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Old 08-20-2013, 10:08 AM   #1
Inane Cathode OP
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Joined: Jan 2008
Location: Arvada, CO
Oddometer: 4,601
Some DT400 Jetting Stuff

I don't know if anyone has followed my other thread on my dt400 shocking me through the gas tank and not revving above 4.5krpm, but i think i've narrowed it down to jetting issues. It won't rev cleanly with full throttle, sounds like it's four stroking and the reed cage is soaked with fuel. When you just barely crack the throttle running it it runs VERY smoothly, it's quite amazing how ride-able it is like this. If you 'floor' it for a stretch and go back to cracking the throttle it'll run quite poorly for a few seconds then clear right up and run smoothly again.

Here's what i've done so far:
- Cleaned the carb 4 or 5 times, it's clean, trust me.
- Fixed a vacuum leak around the reed cage with a new gasket.
- Replaced the stock filter (the stock one looked fine but it ran like absolute dog shit with it in there)
- Replaced the reeds with two stage boyesen reeds
- Cleaned and made sure the choke plunger is sealing (i think it is, but i have another one coming anyway)
- Obsessively checked the timing, it's spot on and advancing the way it should (i guess dt400s had crappy cdi boxes)

Here's what i've noticed:

- Tried a plug check after 20-30 seconds of wide open throttle, the whole insulator is white white white. The old plug is gray with burned oil but it's also very white

- Tried a throttle chop test (wide open throttle until you're on the pipe then cut the throttle back a quarter turn or so) i honestly can't tell if it gets worse or surges, it's just crappy all over

- Bike runs much smoother without the air filter or cover, but it does seem to run WAY hotter

- The idle jet i believe is a 40 (can't remember but it's stock) and the main jet is a 155 (stock smallest main they came with was a 160)

- Like i said the whole intake was soaked in fuel when i changed the reeds over, too rich?


Could the petcock or fuel filter be overwhelmed by the amount of gas this thing needs? The petcock flows like a firehose without a line on it, theres no way to test the filters flow, should i just to a straight line with no filter to it?

Lean misfire, too rich to run, yet white plug? I'm confused on what's going on here. I think it's safe to say that it's running crappy enough 1/2 to full throttle that i can't tell if its too rich or too lean. The too rich argument points to the fact that without an air filter or cover it runs smoother (but wheezy) on large throttle openings, and the fact that everything was soaked in fuel when i went to change the reeds. The too lean argument points towards the fresh plug check being bone ass white and dry, as well as the last plug, as well as the main being smaller than stock. Plus i remember that honda shadows (i realize its a different bike but stay with me) run so lean on wide open throttle when you put a pipe on them that the plug looks like it's too rich because theres so little fuel that what is there doesnt burn at all.

Part of me wants to fatten the main jet because the plug says it's too lean, it does have a smaller main jet than it should, and i just put new reeds in it.

Part of me wants to lean out the main jet because the reeds were soaked, it runs smoother without a filter.

The issue i'm having is it's running so terribly WOT that i don't think i'll be able to tell if im going the right direction with the main size or not. I know i should just guess and check, but i'd like to learn something out of this and maybe get an empirical method to jetting instead of spray and pray with jet sizes.

PS How sensitive are these 70s two strokes to main jet sizes, anyhow? I can't imagine two sizes making a giant difference in rideability.
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