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Old February 11th, 2009, 10:09 PM   #18
TheDuck
Da Duckman
 
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Name: Duckman
Location: Pensacola, FL
Join Date: Nov 2008

Motorcycle(s): '08 250R, '06 Chopper, '80 Puch Magnum II, '01 SV650, '99 CR250/CR500AF Sportbike. '02 CRF450/CR500AF Dirt

Posts: 480
Welp, talking with my buddy again about the intake runners, I misunderstood him the first time. He said that the head, intake boot, carb, and any velocity stack or other tube all add up for the total runner length. Roughly measured the factory from the inlet on the carb to the intake valve is about 7.5 inches, or tuned for about 10% flow increase peaking at 11,200RPM. This also calculates to a 2nd harmonic at about a 7% flow increase peaking at about 9,500RPM, you can actually see this small boost on most ex250 dyno charts.

My intake stacks that I whipped up are 6 inches long, giving a total intake runner length of about 13.5 inches, give or take, which RETUNED the intake for max 10% flow increase at about 6,200RPM. Aparently there is a second harmonic someplace slightly above idle, I never noticed it, who runs full throttle at that RPM?

All that started to make sense when I attempted to run the bike with the 120 main jets with the intake stacks OFF.

At wide open throttle the bike was a little rich at about idle to 5k, then went really flat and even BOGGED down having difficulty increasing revs and made some smoke from about 5k-8k indicating a rich condition. The top end was a little sluggish, guessing also a little rich.

I put 115 kawi main jets in there without the intake runners attaching the pods directly to the carbs, and the bike at wide open throttle actually had MUCH BETTER bottom end from idle to about 5k than with 120 kawi jets and 6" intake stacks. HOWEVER, the mid range from about 5k-8k was noticably weaker than with the stacks and 120s. The top end was a little better than before It required a different riding/shifting style to make the bike feel snappy.

Anyways, it also started to make sense why I needed to play with the jetting so much when the weather changed, and why the midrange 6k-8k seemed sooooooooo much more peaky than top end. It actually felt like a turbo kicking in, or like a 2-stroke bike when it gets on the pipe, after I get past that hump, the bike would go flat again.

So the moral of the story:

I decreased jet size and am running the bike without the stacks on it now.

Running with these stacks they are a pain in the ass to keep tuned. I would recommend them highly on an engine that ran at the same RPM always, like a generator, lawnmower, aircraft, boat, or something similar. Or even for a drag bike running full bore all the time!

The stacks definately worked, but I expected their volume to help replace the resonance the airbox had. I never considered their harmonic length would be such a contributing factor.

But I still might experiment with some shorties on there about 1-2" just for a laugh.

Shame I didn't have a real dyno to tune this on, I would have loved to have seen the radical difference in the power curve with these 2 different setups.

Now, back on the plan to make the intake runner length adjustable based on RPM....

Its fun playin' in the garage.
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