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TOPIC: Getting my DT400C up to snuff

Getting my DT400C up to snuff 11 Jun 2016 09:08 #21

  • Blackhat250
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Meis figures are correct, I seen 438 last week and it was hot, rejetted & airscrew adjustments and shes running 340-380. and a long uphill seen 405,

great add on this trailtech temp guage . B)

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If you aint mixin gas " you aint haulin Ass

Getting my DT400C up to snuff 11 Jun 2016 13:37 #22

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This morning I verified that my compression release mechanism was working correctly. The kicker lever does indeed activate it. I removed and cleaned it and it seems to be fine. The sealing washer isn't pictured in this photo.



Then I took the gas tank off just to inspect what's all going on under there. Everything looks factory stock. The only weird thing I found was where they changed the wiring to handle the DT250 points setup instead of CDI behind the plastic cover under the oil tank. Not really the way I'd do it, but it seems to work. I sprayed that stuff down with DeOxit to clean it as much as possible.



Finally, I built myself a leak down tester. Last month I'd ordered an aluminum coupler for my truck's cooling system and it ended up being too small. It turns out it's the exact diameter of the carburetor, so that was a welcome surprise. I used a freeze plug in one end and installed a Schrader valve and a brass nipple for my vacuum gauge and we were off to the races. I stuck another freeze plug into the exhaust port on the cylinder. The freeze plugs and fittings were just sourced locally at Napa. If any of you want to order the aluminum coupler, here's the amazon link:

Aluminum Coupler Tube

Here's what I ended up with:



And here's the video I made while testing leak down. I lost just over 0.5 psi over the course of 2 minutes. Is this good, bad, indifferent? Right now I'm keeping a close eye on the level of my clutch side oil because I want to make sure the crank seal isn't breached.



I forgot to add that I did a compression test on a warm engine and got a little over 130 psi just kicking it over a few times with the throttle pinned.

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Last Edit: by luv2stroke.

Getting my DT400C up to snuff 14 Jun 2016 07:31 #23

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0.5 psi in 2 minutes certainly isn't "good", but it's probably not going to give you a seized piston unless you're really hammering it. You'll find different opinions on an "acceptable" leak rate for a 2-stroke. If you have a flywheel/magneto puller, I would try removing that to see if your left crank seal is the culprit. Also, your video mentions spraying water. There was some dish soap in that water, right? You seem to know your stuff, so I doubt this is the case, but thought I'd check.

Great looking leakdown tester there, btw! If that's my bike, I'm using soapy water, and looking EVERYWHERE for the source of that leak. A couple more tips that could help... try upping your pressure to about 6.5 psi. That's not in the danger zone for popping seals, but might give you better luck in finding the leak. Another tip is to use an air compressor w/ a regulator on it... hook up the compressor to the leakdown rig AT ZERO PSI.... then slowly bring up the pressure to about 6 psi. This will be feeding your motor a constant supply of air pressure, helping even more to find your leak. The intake boot and base gasket of your cylinder can be common places to leak, so really try to hit those from all angles, and use a good flashlight to really look over your seams. If nothing is showing up, then it's probably time to access those crank seals to see which one is leaking.

One last question... if you leave it hooked up long enough, will it eventually go to zero psi, or will it "bottom out" somwhere higher than that?

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Getting my DT400C up to snuff 14 Jun 2016 07:59 #24

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I didn't actually run a test to see if it'd zero out all the way. I'm going to have to hook it up again to check my case seals I think. I do own the 27mm x 1.00 left thread puller, so I think that's the right one to pull my DT250 rotor.

Yeah, I was using dawn dish soap in the water. It definitely worked to find leaks when I purposely loosened the spark plug to test it. I sprayed water on the intake, the tester itself, the exhaust port, etc.. and was not finding bubbles. The one place I didn't spray water that I should have was along the vertical case seal. I found a youtube video showing a leak at the frame mounting bolt! I didn't even think to check there. I'm going to be pretty bummed if I end up needing to pull the engine and split the cases. Check out this video:

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Last Edit: by luv2stroke.

Getting my DT400C up to snuff 14 Jun 2016 08:10 #25

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Another issue I'm having is that I think my carburetor may have been modified. I took this photo looking in the throat when I had it apart.



