If I remember correctly, my R100GS had smaller valves than my R100RS (which has the larger carburettors). If you fit the 40mm units to the engine with the smaller valves, I would expect the result would be less low-end torque. Personally, I wouldn't find that to be an improvement. I always found my R100GS engine's performance to be very satisfying as it was delivered by the factory. I put 100 k miles on that machine and have suffered seller's remorse since shortly after I sold it. Big mistake that...
Disclaimer: I'm no expert, but I am experienced.

I want to swap the carbs on a temporary basis in an attempt to diagnose a slight stutter that I have on my 95 GS at 2800 rpms and under. I've been looking for this gremlin for years and have tried: carb kits and adjustments per Snowbum's articles, timing and valve adjustments, new plug wires & spark plugs, new coil.

I'm thinking I can eliminate the stock carbs as the problem with the temporary 40 mm carb swap.

What do you think?

I'm thinking you will find even more issues with that swap, as there is more to the 40mm system than just the carbs. If my bike had the same problem, I probably wouldn't notice, as I spend very little time under 3k rpm. There are those out there that believe that low rpm with an airhead promotes spline wear. Having said that, you obviously could have something out of wack, but many questions come to mind. You didn't say if the carbs have stock jetting, what altitude you are at, what quality of fuel you have, what the plugs look like, etc.

Thanks for the reply and the suggestions.

I'll check the jetting against stock specs, and pull the plugs and report back. I'm at sea level but have ridden in the mountains quite a bit with no change in the engine stumble. I've also run all 3 grades of gas with no change in the stumble. Bike runs fine over 3000 rpms, and always has. It's very reliable - completed a 6,000 mile trip with no issues. Just that annoying low rpm stumble when I'm in the city or passing thru small towns. I have an 83 R 100 RS that runs very well, wish the GS ran as well. I exclusively ride the GS now.

What about this: Could I swap out the heads of the GS with the RS heads? The RS heads are new and the exhaust valve seats were replaced with the new unleaded gas resistant ones by Oak, and they have less than 5000 miles on them. That way I would be able to run the perfectly functioning 40 mm carbs.

Would I have to change out the pistons also? And cylinders? Anything else?

I'll swap the entire engines if necessary, but that is quite a bit of work.

Thanks again for the post and suggestions.

I'm eagerly awaiting suggestions on the heads and other parts swap with the RS.

- Thanks