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78 DT400E Air Screw

  • RamadiRat
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78 DT400E Air Screw was created by RamadiRat

This surely seems utterly stupid but just to make sure I'm not really confused: Adjustment of rich/lean is only accomplished with the air screw - which (of course?) on my bike is on the left side. It is in fact on the airbox side (not the intake side) of the carb - so, screwing it in enriches while screwing it out leans her out... Is this correct?

I do understand the possibility and affect of a worn needle, but since those don't seem to be available.. I thought I'd focus on this basic adjustment first.
25 Apr 2023 12:08 #1

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Replied by SKYDANCER46 on topic 78 DT400E Air Screw

Float level can greatly effect lean/rich condition too.

1970 DT250C
25 Apr 2023 12:28 #2

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  • RT325
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Replied by RT325 on topic 78 DT400E Air Screw

Yes in for rich & out for lean. Should never need to go far from 1.5 turns out unless there's another issue adding to the richness.
25 Apr 2023 17:28 #3

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Replied by RamadiRat on topic 78 DT400E Air Screw

Thanks - I'm going to go back into the carb and closely examine the float level, see if the current parts are genuine Mikuni (I assumed they are... not smart), etc.
25 Apr 2023 19:22 #4

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Replied by RT325 on topic 78 DT400E Air Screw

I wouldn't worry too much about float level there's a fair range between very wrong & very right, as in very low or very high to the point of making it run lean or rich. I ran the video again & would like to hear it held on full song for long enough to hear if it's limited & stays rich or tries to clear at the top end--then goes rich again when bought back down to idle. Pilot jet definitely right home in the carb body. Should feel positively seated but don't have to swing on it unless the jet thread is damaged & giving a false impression of tight. Look for something unusual. Everything goes through the main jet to get there except the pilot & choke. So choke is seating [never seen one fail] have a rubber pad on choke piston but fail safe in my opinion. Can you feel choke bottom out under spring pressure & hace a little free movement on the lever [or are they a pull button knob in which case No play]. Nothing ignition going on in my opinion, just very rich. Pulses sound too possitve to be a choked up exhaust.
25 Apr 2023 22:52 #5

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Replied by Andy_C on topic 78 DT400E Air Screw

For what it;s worth, on my DT400C dropping the needle one groove from stock made a huge difference, with it in the stock position it would stutter under acceleration - plug colour looks good so I have no concerns it is running weak.
The air screw on mine is currently 1.75 turns out and has minimal surging, any less than 1.75 it tends to surge / buck. It seems to be very sensitive to the air screw position, so if experimenting with it I would say turn it in 1/8 turn increments and note the affect.
Mine has stock air filter and exhaust, but does have a Rex speedshop conversion, all the carb jets / needle and slide are also stock.
Another thing that can have a big effect is the spark plug - is yours old / well used? if so then try a new one, in my experience of 2 strokes a failing plug can adversley affect the running. I did fit an iridium plug to mine, in fact I fit these in all my bikes as I genuinely feel that they run better with iridium plugs.
Just a few thoughts.
Ex Yam DT400C, Triumph Trident 660, Triumph 865, Kawasaki KH 400
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Last edit: 26 Apr 2023 09:46 by Andy_C.
26 Apr 2023 02:04 #6

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Replied by RamadiRat on topic 78 DT400E Air Screw

Here's a video of my bike, warmed up, with air screw at 1.75, held at rev.  Not sure if this is revved enough and/or if what you need to diagnose is a video of her revved while underway.  I test drove this, then backed her out to 2.0 and tested again, I think it was just a bit better, less/no bucking when I come off throttle and seems to rev further without starting to sputter and such.

I'm keen to try to move the needle down a notch per your suggestion given the fact that I can do that without wrestling the carb back off the bike.

I did check the choke as much as possible while all in place, it 'seems' to close all the way?

youtube.com/shorts/rn223hzmsPo?feature=share



Post/Edit: After running a bit, I checked the plug and she is black/sooty so... probably still quite rich?
Last edit: 26 Apr 2023 19:51 by MarkT. Reason: Insert video
26 Apr 2023 10:32 #7

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Replied by Andy_C on topic 78 DT400E Air Screw

Assuming that none of the airways in the carb are blocked and the float level is correct I would be inclined to take the choke plunger out and check the condition of the rubber plug although this goes against what RT325 says, those symptoms are very much like a choke that is not closing off properley.
I once had an MZ that showed very similar symptoms and that was the choke - same sort of design.
If you have had the carb apart I would also pull out the slide again and double check that the needle has entered the needle guide - sometimes it can catch and be pushed up into the space above the slide, meaning that the main jet is not being metered by the needle, that would give similar same symptoms - dont ask I know!
I guess that you have checked that the air filter is not obstructed or over oiled, again would make the mixture very rich.
Finally, if you run it too much when very rich it will eventually write off the plug - again my MZ experience, when I had the choke problem I wrote off a couple of sprak plugs - yes you can clean them, but for some reason after a few cleans when badly fouled they stop working.
You will get to the bottom of it.
Can I ask what silencer you have on your bike, it sounds a lot louder than my DT 400 C, just curious.
One final thought - is there any fuel leaking out of the overflow pipe on the carb? If there is your floats are sticking or the float valve is not shutting the fuel off - again would make things very rich.
Ex Yam DT400C, Triumph Trident 660, Triumph 865, Kawasaki KH 400
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Last edit: 26 Apr 2023 12:21 by Andy_C.
26 Apr 2023 12:17 #8

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Replied by RT325 on topic 78 DT400E Air Screw

Thanks for the video. Would be good to hear it rung out down the road to hear if it clears at revs. I'm terrible i know but i rev them hard to clear them, or head down the road hooking gears until it clears. Will it red line sitting still. Maybe i shouldn't even suggest that. Sorry if i'm trying to install my bad habits on you. Just go with what you feel safe with. Bit hard to see the red line with the red light in the way but looks 6.5 which isn't that high & should redline easily.
26 Apr 2023 15:31 #9

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Replied by MarkT on topic 78 DT400E Air Screw

Still sounds like timing is way off or something to me. But does sound better than before so RT325 might be right. Ride it and see if she clears out. You can spend a month doing what you're doing and if it's "loaded up" it won't ever get better unless you ride it and get it good and warmed up

Checking choke plunger is not a bad idea either. Carb doesn't need to be removed for that.
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
26 Apr 2023 19:55 #10

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