I am still curious what the red port I've highlighted does. It's hollow and goes back into the carburetor but I couldn't get carb cleaner to exit anywhere when sprayed in there.

Then I got curious and tried to find a diagram online and noticed that other peoples DT400 carburetors have a brass plug in that left air passage. Now I'm concerned that mine is not 'right.'

Here's a photo of one that's on ebay right now. Others on there also have the brass plug in the air passage.




Can any of you shed a light on why someone would have drilled out the plug?

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Getting my DT400C up to snuff 15 Jul 2016 15:56 #26

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luv2stroke wrote: I am still curious what the red port I've highlighted does. It's hollow and goes back into the carburetor but I couldn't get carb cleaner to exit anywhere when sprayed in there.

I can't answer that but here is a link to a Mikuni manual on our carb.

I too have an enormous issue with the broncin' buck. I am to the point that I think the carb is the absolute reason for it.

And unlike some of the others who say this is normal for this bike, I don't agree. I owned one brand new in 1975 and drove it for three years. I never had any surging issue with the bike. I suspect some d***head modified the bikes having this issue. Most likely a performance change, an engine port, a carb parts change, who knows. I think the surging can be completely eliminated, but it is a very difficult process.

Good luck with yours.

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1975 DT400B
Indialantic, Florida

Getting my DT400C up to snuff 16 Jul 2016 15:28 #27

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if you have the bucking and jerking on a 1975-76 DT400 when rolling off the throttle, I suspect it has a 3.0 throttle slide cut away which is a bit lean,

changing it to a 2.5 throttle slide would help in this area. another way is get what is a UFO installed on the throttle slide.

if you do this it will require a leaner pilot jet but if it all done right the bike will throttle down much smoother.

Racerclam offers this service installing a UFO along with some other very trick carb mods.

you can also go a little richer with a richer pilot jet but this only masks the problem a little and makes deceleration a bit smoother but the jerking will still be there a little. I would go up 1 jet size or maybe 2 sizes up if it were me.

it is better to tackle the lean issue due to throttle slide itself with either the UFO or 2.5 throttle slide.
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2 1968 DT1s,1970 RT1M,1970 DT1C,1971 RT1B,1971 Honda Z50K2,1974 DT100A,1974 Honda MT250,2 1975 DT400Bs,1975 Honda CR250M1,1978 DT175E,1979 Honda CR250R.
Dave
Jesus is Lord!
Last Edit: by Bigdog302.

Getting my DT400C up to snuff 12 Aug 2018 12:52 #28

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Alright, Alright, Alright!

Its been a couple years but I've had time finally to work on my bike again. The bike had sat so I cleaned up the tank and the carb. I took it for a spin and it took off like a scolded dog but still suffered from the same issue. When accelerating it ran great but when letting off to maintain a certain speed, it would lurch really badly.

In a moment of clarity I remembered a HS classmate of mine was a motorcycle mechanic. I hadn't talked to him in 7 years but I gave him a call. He impressed upon me how important air leaks were to avoid. Like luv2dtroke, my intake joint was really cracked. Upon looking i could see where several of the cracks went all the way through. I swapped intake joints with my other bike, in much better shape.

It ran great, no more lurching! I could run it at a steady 30 mph, 40 mph, 50 mph and it was smooth! Well, as smooth as a 2-stroke 400 with motoX tires can I guess.

Now I have to get the other intake joint air tight. I was thinking rough up the rubber a little and applying a rubber coating (flex seal).

vic

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Getting my DT400C up to snuff 13 Aug 2018 10:56 #29

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I've had 14 dt400 b and c's that I have done and no 2 were exactly the same but all but one had this bucking on deceleration. I also believe its carburetion because if you pull the clutch in it will drop to idle and not detonate so it is probably got to do with the velocity of the air going thru the carb when the engine is revving during deceleration. It must be picking up fuel to cause a lean detonation or bucking. I find these bike to be very sensitive to temperature changes. it was 38 centigrade here the other day and it was running lean and hot and later that nite when it cooled off it was good. I think it was a head temperature issue. All you can do is make sure the choke is sealing good and I find you have to keep the idle as low as possible and make sure the needle and seat is adjusted and working correctly.
Doug

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Getting my DT400C up to snuff 13 Aug 2018 14:43 #30

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I find closing the airscrew slightly or upping the pilot jet size ,calms the bucking on over-run,,
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