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Inane Cathode 08-20-2013 09:08 AM

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.

dhallilama 08-20-2013 02:49 PM


Originally Posted by Inane Cathode (Post 22140321)

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.

not terribly... i have to go up/down 2 on my '77 IT 250 depending on the season and it's a mild difference.

too rich vs too lean, one really easy way to tell: get on the throttle. listen to the bike and see how it feels. now engage the enricher ("choke") and do it again.
better with the enricher on? you're running lean.
worse with the enricher on? you're running rich.
no change? either rich or the enricher is hosed and on all the time.

stainlesscycle 08-20-2013 03:37 PM

you're probably not gonna like this answer. when jetting has no effect, it's ignition. the cdis ad stators on some dt400's just plain sucked. i checked your other thread- the 1975 cdi is the basically the same junk cdi that's on mx250a/b. sucks. if it was mine, i would go about 4-5 steps up on the main and see if it runs any different. if it doesnt, it's probably the cdi or stator

seems like it runs good on the pilot :rofl

Inane Cathode 08-20-2013 07:43 PM

Well, checked and rechecked the CDI, it's doing what it's supposed to, and the pickup has the right resistance. They might be garbage but mine works just fine.

For the choke trick with jetting, it gets worse, but it doesnt just open the enrichment jet, it also opens up a gigantic air jet (like a quarter inch) so i could see it at least not doing anything. It's crappy WOT, and the same kind of crappy, just at lower rpm with the choke out.

Checked around, i've been told the main is a 170 stock on this year, i've got a 155, so theres that. I'll try 170, and see how it is, if its a lot worse than i guess the 155 is still too small. Hard to find jetting data for these.

JeffS77 08-20-2013 08:47 PM

What size needle and needle jet ? Correct style needle jet ? Primary and not bleeder ? What is the slide cut away ? Float level good ?

Factory carb ? Bigger ? Smaller ?

All things to consider..I do not know the history of your bike so these things may all be stock or may not be. But worth checking out.

anotherguy 08-21-2013 05:27 AM

From what I can see from here you need more main jet.

When jetting work from the bottom up,that is start with the pilot/mixture screw circuit and go up from there. Fueling is additive,all the circuits have an effect as the throttle opens wider so starting top down or any other way only leads to confusion.

Oh and I despise inline filters. As long as the in tank screen is in good nick and you use fresh gas/oil it should be fine. If the tank is rusting fix it those little Chinese things have caused me more issues with WFO fueling I won't even dyno a bike with one anymore.

Twin-shocker 08-21-2013 05:57 AM

Its either the CDI as posted previously, or possibly a very badly clogged exhaust. To check CDI put a timing light on the bike, and rev the motor and see if the flash gets erratic at higher rpm.

In terms of needing different size jets, what you need to ask yourself is why would a PO fit the wrong size jet if the bike was running well?

If it is CDI problem, you will find cheap Chinese pit bike AC 5 pin CDI's will work on most bikes using an AC system.

Inane Cathode 08-21-2013 07:54 AM

Not sure how to check for a clogged exhaust beyond what i've done so far, took the little silencer cap thing off the end, looked in there, turbine core is unplugged, it's got a wicked pop at idle and it's all coming out the end, honestly doesnt seem like its backing up at all. With a timing light on it at idle it'll sit flashing happily about 20-30 degrees BTDC. When you rev it it'll start to move towards advance about 10 degrees BTDC, then high RPM it'll whack over to full advance (where you set it to in the first place, actually) around 4-5 degrees BTDC. It's not any more erratic than any other ignition i've seen. Its for the most part spot on, but you do get some 180* ignition pulses every now and then.

I don't like inline filters either, but the tank is old, and theres quite a bit of rust in there. It doesnt feel like it's running out of gas, it'll just keep running like it does as long as i have the throttle pinned, back off and it'll still do it for a bit then clear up and putt nicely.

Dunno the needle or the needle jet size, i'm assuming its stock (doesnt look funny or different than the fisches or used parts i've pulled up on ebay). Stock carb (vm32) stock slide, stock cutaway. Float height is good, starter jet is stock, main jet is a 155, been told it is supposed to have a 170.

How do i Actually check a blocked exhaust. I can picture in my head the exhaust stream backing up under heavy load, flooding the motor, then clearing out when you back off the throttle. It sure does sound cleared but i dont know for sure.

The chinese CDI idea is very intruiging, is it literally as easy as just plugging the appropriate wires in from the harness to the box? Are the timing curves appropriate? I'd be shocked and amazed that it just so happens the ignition coil and pickups of a 1975 yamaha just happen to work with a modern AC ignition sysstem. I googled it a bit and found a yamaha zuma 5 pin cdi for two strokes, would that be an appropriate fix or does my CDI sound fine. I don't want to rip out stock equipment and mess up the harness for no reason.

