fuel information

1 reply to this topic
  • kev94mx

Posted December 11, 2000 - 05:48 PM


A 426 racer told me today that he was having problems with his sparkplug fouling. He said he tried all different settings on the carb and nothing helped much. The weather was pretty cold and he usually runs 93 octain pump gas and he thought he would try 87 and the problem went away. He also told me that it runs best on 93 in summertime. Today it was arond 55 degrees and i ran 89 and it ran great. So his idea must work.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • mikeolichney

Posted December 11, 2000 - 06:42 PM


Thats interesting, not what I would expect. The cool air is more dense, which would normally require a higher octane rating than warm, less dense air to prevent detonation. Basically, octane rating is a measure of the fuel's resistance to detonation. Thats why as you go up in elevation (like here in Colorado), the octane rating of the gas sold at the pump is less. The engines don't need the higher octane rated fuel to prevent detonation because the air is less dense up here. (Less oxygen molecules get sucked into the cylinder, hence our bikes don't put out the horsepower you sea-level guys get. Fortunately the 426 has plenty of HP to spare for a guy like me). I have been told by a reliable source (he helped develop the research octane number you hear about) that the lower octane rated fuels actually give off more energy when they burn. He says buying an octane rated fuel considerably in excess of what is needed to prevent detonation is counter-productive to developing maximum power. What high octane fuel lets you do is raise the engine's compression ratio (by milling your head or using a thinner head gasket) without causing detonation. Also, pump fuels are notoriously inconsistent. The efffects you are seeing may have nothing to do with octane rating. Fuels have oxygenating additions like MTBE and ethanol which can cause problems regardless of octane rating. I avoid these fuels like the plague. I use pump gas, but I buy it in a nearby county that does not require that oxygenated fuel be sold in the pump.

Related Content


Yamaha YZ450F 2017 by Chris.GVS

Yamaha YZ450F 2017
  • * * * * *
  • 1 review
  • Last review by Tommyk_55
  • On January 15, 2017

Megabomb Fitment by 288yz450

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 1 reply

Should I buy this bike? help....... by Numsa Hul

Dirt Bike   General Dirt Bike Forums   What bike should I buy?
  • * * * * *
  • 10 replies

Snake pit oct 30th by The Anvil

Dirt Bike   Dirt Bike Regional Discussion   California
  • Hot  293 replies

BIGBORE FOR YZ450F 2006? by Taattie94

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 5 replies

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.