bananaman Posted August 24, 2014 Report Share Posted August 24, 2014 Hi Guys - hoping someone has some advice please - am restoring a TY250 twinshock, 1978 and have got a new makuni carb from Alan's performance and it just will not rev out. Starts fine, but every time you open the throttle it just will not rev and makes like a 4-stroking sound. Tried moving the needle about but just cannot seem to cure it. Anybody managed to fix this sort of problem? Cheers. Brian. Quote Link to comment Share on other sites More sharing options...
power175 Posted August 24, 2014 Report Share Posted August 24, 2014 sounds like it is running to rich, is the mixture setting correct or the spark plug gap correct. it may also be because of a sticky choke or the engine is flooding. -power175 1 Quote Link to comment Share on other sites More sharing options...
bananaman Posted August 24, 2014 Author Report Share Posted August 24, 2014 Hi mate, Yes, I did wonder if that maybe the case, the carb is new and is suppose to be jetted up to suit. I removed the air filter, which did improve for a couple of minutes but still 4 stroked a lot. I wonder do i need to change a jet to a smaller one or maybe change the float arm angle? We checked the spark plug and tried a new one, but that made little difference. The 2 stroke mix is 100ml of semi synthetic off road oil to 5ltrs thanks Brian Quote Link to comment Share on other sites More sharing options...
zerorev3rev4 Posted August 24, 2014 Report Share Posted August 24, 2014 does it rev up with choke on Quote Link to comment Share on other sites More sharing options...
bananaman Posted August 24, 2014 Author Report Share Posted August 24, 2014 no, just kinda gets worse and the bike stops. The carb only has a throttle stop screw and an air jet. Thanks Quote Link to comment Share on other sites More sharing options...
majesty Posted August 24, 2014 Report Share Posted August 24, 2014 Has the bike been standing for a while Brian ? It sounds like it might need the crankcase oil seals replacing Quote Link to comment Share on other sites More sharing options...
zerorev3rev4 Posted August 24, 2014 Report Share Posted August 24, 2014 might after try a smaller main jet , you could try one from your old carb Quote Link to comment Share on other sites More sharing options...
dadof2 Posted August 24, 2014 Report Share Posted August 24, 2014 First check the ignition timing with a strobe light. I don't know exact figure for this bike but 2.5mm BTDC should be near enough. Check the carb float height. Check the cold start enrichment assembly is clean seals undamaged and seating properly. Sounds like it is running rich. Try turning the petrol off and see if it will rev out properly (4 stroking ceases) as the fuel runs out and the level in the carb drops. Quote Link to comment Share on other sites More sharing options...
majesty Posted August 24, 2014 Report Share Posted August 24, 2014 Daft question but, I assume you have got the oil pump disconnected/removed and the manifold oil inlet hole sealed ? Quote Link to comment Share on other sites More sharing options...
woody Posted August 24, 2014 Report Share Posted August 24, 2014 What size main jet is in the new carb Quote Link to comment Share on other sites More sharing options...
bestrcpilot Posted August 25, 2014 Report Share Posted August 25, 2014 Check the reeds. A broken or not sealing reeds will cause the motor to run rich. Quote Link to comment Share on other sites More sharing options...
bananaman Posted August 25, 2014 Author Report Share Posted August 25, 2014 hey guys - thank you very much for all the help. The engine had a professional rebuild and rebore with new seals etc. it had two new reeds put in one i noticed was upside down so i have turned it over, and then the bike started. i have checked the float height and is okay. It seems when you open the throttle the engine sounds like it 4 strokes and stutters, but is fine just on tick over although it sounds like it is hunting for fuel a little bit. I wonder if these knew type carbs, which seem to use different jets to the old ones and do not even have a inlet anymore for the old style oil pumps, may not suit the TY250's? A friend said to me a delauto carb from a gasgas works okay, but i have had no experience of this?! Any more advice would be greatly accepted.. Thanks again Brian Quote Link to comment Share on other sites More sharing options...
zerorev3rev4 Posted August 26, 2014 Report Share Posted August 26, 2014 you need to determine whether rich or lean , the choke not helping makes you think its too rich already if this is the case you should find the plug black and wet ( causing miss firing and duffing plug ) and possibly black wet from exhaust ,, if its lean the plug would be clean and grey or whiteish the engine would run hot and possibly pink ( abnormal knock ) if you cant determine which it is then you best buy some jets larger and smaller and try them and see which one improves the problem its only a case of jetting the carb to the engine to make it run right as long as the carb size isnt way different Quote Link to comment Share on other sites More sharing options...
woody Posted August 26, 2014 Report Share Posted August 26, 2014 Did it run ok with the original carb. Did it rev out ok. If yes and you refit that, does it still run ok. If yes, then it's definitely your new carb Allens are mainly motocross/enduro and have little or no experience of trials, or at least they hadn't when I bought a carb from them. The jetting in a carb for trials was and presumably still is, based upon feedback from customers, which doesn't necessarily mean it's going to be right. Mikunis are very hard to jet from scratch as they have a massive range of needle / needle jet combinations and if the mixture is wrong in that particular range it's difficult (impossible for me) to know whether its the needle that's wrong, the needle jet, or both. The original needle jet (emulsion tube) on the TY was an N-8 so you want the equivalent of that for your carb and the needle to go with it. There are several different series of VM roundslide carbs and not all use needle jets in the N-8 range - see on Allen's website, the range is mind boggling. I think the carb with the pull out choke is jetted differently from the carb with the lever choke. If you tell Allens that the original carb, Mikuni VM26SS used an N-8 and a 4V1 needle, they should be able to check you have the right combination. There are two types of needle jet / needle and they must be of the same type. If one or the other is the wrong type it won't fuel properly (don't ask how I know this...) Also, the original main jet size of the old carb won't correspond directly with the new carb as the TY would have had autolube and I think that meant a bigger main as they were around the 160 range standard - that's too big on the new carb for pre-mix. They also had a funny numbering system as they can be as high as 280 but they weren't actually that big. That's speedway dope size... You're more likely going to need around 120 / 130 for the main. Bultacos / Ossas use a 130 generally so the TY250 should be around the same, pilot around 30 / 35 and slide 3.0 So what main jet is fitted at the moment out of interest Quote Link to comment Share on other sites More sharing options...
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