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Issue with 89 Yamaha moto 4 250cc


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My first thought is it might be running too lean by mixture setting or could possibly have an intake manifold leak. Im assuming oil is good and filter changed and getting good oil flow. When it shuts down and its hot will it start back up,  Also make sure all your cooling fins are not caked with debris. I dont remember if that model had a cooling fan i dont think it did, but if its there check for proper operation.

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If you have a timing light put it on and watch to see the spark isn't dying when it gets hot. Check the idle mixture, make sure that turning it either way has the effect it's meant to. Check the fuel flow to and through the carb is sufficient. Undo the drain bung on the carb and make sure it keeps flowing even after the bowl is drained.

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Posted (edited)

Yes it’s a mikuni, but the funniest thing happened, I put new gas in it and I couldn’t get it to die when driving. What a dumb solution. But I did find out that when idling it bogs down and dies after a minute or so, I’m thinking it could be getting too much gas so when adjusting the fuel screw do I tighten it or loosen it for restricting fuel?

Edited by Need-help-with-quads
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Gw's right about in being leaner, but try turning the mixture screw in and out making sure there is a sweet spot in the middle, and that winding it too far either direction makes it run bad.. That checks the float level's not too high or low, that there are no bad air leaks, and that all the jetting is about right. Checking it's responsive in both direction often reveals a problem.

And yeah, stale fuel is something to watch for.. We generally do suggest stale fuel if it sounds like the bikes been parked up for a while.. Good work changing that...

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Spray some carb cleaner around the intake where the carb attaches to the engine if it speeds up that is where your air leak is, you might also look for open vacum ports on the carb, and makes sure all the vacum hoses are attached to where they are supposed to go. Not sure if your model has a vacum operated petcock, some times they will leak.

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I think the mixture at idle is too rich. That would explain it idling cold but then cutting out once it warmed up. And winding the idle mixture screw in doesn't make it too lean by the sound of it... Both things point to a rich mixture.

Check the air filter is clean and oiled, and not over-oiled. Check the breather hoses on the tank and carb are fitted correctly and not blocked. The dieing when warm could also be a tight valve clearance so it would be a good idea to check those.

The idle mixture needs to be adjusted at the slowest idle speed it will idle steadily at. So you slow it with the idle speed screw then wind the mixture screw in and out till it is in the place that gives the highest idle speed, then you wind the speed screw out to slow it right down and you adjust the mixture to it's best idle again, then slow it, then adjust it, and you keep going around in a circle slowing it and adjusting the mixture. Once it's idling slow and steady then you can speed it up if you want to but leave the mixture where it's best at the place where it will run nicely with the least amount of butterfly opening.

If it doesn't come right, if the idle can't be adjusted in both directions with similar rev changes, and/or it keeps dieing once warm then I'd check the float level, which can probably be checked with the carb in place using a bit of clear hose, and I'd check the jetting.

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Engines, if they are too rich, tend to chug and run irregular and rough for some time, either a second or three or more than a minute, before they get slower and slower and finally die.

Engines that are too lean tend to idle quite regularly except every few seconds they give one single miss, then go straight back to a steady idle till the next one miss. They can sit idling with that intermittent single misfire sometimes without dieing, but if they are going to die it will generally be idling fine then suddenly cut right out.

Doesn't always show up like that, but mostly a single cylinder engine with an idle mixture that's not right will behave like that..

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Hey how should I reset the screws cause I wanna try that and then readjust it, I know the fuel screw is 2.5 turns but what is the idle mix screw? Also I had no idea about the float height all I did when I rebuilt the carb was screw it in as far as I could (the carb was old and had a little bit of white hard build up). And now it idles fine after I messed with it but now I want to reset the screw Bcs when I go into reverse it cuts out and dies and in first gear if I slam the throttle it also cuts out

Edited by Need-help-with-quads
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Are you sure the choke plunger  is going right in ?

I'd double check the idle speed screw and the initial setting of the idle mixture screw for that bike, and then try adjusting it to best slow idle, which would be a bit slower than we'd mostly have  it idling at. They need adjusting right down to a slow idle and the best mixture setting, then sped up with the speed screw to a good idle speed. It can happen that people get into a circle of adjusting the mixture too lean or too rich and then speeding it up to compensate..  That circle can end up with things being adjusted right to the end of their range, and then the spring holding the screw from vibrating out, becoming loose, and the screw dropping out..

If it didn't come right then I'd pull the idle mixture screw out and make sure it's O ring and spring and washer was on right, and that the taper was in good condition and looked the same as the original(check the shape carefully !),, then pull the slide and check/inspect the discharge tube down in the venturi was poking up a little, and I'd check the slide needle was secured in the slide properly. If all that looked right I think I'd take the carb off and check the float height and jets hadn't been muddled, though it's generally pretty difficult to get them muddled.

When the carb is set up right the idle mixture screw should be about two turns out as a starting point, and then you adjust it when it's warmed up. It shouldn't go right in to a stop without leaning the mixture so much the engine dies, and you shouldn't wind it out till the retaining spring has stopped holding the screw, and at that stage it should be too rich.

 

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I just pulled up a diagram of your carb. i cant really tell which screw you are pointing to with the screwdriver, but the fuel mixture screw is # 16 on the diagram, and # 7 is your choke plunger, make sure its not stuck. That carb does not look like its been rebuilt recently, you might want to pull it and make sure everything is right.

 

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