Carb upgrade?

[Luddite]

We got a great BMW shop here locally. One of the techs is walking me through some carb issues. To be Frank, he's not a big fan of flat-top carbs. Any thoughts/experience upgrading carbs? It's a 79, 203 carbs

[Tony Smith]

I really do not know why the flat top Bings are so unloved. In good condition they work pretty much as well as the dome top ones and they will NEVER develop a leaky crimped seal such as the one I have glued two small BMW buttons over.....

Yes Bing and BMW decided that the dome tops improved some things, but it is reasonably open to question by how much.

I would be inclined to rebuild the ones you have, but if you really, really want dome tops - then 1st call would be to buy a pair that were actually fitted to an R65 and refurbish them, or just buy a pair of 32mm Bings and refit them appropriately.

If you want to be different and have bragging rights - fit Amals - I mention this only because I spent last night looking over a brand new Amal carb that a friend has just bought and they have seriously improved since I last handled one (Spanish Amal fitted to brand new Bultaco and worn beyond tolerance prior to first start (or thereabouts).

OK Amals are still crap. But they are cheap.

[marcmax]

If you really want to "upgrade" you can go to Del'Orto or Mikuni carbs. Rocky Point Cycle has kits for both of them. http://www.rockypointcycle.com/c-bmw12_mikuni_carb_kits.html

BUT . . . . .  you better have really deep pockets.

[Luddite]

Thanks for the insight,fellas. They've been rebuilt. Did the color tune idle adjustment, which seemed to work OK, checked for air leaks, etc. (normal battle of the carbs bull[BLEEP!])...Thought I'd move the jet needle up (or down) a click and found that the ring thing that holds the diagram in had popped out, which may explain a lot. Thought I'd ask about the carbs, anyway...

[montmil]

Soak that "ring thing" in very hot water before attempting to refit. Makes like so much easier.

[Mike V]

Luddite,

I'm curious - ask your tech exactly why he isn't a fan of the flat tops.  I would be interested to know the specifics.

I'm with Tony, the flat tops get a bad rap too often.  I've had flat tops on my R100/7 since I purchased it new in 1978 and after only +85,000 miles I've had no problems at all with them.  I've only rebuilt them 3 times.  Your R65 was engineered for the flat tops. Changing that set-up may lead to other issues you aren't anticipating.

And, they make a perfect coaster for a cold beverage while adjusting valves.

Seriously - I'm curious what kind of negative empirical information your Tech has to share.  The only thing I can think of is the diaphragms are a little more challenging to replace but that's not a performance issue.

-Mike V.

[Barry]

Mike,

The only thing I've heard said about flat tops that I would judge as rational criticism is they lack a return spring but even that feature is shared with some models of the standard Bing CV's.

My flat tops have been no trouble and I appreciate the flat part as a rare bit of alloy to polish up to a nice shine.

[SoloLobo]

I've been a "Bing only" guy for the last 26 years... my recently acquired R65LS has Mikuni's and I'll say that when Mikuni's are properly set up they return better starting (cold and hot), better pick-up and are really fun!

[Bob_Roller]

Only issue that I have heard about the flat top carbs, is the plastic ring that secures the rubber diaphragm to the slide .

I guess it's difficult to get it installed correctly if it comes loose, or you replace the diaphragms .

[Luddite]

He didn't say anything specifically, he spoke darkly of the late '70's as being a bad time for BMW generally. The late '70's was a terrific time for me by the way.I nodded knowingly because I wanted him to like me. I think he's point was that I'm only gonna get so much out of these carbs, which I already knew. That's part of the R65 charm, right?
  That said, I'm ready to toss this one off a bridge. I've got a profound lack of acceleration (worse when cold) that would indicate a too rich mixture, but I have the screws set at the rec setting (half turn). FULL DISCLOSURE: I went up to a 150 on the main jet...

[montmil]

 That said, I'm ready to toss this one off a bridge. I've got a profound lack of acceleration (worse when cold) that would indicate a too rich mixture, but I have the screws set at the rec setting (half turn). FULL DISCLOSURE: I went up to a 150 on the main jet...

If "the screws" you're speaking of are the air mixture screws on the bottom of the carb body, you're screwing in the wrong place.   The small brass ones are the Idle Air Mixture screws; idle being the operative word.

Confirm the enrichment bodies lever arms are fully closing. These little "choke" assemblies are actually mini-carburetors. if the enrichment levers are still partially open, you will definitely have an over-rich mixture. Also check cable length to the enrichers. Give just a hair of slack.

Your main jet does not come into full play until the engine revs are WFO. What is controlling the mid-range -where most of your riding is done- is the needle jet and the needle's position plus the carb slide's cutaway.

If the bike was mine, I'd first check that the enricheners are truly off. Second, I'd confirm the needle jet size, it's cleanliness, the cleanliness of the atomizer assembly and the correct position of the needles.

Remain calm and carby on.

[Barry]

What ever's wrong, it's not the fault of the flatops as a carb model.  With correct jetting and properly set up they should provide instant starting and good carburation right through the rev range.

How do the plugs look ? The insulator colour is not a a good guide these days. I judge on the base ring of the plug (the bit the side electrode is welded to).  The base ring should have a very light dusting of carbon at most. Any more and the mixture is too rich.  Only problem is use of the choke stuffs up the reading in no time and the carbon laid down by the choke doesn't burn off quickly so you either need to start without choke or put clean plugs in when it's warmed a little.

[montmil]

Forget to suggest you confirm the vacuum take-off ports on the bottom of the carb body -not on the float bowl- are properly sealed with either a tiny machine screw or a rubber cap.

[Luddite]

you guys have given me a lot to think about, But I'm confused: I have two screws on the bottom of the carb body, the vacuum ports (thanks for the tip, by the way) and the mixture screws (#7 in the Bing IPB).
Where are the small brass screws?
The first thing I will check is the enrichers,however. I had these disconnected (obviously) when I took the carbs off....start with what you fixed, right?
I'll keep you posted....

[Barry]

Vacuum port is nearest in the pic but the screws were never originally brass. The thread size is 3.5mm x 0.6mm

It's important that the enrichers are correctly assembled and there is no better way than to understand how they work. The pic shows the enricher in the off position so there is no fuel hole in the disc lined up with the fuel inlet at the bottom and the pear shaped slot at the top is not aligned with the outlet to the catb venturi.

To be sure the enrichers have not been mixed up is very simple and easy to remember. The pear shaped slot should be at the top of the disc and point towards the cylinder head.