Yamaha DT400B

Q:  I’m getting desperate here on the garage floor with my DT400B twin shock. Over the last two years since I bought it, I’ve tried everything I can think of to stop a nasty chugging action when the throttle is shut. It seems to be more violent the hotter it is and the longer/higher the previous road speed has been held.

So far I’ve tried, new crank seals, ignition coil, Rex speed shop magneto ignition — set as standard at 2.7mm BTC — Boynson reed valves, a carb service (after trying all sorts of jetting variations). Ron at Fahron cleaned up the squish band and porting and fitted Wiseco first o/s piston and fitted B10ES spark plug in place of standard B9E. Finally, I converted to 33mm choke Keihen PWK from a KDX 220… which may have softened the effect, but not cured the problem. I’ve put this down to being able to maintain a tickover with the slide in a very low position. Air screw is set at 1½ turns. This couldn’t be done with the standard carb even with variations in jetting.

As I’ve been unsuccessful in my bungling to either cure it myself or find a firm within reach of Sheffield able to diagnose a cure, I’m hoping the oracles at CMM can point me in the right direction. I’ve enclosed a few photos to show the bike in regular use, trail riding up here in the peak district — not easy when going down a steep slippy slope with it chugging away! Don’t worry – original lights safely stored away.

A: The only thing I can think of with this one is that the chugging action is being caused by some variation in the running of the engine, and as the ignition system is presumably okay that probably leaves us with the carburettor. When the throttle is shut there should be just about nothing going into the engine as the vacuum in the inlet tract is high, which suggests that sometimes something is going in, which in turn suggests an air leak between carburettor and combustion chamber.

You have already replaced the crank seals, so that can be discounted, so that leaves us with some other air leak, possibly at the carburettor mounting. Good luck!