dowsonm Posted July 27, 2016 Report Share Posted July 27, 2016 Looking for some advice with my 2010 Evo 300 4T. The bike has been running fine for some time, but at a trial few weeks ago the carb had come loose from the engine when the rear boot to the airbox had failed. I got it reconnected and it did run for a while, but stalled in a section so I retired. I have replaced the boot to the air box and stripped the carb to blow out the jets and also fitted a new mixture screw. The bike now starts OK, especially when cold but as I ride it, if I hold the throttle slightly open the bike will judder and stall. If you get past the low revs it will rev OK. After a few minutes at tick-over the bike will usually stall. Another thing I did notice was that when I have the bike on a paddock stand it ran OK, but when I then lifted it off, the movement of the bike seemed to start the symptoms – maybe something to do with the fuel supply or float? I have tried the adjust the air screw using a type of right angle screwdriver – is it possible that the wrong setting (I was running it at 2 turns out) can make such a difference? All feedback welcomed. Thanks Mike Quote Link to comment Share on other sites More sharing options...
billyt Posted July 27, 2016 Report Share Posted July 27, 2016 (edited) Mike I currently have a 2016 Beta Factory Evo 300 4T There is no air mixture screw on this carb. The adjustment screw underneath at the engine side is a fuel mixture screw. Not sure where you live? Beta USA sells an aftermarket brass fuel adjustment screw with a big thumb wheel on it. I highly suggest this replacement fuel screw as the stock fuel screw is a SOB to get to. There is an idle screw adjustment that only controls the throttle butterfly valve. Did you put the rubber boot that goes from the carb to the air box on correctly. This does NOT go on either way. It is a mono directional air boot. If you install it back on wrong you can get air leaks in all sorts of places at the carb. What do you mean by "I fitted a new mixture screw"? Edited July 28, 2016 by billyt Quote Link to comment Share on other sites More sharing options...
billyt Posted July 28, 2016 Report Share Posted July 28, 2016 Here you go..... 1 Quote Link to comment Share on other sites More sharing options...
dowsonm Posted July 28, 2016 Author Report Share Posted July 28, 2016 Thanks for the reply billyt. By "mixture screw" I meant the fuel mixture screw and I bought a standard one form the UK importers. I agree it is a pig to get at so will consider getting the US version or I think Jitsie also make one. Can you say that if this screw is set incorrectly that it has a significant impact on performance? Also, how can you tell which way round the air filter boot goes? I spent a lot of time looking at the one I removed and the new one and concluded that it was symmetrical - but happy to be shown the correct way round. Thanks again. Mike Quote Link to comment Share on other sites More sharing options...
petert Posted July 28, 2016 Report Share Posted July 28, 2016 I have the Jitsie one on my Evo and to be honest that USA made one looks better/easier to use. Quote Link to comment Share on other sites More sharing options...
timp Posted July 28, 2016 Report Share Posted July 28, 2016 I would buy that USA made mixture screw if it was available in the UK. I seem to remember mine is four turns out. Quote Link to comment Share on other sites More sharing options...
billyt Posted July 29, 2016 Report Share Posted July 29, 2016 Four turns out. WOW that is a lot. For that amount you would have to need a very rich condition at idle. Two turns out plus or minus half a turn is a good straining point. Keep in mind this is a FUEL screw NOT a air screw. I can't state enough about how important the carb to air boot directionality is. Put it on incorrectly and the carb will pull away from the front air boot causing air leaks etc. Quote Link to comment Share on other sites More sharing options...
carlos Posted July 29, 2016 Report Share Posted July 29, 2016 One side of the air boot is a little bigger than the other, should be easy to see the difference. I replaced the fuel mixture screw on my 2016 evo factory 300,4t with one made by Keintech. Should have used some teflon tape or blue locktite because it fell out three rides later! I'm running the stock fuel screw at three turns out. I had a stalling issue going up rock walls. Now running a 123.8 main jet, clip at the bottom of the needle. Revs out okay. Still a bitch to start sometimes. May go to a 125 or 127.5 main. Sometimes i will stall it when bike is leaned at an angle or going down a steep hill. I am running the idle fairly high. Quote Link to comment Share on other sites More sharing options...
