Posts
165
Joined
2/20/2009
Location
Wirtz, VA
US
Edited Date/Time
2/26/2012 5:40pm
What is the best way to keep this bike from detonating? I'm an A rider and the bike has a fmf fatty pipe, stock jetting, 93 pump gas mixed with motul 800 50:1. Some guys just say run race gas and some say just retard the timing. Im trying to keep from running race gas because $9 a gallon is a bit crazy if you ask me. What is the best fix to this problem?
I had YZs for years and tried everything to get them to not detonate, race gas is your best bet, especially at the A class level.
How does the bike idle? A properly jetted MX bike should not idle on the stand w/o the rider goosing it. If it idles smoothly, you should adjust your fuel mixture screw so that it will stall out with no throttle input. Turn the mixture screw in to achieve this setting.
Throttle 1/8th to 1/2. Hold the throttle steady at different positions and listen to the engine. It should pick up cleanly and hold RPM w/o inceasing. If RPMS do not flatten and continue to climb, you are lean on your pilot and main.
Crack the throttle from these positions to fully open and release, the bike should quickly pick up and return to low rpm. If it bogs or hesitates, you are rich. Decrease pilot and main. If it picks up quickly, but hangs on RPM and hesitates to drop RPM for a second or two when you release the throttle, you are lean, decrease pilot and main.
Throttle greater than 1/2. Hold the throttle at positions greater than half. The RPMs should climb and then hold, but not sound like it peaks and then drops RPM. If you feel it drops peak RPM you are lean on the main and/or needle position. At this position, the RPM should remain constant (producing a cackling sound kind of like da-da-da-da-da-da) and then return to normal RPM quickly after shutting throttle off. If it seems to labor to reach peak RPM and does not sound crisp and crackly, you are probably rich on the main and/or needle.
The Shop
You do realize running race/high octane fuel requires a jetting change also?
So why wouldn't he jet the bike correctly to keep it on pump gas instead of running race fuel through his stock engine and jetting to match the race fuel?
Running race fuel is not a bad solution. It's just expensive and still requires jetting changes.
Dialing the jetting in is technically the right way to do it. And I don't know why you would not jet a 2 stroke regardless of how it was running. Maybe i'm just more ambitous and care more about free performance than the average rider...
However, your jetting instructions from above (although great) are A LOT more in depth than 99% of people or going to go, not to mention if your jetting is that precise and you want it to stay that way you have to change/check it almost every time you ride. Especially if the weather changes a lot, or you go up/down in elevation etc. etc.
this condition is talked about at length of on thumpertalk....
In '03 I had a YZ 250, we drove all the way from Idaho to Lake Havasu for the WORCS event. My bike wouldn't run for shit so I took it over to ask the Zip Ty Racing guys for any pointers, the first thing the mechanic did was turn down the idle so the bike wouldn't run unless you were on the throttle. He said there are MANY reasons why 2 strokes shouldn't just sit and idle.
You're welcome
Pit Row
Playing with the timing is a nice tuning method as well. Advance timing requires higher octane, but the motor will have more explosive power off the bottom. Retard the timing and you can get away with less octane and bike wont hit so hard. I would suggest reading up on TT before jumping in head first though.....
Every one of the 50 or so YZ's you've owned came with standard US jetting specs, and yes, they are set to idle because the slide is slightly rasied and the comprising jet setting allows it. How many idiots do you think would try to bring their new YZ back to the service department if it came with race spec jetting and didn't idle worth a shit? All of them...
Yes, i'm aware you can't jet on the stand, but this method gets it really close and often saves alot of time because it is close by the time you make your track adjustments. If it's jetted fairly well on the stand, it's usually nice and rideable w/o other changes.
All old school wives tales start from a kernel of the truth.
And 2 strokes SHOULD not idle on the stand for exactly the reason he stated.
Talk to any motor builder and they will tell you not to mix crap gas with your race gas..its just stupid..
If the engine is set to idle on a stand, when you release the throttle after say charging down a long straight, the engine returns to the idle RPM. At this idle RPM, HP & torque are still being made which drive the bike foward and are counteractive against the brakes being applied. This also makes the bike harder to lean and settle into the rut on entry. When the slide height is set correctly to not idle on the stand, when the rider shuts the throttle off the engine will only be running at the RPM at which the rear wheel is turning (keep in mind it's not a 1:1 ratio because of the tranny and it's much lower than and idling RPM - just enough to keep it sparking and burning the mixture being sucked in) and this is what helps because the engine is acting in unison with the speed of the wheel controlled by the rider. The engine is not fighting the brakes by trying to idle and produce power. You are basically coasting the engine until you release the brake and get back on the throttle.
Keep in mind there are alot of physics behind this and the changes would be very hard to notice unless you were a pro caliber rider or tester.
www.tdc2strokeperformance.com
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