loss of power

[killer400ex]

rebuilt it last summer, ran amazingly well this winter, and its still crazy when i first start riding, the problem i have had in the past (before the rebuild) and now is that after i let it warm up for about 5 mins and start riding its crazy to ride, and it seems that after it starts to warm up more and more it looses power, now its not a huge loss but i can notice it. i am thinking it is either jetting or air fuel screw kinda problem, ive allwase had trouble with getting the jetting/ air-fuel mix just right but after alot of trial and error this winter i had it spot on, like keeping up with most 450's but like i said, its starting to warm up and the hotter it gets outside the worse the power loss is, i cant quite remember how it goes, now please correct me if im wrong but i thought it was more fuel in the winter and less in the summer, my guess is that after it starts getting hot it is running a bit rich

[killer400ex]

any input would help alot since i have a race this wed. and am up against a few 450s

[JOHNDOE83]

Jetting.... get some large jet sizes, put the biggest one in so it sputters on the top end, then back down jet sizes till it stops.

loss of power could be from getting to hot, jetting may or may not solve the issue, but its a start.

When it comes to jetting, my moto is to "go big or go home"

you would rather have slightly to much fuel then not have enough, espeacially with big bore kits.

atleast on the main jet.

[honda400ex2003]

what is your jetting first and how does your plug look? steve

[flyboy1294]

Originally posted by killer400ex
rebuilt it last summer, ran amazingly well this winter,

In the winter, the air is colder. Colder air is more dense. Therefore, you could fit more air into the combustion chamber during the winter, so you needed more fuel to get the mixture correct. (This is why NO2 systems work; they supercool the air causing it to become much more dense, resulting in more air fitting into the combustion chamber, which is then mixed with a greater amount of fuel, thus causing a better power output.)

Now that temps have started to go up outside, the air has became less dense, so you are getting less air into the chamber at any given time. Your fuel mixture remains the same though, so you are getting more fuel than is needed for the correct ratio.

I am not saying I know for certain, but based on the information you provided, I would guess that you have became a little bit rich on your main jet.

[killer400ex]

Originally posted by flyboy1294
In the winter, the air is colder. Colder air is more dense. Therefore, you could fit more air into the combustion chamber during the winter, so you needed more fuel to get the mixture correct. (This is why NO2 systems work; they supercool the air causing it to become much more dense, resulting in more air fitting into the combustion chamber, which is then mixed with a greater amount of fuel, thus causing a better power output.)

Now that temps have started to go up outside, the air has became less dense, so you are getting less air into the chamber at any given time. Your fuel mixture remains the same though, so you are getting more fuel than is needed for the correct ratio.

I am not saying I know for certain, but based on the information you provided, I would guess that you have became a little bit rich on your main jet.

thats what i was thinking originally, but i just wanted to be sure, it dosnt sputter at top end it just isnt as snappy through the gears, like when i first start riding after i let it warm up i can crack the throttle in 3rd gear and have the front end come straight up on me like im in first, but aftr it warms up it seems to hesitate and not pull as hard, the problem im having with this whole thing is that it seems to do the same thing either way i jet it, it will get hott and knock if its too rich and it will do the same if its too lean, i would love to have it dyno tuned for pin-point accuracy but nobody around here does it. i have it running decent right now but not where i want it

[gt400ex]

From the info you have given, and that jetting hasnt really helped much, I would say that its probably just do to heat. Try adding a Spal fan or an extra cooler. An extra cooler is what I added to help my 440 from loosing power after it got hot.

[killer400ex]

yeah i think im gonna try a diff synthetic oil and either put a spall fan on or just use a bigger oil cooler with a spall fan