R75/5 Carburetor question?

I've been attempting to get this 71 R75/5 dialed in. The right plug indicates rich. I rebuilt the carbs, but did not change the jets or main needle. Floats and float needle, diaphragm, and gaskets. It takes about 10 miles of riding before she's completely warmed up, then the idle settles in. I can synch the carbs, and it runs pretty good for awhile, then the right carb starts running rich again. I usually build British Bikes, and these carbs behave like a set of amals with worn sleeves/slides. I am wondering if the left carb was replaced sometime in the past, see photo. Would it be worth it to replace jets and/or needles? Can the slides or sleeves be worn? Both pistons act the same when I lift them and let them drop. Amals are less than $400 for a set, and if it had amals, I just replace the set. $1200 is a little steep for a set of bings. I'm open to suggestions. I have also done a top end, the compression and timing are good. Thanks for all replies. Phil

What are the carb numbers? Looks like one ends in 19, the other in 10. Matched carbs would be L/R 9/10 or 19/20.

They are 9/10. Numbers are right. It just looks like the left one is newer. That's not glare on the right one, the left one has a black label.

The 9/10 carbs should be good carbs to work with...some of the earlier ones were crude and had problems. Something that is over looked is the wear of the needle...it bangs around in the jet and wears thinner. That will allow more gas to flow through the jet.

Thanks 20774, I’ll start with new needles and jets.

Needles

10drum said:

Thanks 20774, I’ll start with new needles and jets.

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Agree. Same with Dellorto's. Sudden drop in fuel mileage or running rich is usually the needle jets. As 20774 states, the needle moves around in the needle jet and over time, both wear, increasing fuel flow through it.

RPGR90s said:

Agree. Same with Dellorto's. Sudden drop in fuel mileage or running rich is usually the needle jets. As 20774 states, the needle moves around in the needle jet and over time, both wear, increasing fuel flow through it.

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It makes sense, and the left carb has a newer looking label, which may mean it it was changed for some reason. I've got them both apart, and I can't tell any difference in the wear, so I've ordered needles and jets for both. I'll report back in a couple of weeks. Thanks for the replies.

Any idea what mileage is on the carbs?
As the jet needle bounces around it causes the needle jet to become oval and not meter fuel properly.
Your carbs are mismatched by number, label and casting.
The black label carbs were a PITA to tune because of poor manufacturing practices. There are plenty of articles out there supporting this.
You may be further ahead to get a used matching carb, preferably without the black label.
Good luck with your project.

Woodgrain