High idle when hot!

[Shaliza]

Hello. I understand this is a common issue, but I haven't yet quite found the solution for my bike.

I have a Magician 250 that idles fine when cold/warm. But after 30 min or so of riding, it idles very high. There is a small tear in the hose that connects the airbox to the carb - it happened when I was removing the carb for tuning. But I didn't think it would cause this problem. I will get it patched tomorrow, but in the meantime, I would like to get your opinion.

My bike: Magician 250
Carb: Stock PZ30
Main jet: 110
Pilot jet: stock

[Megadan]

Three questions.

Can you adjust the idle speed down when it is hot? Idle speed should be set with the bike at operating temp (hot).

What is the idle mixture screw set to?

Does the idle hang at all if you give it a little rev or if you come to a stop, or does it immediately return to it's idle speed?

An air leak before the carburetor, such as in the intake tube, will potentially cause a lean condition if enough air gets in. That won't directly affect the idle speed, but can cause a bike to get hung up or idle at a higher RPM. If plugging that hole doesn't change anything, then that makes my first two questions relevant.

[Shaliza]

I re-adjusted the idle screw after I took it for a good ride. It idles fine now when it's hot. On the flip side, it's a little harder to get it started when it's cold. It won't start with the choke off (ambient temperature 75-80F). I will start with the choke on, but I have to keep my hand on the throttle and rev it to keep it from dying.

The air /fuel mixture screw is set to 1.5 turns. I also have the needle shimmed with 2 washers.

Idle hangs a bit when I rev it. But it's not too bad.

[linemanxxlog]

Have you adjusted the valves? If you haven't it won't idle properly until its warmed up.

[Shaliza]

Originally Posted by linemanxxlog

Have you adjusted the valves? If you haven't it won't idle properly until its warmed up.

Maybe this is what I need to do! The bike ran fairly ok out of the box so I didn't think of checking/adjusting the valves. I will do that and report back. Thank you!

[Megadan]

Carbed bikes will never idle quite right when not up to temp. The cold engine doesnt atomize fuel properly so it needs to be richer, thats what the choke is for. You just never want to run with the choke on for more than a minute or so. Nursing the throttle til it gets hotter is also a normal part of the carbed bike life. Once I can get either of my bikes to hold an idle, even if its a bit low, I start riding it. I am just easy on the throttle until they get warmer.

Hanging idle generally indicates a slighly lean condition in the idle mixture. You may have to do a little more adjusting with the mixture screw.

[Rangerscott]

Diffinitely check the valve clearance. I bought a used apollo 250cc that was manufactured in '14. The rocker arms were barely off the valve stems.

[Shaliza]

Valve clearance was indeed the issue. There was no clearance on my Magician. I set the both the intake and exhaust to 0.05 mm.
I previously had to use the choke to get it to start even during warm weather. But now it starts straight up; no choke needed.
Thanks to everybody for the help!

[Rangerscott]

Yuppers. Tight valves can do more harm than loose valves.

[dpl096]

Glad u caught the issue and corrected it prior to a catastrophic failure

[clark]

Wow! I have the same issue with my Magician 250, i.e. idle at cold start would be very high once engine warmed. If low idle set when engine warm, I would have to throttle the engine w/ choke upon start, and hold throttle.
And thus... this AM will be adjusting the valves!!!
Thanks for the diagnosis on the idle issue.
Update...
I adjusted the valves... from no clearance / tight, to .03mm intake and .05mm exhaust.

Amazing difference!

I actually hear the valves "ticking" a little now, vs before... no tick.
The idle is now stable at both cold and hot engine. No choke needed upon cold start.
Previously, there was a slight backfire on deceleration... now no backfire.
Unsure, but power and acceleration may be slightly better also?

I had no idea a minor valve adjustment would be so noticeable.

[Megadan]

Originally Posted by clark

Amazing difference!

I actually hear the valves "ticking" a little now, vs before... no tick.
The idle is now stable at both cold and hot engine. No choke needed upon cold start.
Previously, there was a slight backfire on deceleration... now no backfire.
Unsure, but power and acceleration may be slightly better also?

I had no idea a minor valve adjustment would be so noticeable.

Think about it this way. Metal expands when it gets hot, so the small gap at the valves gives a little room for the valve stems to get a little longer and take up some of the gap, same with the rockers, pushrods, etc. If there is no gap when the engine is cold once it warms up the valves will no longer seat and allow compression to escape out of the intake and exhaust, along with the fuel that was supposed to be in the cylinder. When combustion happens, more of that pressure escapes - often igniting some of the fuel that got pushed out creating backfires and afterfires and generally making the engine less powerful. Now that they stay sealed with the engine hot you get all of the combustion where it is supposed to happen.

[timcosby]

An air leak before the carburetor, such as in the intake tube, will potentially cause a lean condition if enough air gets in. That won't directly affect the idle speed, but can cause a bike to get hung up or idle at a higher RPM. If plugging that hole doesn't change anything, then that makes my first two questions relevant.[/QUOTE]

air leak befor carb will only get you unfiltered air not a lean condition. air leak after card in between enginr and carb will cause lean condition.