Main Jet Part Numbers

[Bob_Roller]

I finally got around to replacing the main jets on my '81 R65 from 145's to 138's .

The 138's, I found in the grab bag 'o parts from the project LS I got about a year ago .

In the Bing Carburetor manual, there is a conversion chart to compensate for altitude and temperature differences from how the bike is set for, which is sea level and I think 60 F.

The correction factor for my conditions in summer (1130 ft elevation  and 105 F.) work out to a 135 main jet size, don't know if I'll go that small, but I can't seem to find a BMW part number for this size jet .

I'd to see if this size is even available .

Experimenting, trying to get the fuel comsumption back to where it was before the fuel supply here had 10% ethanol added to it .

[Barry]

Bob

Motobins stock 135's.  Haynes list them for some models of R80.

Having said that at moderate speeds I think fuel consumption is more influenced by float level, idle mixture and the needle/needle jet than by small changes in the main jet.

[Bob_Roller]

After seeing the various posts about fuel consumption, it looks like the North American version R65LS, came with 138 main jets, from what I have seen from what the riders of these bikes have posted, they seem to get in the range of about 5 mpg more than my bike ever did .

I checked RealOEM, and couldn't find any 135 main jets listed for any R65 or R80 .

Like I said, just experimenting with it, to see if in real life application, it makes any noticeable difference .

Most of my riding, is at 65 mph or higher, only about 2 miles to the nearest expressway, so slow city riding doesn't make up a very high percentage of my average riding .

With the arrival of the 'hot season', I've done the yearly switch to  premium grade fuel, 91 octane by our rating system, to prevent detonation due to high air temperatures .

Trying to see if I can squeeze a few more miles out of the slightly more costly fuel .

With the oilhead, I get consistently 20-25  miles more per tank full using premium grade fuel, over regular grade 87 octane fuel .

[Barry]

Haynes list the 135 for the 83- 85 R80GS, R80ST and R80RT UK models only so maybe that's why they are not listed in the US.

I suppose the main jets will have an effect on economy at 65 MPH.  Even still I bet your not using much more than 1/4 throttle.  I found it quite enlightening to mark off 1/4, 1/2, 3/4 and full throttle positions using some masking tape. The throttle action is not linear though which I think means that at an indicated 1/4 throttle it's actually not even that far open. I found that I was using no more than an indicated 1/8 throttle for 50 mph cruising on country lanes. This and my previous experience with other bikes has taught me that pilot circuits can have a significant impact on overall economy as they are still contributing to the mixture up to 1/4 throttle.

[Bob_Roller]

I'll try marking the throttle with tape, and see what position the throttle is in for the usual highway speeds .

[Patrick_Krivacka]

I'll be following this thread with great interest. I'm very disappointed with the mpg of my r65, and gaining 5 to 6 mpg would really be desirable. Do I sound like a broken record?

[montmil]

I'll be following this thread with great interest. I'm very disappointed with the mpg of my r65, and gaining 5 to 6 mpg would really be desirable. Do I sound like a broken record?

What sorta mileage are you getting, iman?

Monte

[Bob_Roller]

I marked the throttle housing and handgrip for the ride home yesterday, at 70 mph I'm at half throttle .

There are some differences in the needle jet and the needle .

The carbs on the '81 R65 have a 2.66 needle jet, and a 241 needle, the LS has a 2.64 needle jet and a 251 needle .

I thought the jet numbers was the area of the opening in square millimeters, don't know how the numbers are arrived at for the needle itself, as they are tapered .

Looking at the manual from Bing, the needle and needle jet are the most controlling factor at about half throttle, just as the graph that Barry has posted .

I'm not so sure now that changing the jets will give me any real change in the usual riding that I do .

[Lucky_Lou]

Looking at Barry,s chart it looks like the main jet isnt an issue unless your hammering it.
Lou

[Bob_Roller]

After Looking at the graphs, I'm not expecting any serious benefit from the jet change .

The jets didn't cost me anything,  like I said just an experiment to see if it would yield any positive results .

[Patrick_Krivacka]

Monte, I'm getting 40/42mpg. not so bad,but i was expecting more like 45/48, with the occasional 50 mpg, based on pre-purchase research. One of the things that got me back into cycling, after a several year break, was the gas savings. None of my bikes since getting back into it have gotten less then 60. But I didn't enjoy or 'bond' with any of them like I have this bike, so there you go.

[Barry]

I thought the jet numbers was the area of the opening in square millimeters, don't know how the numbers are arrived at for the needle itself, as they are tapered .  

For needle jets at least I'm fairly sure the jet sizes are the dia in mm as I made up a mandrel to test if mine were worn.

Main jet numbering is different again though because fuel flow vs jet diameter is not a linear relationship. The only jet sizes I know for certain are the 2 fitted to R45's 105 is 0.95mm dia and 123 is 1.05 mm dia so the numbers are not related to dia. I think the numbers represent linear increases in fuel flow so for example using fictitious sizes a 110 jet flows 10% more fuel than a 100.  

BTW how fast could you go at 1/4 throttle.

[Bob_Roller]

I'm not sure what speed I would be at for 1/4th  throttle, next time I get out I'll check it

[montmil]

... I'm getting 40/42mpg. not so bad,but i was expecting more like 45/48, with the occasional 50 mpg...

At 40+ mpg, you're pretty durn good. Guessing a long highway run at steady revs around 65 mph may raise your fuel average. It's all in the wrist, iman! My wrist is too twitchy to get more than the occasional 40.

Monte

[Patrick_Krivacka]

yeah, I'm slowly learning to let it go. with that said, I'm really interested to see what Bob and Barry work out. If I could get 5 more mpg, I'd shut up (about mpg's, anyway)