Hi all, I have a 2005 Baja 250 4-shroke and it has a weird problem. It runs perfect when it's cold, but when it gets up to operating temp (about 20 mins worth of riding), sometimes will will just sit there and bog down with absolutely no power and it will die unless you feather and play with the throttle, then after about 10-15 seconds, it will take off and run normal. It will continue to run normal until you slow way down close to a stop or stop, then it starts bogging out and doesn't want to do anything again when you hit the throttle (doesn't matter how little or how much throttle). If it does die, it fires right up with no problem.
I haven't tried anything yet to fix it. Not really sure it's a dirty carb problem since it will take off and run like it should after it throws its little fit. Just wanting some ideas first or find out if anyone else had this problem. I was going to adjust the carb just to make sure it's where it should be, but I can't find an air/fuel mixture screw on this thing. All I see is the idle screw.

doug

 

Pull off the fuel line to the carburetor and let it drain into a bowl. Does it flow freely or does it trickle out? Does the flow slow down after a cup or so of fuel comes out?

I'm wondering if you're running out of fuel. A plugged filter, petcock valve, or plugged gas tank vent will cause the quad to stall out after a while, but when the engine dies the float bowl starts filling back up albeit at a trickle. Then the cycle repeats.

 

Welcome to the forum blackbettyz28!

The fuel screw is usually located on the bottom of the carb...Usually very tough to get to.. a piece of vacuum line on the screw some times works...or needle nose pliers can some times get to it..

 

right on, thanks. I'll look for it there.
As for the fuel flow, I've opened the drain, not the one going to the carb, the one with the screw and let it flow for about 10 seconds, not as much as you said and it flowed goood. I'll try flowing more.
New update though: I was wondering if the altitude was hurting it too. Because where I was riding at "the rim" is 7900 feet according to their website, so I took it to the desert today not far from my house and is probably only a couple thousand feet and it rode a lot better. My kids rode it and they said it only shut off once and did the bogging-down-no-power thing once and they rode for well over an hour, far far better than it usually does. Maybe I need the dial jet thing on it for the vast altitudes I'll ride it in depending on the time of year. ??? IDK.

doug

 

blackbetty Have you figured this problem out I was cerious because mine does the same thing only it does not die. you can hit the throtle wide open and it shutters for about a second and then it revs up. I thought it was the CDI unit not allowing the revs to ramp up properly. Because I have heard that the stock CDI's are this way so people dont pull wheeles with them, but I think it is anoying. I am working on replaceing my CDI with an aftermarket one to try and fix this, but I dont even know if it is the real solution. would be nice to know before I go spend around $100 for something a trun of a screw would fix.

 

No, I haven't. But I have determined it's an altitude thing. Where I was riding is well over 7000 ft and I have since rode around here in the desert 2-3 times which is about 1000 ft. I just took it out last weekend for about 3-4 hours and never had any problems. It seems like as soon as I go camping on the rim up north, that's when it decides to be all hesitant. It doesn't die all the time when it acts up. My kids have got the hang of it to where they can feather the throttle and get it going fine without dying.
I suppose I could adjust the fuel mixture by what I hear to fix it for high altitudes, but I CANNOT find that darn fuel mixture screw. I don't see it anywhere. The owners manual mentions it for 2-cycle, but not 4-cycle engines, so I'm not sure it has one.

 

hey, look at this previous thread, unless you already found it, and it will tell you about the altitude adjustments.

http://forums.atvconnection.com/gene...justments.html

 

Many thanks

Since I have lived in Michigan all my life I have never realy thought that the carb might not be quite adjusted properly. I just assumed that a carb was adjusted enough to make the engine run. All this jetting and adjusting is new to me. I just hope adjusting many of lawn mower carbs will help me adjust this one. I have also noticed there is a great carb adjustment form on here that makes this task look quite easy.


So I now have anouther question. If the throtle responce is all controlled by the carb. Then what exact roll is the CDI playing?

