Main Jet Size

[Adrian]

Hi again - I'm about to buy all new internals for the Bings. The kit from Motorworks has a 148 Main Jet - the old main jet I took out is a 138. Will it be okay to use the 148 - will it make any difference to the running of the bike? The Needle Jet size is the same. Thanks .......

[nhmaf]

Seems a bit of a jump to me - I'd expect that some adjustment may be necessary.

What are the numbers stamped on the side of your carb
usually something like 64/32/136, or similar ?

[Adrian]

What are the numbers stamped on the side of your carbusually something like 64/32/136, or similar ?  

Hi - the numbers on the carby are - 64/32/348

Thanks .......

[nhmaf]

OK, so according to my Bing book, hat would have been configured from the factory with a 138 main jet and a 2.64 needle jet, 251 jet needle, and a 45 idle jet, with the needle clip at position 3.  

It looks like Bing did go as high as 145 on the main jet with the R65 but with a 2.66 needle jet and a 241 jet needle and a 40 idle jet.  

So, I suspect that it can work, but you may need to play somewhat with fuel bowl level , air mixture, and clip position.  There is a fair bit of overlap in the main jet/jet needle sizes used between the various R65 models years and the R80 carb setups.   It may end up running a little more rich in mixture on average if you don't change the other parts, but it may not be an objectionable amount.    I haven't messed with Bings very much outside of their usual factory setups, myself.

[Adrian]

Hi and thanks nhmaf - I'm going to buy the kit as is but will buy separate Main Jets that are 138 in size. Its still cheaper than buying all the parts individually. This way - hopefully there wont be any faffing about with settings beyond the normal tuning requirements .......

[Barry]

I think you are right to stick with the 138.

Bing jet sizes are something of a mystery and I can tell you for example that those jets are not 1.38mm and 1.45 mm. They are smaller than that. I have some data that can be extrapolated which suggests they are approx. 1.2mm and 1.28mm respectively. The difference in the area of those jets can then be calculated with reasonable accuracy and the 145 comes out at 13.8% bigger than the 138. That's more than what is considered 1 jet size increase and seems a little too much to disregard to me although I agree you could compensate to some extent with fuel level adjustment.

There is a view that as the carb rarely operates on the main jet then the size isn't critical below 3/4 throttle but in practice it doesn't work like that.  Although the needle jet effectively meters the fuel below 3/4 throttle all of the fuel has to pass through the main jet first and there is a resulting pressure drop. Changing the main jet size alters that pressure drop so that in practice a larger main jet would richen the mixture even when the carb is only operating on the needle.

[Bob_Roller]

I replaced the main jets on my '81 R65, I went from 145, to 138 .
I got a few spares when I got my '84 LS .
The US model '81 R65 was jetted the richest from what the Bing manual says .
I was experimenting to see if I would see any appreciable increase in fuel mileage, since 10% ethanol has been added to the fuel supply and I have lost 5-6 miles, 8-10 km per US gallon .
Also with the extreme heat here in the low deserts of southern Aridzona the density altitude, or the decrease of air pressure due to the less dense hot air .
Phoenix is at an altitude of 1130 feet 340 m, at an average air temp of 105-110 F, 41-43 C during our ' hot season ', it's like being at around 6,000 feet, 1800 m in elevation .
The conversion factor in the manual, would make the correct jet size a 139, so  a 138 was close enough .
After I did this, I saw no change what so ever in engine operation, or any noticeable increase in fuel mileage .
So, after saying all this, I really don't think you'll see any noticeable change with your main jet size change .
With the average use of an R65, I don't think the main jet metering comes into play very much .

[Adrian]

Okay thanks guys - I stuck with 138 jets just to be on the safe side .....

[HOFFI]

I'm getting 135's due to availability issues. I have been assured by the dealer who also happens to be a builder and service agent that it shouldn't be an issue. But views on the issue will be appreciated.

[nhmaf]

I think that is close enough that you likely won't notice a difference, or at worst, might need to tweek the air mixture screw a little bit.

[Ed Miller]

Too bad it's so hard to find a place to do full throttle tests.