Cure for backfiring Yamaha Thundercat

Cure for backfiring Yamaha Thundercat

 

Cure for backfiring Yamaha Thundercat

By MCN -

Parts & accessories

 05 October 2011 14:39

I have a Yamaha Thundercat on a 1998 plate which is in mint condition and only has 16,000 miles on the clock. The bike has been great, but last month it seemed to backfire a few times when approaching junctions. The next time I took it out it started to cut out every time I approached a junction. I took it to a mechanic who cleaned the carbs and reset the needles. He also replaced the spark plugs and adjusted the idle speed. He said this had solved the cutting out, but the bike now tends to race at 3000rpm for about 4-5 seconds when I stop before settling down. He recommended stripping down the carbs again and checking the inlet valves as he thought they might have closed up.
Leigh Wilson, Newcastle
 
You’ve described the classic symptoms of an air leak. When you close the throttle air still enters the inlet tract and combines with the fuel/air mixture to make it leaner, which causes the revs to rise. It often happens on older bikes when the rubbers around the carbs become brittle with age and crack.

In the case of Thundercats they have one of the most complicated air duct systems in the world with plenum chambers and pipes going off in all directions to stop the airbox over-pressurising. Hunting down an air leak in that lot can be a mission, but not impossible. The first thing you need is a manual with clear diagrams of the set-up, and then a slave fuel tank that allows you to get it running with the standard tank off the bike. Once you’ve got it warmed up and running, blip the throttle to get the revs to ‘hang up’ while spraying WD40 over the various joints. The WD acts as a (very) temporary sealant over any hole, so the revs should settle down for a few seconds while it’s sealed off.

The issue is that the mechanic has fiddled with the idle speed, but if you initially blast the whole area with the can, then use the straw on the nozzle to be more precise you should track it down.

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