Greetings all, new member. Just did my newbie post in the new members section. Have a question about my '72 R75 and an issue with sluggish acceleration. I'm new to BMW (coming from Honda SOHC world) and i'm just getting the hang of it. Have gone through the bike overall to check things out, replacing missing or incorrect items here and there (clamps, plugs, etc.). Bike appears fairly unmolested but not much known about its history so i've done the maintenance items changed oil, air filter, trans fluid, drive shaft and final drive fluids. New petcocks and fuel lines along with tires.
Bike has been cranking and idling well (I believe having NO idea how well one of these idles); however, its sluggish on acceleration especially at cruising speeds (around 60). I understand these bikes are not quickest or fastest around and i'm not looking for that. After warm up of ten minutes or so, the bike hesitates and shudders with any significant twist of the throttle. However, while twisting the throttle and reach down and put the choke at about halfway the bike immediately smooths out, accelerates very strongly and takes off like a champ. It will cruise like this perfectly until you of course come to a stop and then its idling in the 3k range. Pull off the choke, bike drops down to a few low and steady idle, but if you go to accelerate again there is sluggish response.
I'm coming from the world of Honda SOHC's and at present in the dark about the Bings. While I get up to speed I took the bike to a local shop that I have relationship with, and coincidentally the owner rides a beautiful R90S. He gave the bike a once over for me and set the timing, rocker clearance and endplay and checked out the carbs. He said the diaphragms were good and passages all looked clean. He synced and dialed it and it is cranking and idling beautifully. He told me to try it out and see how it was running and bring it back if any issues and he'd sort it out without charging me again. He did not do a full clean of the carbs or a rebuild, as he was wanting to try the other things first just to make sure and save me some $$ on rebuild parts if not necessary.
So, I'll be taking it back to him with the findings of the ride this past weekend, but I wanted to come to the well here to see if anyone had thoughts. I'm assuming it's probably needing a carb rebuild at this point? It just seems odd to me that its running generally so well otherwise.
An additional odd thing regarding the throttle and i'll do my best to not butcher my description of this. There is a point towards the end of the rotation of the throttle that I continue to rotate the throttle the engine actually decreases in rpms or bogs down further. It's hard to tell if its just slowing back down or the mixture is going exponentially fubar that it bogs further. So, put another way, as I'm twisting the throttle the engine revs increase in step with the twist/opening of the throttle to a point near the end of travel for the throttle. But then as the throttle is opened beyond that point the revs fall off for those last maybe 10 degrees of rotation. I understand that's a horrible description. I'm not sure if this is problem with the cables or throttle assembly or if related to the overall poor running but i throw that out there too.
Thank you for indulging the rookie questions.