Vintage BMW Motorcycle Owners

Over the weekend I had the pleasure of rebuilding my right carb. By all appearances, it went well. The leak was fixed. The engine started and ran after assembly! It is, however, behaving in a peculiar manner. The initial idle seems reasonable 6 or 7 hundred rpm-ish but after blipping the throttle it hangs at nearly 3k rpm and will not idle down. Before I seek outside assistance does anyone have an initial thought as to what I might check? There are lots of online resources I have found but nothing on point.

Pictures added just because pictures are fun.

High idle could be the result of a few things. You rebuilt the carb, so the usual synching steps are needed. Be sure you don't have any air leaks. Another possibility is that the advance weights are hanging up. If it wasn't doing it before the rebuild, it might not be the issue now.

The engine/carbs must be warmed up after a 20-30 minute ride before you begin the synching steps. Since it takes a bit of time to run through the steps, you'll need some cooling air blowing over each cylinder.

Hucky's BMW still has those 2 (R90S) Dellorto carbs for sale for $900....were $1400.
I would like to buy them but then I have to find a R90S to buy. Humm

twocams

So I suppose I should be encouraged that the bike started and ran then. I’m considering just rebuilding the left now rather than wait. I have new throttle and choke cables on the way so I’ll proba just do all of that first then work on the synchronization.

Regarding new carbs, my originals seems to be in great shape and given the price of that pair is 40% of my total investment in this machine I can’t justify replacing the current carbs.

First off congratulations on gettin the "S" We don't have enough info on how it ran other than "leaking" Looks like you did the re-build as good as possible. As you know there is a ton of O-rings in there. Those carbs are just a basic slide with a pump...nothing exotic. Looks like you've got duel plugs so most likely it's electronic....skip that part. Go back and do motor basics valves / timing etc. check for leaks buy squirting some WD-40 around the carb when running. Don't for get the throttle cables. Nothing wrong with re checking your work. Be sure to make sure the pilot jet is open. For all "S" guys.... the source for all Dellorto stuff is https://store.bevelheaven.com/index.php?p=home
They have everything you need along with the tuning manuals. And buy the way.....a pair of new carbs from then is $605.00. I've got a new set going on a "RS" I'm sure they will help you on all questions.

I'd double check the ignition advance is returning. If it sticks it can prevent coming back to idle.

The WD40 idea for checking vacuum leaks is a good one.

As above, check the carb sync. After it runs for a 20-30 seconds are both pipes equally hot? If one is cooler, that cylinder is not pulling it's weight. If one cylinder runs cooler but the carbs seem to be in sync, try to swap the carbs to see if it is the carb or the cylinder.

Mike

All good advise. I really like the way you have laid everything out during the rebuild. I need to do the same thing soon.

So I have decided to extend the project a bit and replace the throttle and choke cables as well as replace the float valve. Based on the condition of the old valve I'm confident that this was the source of the fuel leak. I've also replaced the rubber sleeves on both air funnels on each side. So here is another issue. It appears the choke cables are out of spec, lengthwise. Once installed the chokes only actuate about half way. Either the cable is too short or the housing is too long. Since I have extensive experience as a bicycle mechanic cables are easy to deal with, except in this case where each end is swaged in place. Thus no option to alter the length of the cable. Does anyone have the correct housing measurement in millimeters? I figure I can carefully Dremel the housing to shorten but I'd need a ball park measurement.

In digging in to this bike yes the heads are dual plug. Is this a mod? And Yes there is a Dynatek electronic ignition that was installed somewhere along the way. Kind of disappointed as I had hope to have traditional points for the old school cred and the ability to have motorized transportation after an EMP attack. . .

So, update. Decided to retrace some steps and confirm proper assembly. I also had an illuminating phone chat with Rusty at Max BMW. Wow what a resource and such a nice guy to talk to. Turns out, as with anything over 40 years old, things may or may not be stock. Now I am a firm believer in put things back together the exact way you found them but that doesn't work so well if it wasn't correct in the first place. Now with a better understanding of the Magura choke lever things buttoned up nicely. Tidy up the cable and boom, the bike starts right up. Basic carb settings seem to have nice balance, simple tweaks to get the idle reasonable and off I go on a nice test ride. Now the way it was running before I wasn't going to venture too far from the house, but today, no problem. Nice smooth idle and perfectly normal throttle behavior. I put 20 miles on the clock on some gentle windy roads and I think I've got a rider for the season.

More pics only because the make things more interesting.

Wow .... beautiful bike. Enjoy!