Is it there just to tell the spark plug when to fire (acording to the fly wheel) and to limit the revs the engine can produce?

Because if this is all a so called High performance CDI does then it almost seems like a waste of money. And the only thing you are gainning is the rev limiter eliminated. The reasion I ask this is that some of the CDI companies claim these ramp up the RPM's faster then a stock, and that CDI's have a low speed sensor to decrease throtle resonse eliminating the average user from poping a wheelie when not intended to. Now this is making me wonder if all of this crap they say is just to get you to apend more money when it is just an adjustment of the carb.

Thanks again, I am glad I found this form it has already helped me out alot in understanding a lot of the questions I have had with these things.

 

The CDI produces a moderately high voltage spike (100 to 300 volts high) which is applied to the ignition coil primary. The ignition coil is just a high frequency transformer that steps up the voltage by roughly 100 times to 10-30 thousand volts to fire the spark plug.

To produce this output spike there has to be a power supply inside the CDI to make 100 to 300 volts DC and store it on a large capacitor. This high voltage supply runs off 12 volts DC in some CDIs, or more commonly it is runs off an AC high voltage winding inside the stator.

At the proper time (using a trigger winding mounted outside the flywheel, triggered by sensing a raised steel bump on the flywheel, the CDI dumps the several hundred volt charge on the large capacitor onto the ignition coil primary.

Thus the CDI has three sections: The high voltage power supply (up to a few hundred volts) which charges up a large capacitor, the trigger sense circuitry, and a high power high voltage switch to dump the capacito charge into the ignition coil primary.

Having taken several of the cheap generic CDIs apart and reverse engineering them I can tell you with certainty that there is no active rev limiting circuitry in there. Some specific CDIs do have active rev limiters (eton for example) which can be adjusted with a multi position switch, or have external rev limiters which are wired across the high voltage AC stator winding.

But I need to clarify this a little bit. When I say no active rev limiter I mean there are no parts inside whose sole purpose is to keep the engine from spinning any faster than a certain speed. But the CDI design has been engineered for simplicity and for low cost, and those design parameters may reduce engine performance from optimum.

The problem is with the ignition timing advance. As the engine speed increases the spark plug has to be fired earlier because there is a fixed time delay from ignition until the flame front expands and pushes down on the piston. This is why at idle the timing is set a few degrees before top dead center. At higher speeds the ignition has to happen earlier in order to keep the expanding combustion gases from literally chasing the piston down the combustion stroke. The simple CDIs use the fact that the trigger voltage generated by the stator is directly proportional to how fast that raised bump on the flywheel passes by the pickup coil. The amplitude of the trigger voltage affects when the CDI actually fires the spark plug. The timing advance curve generated with this approach isn't optimum but it is simple and cheap. And it tends to run not quite advanced enough at high RPM - hence diminished performance at high RPM.

The performance CDIs use exactly the same method above for starting up the quad, then it uses a microprocessor inside to calculate the engine RPM from the time interval between the last two ignition triggers. Once it knows this then the micro takes over from the manual timing and applies a timing advance curve based on a custom lookup table programmed inside. The next spark is generated by the micro at the right time based on the timing of the previous two trigger pulses. The micro is also calculating the engine RPM on every revolution using the last two trigger pulses for use in adjusting the timing of the next spark. And on it goes, but now every spark is timed precisely based on engine RPM and a customized timing advance curve.

Are performance CDIs worth it? Maybe. I don't really know. Many have reported higher top end speed. I have never tried one since I don't ever need to go that fast. I'm a natural born skeptic and wonder how on earth the CDI could be optimized when I see adds that claim they are for a variety of quad engines. To me an optimized advance curve could only be for a single engine - not several - but I may now be outside my area of expertise.

 

Wow, thanks man.

That is some good info. I love to learn about how this stuff works since I have worked on cars for years, and am currently a service tech working on electronics. But 4 wheelers are new to me. Any info to help me out is apreciated.

It sounds like the CDI realy would not work unless it was tuned to that spicific engine.