1991 Yamaha TDM850

Q:I’m having trouble getting my bike to run correctly. It pops and splutters, not in the early mornings though, mainly at idle (12,000 to 15,000rpm when warm). I have been on the TDM forums, changed the emulsion tubes, checked needle heights, diaphragms and fuel filter. I’m changing the exhaust seals next, but I doubt even that will work. What should I consider? The float height is spot on, the choke plunger doesn’t look like it’s leaking and I’ve set the idle screws to the manual. I’m ready to send it to a garage, but I hate spending money on something that I can do myself! With a little help, maybe? Merv Martin

A:The first thing I should say is that your idling speed of 15,000rpm is probably a little too fast. Okay, 1500rpm! But seriously, even that is a bit too fast as it is meant to be 1100 ± 1000rpm. The symptoms you describe indicate either an ignition fault or an excessively weak mixture. You say it is okay early in the morning, and I assume you mean when the engine is cold and hence the choke is on. First, make sure the ignition system is just as it should be and it might be a good idea to change the spark plugs which should be NGK DPR9EA-9 or DPR9EIX if you splash out on iridium-tipped ones. Assuming, as you say, everything within your carburettors is correct in terms of fuel levels and jet specifications, then you should check for sources of air leaks. This includes the mounting rubber tubes, which should be pliable and properly clamped and a few O-rings which should be changed as a matter of course, especially the two at the joint between the diaphragm cover and carb body. It is possible to fit the cover with the diaphragm not correctly seated so check this. A few owners claim that the jet within which the needle slides, part number 3GM-1414-194, wears and causes problems but I do not think that is the issue here.