Forum bmwbike.com

If carbs are not balanced properly, can this lead to one cylinder running hotter than the other?

If the centrifugal advance mecanism is worn and static timing difference between the 2 cylinders is offset by 3 to 5 degrees (assuming one is dead on the mark) can this cause one of the cylinders to run hotter?

Hello Pierre,

I had a problem with one side of my R6 running hotter than the other. The first thing I did was to change the plug on the cold side. No improvement. I then put in new needles and jets and finally new points, coil, cap, rotor, wires, etc. Still no improvement. I was going crazy! It turned out to be two consecutive bad plugs. At least I know my ignition and carbs are in good shape. Another problem was created, however, which I will describe in a separate post.

How much colder is the one side? What do the plugs look like? To answer your question, yes, I think both could unbalanced carbs and timing differential COULD cause a temperature differential, but if it is a big difference, I would suspect something else.

Best regards,

Bruce

Hallo,
which cylinder was getting hotter?The right one? If yes:take a bigger jetsize to the right carb, BMWs needs a different jetsize for the both cylinders, the right one has to be 1 number bigger.
Otherwise: unbalanced carbs means that one cylinder works more than the other, so he gets hotter(like we do).
3-5 degrees difference: not ideal, but (what a pity) normal in our days of worn out and bad rebuilded advancer mecaniscm( when I remember right, BMW tolerate 2-3 degrees difference). If you had 5 degrees difference you can take very fine sandpaper and egalise it or(quicker and normally good enough) put the "F" mark in the middle of the two cylinders.
5 degrees it not very much difference.
rolf

Thanks Rolf, much appreciated. I will wait to change the jet size before I actually balance the carbs. I was not able to do so because my float needles did not make good contact with cover seating and I had petrol coming out of the idle air adjustment on both carbs.

I did not know that the right carb needs a jetsize one number bigger than the left carb. What is the reason for that? The Bing catalog shows only one size of jet.

Thanks Bruce. Actually I am still in the fine tuning stage. I was incapable of balancing carbs yesterday as both my idle mixture screws were leaking petrol (Bad float needle to lever top contact). I popped the question in anticipation as I noticed the right cylinder was running hotter when I let the engine idle for awhile after the first start since 6 years! It had not run for a 4 year period when I got it and it took me 2 years to restore it. I have ordered new parts from Mark and wait impatiently to get it going proper.

Hi Pierre,

I think when you get the carbs sorted out your problem will go away. Leaky floats can ruin your day. My R6 has R50 carbs...so the floats are inclined. When the bowls are dry and I turn on the petcock the floats will sometimes stick and flood everything. A couple of taps on the bowl solves the problem. No problem with the bike running.

Best regards and good luck.

Bruce

Hallo Allan,
I do not the exact reason for that, only some ideas(angle of the inlet-tube, less cooling(why?)and so on).
Nevertheless: you can read it as a fact in nearly every magazine of that time that:BMW(Zündapp too) needs a bigger jetsize for the right cylinder and BMW (Bing too) ignore it.
Short: I do it , it runs better and I do not know why.
rolf

Rolf,

I checked with Bing and they confirmed that Zundapp did jet the right side richer on their twins but knew of no other manufacturer that did so.
I for one have balanced my carbs and put in a new centrifugal advance.....the right cylinder still runs hotter!

Pierre