Suddenly lost 20mph top speed...

Carl_G · Renault 5 Driver Joined: 29 Aug 2010 Karma :

On my first stage of a 280 mile trip down to south Wales, down the A34 to Newcastle to stay at the in-laws, the same bike that two weeks ago sat happily at 85 mph on the motorway refused to get above 65. Even when pinned open in 5th on a flat road.

The bike is a '93 XV535, its power delivery has been a bit "lumpy" for a couple of weeks and there may be a new metallic tapping noise from the engine. Though frankly that may be imaginary/always there previously... I suspect that the acceleration has also got worse especially over 40 mph.

Anyone want to throw in a theory why this might be, and more importantly how I might fix it?!
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CBT 11/09/10! Theory Passed 12/10/10! Mod 1 Passed 08/03/11! Mod 2 passed 13/04/11!
1993 Yamaha XV 535 (SMIDSYed) 2008 Suzuki GSF 650 (Hire Bike) 1993 Yamaha XV 535 (Resurrected!) 2004 Triumph Speedmaster 790cc (Payout from SMIDSY!)

jimbothe · World Chat Champion Joined: 29 Sep 2006 Karma :

When was the last time you serviced it? Have you pulled the plugs out the check their condition and colour? How is the air filter?
Have you run it low on fuel?

SoND · World Chat Champion Joined: 10 Jul 2005 Karma :

How is the service history?

Fuel/air system could be clogged. Fixed by changing fuel + air filters and cleaning the carbs/injectors.

Old spark plugs don't help.
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Carl_G · Renault 5 Driver Joined: 29 Aug 2010 Karma :

It was serviced about about three months ago, haven't had chance to check anything because I arrived in the dark, it ran out of fuel at a junction this afternoon and ran for about 400 yards on the reserve to the nearest petrol station. Though the reserve got a longer run a few weeks ago...

jimbothe · World Chat Champion Joined: 29 Sep 2006 Karma :

I would still pull the plugs again and check the colour. Is it firing properly on both cylinders? When you say the power delivery has been lumpy I'm wondering if one of the cylinders is dropping out. Most probably the front one IIRC.

Check the wiring to the coils as well. The vibration on the XV series has always been quite severe and there is a possibility it could have dislodged or chafed a wire or two.

Without actually seeing or hearing it it's pretty hard to diagnose but it's worth a saturday pulling the basics apart and just visually checking. Thumbs Up

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