motor RPMs high when idling

Kamalazooz3

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So my motors RPMs spike out of the blue when idling and I feel like the motor is going to blow up and sometimes its normal it high idles mostly when I'm going ~20mph, I've tested for a vacuum leak with WD40 but can't seem to find one and the spark plug is BLACK even though I'm using 32:1
 

weefek

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Did you try checking the main crank seals for a leak?

How many miles on the engine?
 

Chainlube

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So my motors RPMs spike out of the blue when idling and I feel like the motor is going to blow up and sometimes its normal it high idles mostly when I'm going ~20mph, I've tested for a vacuum leak with WD40 but can't seem to find one and the spark plug is BLACK even though I'm using 32:1
Does the throttle cable have any tension on it when the bike is not running? It should be a little loose in the connector.
 

2speedboi

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Did you check ALL the bolts before firing up? Loose bolt along any vacuum sensitive parts will result in this.

Could be a bad bearing seal acting up.. note yourself If it gets more erratic
 

Chainlube

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Also. Correct grade of fuel?
Actually the correct grade of gas for these little engines is regular 87, but with the ethanol removed or ethanol free. Most everybody buys higher octane fuel because it doesn't have ethanol, but lately, unless you buy the highest octane fuel, there's ethanol in it.
94 is for high compression, high performance engines, which none of these are. I don't see anyone milling the jug and head enough, and still have running engine, to make more than 8:1, or even 7:1. Where as premium fuel needs compression ratios in the 9s or higher. It just doesn't burn fast enough.
 
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