DR 250 erratic idle

I have a 1990 DR 250S that idles erratically. If I turn the idle screw in, the idle stays the same no matter how far it turn it, until the throttle is blipped, then it settles at 4000 rpm. Then if I turn it out slowly, it'll finally drop to about 1200 again but it tends to stall easily. In adition, if I don't turn it down far enough, it will stay reved up when shifting gears. There is no in between. The bike runs great otherwise. Any ideas?

I'm not sure which screw you're referring to.

There is a air mix screw which controls the idle/pilot circuit and there's also an idle adjuster (the big thumb screw) which physically changes the position of the throttle stop.

You may possibly have dirt in the pilot circuit which is very common.

You need to post some more info... did this just start happening. did you modify anything, is the bike new to you ect:

Heres a thread that addresses what I believe to be the same problem you're trying to explain.

http://www.thumpertalk.com/forum/showthread.php?t=699759&highlight=revs

I read the thread you suggested, but I don't think the choke is the problem. I have adjusted the air mix screw and that doesn't seem to affect the problem I'm dealing with either...It makes a difference with the way it runs but doesn't cure this particular problem. I recently bought the bike but it didn't run...the carb needed to be cleaned. I cleaned it thoroughly and now it starts easily and runs great other than the idle problem. The bike is completely stock except for an FMF muffler and the snorkle removed from the airbox top (it did the same thing before the muffler was replaced, and wouldn't run well with the snorkle in place with the stock or aftermarket exhaust). It has a 135 main jet but the pilot jet is stock. The plug electrode is a nice tan color which means the jetting should be ok. I have adjusted the large thumb screw (idle screw) but the problem I discribed earlier remains.

I thought of that but it does the same thing with the stock or aftermarket pipe. Also, I checked the link regarding the way to check if the pilot should be changed and the bike runs best with the air screw turned out 1 full turn from seated, so it should be ok.

Make sure the throttle cables are in good condition and not binding up. Check for a bit of slack in both cables with the throttle closed (off). Rotate the handlebars lock to lock and see if you still have enough slack.

You should have the vacuum slide carb on a stock 250s, make sure the plastic throttle slide moves up and down easily with no rough spots. Check the rubber diaphragm on top of the throttle slide for rips or tears. Make sure it is seated correctly under the cap and not pinched.

Other things to look for are air leaks around the carb boots.

Clean or replace the pilot jet. Check the pilot mix adjustment screw, remove it and spray cleaner in the passage. look for a jet of cleaner spray coming out a tiny pin-hole on the engine side of the throttle butterfly plate. Make sure you have the spring, metal washer, and rubber o-ring (in that order) on the pilot mix screw.

I have check all of the above several times and everything looks fine.

Look at http://fiche.ronayers.com/Index.cfm/Module/Main/TypeID/26/Type/Motorcycle/MakeID/2/Make/Suzuki/YearID/31/Year/1990/ModelID/6650/Model/DR250S/GroupID/281163/Group/CARBURETOR_1990-1991

Ref# 19 in the picture shows a small o-ring on the plastic float bracket. Replace if worn or missing. Part numbers in that diagram do not seem to match the parts list, so be careful if you do try to order it.

Have you already checked the fuel petcock on the fuel tank? Plenty of info here on older posts about them leaking fuel into the vacuum line and causing all sorts of poor-running "carb" symptoms.

When I cleaned the carb, I inspected all the o-rings and everything looked fine. The float level was a bit high, so I adjusted that as well. The petcock checked out too. I even unhooked it an plugged the hose just tomake sure.

I'm not sure this is happening, but it almost seems like when the idle screw is turned in, the throttle plate moves slightly (as usual) and possibly unblocks a small passage in the carb body which in turn lets in more fuel and creates additional vacuum which causes the slide to go up without the throttle plate being opened any further. Does that seem possible?

Best idle is with the stock jet (37.5) or possibly a #40 with the idle mix screw set between 1.5 to 2 turns from the lightly seated position. Then you would adjust the idle speed screw for a steady speed of 1800 rpm, once the bike warms up. Some fine tuning of the mix screw will be needed to get a stable idle speed. If you have a blockage or incorrect idle jet installed, you may have to adjust the idle speed screw too high to keep the bike from stalling. If it adjusted too high, it may cause the idle speed to wander around by itself. This will happen easily if the idle mixture is too lean.

Keep in mind there are two different series of pilot jets for the mikuni carbs. One has holes near the top (cross-drilled holes) and the second type does not. People have bought the wrong kind and had problems with the idle speed. If your pilot jet is stock you should not have this problem. I believe the stock vacuum carb had the pilot jets without holes. It has been a while but I think this was the correct type: http://www.motorcyclecarbs.com/MIKUNI_PILOT_JET_N151_067_40_P2614.cfm?UserID=788499&jsessionid=7e302939181187803788882

The carb is squeeky clean and it turns out that it has a 40 pilot jet. I tried adjusting the air screw and it still runs best at 1 turn out. Anything futher out and its extremely hard to start and any further in and it runs like crap. Either way, the idle problem still remains.