billyt Posted July 29, 2016 Report Share Posted July 29, 2016 (edited) I have had three Beta Evo 4T 300 thru the years. My experience with the Beta 4T has had the Mikuni carb off the bike many more time than I care to think about. I struggled with keeping the carb on the bike and not coming loose from the rubber boots. If the carb comes loose it can run lean forcing you to richen up the jetting and/or opening the fuel screw more than necessary. The Beta 4T comes lean from the factory to pass emissions testing. The needle clip has to be lowered by one notch to richen up just off of idle. As for the rubber boot from the carb to air box. The diameters are the same on both sides. It is the angle that is different. The rubber boot runs VERY close to the shock. As the shock moves up and down and the boot flexes it has a tendacy to dislodge the carb from the front rubber mount going from carb to engine. If you put one cylindracal end of the rubber boot on a flat surface such as a piece of glass and check the angle and then check the other end you will notice that it is not symmetrical, far from it. I believe the more obtuse angle goes toward the carb an the less obtuse angle toward the air box (or maybe it is the other way around). Once you get it the right way around you will see how it aids in clearing the shock better and affording better clearance between shock and rubber boot. The carb mounting is so much easier and secure once you get the rubber boot direction sorted out. Edited July 29, 2016 by billyt 1 Quote Link to comment Share on other sites More sharing options...
carlos Posted July 29, 2016 Report Share Posted July 29, 2016 Good to know, takes me about an hour to remove and replace the carb. It is a tight squeeze. Quote Link to comment Share on other sites More sharing options...
petert Posted July 29, 2016 Report Share Posted July 29, 2016 I have had three Beta Evo 4T 300 thru the years. My experience with the Beta 4T has had the Mikuni carb off the bike many more time than I care to think about. I struggled with keeping the carb on the bike and not coming loose from the rubber boots. If the carb comes loose it can run lean forcing you to richen up the jetting and/or opening the fuel screw more than necessary. The Beta 4T comes lean from the factory to pass emissions testing. The needle clip has to be lowered by one notch to richen up just off of idle. As for the rubber boot from the carb to air box. The diameters are the same on both sides. It is the angle that is different. The rubber boot runs VERY close to the shock. As the shock moves up and down and the boot flexes it has a tendacy to dislodge the carb from the front rubber mount going from carb to engine. If you put one cylindracal end of the rubber boot on a flat surface such as a piece of glass and check the angle and then check the other end you will notice that it is not symmetrical, far from it. I believe the more obtuse angle goes toward the carb an the less obtuse angle toward the air box (or maybe it is the other way around). Once you get it the right way around you will see how it aids in clearing the shock better and affording better clearance between shock and rubber boot. The carb mounting is so much easier and secure once you get the rubber boot direction sorted out. I run the Beta Factory exhaust on mine I wonder if that makes a difference? I've never had to touch the carb on mine or use the hot start. Plug is a lovely brown colour too. Quote Link to comment Share on other sites More sharing options...
billyt Posted July 29, 2016 Report Share Posted July 29, 2016 Due to you not touching the carb the rubber boot is on correctly from the factory. Quote Link to comment Share on other sites More sharing options...
krazyosucowboy Posted July 30, 2016 Report Share Posted July 30, 2016 No experience with Evo 300 4Ts but most fuel screws have a series of tiny washer, spring and o-ring in the needle end. If those are missing or installed in the wrong order they can cause the type of problems you describe. Quote Link to comment Share on other sites More sharing options...
billyt Posted July 31, 2016 Report Share Posted July 31, 2016 No washers on the Beta 4T carb fuel screw. Just the jet, spring and O ring. Quote Link to comment Share on other sites More sharing options...
dowsonm Posted August 1, 2016 Author Report Share Posted August 1, 2016 Billy - the 2010 bike has a washer in addition to the O-ring so maybe they removed it on later models. It is shown on the parts diagram: Thanks for the feedback so far. Not there yet, but still trying things to improve the bike's running. Quote Link to comment Share on other sites More sharing options...
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