Inane Cathode 08-21-2013 09:15 AM

Alrighty, put in the 170, runs exactly the same if not a tiny bit worse. Loosened the exhaust flange way out so most the exhaust wasn't even going into the pipe, still runs the same (just way louder).

I don't know exactly why it runs somewhat ok (actually does start to cut out at higher rpm) when you crack the throttle, but i think through the process of elimination it has to be the cdi at this point. I put the timing light back on it with an eye to erratic spark timing, ran it up to higher rpms and it does actually kinda go bonkers. The mark jumps all the way back retarded at higher RPM when it *should* be at full advance, then forwards, then disappears altogether, then goes 180 out of time.

The problem is, the only dt400 cdi i can find is $550, and is as old as what is already in the bike. So, that chinese CDI idea, is there a reason an AC powered CDI unit for a two stroke pit bike wouldnt work in the DT? I'm assuming the ignition curves would be different. A few years before and this bike was running points with no ignition advance so it can't be so bad. Just not sure if some random AC CDI box is going to jive with the electronics already on the bike or not, does anyone have any experience with them?

Tim McKittrick 08-21-2013 12:39 PM

I spent the last one and a half race seasons chasing a similar issue with my Honda RS250 road racer. It turned out to be a weak coil- the CDI and powervalve computer were operating just fine but I was getting inconsistent spark when a load was applied to the engine. On the work stand it seemed fine- but on the track it would cut out and run all boggy. Plug chops revealed good jetting, ohm tests of all components showed nothing. EVERYTHING looked as it should, and tested fine ON THE BENCH. A fellow racer finally noted a tiny flaw on one of the input wires to one of the coils, which eventually (once the HRC unobtanium was found) lead to a replacement of the coils and wiring connections.

Problem solved.

A replacement coil would possible be easier and cheaper to obtain than a new CDI for your 400 and might cure your problem- it certainly sounds like it's not a fuel or exhaust issue to me. If nothing else it'll be a cheap item to rule out- the ebay link below is a whopping $12.00

A few quick links, via uncle Google:

stainlesscycle 08-21-2013 02:18 PM


Originally Posted by Inane Cathode (Post 22148122)
The problem is, the only dt400 cdi i can find is $550, and is as old as what is already in the bike. So, that chinese CDI idea, is there a reason an AC powered CDI unit for a two stroke pit bike wouldnt work in the DT? I'm assuming the ignition curves would be different. A few years before and this bike was running points with no ignition advance so it can't be so bad. Just not sure if some random AC CDI box is going to jive with the electronics already on the bike or not, does anyone have any experience with them?

i have chased similar what seems like jetting problems on many early cdi bikes. the ignition sucks at high speed. and things get all wonky. usually the spark is not strong enough to ignite the your jetting/plug reads will get all inconsisitent. there is no reason that dt400 shouldn't run decent with stock jetting. if you've been through the carb/no air leaks, and static timing is right, it's usually ignition. give it more than just the fuel from the pilot, and it don't know what to do with it. i bet it runs great on the stand, but sucks trying to pull any load.
to check the exhaust, run a wire pulling snake through it a couple of times. it should not come back without anything on it... i take the pipe off and smack it on the ground a few times, and listen for stuff rattling in there....i don't do this on nice pipes

i swap my 73-75 mx100/125/175 with stators and cdi's from it125/dt175 (1979ish). it costs about $100.00 for everything, and they are super reliable.... i have not investigated your stator setup, but i'm guessing it's the 2 source coil dealio (1 high, 1 low). if it is, the only cheap way is to figure out what late 70's/early 80's yamahas shared the same crank stub. all you really care about is matching a flywheel up (the 2 source coil flywheel will not work with anything but them crappy ass cdis. ) i'm gonna make a wild guess but by about 1978/1979 or so and up dt's/mx/yz/it's all got consistent on their ignitions. there's 4 basic types small bore and big bore, and inner and external rotor. if you start mixing and matching you will see what works. your best bet would be find 1978/1979 dt400 stator/flywheel/cdi. they got real reliable after 1978....

if you give me the measurements of center to center of stator plate mounting bolts, i can at least tell you which stator/flywheel combo will work...the cdi side is easy - any yamaha post 1978 willmost likely work as long as it has at least, a red, orange, black, and white/red - that don't get you lighting, but it does get you running....

you can use the chinese cdi's, as long as you don't care about ignition curve, 99% of them have no curve at all, even though they have a pic of a curve on the cdi :) (careful with that ping eugene.)..

there is a 2 stroke moped cdi i have used in the past, works awesome, has a real timing curve.

either way, none of these cdi's will work with your 2 stage stator.

you want the stators that look like this:
1 lighting coil, 1 source coil, and 1 pickup.

anotherguy 08-21-2013 06:28 PM

If you're any good at understanding how to wire something up these guys can hook you up cheaper than you think.

