Carb conundrum

[equipo5]

What jets should I use? I'm a bit confused with this. Any advice appreciated.
Ive got R65 carburetors on my R75/5. Not sure why but it was sold as a positive.

The carbs are 64/32/359&360

Presently on the bike are:-
Main jets 135.
Idle Jets 45
Needle Jet (rt) 2.70
Needle Jet (lt) 1.66

Cant tell what the Needles are and not sure if they are the same but they both are on the third clip position.

Looks like it's buy new carb kits with needle and jets but what do I use and does it really matter?

I'm thinking the R75/6 needle and jets seems to be the best choice but should I use the original R75/5 sizes?

Being a bigger engine it would need larger jets but hold on, excepting the idle jet, the R80 & R65US the jets and needles are the same! Maybe I should stick with the R65 sizes.

I dont know, its all so confusing.

Thanks in advance.
Equipo5

[Kurt in S.A.]

My take: Call Bing for input. I'd start with the R75/5 elements and see how the bike runs...then go from there.

Kurt in S.A.

[SteveD]

Yep, it matters. I'd get two new 241 needles and two new needle jets that are the same. I'd get 2.68 x 2 and 2.70 x 2 and start experimenting.
They're cheap from Motobins.

[Duane Ausherman]

You should always give us the problem. Is it running well? Does it start easily? Do you have evidence that it is lean or rich?

I am sure that you must have a problem of some sort. If not, why are you bothering to fiddle around?

[Deleted User 62]

What jets should I use? I'm a bit confused with this. Any advice appreciated.
Ive got R65 carburetors on my R75/5. Not sure why but it was sold as a positive.

It is a positive, the early carbs your 1970 R75/5 came with were problematical at best and lacked a vacuum take off for balancing. Bing upgraded the design several times over the years. Also, those R65 carbs are around 10 years younger than the originals. Never a bad thing!

[Dan in IL]

I hope this isn’t too late, I just got around to reading this thread.

A few years back I put a set of 1000cc cylinders on my R75/6. At first I didn’t make any changes to the carbs. It seemed to be running okay. Then one day when passing a car on the highway I realized that above 5000 rpm’s the engine wasn’t pulling as hard as it should be.
The first thing I did was raise the needles one notch in each carb. Wow, that made a difference! I left it that way for awhile then found out my gas mileage went from 47 to 35 mpg. Of course I wasn’t too happy about that. Next I got a set of main jets for a R100/7 and put the needles back where they were. Gas mileage went to 42 mpg and I still had plenty of top end power.
I sold the bike shortly after that, so that’s as far as I took it.
Just thought this would give you some idea’s about tinkering with the carburetors.

[equipo5]

Thanks Dan and everyone.

I've settled for a post 83 R80gs Carb. recondition kit and will fiddle from there, .. once the bike is on the road.

I know this is quite contentious, but I'm keen to use a "colourtune" to help sort out the jetting. Mainly because ever since I was a kid I've been curious about them plus I know nothing about tuning carbs., and seems a good place to start. There is plenty of bad press about the threads but I guess its a matter of being careful. It just seems a too good a toy to pass up.

When the accountant coughs up, I'll post a report on how it all goes.

Thanks for everyone's input.

regards
Ian

[Major Softie]

"Because I want one" is the best reason I've ever heard to have a ColorTune.

Might be the only reason...

[barryh]

My experience 30 years back with a colortune is that following the instructions for a completely blue flame will end up with a mixture that's too weak. So I'm dubious but I would be interested to see if if they are any good for indicating a difference in idle mixture side to side.

[Roy Gavin]

IMHO Colortunes work perfectly - Bing list them on their site, so they must think so too.

To set the mixture side to side you have to move the Colortune plug from one side to the other, and if you do this you will find that they work perfectly.

But they are only for setting the mixture, not for balancing the carbs - I have two Colortunes, and did a bit fiddling to see if they were any use in the balancing process, but they were not.

SU recommend using them too, and go so far as to suggest putting the vehicle up on axle stands and running the motor against the brakes, to check the needle type and position and the piston spring strength, but I haven't tried that on a bike with Bings.