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

DT250 not running rich anymore, but I'm scared... 16 Apr 2019 07:36 #11

  • JimGoracke
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I think that may be it! I don't remember running a brush through that hole, and the carb was pretty covered in varnish.

Just to satisfy my need to understand the inner workings...
That air bleed meters air for the main jet, so if it is clogged as I open the throttle more fuel would be drawn from the jet than there should be because there is no air to reduce the vacuum. Does that sound correct?
Would that also affect wide open throttle? It seems like that little air bleed wouldn't impact the fuel draw when the throttle is wide open and the maximum amount of fuel is being sucked through the main jet.

Thank you so much for your help!!!
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1976 DT250

DT250 not running rich anymore, but I'm scared... 16 Apr 2019 12:30 #12

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You ask about the tool tray obstructing the air flow and yes it does. Also you cannot use a twin air filter or a heavily oiled sponge filter of any kind The best filter I have tried for these bikes is a lightly oiled K&N The airbox system is good to keep dirt out but it also keeps air out. There is a noticable difference in power and intake noise with the tool tray removed
Doug

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DT250 not running rich anymore, but I'm scared... 16 Apr 2019 13:45 #13

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I’m running the K&N. It’s been OK with the tool tray removed. Maybe I’ll just remove the rubber part of the intake from the air box so I can keep the tool tray. I don’t expect to cross many deep streams, so I can’t see any harm in it.
All of those yellow DT400s in your avatar make my head spin. I’d like to paint my 250 yellow, but I hate to change it from original.

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1976 DT250

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

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Jetting is a fickle art " ,your getting there, i work the TTO plug temp sensor, so i can see the difference in jetting and Air screw adjustment, i work old socks on the air filter cage, works for me,[ one full sock and half a second sock ,, main is 180, RUNS AT 360f , And if you close the air screw 1/4 the temps drops to 340f, , amazing to see the difference adjustments make ,
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If you aint mixin gas " you aint haulin Ass
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DT250 not running rich anymore, but I'm scared... 16 Apr 2019 14:07 #15

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go really light on the oil on the K N also if your on pavement take it for a ride with no filter in place Thats how I finall figured it out. Its unreal how much dust those bikes collect in the intake area. I know what you mean about the yellow and black and the other nice colour is the 72RT2 Raider colours

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DT250 not running rich anymore, but I'm scared... 16 Apr 2019 19:10 #16

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JimGoracke wrote: ...............................
That air bleed meters air for the main jet, so if it is clogged as I open the throttle more fuel would be drawn from the jet than there should be because there is no air to reduce the vacuum. Does that sound correct?
Would that also affect wide open throttle? It seems like that little air bleed wouldn't impact the fuel draw when the throttle is wide open and the maximum amount of fuel is being sucked through the main jet.
......................................


<sigh> I'm not sure if I should go into the answers to your questions...

The answer is "it depends"... Is the main fuel system the "primary" type or the "air bleed" type?

Here are the two main types of emulsion tubes (aka needle jet aka main nozzle) used in the Enduros. The top one is the "air bleed" type and the lower one is the "primary" type. Primary type is most common in early Enduros... later models sometimes have the air bleed type.



On the "air bleed" type the air coming in from the air jet is designed to mix with the fuel through the series of little holes in the tube. A bigger air jet will result in a leaner mixture. At low RPM, the bigger air jet will have little effect on the mixture strength. As the RPM increases, the bigger air jet will lean out the mixture progressively more. So you can see it gets complex... if you needed the lower RPM to stay richer and needed to lean out at higher RPM, you'd use a bigger air jet passage.

On the "primary" type... which is most common on these bikes... the air coming in from the air jet is not designed to mix with the fuel like it does in the air bleed type. The air enters into the hole in the back of the tube and comes out an annular channel around the fuel passage in the center of the tube. More air (bigger air jet) on this design results in a RICHER mixture. And again, the difference in mixture strength is small at lower RPM and the larger air jet richens the mixture more as RPM goes up and more air is flowing.

(I learned this from many years of experimenting and tuning... and what I said about "more air = richer" on the primary type can be proved with a simple experiment if someone doesn't believe it.)

The picture I posted earlier of the pressed in brass jet with the tiny air jet passage is not typical on a Primary type carb... usually the passage is pretty large and drilled right into the carb body with no brass jet needed or used. Air Bleed type carbs are more likely to have small air jet. Again, air jet (or drilled hole) I'm talking about is located in the lower center of the carb bell. (red arrow below)






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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... 17 Apr 2019 12:04 #17

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Fickle art indeed...
That sensor is really cool. I just ordered one:) It'll make me feel better about suddenly going lean and burning a hole in my piston.

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1976 DT250

DT250 not running rich anymore, but I'm scared... 17 Apr 2019 12:12 #18

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I appreciate you taking the time to explain.

Mine is the primary type, so a clogged air jet is not likely as that would make it run leaner rather than richer.

Your explanation makes sense. In the air bleed type the air is mixing with the fuel, so more air means less fuel and a leaner mix. In the primary type, the air is helping to draw the fuel up the tube, so more air means more fuel and a richer mix.

I checked the passage just for fun and it was clear. I went ahead and ran brushed through everything again but it's still the same.
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1976 DT250

DT250 not running rich anymore, but I'm scared... 17 Apr 2019 12:23 #19

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Quick update...
I did a plug check and it is still pretty black, but not wet. I turned the air mixture out to 4.5 turns out and it runs even better. I'm going to go down to a 140 main to see if that makes a difference.

I understand how the circuits overlap, so I know the pilot jet should not impact 1/3 throttle and up, but it seems like if I moved down 4 sizes in the main jet, I should go with a smaller pilot jet. It is still the original #30. It starts on the first kick and idles perfectly, so should I just leave it or would going smaller help with my richness across the board. I hate having the needle on the top groove.

I need to get more new plugs to keep testing. Anyone know a way to get plug ceramic clean enough to reuse for a plug check?

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1976 DT250

DT250 not running rich anymore, but I'm scared... 17 Apr 2019 15:50 #20

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The emulsion tube is definitely more complex than I thought. How are they typically held in? When I cleaned my carb I tried pressing it out (from the main jet end, pressing up). I even heated the carb up first, and pressed it smoothly in the vice. I put more pressure on it than I liked with no movement and finally gave up. I can see what looks like a brass pin going into the aluminum, sideways just above the main jet. Does that need to be drilled? Extracted? Pressed from the top down? I don't like thinking there's goop still lodged in there, but don't want to destroy the carb either.

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