Tommy Boy Posted November 2, 2012 Share Posted November 2, 2012 Hello all and firstly thank you for having a look at this thread. I'll just give you all a brief insight on what ive done so far just incase i'm looking in the wrong place. So i started off with a rattly old SR125 with high milage so i decided I'd give it an engine change. I sourced an newer engine complete with carb with a low unconfirmed milage. I couldn't get it running at first, then i discovered the electrics were different. After replacing the generators over, i still had no luck. I then looked at all the other easier possibilities like compression, spark ect before i narrowed it down to the carb. I stuck the old carb back on and hey presto the thing fired into life after i bump started it. A while later I decided id probably get more out of it if i tried getting the newer carb fitted and running. So i counted the turns on the old and messed about with stripping both of them, cleaning them and everything i shouldnt really do, then once i reassembled both carbs neither of them would work. Now here's the latest. With the new carb cleaned and with the 108 main jet from the original carb fitted (new carb came with a 102. Anyone know why/if this will cause issues) and the throttle screw mostly out and the pilot screw 1.5 turns out i tried bumping the bike. I got alot of spluttering. I tired multiple times each time either turning a 1/4 in/out or with the choke half/fully out. The most i got out of it was about 4 seconds of the bike running before cutting out and throwing loads of petrol out of the air filter drain tube. After doing a lot of reading i think the fuel pouring out of the air filter is related to do with the fuel level in the carb/float height. Can someone please confirm this? Will this cause a rich mixture and prevent it from starting? As far as I'm aware the fuel inlet valve and needle aren't worn. The floats are in good condition but im pretty sure their height is the issue. I read in the haynes that you can do a check without taking the carb off but it involves clear plastic tubing and the black bit of pipe that hangs from the drain part of the carb is missing. Checking the float heights I can do but it just doesn't seem like a very accurate way of doing it. Any way what do you lot think? Am I looking in the right places? Where should I start. Just to confirm - when trying to jump the bike, it very rarely starts, mostly splutters, and when i got 4 seconds of revving out of it it cut out then dumped a load of petrol on the floor from the air box. all the jets are clean and the floats aren't damaged but i think the heights definitely need looking at. Does this sound correct and if so what the most accurate way to measure? Link to comment Share on other sites More sharing options...
jimmy Posted November 2, 2012 Share Posted November 2, 2012 Firstly mate, Yes, if the float height is wrong it will just flood petrol though the airbox. Do you have a manual, if so strip, clean and reassemble the carb to the factory settings. What year is the bike, and what year is the newer engine. When changing over the electrics did you fit a newer CDI unit. The fitment changed around 1997, I think. Now I dont know if the electronic brain inside the box changed but the CDI is not functioning correctly your bike will do everything you suggest in the opening paragraph. As I say Im not sure if the internal of the unit changed at the same time as the wiring fitment, and indeed the new enging mapping may well have the same advance, retard curve of the auld engine, but summie else to think about once you sort the carb. seemingly 50% of all carb issues are electronics and visa versa Link to comment Share on other sites More sharing options...
Tommy Boy Posted November 2, 2012 Author Share Posted November 2, 2012 Hello Jimmy and thanks so far. As far as the electrics go on these bikes, the only thing that's different engine wise is the generator (and the fact the carb has a heater). My bikes a W reg (94/95) and the engine i think is A 99 But I can't remember. It's certainly the newer model. As far as I'm aware as long as the new engine has an old generator fitted, the bike should run ok. I've kept all the old electrics on the bike the same so the cdi, distributor ect are the older type. I did have the bike running ok with the new engine, and the old carb fitted so I'm assuming its just the settings on the carb that's causing me grief. Anyway i'll check out the float heights, adjust and put every thing else back to factory settings. Hopefully this will moves things forward. Cheers! Link to comment Share on other sites More sharing options...
dt502001 Posted November 2, 2012 Share Posted November 2, 2012 +1 on the float height the different jet size is due to the ever tighting emissions. As for the electrics if the stator and cdi are a match set,and from what I read it thats what you have done,all is good. Most likely you will have already set the float height by the time you read this but, float bowl removed holding the carb at about a 45deg angle( eng side low a/f side up) the float will close the needle the float arm should be level or slightily larger gap at the farthes from the needle with the bowl mating surface. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted November 2, 2012 Moderator Share Posted November 2, 2012 1: Check the inlet valve clearance is there at TDC 2: Stick a couple of teaspoonfull of oil down the plug hole and try again Link to comment Share on other sites More sharing options...
