Just wondering if anybody could let me know if a bike with a big bore needs to run richer jetting? I ask because ive got a yz250 bored out and i took apart the carb today to clean it and the former owner has a 65 pilot and my local dealership says it should use a 45 stock. The bike doesnt run right at all and the other day i forgot to turn the gas on and right before it ran out it ran great! Im gonna take a look and see what the main is but do you think i should run the stock jetting? Its completely stock other than the big bore.
Does big bore need richer jets
- Thread starter ldub128
- Start date
dirt bike dave
Sponsoring Member
- May 3, 2000
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FWIW, the only big bore I ever had ran much better with stock jetting than the massively overly rich jetting recommended by the big bore maker (LA Sleeve).
The bike in question was an '84 KDX200, bored to a 240.
Think of it this way - the big bore sucks in harder and gets more air and fuel through the same size jet. This greatly reduces the need for a bigger jet.
The bike in question was an '84 KDX200, bored to a 240.
Think of it this way - the big bore sucks in harder and gets more air and fuel through the same size jet. This greatly reduces the need for a bigger jet.
The big bore probably does not want stock jetting, but it doesn't sound like what you have in there is working either. Yes, most likely the jets will be larger than stock by some amount. Use what you have as a baseline, and jet it. There are plenty of good guides around to help you along the way. It's not a guessing game, don't just slap way different jets in it to see how it runs. That's usually a very bad idea...
*edit*
Dave beat me to it. Good post too! I do think the amount of air it can take on should increase more in comparison to the larger amount of fuel it can pull through the jets. IMO larger jets should be needed, but perhaps not as large as one might think.
*edit*
Dave beat me to it. Good post too! I do think the amount of air it can take on should increase more in comparison to the larger amount of fuel it can pull through the jets. IMO larger jets should be needed, but perhaps not as large as one might think.
I have jetted a 134 and a 265?, both EG's, both ran great on stock jetting for a baseline. Use oem jets also.
it is called volumetric efficiency.
True with addition of pipe, porting etc. Many times you end up a bit leaner due to the more efficient engine. But start at stock and work up slowly doing a few plug chops. Easier to run a bit rich than lean and seize the motor.
Also if the previous owner used a different oil mixture ratio, it may be jetted for that. LIke a 24:1 mixture or 20:1
True with addition of pipe, porting etc. Many times you end up a bit leaner due to the more efficient engine. But start at stock and work up slowly doing a few plug chops. Easier to run a bit rich than lean and seize the motor.
Also if the previous owner used a different oil mixture ratio, it may be jetted for that. LIke a 24:1 mixture or 20:1
