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84 XR250 idle

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Hey guys, this is my first post. I like this site, it has lots of very helpful information. I am rebuilding an old  84 XR250 that I purchased not running. Had to rebuild the motor completely. I now have it running but the only problem is the idle. The bike start very easily, throttle is good from 1/10 to full but it has a hard time idling, I got it now almost there. The slow jet had a 48 in it when I first opened it and I changed it to 45 which is standard for the bike. The bike will idle but after some time or if I play with the throttle a bit the idle speed just goes up, like way high and stays there until I cut off the engine. It is a twin carb and I know that they are hard to tune, I've read bunch of threads about it, it helped me get it to where I have it right now which is really close. Anybody knows why the idle would increase on it's own and stay that way. By the way I changed both needles in the fuel bowl, new gaskets, adjusted fuel bowl at 18mm, cleaned jets, etc... carb looks new

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A hanging idle can indicate an air leak. Make sure the front and rear rubber carb boots have no cracks and are sealed well with the carbs. Make sure the o ring between the head and intake manifold is in good shape. #15 in the diagram.

http://www.servicehondapsn.com/fiche_image_popup.asp?fveh=131776&section=2487526&year=1984&make=Honda&category=Motorcycles&dc=5924&name=CYLINDER+HEAD+84%2D85

You can probe the areas with a long piece of fuel line with the one end held to your ear to listen for leaks or use a carb cleaner spray and spray around the areas and listen for the engine idle to change.

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my intake boot is brand new, I suspected that could be the problem, the original one was broken, brittle, and old, got a new one from ebay, there are not cheap items, came with new gasket as well, that didn't fixed it tho. sorry, I forgot to post that in my first post. Any other ideas of what could do this, I'm running out of ideas, I tried so many things and still not idling well yet,   

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You say you adjusted the fuel bowl to 18mm. I assume you mean the float. That sure seems like a lot. Where did you get that spec?

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Yes, I meant the float, I have the Clymer book, it says 18mm is float height to spec (on both carbs). I read some thread that recommend 19mm for crisper throttle response but for right now I am just trying to get all the chances on my side by sticking to stock. I just went starting it again, it idles well until about a minute then the rpm seems to go from 1000 to 2000, I have to cut of the motor to stop it, I don't want to overheat it so I don't let the engine rev that high for too long. I think I will try to play with the float height a bit and see if I get any changes. it is mingboggling to me that the rpm goes up like that, I just cant figure out what it is.

 

And yes, 18mm seems like a lot. I also have a yz400f that runs great now, the fuel bowl adjustment was to 9mm, quite a difference, lol

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When I say that seems like a lot I mean it seems like that would make for a low fuel level in the bowl, making a lean condition and causing the hi idle. You're talking about going even leaner on the fuel level at 19mm.

Did you retune the fuel screws after swapping in the leaner 42 pilots?

Edited by Trailryder42

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I've got an dual-carb 85 with stock jetting and float height and it idles pretty good (stock 85 main jetting is a little different from stock 84, but the slow jets are the same).  The change in idle speed sounds suspiciously like what happens if you activate the "choke" lever (the "choke"  is actually an extra air path).  When I last went thru my carbs, I replaced the o-ring at the top of choke linkage rod because the old one looked bad. Don't know if this presents a leak path, but it should be an easy thing to check. 

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It does sound like an airleak. I have rebuilt the same model and something that cost me alot of time mucking around with was there was no spacer between the inlet manifold and the motor.  Also maybe a silly sugestion but can you just turn the idle down as the engine warms?  I always turn mine up for starting then progresively turn it down over the next 5 minutes or so as the engine revs increase.

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I solved a surging idle by opening the fuel screw 1/2 turn.  I would check the fuel screw, o-ring, metal washer, spring, and its passages.  

 

Also, is the secondary carb fully closing and staying closed at idle?  The dual carb setup is really a two barrel carb with a primary and secondary.

Edited by Baja Rambler
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Thanks for all the advices guys, really appreciate it. Will inspect the carb this weekend. Hope this will fix it. Will let you know what happens, thanks again

 

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