Dangerous flat spot at 4K rpm

[Malcolm Leick]

Just got back from a 21 day / 6750 mile trip to and around Turkey. About 1200 miles into the trip the engine developed a flat spot at around 4100 rpm. The engine would stop accelerating for a few seconds like a limiter had cut in, and then suddenly surge forward with no extra twist on the throttle. It was a real problem because I would pull out to overtake, lose acceleration half way through the overtake, and then shoot forward as I was pulling in behind the car in front.

Things got even more complicated when the engine started choking up intermittently all together just past Istanbul (where my AA breakdown cover finished!) I was convinced I must have had some dodgy fuel at some point. Eventually I suspected the engine dying might be a separate issue related to the side stand switch, although I had been no where near mud since the beginning of the trip. I got the bike into a BMW service centre in Xanthi and they confirmed the switch was loose. They replaced a washer and tightened it up FOC.

I thought all my troubles were over as the bike ran much more smoothly afterwards, and I had no more problems with the engine dying. However the flat spot returned within 24 hrs and persisted all the way back to home. I just had to make sure I dropped right down through the gears to get the revs up to 5k plus before overtaking.

Any thoughts? I have to get the bike in for a service but It would be useful to have some ideas. I had a new clutch fitted the day before I left. (I wonder if they loosened the side stand switch when they fitted the clutch!)

I'll put a up a ride report in the next few days. Brilliant time despite the engine problems. The bike is the F650 GS twin.

 

[Malcolm Leick]

The ride report is now slowly emerging here

http://www.ukgser.com/forums/showthread.php?p=2453926&posted=1#post2453926

 

[Mr K]

Was it dusty? Air filter?

 

[mr badger]

Hi Malcolm

I'm sure I followed you in my car yesterday. M25 or M11. Remember seeing a well laden GS with a small green gas can on the back - made me feel all wanderlusty!

Will enjoy the report.

 

[Malcolm Leick]

Hey that probably was me, riding back to Nottingham from the Chunnel.

You should have beeped! I was pretty shattered but hopefully going in a reasonably straight line. It was my major sponsor's birthday, so I had to get back, buy her a present, and cook her a birthday tea.

 

[Malcolm Leick]

If it was dust I don't know why it would kick in at specific revs. A Belgian biker I met in Germany suggested the distributor. My mechanical knowledge is very limited but I have it booked on for a service next Thursday when hopefully all will be revealed.

 

[Dean]

Hey Malcolm,

Sounds like you had a great Trip over a few weeks great pictures ..... hope you get the problems sorted soon

If you dont mind me asking how did you get on with the TKC's on tarmac on the F650GStwin ? and what engine bars do you use, how do you rate them?

Dean

 

[Malcolm Leick]

The TKCs are fine in the dry but a bit twitchy in the rain. It wasn't really an issue on this trip as on the two days it rained I was on the motorway so no bends!

To be honest, on the way out I was cursing the tyres a bit because the rear was brand new and I was getting some of that wobble others have mentioned at 85+ mph. By the time I had knocked off the first 1200 miles it was less of an issue, partly because the tread wasn't quite as deep, but also because I was off the very fast roads. By the time I was up to 90mph in Turkey it was no problem at all. I was hitting 95+ on the run back to Calais, and again no problem.

I never got anywhere near mud because it was so hot and dry. The main problem was rough and broken roads, particularly where tons of chippings had just been dumped. I have never ridden on these kinds of surfaces with other tyres so it's difficult to compare. The other surface I did a lot of miles on was roads just before the tarmac goes down, which you can see in the pictures. Again, I have been running TKCs full time for the last 18-20,000 miles so I can't compare with anything else.

There is probably only about 1000 miles left on the current tyres and I'm almost certainly going to put Tourances on for the winter. I don't expect to be doing any heavy off-roading, and I'd prefer a smoother ride for a while, with more grip in the wet.

The German guys I met who were heading for Africa on the KTMs had TKCs strapped to their bikes but they were running on more road biased enduro tyres. Not sure what they were, but they weren't knobblies of any sort. Smoother tread. They were saving the TKCs for Africa.

All comes down to what sort of terrain you are covering. There are endless threads here devoted to the subject. Ideally I would have two sets of wheels, one spoked with TKCs on, and a set of road biased tyres on my alloys. If you are doing thousands of miles of roads before hitting the mud then take two sets of tyres (or more). You are going to wear out at least one set anyway.

The engine bars are Adventure Spec. They have taken loads of hits, both off road and a couple of times on tarmac. They have been brilliant. Probably slightly off centre now but still doing a great job. Likewise the Adv Spec bash plate. I used to have a BMW bash plate but split a weld on a rock. This one has taken some big hits and survived.

Hope that's useful. I'm no expert.

 

[Dean]

Thanks for the info

its the twitch in the rain that bothers me, Im on tourances now last were original tombstone deathwings very twitchy on the wet (on my F650)

Tourance is a lot better (trouble is i'm waiting for the twitch so maybe me being to cautious) went for a tyre deal with a fellow GSer same tyre sizes, so my rear section is 150mm same as 1150GSA. I agree you need different tyres for the best of off and on Road

I like the adventure spec crash bars they sit nice and high they list them as for the 800 so give the apearance of sticking out more on the f650 as the the F800gs has more tank plastic (wider) my givi's have had a few slow motion knocks no doubt will change sooner or later.

Thanks again for the info and be interesting to see what the cause of that flat spot is and a painless resolve