VULTURE Posted November 13, 2014 Report Share Posted November 13, 2014 Hi. Thought I try on here first before giving in and paying someone to look at the bike. I have bought a Yamaha RS100 1978 vintage and I'm having trouble with a lack of performance,it should easily pull me up to 60 mph on the flat but I'll be lucky to see 30!. I've checked everything I can think of,points have been cleaned and gapped,carb cleaned out and reset to factory specs,engines had a rebore and new crank seals,everything is as it left the factory with the exception of the exhaust but I have the original one as well no difference,the bike starts first or second kick and ticks over spot on hot or cold. Spark plug colour is spot on as well, the throttle works well until 1/3 open then nothing more after this,,have replaced air filter. I'm stumped now ,any ideas anyone. Cheers. Vulture. Quote Link to comment Share on other sites More sharing options...
TriBsa Posted November 13, 2014 Report Share Posted November 13, 2014 (edited) Assuming Crank Compression is OK with new seals and the rings have bedded in, sounds to me like it could be the Carb. When you say you have reset the carb to factory specs, do you mean you have installed a new needle and needle jet or just fitted old ones with the correct numbers on? Needle Jets can wear greatly (needles less so). Is the main jet also new and in spec? Are the Reed Valves OK and not cracked/broken/bodged? Just remembered they've got them. Edited November 13, 2014 by TriBsa Quote Link to comment Share on other sites More sharing options...
88b Posted November 13, 2014 Report Share Posted November 13, 2014 Have you set the ignition timing correctly. Perfect timing is critical , ideally done with a dial test indicator through the spark plug hole and a light on the points. I've also seen crak seals fitted the wrong way around Quote Link to comment Share on other sites More sharing options...
amateur Posted November 13, 2014 Report Share Posted November 13, 2014 (edited) Is the exhaust blocked with carbon & oil? Or does that only happen to British 2-strokes and MZs? I found that my old 2-strokes always ran better with a cleaned out exhaust and silencer. The recommended method is to detach silencer and exhaust from the bike, use an oxyacetylene flame at the exhaust end to set fire to the oily gunge, then keep it going with pure oxygen. This results in the most amazing smoke plume coming out of the silencer (and wrecks the chrome plating), and you can see the burn travelling along the pipe and silencer, when it all cools down, you just tap out the crispy ash. Not having had oxyacetylene when I owned my two-strokes, I used a blowlamp to set the sludge alight and an old vacuum cleaner to blow air through. The results were the same Edited November 13, 2014 by amateur Quote Link to comment Share on other sites More sharing options...
Rimfireboy Posted November 13, 2014 Report Share Posted November 13, 2014 Check if the main jet is clear, old petrol and 2 stroke oil can turn into a gummy residue and block jets. If the main jet is clear try lifting the needle one notch and try it again to see if there's any improvement. If there's no improvement check you're ignition timing and also check for air leaks on the inlet manifold. Are the exhaust baffles blocked? These engines are so simple, it can't be anything too difficult to diagnose. Quote Link to comment Share on other sites More sharing options...
Codeye Posted November 13, 2014 Report Share Posted November 13, 2014 Air filter plus all of the above Quote Link to comment Share on other sites More sharing options...
Twistedsanity Posted November 13, 2014 Report Share Posted November 13, 2014 Reed valves Quote Link to comment Share on other sites More sharing options...
shaun4860 Posted November 13, 2014 Report Share Posted November 13, 2014 I don't suppose they have had it restricted? Quote Link to comment Share on other sites More sharing options...
TriBsa Posted November 13, 2014 Report Share Posted November 13, 2014 I don't suppose they have had it restricted? To what? It's 97cc! Quote Link to comment Share on other sites More sharing options...
Gordon R Posted November 13, 2014 Report Share Posted November 13, 2014 Some had a washer in the exhaust to restrict the power, but the OP has tried two exhausts - so unlikely that both are restricted. Quote Link to comment Share on other sites More sharing options...
TriBsa Posted November 13, 2014 Report Share Posted November 13, 2014 Some had a washer in the exhaust to restrict the power, but the OP has tried two exhausts - so unlikely that both are restricted. Thought that was the 125's Gordon, but if so thanks and apologies Shaun. Quote Link to comment Share on other sites More sharing options...
