Hi idle runs high, under no load - air leak?

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soup325

Member
Dec 11, 2010
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16
Canberra
when the engine is warm, the idle rpm rises to 4000rpm at no load. it also bogs down for a few seconds, when you hit the gas. from a stop.

is this a typical air leak?

so far, the gunson colour tune indicates a nice blue flame.

running a 95:1 fully sen ELF HTX909 castor mix.with 98RON unleaded.

I have tried my best to find air leaks around the carb and intake side of things. I have not inspected the rubber crank seals yet. tonight's job.

i have a 2 stroke, 48cc, china girl. dellorto 16.16 sha carb, with reed valve. running with the ivan and co's modified cdi unit.
 

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
If you haven't found any evidence of a vacuum leak in the intake circuit then the crankshaft seals are the next place to look. Your symptoms are consistent with a leak which is allowing excess air into the engine.

You also want to assure that the throttle is returing to the idle position when released. A sticky throttle cable or a binding throttle slide can give you a fast idle condition too.

Tom
 

soup325

Member
Dec 11, 2010
181
0
16
Canberra
I found the leak in the crank case rubber seal on the magneto side.

It wasn't till I removed magneto itself, that i found the leak.

The magneto must have held some of the pressure in the crank case, to enable the engine to run half decent.

I checked the throttle cable a few weeks ago to make sure it wasn't that. I'm looking forward to my new seals.
 
Last edited:

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
Good for you, Soup.
Use care when installing the seal. Lubricate it and the crankshaft with 2 stroke oil and carefully position the new seal in the bore. Use a socket that is just a hair smaller in diameter than the seal and tap the seal in making sure it goes in square and is bottomed out in the case.
I hope you marked the magneto rotor (magnet) before you removed it. They will go on two ways but only one way is right. Backwards and your engine won't run. With the piston at TDC, top dead center, the shaft key should be at about the one o,clock position.

Tom
 

soup325

Member
Dec 11, 2010
181
0
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Canberra
Just reporting in that all was good in the re-installation of the rubber seal.

thanks, 2door. I did note which way the magneto went. there was burnt rubber on one side of the magneto, so i could not put it back wrong.lol.

I have read many a stories of people putting them in backwards.

anyway, Im back loving my bike now.