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What causes these engines not to start when hot

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Dam engine won't start for shit when its hot. Just wondering the main cause? I have already ordered a new CP piston and plan on checking the head when i tear down but was wondering what people see as the main issue?

The only way to start is to hold the gas wide open and keep kicking until it cranks over. This gets very tiring and I dont have the energy for it anymore :smirk:

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Valves not adjusted properly. Sounds like you're losing compression.

I think compression is gone thats why i went ahead and ordered the piston

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Spounds like you are too rigch on the pilot. A piston or valve problem would make cold starts harder, not hot starts.

I bet you richened your pilot circuit to deal with decel popping or a bog.

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.

As far as the bog goes, properly setup the AP

If you have decel popping, you have a race bike, not a play bike and expect to live with it.

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I have a 45 pilot and a stage 1 jet kit just recently done. Also did a quickshot 3 and fuel mixture screw

Valves were checked probably 10-15 hours ago and they were spot on, as mentioned I dont think its valves because it will start cold with 1 kick.

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Your pilot sounds about right. What size main jet is in there? Is your hot start working properly? What year is the engine? From your signature its a 250f in a 150r frame?

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Your pilot sounds about right. What size main jet is in there? Is your hot start working properly? What year is the engine? From your signature its a 250f in a 150r frame?

Yes its a 06 250f in a 08 crf150r frame thanks to Service Honda.

I bought the motor sight unseen and with unknown hours. The main jet is dyno jets recommendation for under 3000'. I'm at about 700ft above sea level.

The hot start is working.

Just going to do the top end and check the valves at the same time. Just have to yank the motor out because its way tight to do it in my frame.

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Do the test EXACTLY as I wrote it. Stop guessing.

My mechanic did that when we installed the 42...he recommended a 45...I installed 45 and set the fuel screw like you mentioned. Still sucks!

Ill update once the top end is off on what we find.

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Then the starting issue is the rider, not the bike.

Do not fan the kicker, a slow, steadly increasing effort press, not rapid stomping. If after two presses it does not start, engage the hot start.

Every bike I set up, takes two easy kicks cold, one hot to start. Every time. These are tiny engines, you can use your hand to start them. Even a Harley is a easy start. 90 pound girls can do it no problem.

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So you are saying in your instruction to turn the Fuel Screw to obtain the maximum RPM and retun the idle. Would an ambient temp from 75 to 90 degrees effect this adjustment much?

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Read the instructions, follow them exactly. If they are not clear, read them several times. Understand the concept before applying.

Every 10 degrees does make a little difference.

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Got it ! Meter the mixture at the lowest idle setting with a stable idle within the 3 to 0 turn parameters. Makes better sense than the higher RPM technique. Much finer calibration. The lower the stable idle a mixture can be made, the more reliable and easier the starting should be, considering everything else is in a good state of tune.

Thanks for the lesson!

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