They built a boost sensitive ignition for a supercharged Valkyrie I built last year. 10 separate advance curves depending on manifold pressure. For under $300 USD and plug in easy to install. Took some dyno time to sort it out but your bike is nowhere near as complicated. After sale support is pretty good if you have patience. Thier English is good but terminology can be confusing.

Contact them and tell them what you're running and they can supply something that will plug in and work.

Inane Cathode 08-21-2013 07:42 PM

Here's the diagram for the 75 dt400:

I'm pretty sure its not one of those dual source ignition coil setups. It's got the magic black box pickup that apparently is on galactic backorder including ebay. The 77 and later use a little tiny lighting coil shaped one that's coming out of peoples ears, but not the black one.

I did some reading and noodling on how these CDI boxes work. The more i think about it the more i think the pulser coil is failed/failing. The way the CDI (in this version, i read) works is it uses the amount of voltage coming off the pulser. The slower the rotor sweeps past the pickup the lower the voltage. That means at idle, the CDI sees a low voltage, and retards the timing (i think the voltage amount acts directly on a calibrated thyristor, but i may be wrong). At high rpm, theoretically, the pulser puts out much higher voltage, which tells the cdi that the motor is spinning faster, so it advances the timing. If the pulser coil is failed or failing, it wont put out as much voltage as it should be, and the CDI doesn't think the motor is spinning fast enough for advance, so it goes full retard, which is what the timing light says.

I'm sure i'm over thinking it, but i can see a pulser coil putting out not enough voltage and fooling the CDI into not advancing the spark because from it's data the motor isnt spinning fast enough. What i dont get is that the advance appears to start to move, then snaps back to full retard. I don't get how a thyristor can sort of work, i was under the impression that they either work and work well, or don't work completely and are failed.

Edit: Found the actual service manual for this bike (not a generic clymers) that has an ohm chart for the stator bits and bobs. The source/charging coil is right on spec at 13 ohms or so (spec is 12.5ish). The pulser coil is way off, i'm getting 180 ohms or so, it's supposed to be 90 plus or minus 10 percent. Reading quite a few other threads, noone has one that actually will run with more than 96 ohms or so. Looks like i've got a dead pulser coil, at least. I've got a cdi coming anyway but looks like im up a creek with the pulser coil, yamaha stopped making them in the stone age, and theres not a single one on ebay. Damn.

Edit again: I'm wondering if these are rewindable or not. You can't actually see the windings as it is covered in Magic Black Box plastic, but as long as the wire gauge is correct, and you use the right length, you should be ok, right?

dhallilama 08-21-2013 11:00 PM


Originally Posted by Inane Cathode (Post 22152675)

Edit again: I'm wondering if these are rewindable or not. You can't actually see the windings as it is covered in Magic Black Box plastic, but as long as the wire gauge is correct, and you use the right length, you should be ok, right?

i've rewound a couple pulser coils... one i couldn't find a replacement for, the other i was too cheap to spend the $$$ they wanted for one. both times it worked fine, but i was left with sore fingers for a couple days. i wasn't exact with the wire gauge or the amount of turns... i just turned as much (cleanly) on the thing as i could. worked fine. can't say i'd be excited to do it again, but if the part wasn't easily found...

stainlesscycle 08-22-2013 02:31 AM

would rewind it if possible. i'm assuming you already did a cross reference for the coil, but if you didn't here's the bikes that used that same pulser..
1975 DT250B C - D - I - MAGNETO 400B - C
1975 DT400B C - D - I - MAGNETO 400B - C
1975 MX400B C - D - I - MAGNETO
1976 DT250C C - D - I - MAGNETO 400B - C
1976 DT400C C - D - I - MAGNETO 400B - C
1976 IT400C C - D - I - MAGNETO
1976 YZ400C C - D - I - MAGNETO YZ400C

they do come up on ebay frequently.. if you look at completed listings there's about 15 sold in the last month...

you also may want to check the specs on the later one (77+) yamaha part #1M2-85580-20-00 - it's possible it has the same exact specs and just has a new part number.......

or you can check with electrex and see if they have a new one...

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