If the carb already has a 40 pilot, it has been changed by the P.O. Does it look like the spring on the throttle slide has been cut or modified in any way? Maybe the P.O. drilled the vacuum holes in the slide too big? Is the throttle butterfly shaft worn out and possibly leaking in air?

You could start with a new throttle slide / diaphragm and spring to see if it cures your problem. It sounds like your pilot circuit is OK if you can adjust it too lean and too rich with the mixture screw.

The bike has very low hours/miles...everything in the carb seems tight, the diaphram doesn't have any pin holes and the spring doesn't appear to be modified. This is getting really frustrating. I can adjust the idle screw in numerous turns and it doesn't change the idle at all until the throttle is blipped, then its sky high. I do have a used pumper carb but its for a 350 not the 250. Since the inlet and outlet sizes are different, and the throttle tube and cables are also different, I don't want to spend the money to change everything over and have it not cure the problem.

Unless you have an obvious air leak anywhere in the carb or intake boot, it looks like your problem is probably the vacuum throttle slide. Small adjustments to the idle speed (butterfly) should have the same effect on the vac slide. It sounds like it gets stuck in the closed position, then opens abruptly causing the high idle. You should be able to leave the airbox boot off and start the bike. Watch what the throttle slide does when you slowly raise the idle speed. It should move slowly and smoothly as you turn up the idle speed. Make sure the bike is clean if you try this, you don't want the open carb to pull in any dirt.

The vacuum carb is actually very easy to setup and tune when everything is working correctly. It is less sensitive to jetting errors compared to the dirt model pumper carb. Keep in mind the used pumpers have their own set of problems, the worst of which is a worn throttle slide (body). When the carb body wears out, the carb is junk. I have a "used" one with this problem. Nothing you can do to fix it, I bought a new carb off e-bay to replace it. As you know, you need the intake and airbox boots off the dirt models to get the carb to fit. E-start models need a 1/2" spacer on the engine side to clear the starter motor.

I pulled the airboot off and watched the slide and it did not move when I turned the idle screw in, but stayed up when the throttle was blipped. I took the carb apart again and inspected the slide, diaphram, slide guide and needle. Of all those parts, the needle is the only one that showed any signs of wear (Extremely minor). Everything moves smothly and easily...nothing seems to be sticking whatsoever. Still stumped! The float level still seems a bit high; could that be creating the problem?

Float height is spec'd at 14.6mm. Invert the carb and measure from the top of the float to the mating surface of the carb body/ fuel bowl. Lower the float slowly until the needle valve seats itself and take the measurement. Bend the small metal tab (carefully) that pushes down on the needle valve to adjust.

I posted a cutaway diagram of the BST carb in a different post, maybe it will help. Look for a good spray of carb cleaner out of the small pin-hole idle port just in front of the throttle butterfly. See the red arrow near # 10 below: Remove the idle mix screw and shoot the carb cleaner inside.

#8 is the pilot jet

#9 is the pilot mixture screw

# 4 is the small air hole in the throttle slide that will draw a vacuum above the slide diaphragm as air through the carb throat rushes past it. This hole (or holes) is what will cause the vacuum throttle slide to "rise".

BST_CV_CARB_CUTAWAY.jpg

You can still have a partially plugged idle circuit causing little or no fuel flow at small throttle openings. Once the throttle opens enough to raise the vacuum slide a bit, it begins to draw fuel from the main jet / needle and keeps the bike at the high idle. The only other thing i can think of (last post) is the vacuum slide somehow sticking and not moving freely near the bottom of its travel. I hope you find out what is wrong with that thing.

I gave you some bad info previously and found out something new. Unfortunately, when I did the slide test with the boot off, I was in such a hurry and it was almost dark that I didn't see what was actually happening. Today I tested it in bright sunshine and found that the slide does not move at all when the idle is adjusted higher (as I mentioned before) and does not move if the throttle is turned up slowly. But when the throttle is turned quickly, the slide will move up then comes down just as quickly when the throttle is closed but the idle will stay very high if the idle screw is adjusted to high. Also, when the bike is at idle, I can move the slide up with my finger and the bike does not react at all. Another thing I found was when I blocked the larger jet at the rear of the carb that goes to the pilot jet, the bike would die; but if I blocked the the other smaller one that goes to the needle/main jet, there was no change whatsoever. I also realized that when the bike is at idle, the tube that comes from the crankcase to the airboot blows out air, but when the bike is reved up, it doesn't. Is that normal? I thought the vacuum in the airboot would suck crankcase gases out. I took the carb apart again and found the pilot jet is actually a 45 (my eyes aren't as good as they once were). I have a manual and it indicates the stock pilot should be a 40 and the main jet a 132.5 (mine is a 135). Does this info help?

I forgot to mention that all the passages and jets are clean and clear.

If moving the slide with your finger has no effect on idle, the extra air (causing the high idle) must be coming from somewhere else. It still looks like you may have an air leak at the boot/carb junction or the boot / cylinder head area (rubber o-ring seal). I have used carb cleaner spray with good results to check for air leaks. Just make sure you have a fire extinguisher in case something happens. Do this outside, away from anything flamable. Be careful.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now