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1998 Big Bear running lean, real lean.


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I am working on a basket case for a friend.  It is a 98 Big Bear (YFM350FWB).  It came with a parts machine which is a 96, which I am coming to find out is VERY different from his machine.

 

Anyway, first thing is first.  I checked compression, 150psi.  Valves are set correctly, and the engine is in time.  I cleaned the carb, and there was some junk in the passages.  It is a chinese carb, which I figured from the lack of markings or lettering altogether.

 

The bike runs.  It idles ok, but breaks up badly under acceleration.  Pulling a new spark plug shows it is running lean.  White strap, with no black whatsoever.  So I have the carb adjusted with the mixture screw out way too much (about 5 turns), and the diaphragm needle lifted to the top slot.  It is still not getting enough fuel (or too much air).  It backfires through the carb, and cannot get above idle without carrying on.  The carb boot is in good shape, so it isn't pulling any air there.  I did test this as well by spraying ether around the boot when running, looking for intake leaks.

 

Anything big I am missing here?  I am starting to believe that this chinese carb is the issue.  This quad came with the original carb, which I would like to rebuild and use, but the mixture screw is totally stuck, and stripped way down in its recess.  

 

Thanks for any assistance.

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You've got the slide needle "lifted to the top slot" ? I hope you mean you have the needle lifted, which would be with the clip in the lowest groove on the needle. You want the needle raised, not lowered..

Have you completely stripped and checked the carb right over, checked every jet and O ring is in there, and that they seem to be about the right sizes/in the right proportions between the fuel jet and it's corresponding air jet, that the float level is right, and that plenty of fuel is getting in through the float needle ? It will be either not getting enough fuel, or getting too much air..

If you put your hand across the back of the carb and only let a bit of air in between your fingers, will it rev up then ?

The genuine carbs nearly always run better than any after=market one. I'd try fixing the old one.

I've got the idle mixture screws out before by drilling them. First though, most of the screws are in a sort of aluminium tube, and it's often just a thin walled tube in part. Often at the front side of the screw's tube there is a thin walled bit. If you gently tap that bit, as much as is possible, with a punch and tiny hammer, before you drill the screw out, you can spread the metal and expand the hole slightly. Because the screw is only fat where the threads are, you can expand most of the metal that's around the threads.

If you look at a mixture screw you'll see the threads only go down a certain way before the needle starts, and the spring is. That's the part that's seized and we can tap the tube around quite a bit of that. When/if you do drill,  don't drill so deep that the drill hits the spring. Drill it first with a very tiny drill and stop if it's not exactly in the center. If it's off center, drill that shallow drilling out with a slightly larger drill so that you can then go back to the smaller drill again and drill at an angle with it to correct the centering. Once the tiny drill is getting across to the center position you enlarge the holes and then go back to the tiny one again. Keep doing that. By putting the tiny drill down a slightly larger hole you can realign things. Once you have a hole going down to the right depth and in the center then you drill it out to a size that will just leave a thin wall of threads and then use an easy-out or a screwdriver driven gently in to wind the weakened screw out. The screws are soft and drill easy. Once the center is out of them they loose their grip in the hole.

Oh.. It normally only effects them at revs, but a blocked exhaust won't let it suck air in, and will give a lot of back pressure and so make it spit out the carb. There's a difference though between spitting back out the carb, and backfiring back out the carb. Spitting is back pressure, and backfiring is mainly caused by lean mixture.

Edited by Mech
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You are correct, the needed itself is lifted to the highest position, which means the clip is at the lowest of the 5 slots.  My practice when cleaning carbs is to ensure every hole is able to flow carb cleaner in, and out another hole.  I then blow compressed air through as well.  The pilot jet has no markings.  The hole is tiny, and only 1 bristle of my small wire brush fits through it, which is how I cleaned it.  The main is 125, which I believe is correct.  Great point about choking the carb intake, I'll give that a shot when I put this back together enough to try it.  Likely go at it again Sunday.

 

I need to man up and go after that mixture screw in the stock mikuni carb.  That is where my best hope lies.

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I would try everything possible to get it out, before drilling, that is a tiny screw and would be easy to get into the threads and then the carburetor is ruined. I would soak it completely in a combo mixture of carb cleaner and penetrating oil, and use heat if there is no head left on the screw at all and it down in the hole,  then you have to drill a small hole and use the smallest ease out and it still may be too big, i dont like using easeouts but are sometimes necessary. 

