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Old 08-20-2013, 10:08 AM   #1
Inane Cathode OP
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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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Old 08-20-2013, 03:49 PM   #2
dhallilama
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Quote:
Originally Posted by Inane Cathode View Post
....

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.
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Old 08-20-2013, 04:37 PM   #3
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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
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stainlesscycle screwed with this post 08-20-2013 at 04:45 PM
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Old 08-20-2013, 08:43 PM   #4
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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.
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Old 08-20-2013, 09:47 PM   #5
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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.
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Old 08-21-2013, 06:27 AM   #6
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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.

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Old 08-21-2013, 06:57 AM   #7
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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.
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Old 08-22-2013, 03:31 AM   #8
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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..
1974 DT360A CDI MAGNETO DT360A
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
1976 YZ400C CDI 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...
http://www.electrexworld.co.uk/acata...al.html#pcoils
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stainlesscycle screwed with this post 08-22-2013 at 03:43 AM
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Old 08-22-2013, 08:29 AM   #9
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I don't see any on completed listings, don't know what i'm doing wrong but with the yam number or just '75 dt400 coil' no pulser coils come up. Tried the cross reference, nothin.
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Old 08-22-2013, 09:31 AM   #10
stainlesscycle
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try this search

(dt250,dt400,it400,mx400,dt360) (stator,igntion,pulser) -cover

you could add yz400 in there, but it ends up with some yz400f/yz426f junk.. and you're gonna hafta weed out 78/79 stuff - you could put

(dt250,dt400,it400,mx400) (stator,igntion,pulser) (1974,1975,1976,1977) -cover

but that eliminates the ones without the year in the title...

you can save the search for daily searching/notifications. i bet a stator pops up in the next week or so..
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Old 08-22-2013, 10:08 AM   #11
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What's the feasibility/reliability of swapping out the stator with one from a 77-78? Would i have to change the rotor and the CDI at the same time? Looks like theres tons of 77 and later stators and pickup coils.
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Old 08-22-2013, 10:27 AM   #12
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first i would see if you can find the testing spec for the 77/78 pulser (or even newer..) - if it's the same range, just swap the stators out (or just the pulser). if it's way different range, you may need to swap cdi out - not a bad thing, i don't trust any pre-1977 cdi/ignition anyways... you may need to change flywheels if you change the whole stator out. i dunno if the i.d. of the flywheel changed, i have to swap flywheels on my ignition swaps - the early mx external flywheels were small i.d.. the taper on the crank is probably the same. the yz's had several tapers, to the best of my knowledge, the mx and dt only had small (175 and down)and big bore (250 and up) tapers....

if it was mine i'd slowly swap the entire ignition out to the newest (and most available) ignition i could find. the yamaha trail bikes from the late 70's were produced in huge numbers (especially 77/78/79) so there's a ton of them/cheap parts out there.

yamaha really started making reliable ignitions in the late 70's. they don't seem to fail often... the 73-75 ignition systems had a ton of problems.


i bought a couple of random stators cheap that i thought would work - the ones that don't fit, i just re-ebay... i just check to find which ones there's lots of, and are cheap.
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Old 08-23-2013, 10:55 AM   #13
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Well, i'm an idiot. I had my nice digital meter at work, so i went out and got an analog meter to do the ohming on my pulser. Went by the junk yard today and what would you know that every single early cdi pulser coil i tested on yamahas read right around the same ohms, 180 ish. Almost bought the lowest reading one just to be sure but i didnt.

Got to work where i had my regular meter and ohmed out the pulser with it, it's reading perfect, 88.5 ohms, right smack dab in the middle of where it should be. I looked more carefully at the meter, did some light reading, now i realize that the ohms scale has to be zeroed out with the little black dial i was messing with on the meter thinking "hmm what's this do?" *turn turn turn* "Huh, nothing, weird."

So, i guess we're back to CDI crap again. I'm going to recheck the charging coil ohms just to be sure, but it was putting out good voltage and had good ohmage before.

I've got that 5 wire CDI box coming anyway, hopefully that should help shed some light on whats going on.

Some unrelated news, i got a fine from the complex i live in for speeding on the DT through an empty parking lot. If they think that's fast they aught to see it after i get this issue ironed out ;)
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Old 08-23-2013, 12:57 PM   #14
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Never ever had a problem with a pulse coil. If the pulse coil was bad, then I would guess it wouldnt work at all?

But bearing in mind that unless you get all the coils properly rewound, and can find a new CDI unit which will work, you are going to be working with parts that are 40 years old.

The easy way out here might be to get a points ignition from an earlier bike and fit that? Sure you will need to maintain it from time to time, but you will also be able to fix it very easily if anything goes wrong.
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Old 08-24-2013, 08:50 AM   #15
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cdi = no breaking up when you are in the upper revs, still delivers enough power fast enough for the coil to collapse. points, if not perfectly set, can float and miss at super high revs..
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