83 r65 sudden outburst of revs when hot

[scuba]

Hi everyone,
and finally I have a problem. On my 83 r65 I experience sudden outburst of revs when engine is hot, let's say after couple of hours of city driving. I have (or better to say he does) new throttle cables, bmw original, new choke cables, on bing 64 flat tops, electric eq. e.t.c. And it doesn't happen every time, what’s bothering me. It just goes crazy. I tried to loose cables, to tighten them, to change position, to swear, to pray...  

On my other r75 with same bings it never happened.
Anyone?
Something?
Pls

[Justin B.]

Pull up on the cables at the carb end next time, if you still have the proper amount (or any) of freeplay then the RPM will not increase and it is probably nothing cable related.  If you don't have any freeplay at all and the RPMs increase with the slightest pull on the cable than maybe take a look at the routing.  If you have pretty much ruled out cabling then without touching the throttle kill the engine and restart, did the revs go back to normal?  Often, the centrifugal advance mechanism will stick and cause an abnormally high idle.  This is usually after a good freeway blast but could happen at other times as well,  I have personally had this issue on an '80 R65 and '81 R100.

The beancan can be disassembled and the advance mechanism is "hidden" in the bottom of the cannister.  Pulling the sheet-metal cover out of the rectangular inspection hole will allow you to take a look.  Most likely it will be very rusty looking inside and if you can poke in a scribe and move things the mechanism will most likely feel "notchy".

One other thing, a down-stream air/vacuum leak will cause idle to increase as well so make sure the clamps securing the carb to head are tight and the rubber hose at that joint is in good shape.

Good luck.

[scuba]

Thank you, Justin,
rubber hoses between carbs and heads and clapms are new as well, and tightened. Freeplay on carb ends of cables are ok, i have played with them the first time this happened. It was most reasonable thing to do, since I have replaced old ones with new cables. As you have said, I have pretty much ruled out cabling. Tomorrow I will dissasemble the beancan and clean the advanced mechanism. I was puzzled because I took off these carbs and put them on r75 and "kill" it through traffic at 5p.m. and nothing happened. Must be the timing. I have left the engine to cool down but RMP is still extremely high the moment I press start button. I will have pretty fun tomorrow, hahahaha

[Justin B.]

Normally if the advance mechanism sticks cutting off and back on will "un-stick" it.  What happens is the weights are flung outward at the higher RPMs and this being tied to the points cam advances the timing.  When RPMs drop the weights don't return like they are supposed to which results in the high idle.

[scuba]

Hahahaha,
after whole day of checking and rechecking of cables, advanced mechanism, ignition timing, valves clearance, carbutetters... the answer came on evening news. It was a small affair about sh... lowsy fuel on some gas stations and I felt victim of fraud. Normaly I poured that urine out and filled the tank with new gasoline and WOW! My toy works smoothly as ever before.
But the good part of this is that I washed and cleaned all the stuff under the tank, beneeth front cover and adjusted everything by the book.
So much of a trouble, but thanks anyway for the advice.
Excuse me, but I must go now to hit the road

[Justin B.]

Are you able to take the crap-gas back for a refund?  I think this is the first time I have ever heard of bad gas causing a high idle.  I would never have thought that in a million years...

[nhmaf]

I wouldn't expect bad gas to do that either, unless it had something sticky in it that was gummy up the slides/needles in the carbs such that they wouldn't
go back... :-?

[Justin B.]

Yeah, that's a possibility.  Whenever I think of bad gas around here it's usually water.  Sometimes, the fill points for the underground tanks will leak if their seal is not perfect and after a very hard flooding rain it's possible to get some water in the tanks.  I know that has happened at a couple of stations around here over the past few years.

Scuba, was there any reporting as to what was wrong with the gas?  How did your advance mechanism look on the inside?  It might have needed some attention anyway as all of them I have poked around in were dry and extremely rusty.

[scuba]

Yeah, it was leaded petrol but it's octane number was low. My valves were allready bit lose so rattling was "normal" and I never had a chance to hear it because of high rmp-s. It's not just me who had similar problems. Many car owners expeienced same thing (those who drive on leaded petrol) after just a couple of kilometers after gas station.

As for the advanced mechanism, it's allmost as new because I pay a lot of atention to it but newer the less, I dissasembled it and cleaned it properly. I remember once I had similar problem with land rover when I got something that smelled like petrol, but land rover is far less prone to have such problems than bmw.

And I just don't have the time and energy to go and ask for money for 10 liters of that "something". I just bought 10 liters from another station and now pray for valves and valve seats because the day before yesterday the engine went hot immediately due to high rev's (6-7000). And no, I didnt see anything sticky in carburetters. Today it was good as before and I have had a beautiful ride of some 27km without any problems. Maybe my nerves are thinner than they used to be. I simply love that peace of mashine.

Sorry for bothering you guys and again thank you for the time and energy

[Justin B.]

That's bizarre...

[nhmaf]

Yes, very odd.

When I used to work at a service station, many many years ago, we often got water in the fuel tanks - usually the loss often used ones.
As long as it stayed below 2" deep in the tanks, the fuel pump pickups wouldn't bring it up into the customer's car tanks.  We'd put a putty like material
(I cannot recall the name of the stuff) on the lower 4-5 inches of our tank measuring sticks to monitor it.  If it got too high, we'd have to schedule
a tank cleaning with the fuel distributor.  now, water will certainly lower the effective octane level, but I never heard of it causing
engines to rev high - generally they'd sputter and be prone to stalling.

Maybe the fuel you ended up with contained some kerosene or diesel ?   That would have a lower octane and be a problem for gasoline engines.