Boxerworks.com

For the past month and a half I've been having constant problems with my R75/5 carbs.

They're the early versions, /3 and /4 with the o ring on the outside of the choke (enricher) cover.

At first, bike was running but it was losing power and one side was way too rich while the other way too lean, tried adjusting a couple of times with no success, so I completely disassembled both carbs and rebuilt them with a kit from Euromotoelectrics. In doing this found one of the bowls had the little corner passage blocked and some of the internal passages in the carbs were less than ideal. Diaphragms and gaskets were good but were replaced with new.

Adjusted valves, timing, and checked compression; all good.

Still problems, after warming up bike would not idle, had a couple of trips where I barely made it home. Not overheating, but running mostly on one cylinder.

Bit the bullet on a set of new Noris points, checked the coils, cables and boots; again all good.

Same problem, cold bike worked great but only wanted to be at 3500-4000 RPM when warm, discovered this was due to one of the throttle pistons sticking.

Disassembled carbs again, polished both pistons to a mirror finish, checked for straightness and made sure both needles were in the correct position. Discovered one needle was coming out at an angle because the clip wasn't fully bottomed out on the piston, now they don't stick and the bike appears to be OK, but haven't had time for a test ride.

I also ordered a couple of 13 11 1 338 134 springs as per Snowbum's article, they fit perfectly in the carbs and made the bike idle marvelously but it was absolutely impossible to rev the bike at all without it flooding and stalling. Has anyone else had this problem? I'm thinking maybe the springs I received might be too strong and they don't let the piston lift but the part number is the one that I've seen be recommended.

My next step is doing a test drive, without the springs, to see how she behaves; but now that I bought them if things can be further improved I'd like to know how. Thanks in advance for any tips or recommendations.

According to RealOEM, those springs were only listed for the later models, say 1980-on. Bings manual suggests that same thing, for later version carbs.

Kurt

GBonduel wrote: ↑Tue Sep 23, 2025 2:13 pm
I also ordered a couple of 13 11 1 338 134 springs as per Snowbum's article, they fit perfectly in the carbs and made the bike idle marvelously but it was absolutely impossible to rev the bike at all without it flooding and stalling. Has anyone else had this problem? I'm thinking maybe the springs I received might be too strong and they don't let the piston lift but the part number is the one that I've seen be recommended.

My next step is doing a test drive, without the springs, to see how she behaves; but now that I bought them if things can be further improved I'd like to know how. Thanks in advance for any tips or recommendations.

Although they may have a beneficial effect at idle, that was not the intended purpose of the springs. They were introduced to allow weaker mid range settings as they provide an accelerator pump effect by slowing the rise of the piston when the the throttle is opened. It stands to reason that if springs are added to a carb that doesn't have weak mid range settings then they are going to cause an excessively rich mixture at least briefly. The steady state impact on the mixture shouldn't be that great unless the springs are far too strong but you might try weakening the mixture by dropping the needles one notch even though that is a big change.