Forum bmwbike.com

Hi,I am the proud owner of a '66 R69S
I rebuild the top end with new valves,piston rings etc. Compression is now 152-155PSI
I replaced the complete clutch assy.I checked the rear drive ratio = solo drive
I drilled out the mufflers to 30mm dia.I reset valves,timing & ignition as per manual, fitted new sparkplugs & ignition point. I had the magneto tested = OK
I set the carby needle to the highest setting= 4th hole
Accelleration up to 110km/h = OK. But top speed is only 130 - 140 km/h downhill
My previous'61 R69S used to go to 180km/h
Any advise of how to increase my top speed ? ( short burst only, police not happy ! )
Happy motoring
Horst,Rosebud,Vic,Australia

What size tires are you using? Are they matched to the final drive ratio?

Have you checked the timing to ensure that you actually get to full advance at cruising RPM? The advance unit may have a mechanism that limits the ignition advance, thus preventing optimum timing.

The carb setting of the 4th hole is likely way too rich if I understand you correctly. According to the Bing book, it should be in the 2nd hole. Maybe your carb slide is binding and you're not getting it to the top of the carb body. Also, your main jet might be too small, thus not allowing enough fuel for the high speed demaind. Also, your floats might be set too lean, again, restricting fuel when trying to hit maximum speed.

I also wonder if drilling out the muffler has changed the breathing characteristics of the engine. Usually the muffler is matched to the engine for optimum performance.

Kurt in S.A.

Hi Kurt
Tks for the reply
a. I have a new Metzler rear tyre fitted = 4.00-18.64H ( manual says 3.50-18S )
b. The carb. slides do move to the top c. The needle to the max does improve the top speed slightly d. I do not know the difference or indentification of the carb. needles
e. we touched up the timing cam lobe on one side only , because of small timing variation from right to left
f. The drilling out of the mufflers was recommended by an expert motor cycle tuner ( he build the engine for the fasted Indian, as per film ), maybe I should join the Harley club = now measured 91 decibals
g. I kept the old repair manual from 1961 & all my setting are as per manual
h. where do you live ?If close I could come the a visit
Horst

Horst -

The 4.00 rear tire should get you more top end, at the expense of "snappiness" on the low end. From what I understand, the 4.00 rear tire was meant for the later model R69S and US bikes with the 27/8 rear end. The 25/8 was to be used with the 3.5 tire.

Do you know what the main jet in the carb is...manual says 130 for the 91/92 carbs, but 135 for the 69/70 and 75/76 carbs. Jet needle is set to the 2nd clip position.

You might need some water-wings to get here...I live in the southern United States, Texas to be exact.

Kurt in S.A.

Hi Kurt
The carb's are stamped 1/26/91 & 1/26/92
There are no indentification marks on any jets, my manual also says 2nd position.
I do have all the carb's technical data in my manual, but I don't know to indentify the main & needle jets
The bike was purchased from Hingham M.A in 2007, so it could be an U.S. delivery
Sorry, but no visit = S.A. in Australia stands for South Australia
We do have a 100km/h speed limit, so I am very cautious to test the top speed
We had no speed limits in the country in the 1960, so 160 + was normal!!
In that time the only bike faster than mine was a Vincent Black Shadow.
For all your help, I could email a picture of my BMW, if I had your email address
Horst

I don't see an option to send private messages...that way I could provide me email address directly to you. There seems to be some capability there when I check "My Control Center", but maybe the PM feature is not enabled on this forum.

Kurt in S.A.

The needle affects midrange throttle, and 4 is too rich, but would not affect full throttle mixture.

The 4.00 rear tire is oversize for your mid-60s final drive ratio, and could have the effect of not allowing the engine to reach full rpms before the top speed is drag-limited by wind resistance. I for max speed you should change to the 3.50 tire that BMW intended with your final drive ratio.