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Makotosun

1978 DT400 General Advice resurrecting solid bike

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It won't be the carb.

Common places are the intake manifold (cracked rubber or leaky gaskets), cylinder base gasket, or crankshaft seal.

You can build or buy a test kit to pressurize engine to about 5psi... spraying with soapy water will reveal leak.

Spraying gaskets/Manifold with something flammable with engine running is another way to find a leak. (A friend of mine uses a water hose on low flow to run a stream of water over suspected leak areas, I've never tried that... another friend uses a propane torch, not lit... and probes around with valve on... he says it revs up when leak is found. Never tried that either but I'd probably use it before water.)

If using flammable, do it outdoors in a well ventilated area... with a fire extinguisher handy just in case.

If no intake or base gasket leaks, most likely is ignition side crank seal, available from Yamaha, don't use aftermarket crank seals.
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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22 Apr 2023 09:37 #11

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I use the propane or mapp gas as MarkT says non lit I put a hose on the end and turn it on low I also use that trick on autos too
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Last edit: 22 Apr 2023 10:05 by Ht1kid.
22 Apr 2023 10:03 #12

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  • RamadiRat
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Attached a video of the bike idling and revving.  I could remove the carb and inspect the pilot jet again (I failed to take or post pictures) but I cleaned it pretty thoroughly with Berryman carb cleaner. I did see it's an .80.  I tested for leaks with a propane torch and found nothing, however I did not test the crank seals - I assume they're behind the covers l/r sides.

With the air screw 2 turns out, she seems to stay lit with idle around 1400.. I still suspect an air leak because turning the air screw 1/2 turn either way seems to have NO affect

youtube.com/shorts/WTBQSuOcJz8?feature=share

Last edit: 23 Apr 2023 15:39 by MarkT. Reason: Insert video
23 Apr 2023 12:29 #13

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The old girl is Rich not Lean. Be good to hear it held wide open to hear if it limits out or clears to a point. But two turns out on the pilot screw & sounding "that" rich has me wondering if the pilot jet itself is right home in the body. Either 'that' or the slide needle jet is very worn & its sucking fuel past that at idle & not unheard of. Or needle misplaced in the slide but not sure if that's possible. Some carbs the needle could sit on top of the plate, where it should be under it & retained by the plate. I'd think yours would have a screwed on tower for the cable so can't go wrong.
Last edit: 23 Apr 2023 15:20 by RT325.
23 Apr 2023 15:19 #14

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Other thing 'is' i think your tacho is telling porkies & reading about double if that's possible. Also, #80 sounds big for a pilot jet. But if 'that's' what it is mean't to be then all good & wouldn't go reducing it just yet but look for other possibilities--like i mentioned above.
& another one--choke being held off the seat [unlikely & would be obvious with seal cap misplaced lifting it. Also in theory more choke fuel also gets more air so idle would rise & its not far off stalling [stalling rich] in the short vid.
Last edit: 23 Apr 2023 15:28 by RT325.
23 Apr 2023 15:21 #15

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But wait, there's more lol.--If i expand the pic looking in the top i can convince myself the needle jet isn't square on sighting it against the idle bleed hole. If that's the case has it missed the guide pin in the bottom. Not far enough out of line for that--but!!, is the main jet in there & all sitting right--thick brass flat washer i guess they must have to hold it all down.
Last edit: 23 Apr 2023 15:35 by RT325.
23 Apr 2023 15:35 #16

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That certainly isn't the sound of an air leak and "runaway" revs where it revs to the moon and doesn't shut off with key. Not even close.

My guess is your CDI is bad. Possibly the box, most likely the stator. You could try cleaning all the connections where the CDI box plugs in.
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
23 Apr 2023 15:46 #17

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I think the pilot jet in my 400 is ether a 50 or 55
sounds good with no slaps or bad noises, perhaps too rich like RT said
23 Apr 2023 18:04 #18

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I guess #50 could look like an 80--small numbers etched in. 80 would look big looking through, difference to a 50 would be quite visible.
23 Apr 2023 18:54 #19

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Now I am confused and hope I didn't spread confusion and sorry if I did
I just looked into the manual and the Supplimental manual for the 78 DT400E from this site and it says pilot jet is 80.
Pilot get for my 360A listed as a #60 and I think I went up to a 62.5 or 65 to improve the you know what thing at light and steady throttle.
So 80 on a 400 seems to make more sense than me
24 Apr 2023 14:42 #20

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