Some DT400 Jetting Stuff

Or you're just running rich.

I never realized you were that far up. DOH!

A rule of thumb for the main is 1 size per 1000 feet gained in elevation. You ever look up the stock high altitude jetting for the bike? I cannot seem to find it anywhere online.

I'd start at the pilot by one size down. Tune that circuit. Then on to the needle/needle jet. And then on to the main. Until the bike runs well.

Man I missed the most elemental part of the equation. My humble apologies.

i thought the same thing when i first saw his elevation - but 170 is standard main, and he's on 155... so it's pretty close. (edit - he was on 155 in the beginning of the thread - now 165? if it's burbling on 165 go leaner...)

if it burbles and won't get on the pipe, it's probably still too rich on the main... or your needle jet is all messed up or your needle setting is way off - is the needle jet seated all the way in the carb body, and is it clean and undamaged? i have seen the pin come out of the carb body that holds the needle jet... causes all kindsa wonky problems... i've also seen the split float style carbs loose their pins that hit the float arm and the float flops around on one side and lets all kinds of fuel in... your needle jet could also be all wallowed out ...


so with the correct needle does jetting now have an effect on how it runs? if the pilot circuit is good, have you moved the needle around yet to see if there's a change?

only way for it to use a bunch of gas is (afaik): too high float level, way big main, wrong needle jet/jet needle, cracked overflow tube, blown reeds..

i drop a size every 1000-1500 feet increase in elevation... i'm at 2500 feet to begin with, so i generally gotta go up a size everywhere i go around here..

It burbles the same on the 155 or the 165 (or the 170 for that matter). Same burble, just different quality, if that makes any sense. Can't tell if jetting has any effect outside of that, really. Moved the needle up and down, honestly i can't tell at all if that does anything.

The pin is in fact broken in the needle jet tower thing, but the little shroud is facing the right way, im pretty sure thats just an indexing pin anyway, there's a brass washer sealing things up in there. The little shield/shroud thing is a little bent up (one corner is slightly turned down) but i can't imagine that being a big problem. That's the only damage that i noticed, but i'll yank it out (again) and check to make totally sure its not wallowed out, along with making sure the air jet is clear all the way to the needle jet.

Float height is ok, everythings working in there. No overflowing or anything (put fuel on it on the bench, didnt overflow). Float height is good, reeds are good, overflow is bueno, main jet (i think) is around the right size, needle is correct now, dunno about the needle jet.

It feels alot like a intruder i did with a blocked air jet. Just non nondescript awful running.

There's no tuning to be done yet that i can tell, it doesnt run good enough long enough to get any kind of throttle chop data, and i'm getting seriously goofy plug colors and conditions out of it, along with using too much fuel (i think). There's something wrong with it outside of jetting. I've jetted other 70s two strokes before and it's a slight improvement one way or the other, or a snitch more power, or a little smoother idle. It's vitally important, and its additive like you say (as far as i've read the idle jet doesnt do anything while the throttle slide is open, physically speaking, but that's a different thread).

I'd like to find out what's wrong with it before i start changing jet sizes around that may possibly mask the actual problem it's having. Same idea as not just setting the float height wicked low because it seems to run better with no fuel in the bowl.

pilot jet does have an effect when slide is open, but yep, that's a whole 'nother thread.


if the carb body itself is wallowed out and the needle jet is 'loose' it could pull tons of fuel up. that could be a problem. that little ding in the needle jet? could be a big impact on how the fuel atomizes..

if your jetting has no effect, fuel is getting there some other way.....

how bout this - does the air screw have any impact? when you close it does it stall, and when you open it does the idle increase till it gets flat sounding?

Yup, screwing it in will slowly lower the idle till you have to hold the throttle to keep it going, taking it out all the way increases the idle a bit and eventually it dies that way too.

Runs like a swiss friggen watch on the idle circuit if you just barely crack the throttle.

Searched all over the place for a good used carb, none to be found. Found a couple on ebay for later dt's, but its a different (bigger) carb with different jetting. They had quite different induction parts, rather not have to also try to fix that at the same time. I suppose i can pop down to the salvage place and see if i can pick the right carb out of literally hundreds in the 'carb room'...

Visited the 'carb room' at the local salvage yard. An entire shipping container packed front to back top to bottom with a path down the middle of just carburators. Didnt find a single one that was even remotely close. Almost left dejected until the guy who runs the counter told me they might have one in storage 'upstairs'.

Went upstairs and there were 4-5 blue plastic tubs full of carbs. Searched through all of those and didnt find a single one. Went to walk out of the room and on the corner of a shelf all by itself was a black mikuni vm32 left hand spigot carb with the same number stamping as mine. Woohoo!