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The Big Bears I've worked on have a really small stock pilot jet. I think it was like a 16 or 18 or something. I switched it out for a 22.5 and it started and ran perfect every time. I would bet your needle position isn't helping things. Usually 2 or 3 spots down from the top will give you a better idea what's going on when you blip the throttle.

As for the stock carb... another way to get the mixture screw out without drilling is to take a dremel with a cutting wheel and just cut off the empty part of tube that Mech is referring to. Measure how deep the screw is set with a small flathead and mark it on the outside of the tube. Then just cut flush with the head of the screw. If there's room you can now turn the dremel sideways and cut a small slot in the head of the screw just deep enough that a flathead will fit in. you can now attempt to screw it out or use a small impact to break it loose. If its really stuck a little heat from a torch can help as well.

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Yeah Gw's right about using the easy-out. If you  drill it out too much the easy-out will spread the screw if it's tight and make things worse. Drill it a bit and then try the easy-out. Then if it doesn't turn drill it further. As long as you get the hole real well centered you can drill them fairly thin. I prefer to use an old fashioned egg-beater drill because it's slow and delicate drilling. Heat on the aluminium does help as well.. I've found the most effective part of it though is tapping the outside of that aluminium tube, and if you have thin lube down the hole, the tapping helps it work down the threads even if you don't spread the metal to enlarge the hole.

Lube and tap it then drill it, then use an easy-out, or drive a small steel jewelers screwdriver into the hole..  The screwdriver won't spread the hole as much as the easy-out.. if you choose the right size drill and screwdriver.

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I have actually destroyed a carb in the past using an easy out.  The flutes pull the extractor into the soft brass, actually making it bigger and more stuck.  I then split the carb from the force of the easy out.

 

I was talking to a friend with a milling machine, and he thinks we can get very centered up, and drill out the screw until only the threads remain, and then pick them out.  This is likely what I will end up trying.  Thanks guys for the thoughts.

 

I am going to be disappointed if I end up with the same symptoms.  Is there anything electrical that can manifest itself as a lean condition?  Or am I only chasing fuel/air?

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Being too advanced with the ignition timing, or having too hotter plug will beoth make the porclain white, but the spitting backfiring out the inlet does sound like lean. Try your hand across the inlet.

Oh, and check the air jets are still in the back of the carb. There should be two brass jets you can see right where the air-cleaner fits on.

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I very rarely find one of these out of time, those engines are very resiliant and the valves rarely need adjusting, but this one is older and more than likely been rode hard and put up wet many times. Wrong plug will definately do it, i usually just put in the factory recommended plug.

Then there is the carb most of the after market carbs that i have seen have 25 jets, which may be right since the aftermarket ones are slightly different, but the OEM one is 22.5 and the kits i have bought also have the 22.5, but it may not be correct in the aftermarket carb. I think i would keep working on the OEM carb, see if you can get it to  factory specs. Aftermarket carbs seem to run lean right out of the box even with the 25 jet, so that might be some of your problem. I have noticed the same spitting and sputtering and not smoothe acceleration with them, that is why i have been working to get all of mine back on OEM carbs. 

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The idle fuel jet has to have the right size air jet to go with it, and they both have to be suitable for the mixture adjusting screw, which can have different diameter and taper needles, and, the main and it's air jet have to be matched and suit the slide, slide needle and  discharge/emulsion tube, and both those different fuel ranges, idle and main, both have to be suitable/matched for one other. The low speed air and fuel jets have to suit the air and fuel main jets. And whole carbs, even when everything is set up and mixes well with no flat spots or wasted fuel, needs to matched to the type of engine it's fitted to. Road bike carbs are set up different to work bikes or scramblers. Even twin cam cam engines are different set up to the same engine with a single cam, not because of the cam specifically, but because of the different power band they've used the twin cam or single cam to achieve.

It is best to stick with what the manufacturer has gone to so much trouble sorting out for us.

Oh yeah. I don't think the ignition timing will be out either.. I was just saying it does make the plugs burn whiter.

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I just rebuilt a 96 Kodiak carb for a customer which is almost identical. Did you remove the plastic center block the slide glides on? There is an O-ring in there that gets deteriorated and won't allow the venturi pull the correct amount of fuel up from the jets. The All Balls Racing kit I used came with that O-ring. 

Also did you adjust the linkage arm so when the buttery starts to open it's lifting the slide at the same time?

 

Edited by flyinbrian365
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Ok, back to this pig.

