Bing Main Jet sizes

John Roberts

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The main jets on my '82 R100RS are 160, but acording to the BMW manual they should be 170. The consumption has always been on the heavy side, so I'm intrigued to know if the 160 jet has a bigger hole than the 170. :eek

PS, and Bugger. :mad:
Before pressing the "send" button I checked my facts, and found that the page in my manual was dated 1978, whereas the bike is '82. Haynes says it should be 160. I am now confused. Can anyone tell me what jets I ought to have, please?
 
You should have 160's in your bike John. The 170 was fitted to the original RS (pre 81) with 40mm exhausts.

1.6mm hole in a 160. And 1.7mm in a 170.
 
I am surprised that Realoem is wrong. It lists 170's for up to 1984 R100RS, except for US bikes.
 
160 in the Haynes manual for UK 81 to 84 machines.

Realoem is a bit misleading. The yanks had 32mm carbs after September 1980
 
Would like to know, will a smaller jet necessarily mean better consumption? I mean if I need to open a throttle more because of the smaller jet, wouldn't I be back at square one as far as mileage is concerned?

Cheers
 
You will be running a weaker mixture so will be loosing power and if you go weak enough may be damaging the engine.

Much better to keep the mixture correct and ride slower.
 
Its all about the maximum operating efficiency. Too rich or too lean are less efficient so you convert less of the potential fuel energy into usable grunt.
 
You are going to get best fuel mileage by concentrating on the mid range part of the carburetor meaning the needle and needle jet. Make sure these are new and the correct ones for your model/year.
 
I’m using less then 360º turn in the middle jet as stated in the BMW manual.´
It back fires a bit but at least the spark plugs are not full of carbon deposits….

What is the normal setting?
 
Just got a pair of new jets from MotoWorks. I compared my new main jets with the old and after 70,000 miles they seem identical. I used the shanks of 1.50 and 1.60mm drills, funnily enough the 1.60 wouldn't enter the holes in the new jets but the 1.50 was equally sloppy in both so I reckon they are still as good as new virtually. I thought the buggers were supposed to wear??

Seventy K at 36mpg, 2,000 gallons near enough, that's a thousand gallons through each jet. Funny to think that petrol can wear metal.

But, there, again, what do I know.:P
 
Jets are not measured by a linear unit like millimetres but by the amount of fluid they flow.
For example a 110 main jet is not 10% bigger in diameter than a 100 but will flow 10% more fluid of the same viscosity at the same temperature in the same given time.
Your needles and needle jets are more likely to be worn than main jets and as you spend most time on a part throttle will lose a lot of fuel consumption there.
 
ians said:
Jets are not measured by a linear unit like millimetres but by the amount of fluid they flow.
For example a 110 main jet is not 10% bigger in diameter than a 100 but will flow 10% more fluid of the same viscosity at the same temperature in the same given time.
Click to expand...

I thought that too, but as they happened to be so very close to what Rob Farmer said in post #3 it seemed that what he said was quite logical.
ians said:
Your needles and needle jets are more likely to be worn than main jets and as you spend most time on a part throttle will lose a lot of fuel consumption there.
Click to expand...
Good point, I'll start sniffing around there then. :thumb2
 
Nippy Normans supply Touratech, Wunderlich, Desert Fox and many other makes of accessories.
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