bike ran fine before i tore it apart. Could the pilot jet be just too big? While it was apart i forgot to check the valves. Im leaning to the bigger pilot jet being the problem. What else could it be?

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Put the 38 pilot back in and re shimmed the valves, they needed adjustment. Bike holds an idle now but only after i hit the throttle a few times to clean it out otherwise the engine seems to be racing. It will then slowly come down to a normal idle rpm after i hit the throttle. What could it be now, mixture screw?

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Noticed the header pipe had turn blue close to the head after I made a few high rpm runs. That mean I have a lean condition up top?

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You need to fine adjust the fuel screw and /or use a different pilot. These to work together, the fuel screw being a fine adjust. Blue header is normal.

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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Ive done all that and I'm still having the same problem. Tried adjusting the fuel screw with stock 38 pilot and a 42 pilot ( which is what is recommended from other members of this board). Seems to be a little better with the 42 pilot but still not fixing the problem. Is it possible i need an even bigger pilot(the bike is totally stock)? Or maybe the throttle plate seal as I've seen may be a possible cause.

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I've sprayed brake cleaner around where the cylinder meets the bottom end, the cylinder and the head and both sides of the carb and haven't found any air leaks, where else could an air leak be coming from? The bike runs fine and starts very easily, could the timing be off even though it runs and starts fine?

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I've sprayed brake cleaner around where the cylinder meets the bottom end, the cylinder and the head and both sides of the carb and haven't found any air leaks, where else could an air leak be coming from? The bike runs fine and starts very easily, could the timing be off even though it runs and starts fine?

Hot start plunger

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Sprayed carb cleaner around the hot start plunger and choke, no spike in idle. Should I think about replacing the plate and seal on the slide?

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Im still having issues with this bike. When I bought this bike the motor was locked up. I took it apart to find it there was no oil in it. Broke down the top end and found that one of the buckets the goes over the shim and valve spring was stuck in its bore on the cam tower. I replaced both buckets and the cam tower. Everything else looked good to my eye. Put it back together and everything seems good except for this idle hanging. SInce I put it back together I have replaced the hot start cable, plunger, changed pilot jets, mixture settings triple checked timing and valve clearances, replaced the throttle plate seal, checked throttle cables, etc......I have a friend who also has the same bike, we switched out my carb and cdi box and still no changes.

Is it possible that there is an issue with the head? Bent valve or something like that? The only thing that was switched out was the motor itself.

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