 

So I pulled the clean chinese carb back apart, but this time I pulled the factory one apart as well.  I compared jet sizes and such, and I stumbled upon something very interesting.  The needle sizes are very different, and tapered different.  See my picture below.

 

The chinese parts are up top, and the factory parts are below.  The emulsion tubes and main jets appear the same, but the needles are vastly different lengths, and taper.  I have since installed the emulsion tube, main jet, and needle from the factory carb into the chinese carb, and ran the bike.  At first it was still a little lean at idle, but now I have gotten my mixture screw adjustability back.  I can now idle, accelerate, and rev at high rpms with no backfiring through the carb.  Big step forward.  It looks like the long needle was not allowing hardly any fuel through.

 

One issue I have now, is with the idle screw backed all the way out, the bike idles too fast.  A little more digging required, but it looks like this junk carb may end up working for me.

20240206_235615.jpg

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I've done that before. It's what I normally recommend.. Swap every brass bit and the slide needle from the oem to the aftermarket and they aftermarket often runs good.

There are only a couple of reasons for biffing the oem, badly worn slide or throttle spindle, or seized idle mixture screw(which is I think the most common problem). And the slides have to be pretty worn to be a problem.

I suppose you have checked the throttle cable has free play Bobby ?

As Gw says, look through the carb and make sure the throttle plate is closed right off. And check the choke is going right off. The choke plungers can be different designs in after market. Some have a needle and some are flat ended.

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You guys were correct about the throttle cable binding.  I have it adjusted now that the butterfly completely closes, and idle is now smooth.  Up top is now smooth as well.  1/8 to 1/4 throttle is still a bit rocky.  Playing with the needle height is the fix for that I believe.  I've never had this much trouble messing with a carb before.  I guess it is because it is a bit of a Frankenstein now.  Thanks for the assistance guys.

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There are several overlapping transition points in those carbs.
Under the butterfly when it's closed there are two holes in the venturi. One hole's exposed to full vacuum and the one nearer the air-cleaner end is at close to atmosphere when they are at a slow idle. Both holes are connected. At idle the idle jet and idle air jet supply fuel/air mix to the idle mixture screw. The screw controls how much of that fuel/air mix gets mixed with the other air, the air coming in through the discharge hole under the slide that's nearer the air cleaner. The highly diluted fuel/air mixture then gets discharged through the discharge hole nearer the engine. Then, as the slide rises a little,fuel starts to get drawn out of both those holes which reduces the amount of air being drawn in through the air cleaner side hole, the air that was diluting the idle mixture, and so it makes that mixture slightly richer, and discharges it out of both holes so it really does supply a heap more fuel.  That's the first of the transition points. That transition from one discharge hole to two prevents stumbling just off idle. If we adjust the mixture with the slide too high then fuel is getting discharged through both holes and we have to wind the mixture screw in too far to compensate for that, which then causes a stumble/flat spot when we d try to open the throttle. It's important when adjusting the mixture that we have the slide down as far as possible.
Somewhere in that system is probably where your problem lays.

 

The other transition points are controlled by, first the slide cut away, and then the emulsion tube's holes and the capacity of fuel in the drilling the emulsion tube is in. It's important to clean the tiny holes in the emulsion tubes side, and the drilling it goes in if it has crud in it reducing it's capacity for fuel. The tiny holes in the emulsion tube start off being covered with fuel at low throttle settings and that fuel is drawn into the tube along with fuel from the main jet and discharged past the slide needle, but as the fuel consumption increases with throttle opening the drilling starts to empty and more of the tiny holes are exposed allowing air to be drawn through them diluting the mixture being discharged out past the slide needle. Eventually the main jet is controlling the amount of fuel being drawn in and the tiny holes are mixing air into that fuel so that the fuel being discharged past the slide needle is diluted. That process gives a rich mixture as we open the throttle, but then leans it after a short time when the drilling for the emulsion tube is emptied. The overall effect is similar to an accelerator pump.

At 1/8 throttle it's probably not transitioning from one hole to two holes under the butterfly. Set the idle speed as low as it can be and keep adjusting the mixture till it's ticking over nice and slow on the idle speed screw, and at the best mixture point, neither too rich or too lean. Then use the idle speed screw to bring the idle speed up to what  it's meant to be. Try not to adjust the idle mixture after that. If the jets and the slide cut-away is right, and the float level is right, the two discharge holes should work as an enrichener as you start to open the throttle.

Edited by Mech
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