jaan Posted May 9, 2009 Report Share Posted May 9, 2009 (edited) My Montesa Cota 348 (1978) has allways worked fine without any problems until yesterday but now there's something wrong with the intake. The carb is Amal 2627, and I haven't done any changes to the standard jets and settings. The engine works poorly with low rpm's and I can't find good idling with low enough rpm's. I can set the throttle top screw so that the engine revolution at idling is high but can't get my target speed for idling. The engine will stop when I lower the rpm's. Ofcourse I've tried to turn the air screw left and right but without succes. When I turn the throttle grip at no-load operation the engine revolution increases normally but when turning the grip back the engine revolution decreases too slowly. When riding with higher rpm's the engine works normally. I've dissassembled the carb, wash the jets and carb proper in solvent and blowed them out with compressed air. The slide works freely and the jet needle is in it's place. I've washed and oiled the air filter. There's no air leaks in the intake and airbox tubes. The fuel is fresh. Any ideas what to check next? Edited May 10, 2009 by Jaan Quote Link to comment Share on other sites More sharing options...
feetupfun Posted May 10, 2009 Report Share Posted May 10, 2009 Sounds like the rubber intake connector has developed a leak Quote Link to comment Share on other sites More sharing options...
jon v8 Posted May 10, 2009 Report Share Posted May 10, 2009 Main bearing seals ? Quote Link to comment Share on other sites More sharing options...
jaan Posted May 10, 2009 Author Report Share Posted May 10, 2009 Sounds like the rubber intake connector has developed a leak I just checked it once more but it's in a good shape, no air leak. Quote Link to comment Share on other sites More sharing options...
jaan Posted May 10, 2009 Author Report Share Posted May 10, 2009 Main bearing seals ? I really hope not (even this has occurred to me)! In this case an air-fuel mixture were lean and spark plug should show overheating. However the spark plug don't burn but has carbon fouling which means over-rich air-fuel mixture! When air-fuel mixture is too rich, removal of the air cleaner will improve the condition somewhat. So I at once tried without air filter but without effect. For sure I washed and oiled the air filter so it's OK. Next I'll dissassamble the carb and check all parts once more again. Somewhere there's something wrong! Quote Link to comment Share on other sites More sharing options...
jaan Posted May 10, 2009 Author Report Share Posted May 10, 2009 I've once more cleaned and checked the carb and intake parts. Everythink seems to be OK. New spark plug but no effect. I really don't know this time if the air-fuel mixture is too lean or too rich. Because of the intermittent engine noise I think the mixture is too rich. Spark plug was fouled. However exhaust gases are not heavy. Because of the revolution of the engine fluctuates I think the mixture is too lean. However no poor accelaration and lack of power. When I open the starter the engine revolution decreases and the idling gets normal, the engine revolution no more fluctuate. Is this sign about too lean mixture? Perhaps I've to check the ignition too. I really hope that the main bearing seals don't leak! What kind of job it is to mount new seals? I haven't ever dismantled Montesa engine. Quote Link to comment Share on other sites More sharing options...
jaan Posted May 10, 2009 Author Report Share Posted May 10, 2009 I checked the piston head and cylinder head. Much carbon deposit but not too much. It looks like the air-fuel ratio is not too lean. So there must be some problems with fuel tap, tube or tank. Tomorrow I'll once more check, wash and blow them. Quote Link to comment Share on other sites More sharing options...
dave dix Posted May 11, 2009 Report Share Posted May 11, 2009 Pilot jet or, possibly, condenser breaking down Dave Quote Link to comment Share on other sites More sharing options...
belldane Posted May 12, 2009 Report Share Posted May 12, 2009 Jaan You have not lived until you pull an old Monty engine apart. The seals are very easy to replace but beware the primary gear is a very frustrating job to get off. If it is the engine crank seals that are the problem, which I don't think they are, from the info you have provided and in such a short time from going OK to going bong, I would be looking at other things that people have suggested at your question. Check air leaks, carburation and electrics. Have fun mate!!!! I have done it many times. Quote Link to comment Share on other sites More sharing options...
jaan Posted May 13, 2009 Author Report Share Posted May 13, 2009 --- condenser breaking down Dave I've mounted a new condenser last year. What kind of indications point at broken condenser? Quote Link to comment Share on other sites More sharing options...
b40rt Posted May 13, 2009 Report Share Posted May 13, 2009 I've mounted a new condenser last year.What kind of indications point at broken condenser? Usual indication burnt points, and bike runs when cold but not when hot. Quote Link to comment Share on other sites More sharing options...
jaan Posted May 14, 2009 Author Report Share Posted May 14, 2009 It was both right crank seal and condenser! Two faults at the same time, that's why it was too difficult for me to sole the faults. Fortunately you mates and a really experianced Finnish trial veteran in Finland helped me. Thanks for you all! Quote Link to comment Share on other sites More sharing options...
moment88 Posted May 14, 2009 Report Share Posted May 14, 2009 See comments and questions in the "Yamaha" section, thread "TY80 Starts when cold, won't start when warm" Quote Link to comment Share on other sites More sharing options...
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