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Hey guys, this is my first post on the site so hopefully I don't sound like a noob. I searched a little for 1979 carb tuning and other posts but couldn't find a whole lot. So here goes...
A guy gave me a 1979 DT250 last year and I have been working on it ever since. Most things are in pretty good shape but now I am on to cold start problems. The Mikuni carb it uses has both a pilot air screw as well as a fuel mixture screw. Don't bother trying to educate me on what each screw is, I promise you it has both. I will add a picture.
I had some trouble trying to figure out which ports on the carb are to be plugged but I think I got it right, one of them had a broken cap. It is the port my buddy's 77 DT250 uses for oil injection.
Anyways, it has been running well and not, up and down, as I try to fix each little problem that arises. First it was the spark plug, then I just ended up rebuilding the carb. The pilot screw I got with the rebuild kit has threads that are too big so I am sending it to someone to die it to an M6.
Long story short, I have had troubles getting it started cold with the choke open. I have to kick it over several times and open/pump the throttle to get it warm faster. It will kick over and start up and even idle for a few seconds but will inevitably die. I have to play around with it quite a bit to get it running constantly (seems like it runs fine after it is warmed up a good bit).
Is this to be expected with an old bike? I have a clean air filter, my carb should be darn clean, and I have good compression (maybe I should test that again). What kind of adjustments can I do to get a better cold start? I have played around with the air/fuel mixture screws but it is kind of a crap shoot. What my bike seems to like is air screw in all the way and fuel out about 3/4 of a turn.
On that topic...how do I tune my carb with two mixture screws? And is it acceptable to have the air mixture screw turned in all the way? I tried starting both at a turn and a half out but that did not work. I have also checked for holes in the carb/tubes/stuff with carb cleaner while it is running to see if the idle spikes but haven't found anything. It seems air tight.
And along those same lines, when I am riding it around, it seems to do okay. Actually, it was really good on Saturday. But I am finding that it reaches this point after warming up where it stays idling pretty high after I have come to a stop. I think the clutch plates get stuck* so it will continue to engage with the engine while I am rolling but after I stop it will continue to idle high until I slow it down by engaging the clutch a little again.
Well sorry for so many questions, and thanks in advance for any help. I love the bike and hope to do a lot of motorcycle camping in the Utah mountains this summer so I am looking forward to having it run well.
Notes on pictures: (there is a little mud on the bike from Saturday)
#1 The left side of the carb is where the mixture screws are. Screw left of the idle screw is fuel, right side is air screw. It has a phillips head black screw. Story with that is it had to get drilled out but I couldn't find a replacement so I tapped and died the hole and screw and tapped a smaller phillips head into it so I could still turn it.
#2 Other side of carb has the oil injection. I threw some JB Weld gasket maker on the hole to the bottom right of the carb where there was some plastic broken off (how I received it).
Genuine Mikuni/Yamaha parts, are still made for these carbs, so my first suggestion is this: Have patience, order the right parts, and don't be "tapping and dying" new screws into holes where you have changed the threads. Wait, and do it right.
My guess is that now, your air screw is leaking, or not working or not working as designed, at any rate. And it sounds like you still may have a plugged pilot passageway, if not the pilot jet itself being plugged. Clean the carb again, blow compressed air through all the passageways, and make particularly sure the pilot jet and passageway are free and clear. That's the first step toward a good idle circuit.
Thanks for the suggestions. Yeah, I'm not changing the carb at all, I just cleaned the threads when a shop drilled out the pilot screw because it was stuck. I'm just modifying the pilot screw I was sent with the carb rebuild. I've been having a hard time finding out what carb my bike has since most rebuild kits say 1977-1979 and it seems the pilot screws are different sizes. Bummer with the DT series is they changed the bike up a bit in 1979 and then promptly stopped making it. So it has been difficult finding parts...
In poking around eBay and Partzilla, I see your problem. Not much out there for that unit. I apologize for not understanding right away.
Next step might be to get the numbers off the carb and see if anyone knows anything based on that info. Like, MarkT says he has a box of carbs and parts out in his storage unit; maybe he has what you need...