Tommy Boy Posted November 5, 2012 Author Share Posted November 5, 2012 Messed around with float heights today and still no joy. Managed to get the bike starting but i had to have it on full throttle all the time while turning the engine over to get it fired up, then from then on it would run really lumpy and kick out a load of petrol from the air filter box. If i let off the throttle it would just cut out. Tried both carbs on it and there wasn't much difference when it came to starting it. Like i say, it was just about running on full throttle which is better then nothing but it's no good to ride. Is it time to consider taking it to a bike garage? Link to comment Share on other sites More sharing options...
Moderator Airhead Posted November 5, 2012 Moderator Share Posted November 5, 2012 is my last post invisible? Link to comment Share on other sites More sharing options...
jimmy Posted November 5, 2012 Share Posted November 5, 2012 What last post Once you have checked the valve clearance, check the timing with a strobe, but I still think CDI Link to comment Share on other sites More sharing options...
Tommy Boy Posted November 6, 2012 Author Share Posted November 6, 2012 Sorry airhead I completely missed that. Also would that cause the engine to throw fuel up into the air box? I'll see if we've got any feeler gauges. Pretty sure its 0.8mm for my bike. Gunna look up how to do it now. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted November 6, 2012 Moderator Share Posted November 6, 2012 Sorry airhead I completely missed that. Also would that cause the engine to throw fuel up into the air box? I'll see if we've got any feeler gauges. Pretty sure its 0.8mm for my bike. Gunna look up how to do it now. yes if the inlet isnt closing because of poor adjustment or a broken spring etc etc...The air/fuel will be pushed back into the airbox on the piston up stroke Link to comment Share on other sites More sharing options...
Tommy Boy Posted November 9, 2012 Author Share Posted November 9, 2012 Ok guys got some good news. Managed to get the bike going and running ok(ish). I'm pretty sure that it's within the bikes nature to be a bit lumpy on the idle speed and fluctuate slightly with occasional cut-outs and if this is the case then my problem could very much be sorted. If someone has a second opinion on this please advise. How did I do it? Well strangely enough after another attempt at playing with the carb, I fitted the jets and screws that looked in best shape, then set everything as close to factory setting as I could. After a few coughs and splutters a friend told me to disconnect the breather pipe (the one from the engine to the air box) and low and behold the bike fired into life. I couldn't quite understand why it was this that was giving me the grief but whenever it got reconnected the bike would go straight back to lumpy and cut out. I decided an oil change was necessary as it was reading slightly over and upon draining the oil i noticed that it seemed as if the oil had turned into a petrol mix. It was watery and had no real resemblence to oil any more. Anyway I stuck fresh oil in it and still had no luck with running it however things did seem to improve slightly. So I took the bike up and down the alley a few times with the breather disconnected then stuck it back on after a few runs and the bike didn't seem to mind. I took it for a few more runs up and down and the bike performed very well. So i spent a bit more time and adjusting the carb whilst running and tried to get the idle speed down nice and low without it cutting out. Now I haven't had the chance to let the bike cool and try restarting it but I will let you know how i get on once i try it. My only concern now is that the first part of throttle is a bit lumpy and the bike cuts out every now and then. The mixture screw seems to have no effect on the running what so ever. Does anyone know if it's knackered and needs replacing? Or am i just not paying attention. I screwed the mixture screw all the way in and it had no effect on the speed or tone of the engine. To be honest I'm more then happy with the bike as long as it starts for me next time I try it but I would like to iron out these last few creases. If anyone can assist I'd be very grateful. Cheers all Link to comment Share on other sites More sharing options...
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