Stutter/Bog at 1/4 throttle 04 YZ450F Fixed?

My 450F stuttered (acted rich) at 1/4 constant throttle, 1st and 2nd gears. The dealership could not correct it and claimed that it was natural for the bike. They even offered me a ride on one of the floor bikes. It sounds like jetting huh. But, I had tried all the stock needle clip positions. I even tried a leaner needle but could not get rid of it. With the Throttle Position Sensor (TPS) unplugged, the problem was greatly improved.

I replaced the CDI with a Vortex CDI ignition that has 10 maps. The problem is gone in half the map selections. Horay! :thumbsup: Vortex said that the stock CDI has troble interpolating between the curves and causes the spark plug voltage to drop. This made sense to me because on my WR250 acted rich when the battery drained down and the lights were on. Thus, it's CDI could not keep a hot spark.

I guess the mx riders never use constant throttle and as I get better on the track I seem to notice it less. However, out in a field or trail style riding that stutter was a pain. Plus, since I just got it, I was increasing speed gradually. I was afraid to really crack it. All the Vortex traction maps were fine. One performance map did not have the 1/4 stutter.

Here is the puzziling thing. What is going on that timing can screw it up like that? Why not just keep a constant spark? Why doesn't Yahama fix it? Could there be another issue going on? It is surprising that timing can make you think you have a jetting problem!

I am very happy with the Vortex CDI since it fixed the problem and gives me several different curves.

Glad to here you fixed your problem. I had the same problem back in Feb and went through all the maintenance checks I could find. Valve clearances, cleaned carb, check TPS etc. I ended up buying a JD jet kit and replaced the needle (it had some wear, but didn't think it was a problem as the wear was in the throat of the carb and shouldn't have caused a metering problem). Anyways, just last week I started having the same problem again. First thing I did was check the needle and it was perfect. Checked the jets for any debris and found nothing. I ended up changing from the blue (recomended) needle to the red (higher altitude, hotter and humid climates) and my problem went away. At this point I think it is just a temper mental tuning issue. Granted it is hot here now 110 degrees versus Feb. weather. I have never owned anything (bikes, jet skiis) that required retuning for weather. Just my $ 0.02. Hope it helps. BTW, what does a CDI run in case I end up back in the same boat?

If it was jetting, why would a different CDI fix it?

Stock CDI is 250 and the TPS is 150. The Vortex CDI was 400. I am at 5000 ft and just got the bike in July. My WR never had the problem and was smooth at all constant throttle settings. The Vortex Rep was familiar with the problem and said there was a stutter at the high end too. I could not verify that because I am still new to the bike. The stutter acted rich. Normally, we go one cilp on the needle and two down on the main as a starting point. I could not fix the rich stutter no matter what clip position I tried. They say it may be that the voltage to the spark plug from the CDI was too low, causing a weak spark while the computer was interpolating.

What I may try later is replacing the TPS and then trying the stock map again. My TPS was reading 3.8 Kohms at full throttle versus the spec of 4k to 6k.

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