Vintage BMW Motorcycle Owners

Hello again. FNG here again. I would like to ask what may very well be my first stupid question. As noted in my last post, this is my first airhead. I have two oilheads and have owned a third, but as we all know, that’s a different breed altogether.
So… I’ve got this new to me 74 R90S that has been sitting for at least twenty years. It apparently was very well pickled when they put it away, so everything is in really nice shape. To bring it back to life I installed a new battery, checked all the wiring, (everything works great except the high beam indicator)
I changed Engine oil and filter, transmission oil, driveshaft oil, and rear differential oil. I installed a new set of Bosch spark plugs, and sprayed someMarvel Mystery Oil into the cylinders. I then turned the engine over many times with the kickstarter before firing the bike up. Once I fired it up, it of course smoked like a chimney for a few minutes due to the oil I sprayed in the cylinders, but then that stopped and it settled into a nice steady idle. Oh, I also disassembled both carbs and cleaned them up in the ultrasonic cleaner. They were pretty clean to start with as I believe they were drained before being put away. The only thing I was not sure about was the accelerator pumps. They seemed a bit iffy to me due to their age, but I put them back in anyway.
So here’s my question. When reving the bike up off of idle from 1000 rpm to maybe 3 or 4000 rpm, the bike SEEMS like it’s running on one cylinder. It’s not “smooth” like I would expect it to be. It literally sounds like an old thumper to me. The kicker is, it actually IS running on both cylinders. The exhaust temperatures, and the jug cylinders, taken with an infrared thermometer, are both within 10-15 degrees of each other. I haven’t ridden the bike yet as I’m still going through the brakes and putting new rubber on, but sitting on the stand it just doesn’t seem right to me. I would love to have someone near me, Winston Salem, NC, to compare notes with. If anybody here is within shouting distance I would love to see your bike and hear it run.
Any ideas, guys? Am I just imagining things? My guess is that the bike is running perfectly and I’m just not used to it. Could it be the accelerator pumps not working?
Thanks again for any ideas you can throw my way.
Chris

I am a little too far to help in person.

You have covered the things I would do to bring a long stored bike back to life.

So two things come to mind. First is Airheads are picky about valve adjustment. When a valve gets out of adjustment, idle is the first thing to go.

I bet your problem is as simple as synchronizing the two carbs. In this case I would bet one cable is pulling one carb off of idle faster than the other. Cables have to be adjusted so there is enough slack that when the handle bars are turned they don't pull and the carbs are actually in the idle position. If you have a good ear, you can do this but most of the time some kind of tool is used anything from sync tubes filled with alcohol, paired vacuum gauges or even some of the "electronic' tools.

As far as that goes, once you get the bike running and you have ridden it a bit, a spark plug color check after a good long ride will tell you if you need to do any fiddling with any other adjustments. Really there are not many adjustments to make on carbs.

As I have not had a lot of experience with Delorto carbs, I can't guess how one would know for sure an accelerator pump is bad or not. First I would guess is to make certain the cables are pulling off at the same time, if they are then I suppose you might feel something under hard acceleration if one is not working.
Good luck, Keep us informed how you make out. St.

I'm not as familiar with the carbs on the R90S, but are their internal rubber bits in them? I would not take a Bing CV carb and dunk it in an ultrasonic tank when thinking about the o-rings on the butterfly shaft. Maybe it's OK, but that would bother me a bit. Again, don't know about the Dell Ortos.

OKAY, Kurt makes a good point, I would suspect the ultrasonic cleaner solution would make a difference in action on o-rings.

I can't say for certain as I have never used one myself.

If you have the carbs back on the bike and it is running, you can check for vacuum leaks using some kind of spray. Just pick something that won't eat paint if you do, don't ask I how I know this (carb cleaner got on the inside of my fairing lowers).

At idle a change of rpm if you hit a leaking seal will be noticeable. The mentioned butterfly shaft seals are good spot to check as well as the fitting between the carb and the head.

I would think however a vacuum leak would give poor and unadjustable proper idle rather than manifest as a cylinder drop when the throttle is applied? St.

As I said, the Dells are different...aren't they "just" fancy slide carbs? If that's the case, there's no butterfly like on the CV carbs. Certainly, checking for leaks after overhaul and being removed from the bike are important.

Thanks, guys! You are correct in that the Dellorto’s have no butterfly to worry about. Just a big ole fat slide. I’ve tried the vacuum leak test by spraying all around the carbs/intakes with brake cleaner/carb cleaner, and everything seems good and tight. I would love to synchronize the carbs, as I have both a gauge setup and a homemade twin cylinder tester I made years ago for the oilheads. The problem there is that I don’t see any ports in the intakes to hook the gauges to. I’m going back out in a few to start the rear tire install and I will look again. VERY good idea to check the carb sync! I just need to figure out how to do it. I guess I could always drill and Tap some small holes in the intakes to hook to, but I want to try to avoid doing anything to take away from the originality. I’ll let you guys know what I find out.
Thanks again!

LOL, it has been a very long time since I had a Delorto in hand so I can't point out the exact location of the vacuum port on them. I am certain there are ports there.

Here is the guy to go to, https://bmwmotorcycletech.info/dell.htm

St.

Synching can be done by shorting out the plugs on each side, one at a time. It's something people are a little hesitant about but can be done safely. You'll need spark plug extensions...Snowbum discusses it under the synching section. You use a plastic handle screwdriver with the blade firmly on the engine fins and then swivel the shank of the screwdriver onto the plug extension. Do it in a firm direct movement. The idea is that you give the spark energy a place to find ground, rather than go through the plug. Read up on it at Snowbum's site.

LOL, I forgot the stock R90S bikes had those awful things called points. Yes, a bike with points can use the shorting method to set up carbs but don't ever do it on an electronic ignition.

Oh by the way, take the F out of FNG, you never need it here. St.

Thanks, guys! I’m just diving into reading some of Snowbum’s stuff. There’s a LOT to digest there!