bronco Posted March 16, 2017 Report Share Posted March 16, 2017 (edited) Hi rubbish that my first post is a call for help..! so I bought the bike last year off eBay. It has a sport filter and sammy miller exhaust. It had an oversized carb on and didn't run. i sourced an origional and had some success, although it had flat spots all over. after some searching on carb tuning it looked like with those mods I need to upjet a little. so std jets are 35 and 92. Earlier models 38 and 95. the info pointed to going up three sizes. So finally I've sourced a 40 and 98 main. after a no spark temporary disaster I've just about got it going again. so it will kick over but I can't get it to run for more than a second or two. needle now on the highest clip. but starting to run out of ideas. So starting with the air screw at factory 1 to 1.5 turns out and the idle totally screwed out. ( I've had the best results at this setting) im starting to think it's sucking a small amount of fuel though but then runs out? ( the new pilot jet is a little shorter than the origional one. should it run a little bit more than that so I can at least have a fiddle with the carb. ( as it did before the rejet) or could it be the jets are miles out now? regards david Edited March 16, 2017 by bronco Spelling! Quote Link to comment Share on other sites More sharing options...
2stroke4stroke Posted March 16, 2017 Report Share Posted March 16, 2017 It is quite possible that your replacement carb had been left lying with modern petrol in it by the previous owner. It that is the case then the passages could be obstructed by the gunge that is left when such petrol evaporates. Not always easy to clear. Quote Link to comment Share on other sites More sharing options...
shakennstirred Posted March 16, 2017 Report Share Posted March 16, 2017 (edited) Ditch the sports filter for now, if you have the stock air box? Or is it one of those cone type that replace the box?. If its this, is the filter bolted to the end of the carb like this pic lots of bikes don't like the filter right on the end of the carb, its better on a link pipe so further away. Its something to do with the pressure wave hitting the back of the filter and getting back to the carb to soon and messing the fuelling up. had it a few times on my road bikes, I moved the filter away from the end of the carb and everything was fine. If its a sports filter in the stock air box disregard what I have just said. Then start with stock jetting and then work from there. Edited March 16, 2017 by shakennstirred 1 Quote Link to comment Share on other sites More sharing options...
thats_a_five Posted March 21, 2017 Report Share Posted March 21, 2017 A couple thoughts: Those jet sizes sound a little big for a 125 but it should still run. I run a 40 and 105 on my 200cc Honda with stock intake and exhaust. On some carbs, the “fuel screw” is actually an “air screw” so it works opposite of what you might expect. DelOrto’s come to mind. Not sure about your carb. If your “Idle (speed) screw” is all the way out, the slide is probably too low, completely closing off the air supply to the motor. For an engine to run, you need spark, fuel and air. With the needle all the way up, there is maximum gas getting to the motor. After trying to start it, is the spark plug wet with gas? It sounds to me like the engine is getting flooded with fuel. Quote Link to comment Share on other sites More sharing options...
bronco Posted March 30, 2017 Author Report Share Posted March 30, 2017 Thanks for the input so far lads. yeah it's been very rich....plug blackened very quickly. Im back to stock jets and needle one above centre so it's now idling ok. although it seems to have the trademark cough when I crack the throttle. Can this be adjusted out or does it need some further mods? regards david not road tested yet......... Quote Link to comment Share on other sites More sharing options...
thats_a_five Posted April 11, 2017 Report Share Posted April 11, 2017 Giving the idle jet a little more gas with the screw or move the clip on the needle one step might help. Quote Link to comment Share on other sites More sharing options...
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