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TOPIC: DT250 not running rich anymore, but I'm scared...

DT250 not running rich anymore, but I'm scared... 15 Apr 2019 07:58 #1

  • JimGoracke
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Hello! After spending an embarrassing amount of time lurking and reading here, I am finally posting. I bought a neglected but remarkably complete 1976 DT250 for $400. I cleaned out the carb and replaced a broken jet setter and it fired right up. It had a bit of piston slap, so I pulled the cylinder and found it needed to be bored. I got a new Wiseco piston, rings, wrist pin and bearing, and had it bored to match the piston. I put it all together and it was running VERY rich. It idled perfectly and was not bad at WOT, but mid range was very choppy. The plug was black, so I dropped the needle to the middle grove, then to the top. It was better, but still rough so I ordered several smaller jets (stock 160 was in it). Well, at this point I didn't have the air box on it. While waiting for the new jets I got the air box cleaned up and installed with a new air filter. When the jets came I put a 150 in. It still started and idled fine, but from 1/3 throttle on it was sputtering and cutting out. I was afraid it was lean so I pulled the plug and it was wet. Seeing it was even MORE rich, I thought I must have put in the wrong jet so I pulled it and confirmed it was a 150. I dropped to 145 and it was still rich. I thought maybe the air filter was too restrictive so I pulled it and left the cover off, and it ran much better. I thought I would order a different filter element and put the airbox cover back on and it was a dog again!?!? I messed around with it, and noticed a strong sucking sound under the tool tray where the air intake is. I took off the tool tray so the intake was open and it ran like a champ. I put the filter back in the air box and it was still good.

So, here is where I am:
-Stock #30 pilot jet
-145 main jet (down from stock 160)
-Needle on second from the top groove (3 grooves higher than stock)

I have checked and double checked the following:
-Float and needle adjustment or leakage
-Timing
-Exhaust restriction
-jet setter o-ring
-Choke leakage

I have two big questions:
1. Has anyone else had a problem with the tool tray obstructing the intake? After studying dozens of pictures I'm pretty sure I have it installed correctly. I can't imagine this is common. I couldn't find anyone else with this issue on the forum.
2 Should I be scared of leaning it out like I have? I'm afraid whatever is causing the rich condition will suddenly go away and it will suddenly be very lean and cause my pretty new piston to melt:( Going down 3 main jet sizes and 3 needle grooves is excessive for a completely stock bike (other than the .2mm boring, and that should cause a lean condition if anything). Does this sound scary to anyone else?

Here are a few pics of the bike. While I waited for it to be bored I painted a lot of the parts. Next winter I plan to paint the frame, engine, tank, and oil tank.

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    1976 DT250

    DT250 not running rich anymore, but I'm scared... 15 Apr 2019 10:04 #2

    • darinm
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    Are they genuine Mikuni jets from a reputable source? Looks like you got a steal at $400, she looks great.

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    1972 Yamaha CT2 175
    1972 Suzuki TS185 Sierra
    1995 Suzuki DR350

    DT250 not running rich anymore, but I'm scared... 15 Apr 2019 11:26 #3

    • JimGoracke
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    I think so. They are the “Genuine” Mikuni jets from JetsRUs.

    I felt like it was a great deal. My wife was not so sure. Didn’t look great when I picked it up:)

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    1976 DT250

    DT250 not running rich anymore, but I'm scared... 15 Apr 2019 15:12 #4

    • RT325
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    Apologies for not reading it all just yet but do they have the )ring on the jet setter thingy & is it good & under tension. if ok on full throttle it won't be that problem. I saw somewhere recently where the slide guide pin had broken off & if 'that' happened then the slide could turn back to front with cutaway forward making it super rich midrange but unlikely once again. Sounds more like choke not seating 'which it will be seating if its hitting the bottom' & not held off the seat by a misplaced plastic seal cap where the choke rod slides through. Ok, i've talked myself out of all of those--so is a very mismatched slide needle & needle jet/emulsion tube. Eyeball the metering bit near the top for an elongated shape & check the needle for obvious wear. After that--i give up at the moment. Oh, one more--if the idle mixture screw has been screwed right in it'll richen it up a bit above idle especially if going at open throttle then shut off into a corner then bringing it on again. Have seen the seat base collapse onto the airbox--not particularly on a DT250 but!!. Restricted exhaust would kill it on full song. Run out of ideas & confused things enough for now. Video is good--we love videos lol.

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    DT250 not running rich anymore, but I'm scared... 16 Apr 2019 04:37 #5

    • JimGoracke
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    I saw that guide pin post as well and checked that. It’s good. I checked the emulsion tube pretty carefully. I even stuck the needle in and held it up to the light to see if it was egg shaped, but it looks to be very round. The needle doesn’t show signs of wear. I’m pretty sure the choke is sealing well. I can’t blow through the pickup tube when it is off. I’m certain the exhaust is not plugged.
    A sewer clean out tool goes right through it, and I tried running it with the exhaust spaced off with nuts.

    It’s running good right now, so a video won’t helpI just worry about running it with a main jet 3 sizes smaller than stock and the needle dropped all the way down. Seems like it could suddenly go lean...

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    1976 DT250

    DT250 not running rich anymore, but I'm scared... 16 Apr 2019 04:58 #6

    • RT325
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    Reminds me of a 200 twin years ago where one side ran rich midrange & made no sense as motor was good & both carbs the same. Turned out something had either got past the air filter or the filter was crumbling & a tiny piece had gone right down in the air bleed to the needle jet & blocked it which made the midrange go rich on one side only. Umm, 45 years ago lol but things ya never forget because it was unusual. In my case the carb had a tiny air bleed hole or drilling but probably different on your carb.

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    DT250 not running rich anymore, but I'm scared... 16 Apr 2019 05:04 #7

    • JimGoracke
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    I’ll have to check that out. There are certainly many tiny passages in the carb. I’ll check the diagram and see if I have something similar.
    Thanks for the tip!

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    1976 DT250

    DT250 not running rich anymore, but I'm scared... 16 Apr 2019 05:14 #8

    • MarkT
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    At what elevation do you live/ride?

    Bikes are jetted for sea level... high elevation can require leaning out.

    P.S. I believe the air bleed RT325 is talking about is in the lower center of the inlet bell and goes forward straight into the emulsion tube hole. With emulsion tube out you can see or test if passage is clear.

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    1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

    DT250 not running rich anymore, but I'm scared... 16 Apr 2019 06:10 #9

    • JimGoracke
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    Hi Mark,
    I'm in Kansas so elevation is about 1500 ft.

    Is it the drilled hole that angles down halfway between the inlet and the emulsion tube? I know I had that clean when I cleaned the carb. I think I'll pull it again this evening and see if something got sucked in there. At this point it takes me about 5 minutes to pull the carb and have it completely disassembled>>

    Thank you for the info.

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    1976 DT250

    DT250 not running rich anymore, but I'm scared... 16 Apr 2019 06:34 #10

    • MarkT
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    Hole goes straight in... Took a screenshot of one from another thread. This one has a pressed-in brass jet with a tiny hole... on your bike it's likely a much larger hole drilled right in the carb body. 2.5mm diameter going by the spec in the service manual.





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    1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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