Gen 5 - Engine Vibration as Ambient temp rises

My Gen 5 Mk1 03/99 has started developing a vibration that is smooth at cold ambient temps but starts then increases as ambient temperatures increase.

At 46 to 62f the Vibration is pretty much non existent but the ambient temperature increases above 68f it starts with a small tingle in the bars and pegs but by the time it gets to ambient 76f and above its like riding a small jack hammer or Harley. Engine temp is fine around 172f at ambient 46f and about 194f when the vibration starts and highest maybe 208f at 76 in traffic.

As an example this weekend in the same ride when the ambient was cold 46f the engine was smooth, when the ambient increased to 71f the vibration got progressively worse until it got to 76f then I hit a wet Mountain range where the ambient dropped to 54f and the engine got smooth again but by the time I stopped for the day at 76f it was rough again...Reving statically on the centre stand could hear a near but not quite miss starting around 4k rpm.

Have confirmed Coolant level is full and plugs have been changed recently but nothing else at this stage, remember a few years ago having a plug lead accidently off once and the motor was still smooth just noticably down on power and fuel economy but in this case fuel economy seems little changed.

Any ideas what it may be or where to start ?

 

This is just a crazy wild guess. I may be way off, even dead wrong. Here goes. Your thermostat just starts to open at 172F. It is partially open at 194F. It is fully open at 208F. When the thermostat is closed there is a lot of back pressure between the thermostat and the water pump. It could be that the water pump is loose or has bad bearings. When the back pressure is high it may tend to hold it in place and keep vibrations down. When the thermostat is fully open, the back pressure is much less. This may leave things free to vibrate. Your ambient temp readings would suggest a different thermostat position.

Remember this is just a crazy theory from a guy on the internet but if you find loose water pump bolts it's free fix.

 

Thanks for the suggestion but suspect it might be Ignition related as mentioned above Reving statically on the centre stand could hear a near but not quite miss starting around 4k rpm but should have mentioned from the back of the exhaust also guys following behind mentioned the strong smell of possibly unburnt gas from the exhaust when it was vibrating at its worse when hot but not too bad when cold.

So far best suggestions I have asking around are, pulse generator, lead ignition coils ( their are 4 ) or a faulty spark plug breaking down as temperature rises, I'm trying to see if anyone has suffered similarly before outlaying good money before troubleshooting.

 

Could you possibly have a sticky float in one carb? The engine would tolerate a rich mixture when it was cold because it would act like a choke. At normal opporating temps it would be too rich.

 

Maggot, 5th gens are fuel injected.

John, when was the last time you replaced your air filter?

-Jake

 

I'm betting money that the intake air temperature sensor has gone out to lunch, causing the bike to think the intake air is a different temp than it actually is, throwing the fuel mixture out the window. This is a probe located in the airbox. I would also suspect the barometric pressure sensor, I recall seeing the one on mine under the rear cowling. A shop manual should show you how to test these things.

 

Ooops!....