VULTURE Posted November 13, 2014 Author Report Share Posted November 13, 2014 OK. Just spent a couple of hours double checking everything,points are set to the gap specified in the manual as is the carb,one of the exhausts is a as new micron and is not blocked up, The bikes only done 9000 miles from new so no wear in the carb or anywhere else for that matter.i only only changed the crank seals in case they were perished,checked the reed valves and carb rubbers all OK. Still starts and runs spot on but no power,still 30 ish wether thrashing it or bumbling along in top. And yes I've checked the brakes are not binding. Cheers. Vulture Quote Link to comment Share on other sites More sharing options...
Gordon R Posted November 13, 2014 Report Share Posted November 13, 2014 (edited) Are you setting the points with a feeler gauge or dwell meter? Is the petrol old or new - might have gone flat. Edited November 13, 2014 by Gordon R Quote Link to comment Share on other sites More sharing options...
VULTURE Posted November 13, 2014 Author Report Share Posted November 13, 2014 Hi. Used a feeler and a dial indicator, petrol is fresh out of another bike which is running OK. Quote Link to comment Share on other sites More sharing options...
neil82 Posted November 13, 2014 Report Share Posted November 13, 2014 don`t rely on the points gap as a timing setting, that is only a start point, if you have a dti to set correct firing point then put a mark on the case and flywheel, disconnect the lead to the points, connect a multi-meter between that lead and a good earth and use that to see when the point open, if that point coincides with your mark then you have the timing right, if the gap is now way out then you have worn points, as to the mixture you will have to do a plug chop, warm the engine up, swap in a clean spark plug and run it up the road, at the point where it starts to run rough, whip the clutch in and kill the engine, check the colour of the plug and that will tell you how the mixture is, if its rich see if there any alternative grooves on the needle to raise or lower the needle (raise-leaner, lower-richer), if compression is ok and the crank seals and reeds are in good nick then I`m sure its mixture that's the problem Quote Link to comment Share on other sites More sharing options...
magman Posted November 13, 2014 Report Share Posted November 13, 2014 Have you replaced the spark plug ? Quote Link to comment Share on other sites More sharing options...
ditchman Posted November 13, 2014 Report Share Posted November 13, 2014 has it got an automatic advance and retard...? Quote Link to comment Share on other sites More sharing options...
amateur Posted November 13, 2014 Report Share Posted November 13, 2014 Or, of course, you could get a four-stroke - much less hassle Quote Link to comment Share on other sites More sharing options...
michufc Posted November 13, 2014 Report Share Posted November 13, 2014 My petrol jet wash was running **** and I cleaned everything out fresh fuel the lot the spark plug looked fine but it would not run over tick over that turned out to be a spark plug. Looks can be deceiving for the sake of a fiver I would get a new plug aswell Quote Link to comment Share on other sites More sharing options...
Andy135 Posted November 13, 2014 Report Share Posted November 13, 2014 the throttle works well until 1/3 open then nothing more after this Cheers. Vulture. Have you checked that you're getting more than 1/3 open at the carb when you twist the throttle? Quote Link to comment Share on other sites More sharing options...
chris31 Posted November 13, 2014 Report Share Posted November 13, 2014 Have you taken out the pilot jet in the carb? It's a small jet to the side of the main jet, probably about 1cm or so down a recess, these are usually a common cause of trouble. Quote Link to comment Share on other sites More sharing options...
69chris Posted November 13, 2014 Report Share Posted November 13, 2014 seems lke most stuffs covered, have you checked the fuel tap is flowing properly and the cap breather is clear ?? Quote Link to comment Share on other sites More sharing options...
drut Posted November 13, 2014 Report Share Posted November 13, 2014 Not very helpful but it is strange that although 2 strokes are the simplest of engines they are also the most difficult to fault find! My experience is being pedantic and checking out the basics step by step,although not quick,is often the only effective solution.As an aside,if power delivery does not increase above 1/3 throttle it would point me to check fuel restrictions,particularly fuel tank/carb filter issues although a fouled/restricted exhaust would give the same issue.You really need to be methodical and check everything out step by step,although some "cheats" such as using butane/propane torch(unlit)to check leaks on carb intake rubbers or carb cleaner aerosol to add fuel can be of help. Quote Link to comment Share on other sites More sharing options...
Gordon R Posted November 13, 2014 Report Share Posted November 13, 2014 I would still be reaching for a dwell meter. Quote Link to comment Share on other sites More sharing options...
HAYBURN Posted November 14, 2014 Report Share Posted November 14, 2014 HI never worked on bikes just cars....check you get full throttle on carb,check points with a dwell meter,check timing do the points have some sort of advance system,bog weights siezed etc. sorry not much help. Quote Link to comment Share on other sites More sharing options...
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