Put it in carb dip for 8 hours, pulled it out and blasted all the black paint off (its a nice pretty aluminum now) and cleaned the hell out of it. Threw it on the bike last night, didnt start it though, i wanted to go to sleep not disappointed. Got some fresh gas in it this morning, wouldnt start cause the throttle cable wasnt in the boot all the way. Fixed that, and it fired right up.

Tooling down the street it runs perfectly. Nice smooth off idle, all the way through the power band, out the other side where it starts to run out of steam. It actually pulls now where before it would just sort of go faster.

So, i did the thing i tell people to never do, just replace the carburator. I still have no idea what was wrong with the old one. Same jetting, same model carb. The only part i transferred over was the throttle slide cap. I held them up next to each other, every part is identical. There must be a cracked casting or something where i can't see it. I hate to end projects without actually finding out what was wrong but, oh well.

When you mentioned the peg that locates the needle jet is gone that made me think it's passing fuel through the body. Stainless mentioned it earlier.

Glad ya got it sorted out. Now ride the fuckin' thing 'til the wheels fall off.

Peg is just for indexing and keeping it from spinning, theres a big soft brass washer to seal things up on that end. Pretty sure it wasn't spinning in there, but i suppose anythings possible.

Might have saved you an awful lot of time if you had read my earlier posts suggesting you tried a different carb? In terms of carbs, you can get brand new 32mm OKO flat slides for $49, which would work very well indeed on a DT400.

Ah, you're right, darn it i should just throw new parts at problems more often

Maybe spending $49 might have worked out a lot cheaper than all the various things you have outlined on here?

I'm just ribbing you, guy. I found a metric butt load of unrelated problems that actually did need to get fixed at some point. I knew that going in, i knew that during the entire process, this bike has been very used at some time in the past so there's a lot of "it will still run, oh well" problems in it. Additionally, the whole idea was to try to find out what was actually wrong with it so i could apply that knowledge to other problems i may come across in the future. I guess down the road if anyone has a bike that's running weirdly i'll tell them to go buy the cheapest carb they can find and put it on heh.

Let me try to remember what all i spent chasing this problem:

Spark plugs (3)
Gasket paper
New fuel
Main jets (4)
Two stage reeds
Propane for the torch i cooked the exhaust with
Electrical connectors for the cdi
Choke parts
New CDI box
Carb
Time spent (free time isnt free, i suppose)

I'm sure I'm forgetting stuff but i think the only dumb purchase was the CDI box, but it did help eliminate that as a problem.
So something around 120 dollars or so, most of that stuff i'll use again anyway, i think i made out ok, haha.

That said it does ping when it gets really hot, prolly dicker with the main a little more but it runs fine now otherwise, hooray!

glad you got it running right again, if even it meant not finding the root cause (other than the whole carb).

Uhg, yeah. Nice to have it running, though. Got it all insured and registered today! I think i'll throw a bigger jet in it and go burn up a new spark plug :)

Fighting pinging issues now, nothing too crazy. Reset the timing, put some premium fuel in it, still pings a bit, but the plug looked too light so i'll go up a couple more sizes :)

Does it ping about 2K RPM and go away? Wide open or partial throttle?

Its worth remembering that if you are using modern fuel, plug colour readings are not an accurate way of setting up carb jetting. Also worth bearing in mind that pinging generally occurs when motor is producing most torque, so in most cases changing main jet size wont make a lot of difference.

Pings all over, mostly 2k and above, most consistently when i open the throttle all the way, and it's most severe when i crack the throttle. It's so evenly making the noise i'm not even sure it is actually pinging, might be an exhaust leak or something?

160 main is a little too big, i think i'll leave it with a 155. Needle feels a little too fat, i might try dropping it a clip to see if that'll straighten that out.

Still does this thing where it runs like crap until it warms up, then it starts with the pinging stuff. Timing is dead on, jetting is basically dead on, not sure what's up with that.

Another bizzare thing i noticed. On the highway at reasonably high rpm when i go to hit the horn it completely screws up the way it's running. Chuffing popping, loses a ton of power. What's up with that? I'm honestly baffled at why it would do that. It must be wired wrong, or something, but i can't think of how. Maybe the ground wire for the horn is hooked up to the kill switch wire to the cdi :huh

I think priority wise, i need to figure out why it's making this pinging/rapping/knocking noise. The noise kicks up like a light switch when you open the throttle, doesnt get louder with more throttle it just either is making noise or is not making noise. Hmm.

Thanks for sticking with me through this thing, guys, i think it's kinda fun. Like a puzzle. Like a puzzle missing 8 pieces and a third of the pieces you do have are from a different puzzle