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R80G/S Jetting


1981-1986 R80G/S Jetting for Dummies

Disclaimer: I am no expert in carburetors (although I remember those darn Venturi equations in university), nor am I racing or do engage in engine tuning. I am just a amateur mechanic like everybody else with the urge to fix things that aren't right.

The intention of this little write-up is to give some information (not too theoretical) and some hands-on advice to improve whatever your problem may be when it comes to jetting and plug color.

It all started when I got my first 800cc BMW, a 1982 R80G/S. It was a basket case, not running, with the emission control installed. During my ressurection, I removed the EPA, refurbished the carbs (64/32/323 and /324) and kept the standard jetting:

Main Jet
Needle Jet
Idle Jet
Needle position
Comment
Bing
150
2.66
40
3rd
US bikes incl. EPA (my bike had those installed)
Clymer
130
2.66
45
3rd
1981-1982 US models acc. to Clymer
Clymer
150
2.64
40
3rd
1983-1986 US models acc. to Clymer
Bing
148
2.64
45
4th
European model
Haynes
150
2.66
40
3rd
1981-1982 US models acc. to Haynes
Haynes
132
2.66
45
3rd
1983-1986 US models acc. to Haynes
Haynes
145
2.64
45
4th
1981-1982 UK models acc. to Haynes
Haynes
135
2.68
45
4th
1983-1986 UK models acc. to Haynes
Bing EU
148
2.64
45
4th
Europe
User 1
135
2.68
45
3rd
05/87 G/S with stock airfilter and exhaust
User 2
148
2.66
45
3rd
early G/S
User 3
140
2.68
45
3rd
reported improvement over 135/2.68/45/3rd
User 4
145
2.68
40
3rd
reported improvement over US stock setting (Emission Control ???)
User 5
138
2.68
40
4th
no EPA, 37mpg fully loaded
Test 1
135
2.68
45
3rd
suggested with W6DC for post-1985 models

I guess you can all wholeheartedly agree with me: how is the simple man supposed to know? Almost every possible combination of jets and needles and grooves has been used and applied over the product lifetime of the R80G/S worldwide. Add the different user experiences and opinions and you end up with a lot of possibly working scenarios.

So, what to do if your engine doesn't feel right and despite you using some of the settings as per above isn't what you think it should be? I will try to keep it simple with three drawings and one diagram I found on the net:

Many of us aren't very knowledgable when it comes to the "inner mysteries" of 'constant velocity' (CV) carburetors. And how are we supposed to do a good job of "tweaking" the jetting if we don't know where to start. A simple diagram is going to help us here:

Without going into too much detail (there are better sources on the net), here a brief summary. As you can see, there are a large number of variables involved in proper fuel delivery:
  1. From zero to 1/8 throttle, the pilot circuit (idle mixture screw and idle jet) is the major contributor, the root diameter of the jet needle plays a role too
  2. In the lower midrange, the starting point of the needle taper is crucial (needle position)
  3. In the upper midrange, the degree (steepness) of the needle taper is significant (BMW has no variants I believe, don't know about Bing)
  4. In the high speed range, the main jet is the biggest contributor

This doesn't consider the tension of the vacuum piston spring or float height and since I don't know nothing about their influence I leave that to the experts too.

How does the above translate into real-life hands-on tweaking instructions. Harry Teicher, Denmark did a good job describing the basics on ADVrider some time ago and I quote:

When the main jetting is to rich, the engine will feel sluggish and acceleration will 'feel' slow. You can check the main jet by quickly closing the throttle from wide open to 7/8 position when the engine's RPM is greater than 4500. If the engine accelerates slightly, the main jet is to lean. A larger main jet is needed. If the engine hesitates or misses slightly, the main jet is to rich. A smaller main jet is needed. If the engine just slows a slight amount, the jetting is very close to correct. Another test is to accelerate through the gears at full throttle. If the engine backfires through the carburetor, misses, cuts out or quits running, the main jet is lean. Increase the size of the main jet. If the engine acceleration seems sluggish, does not react to the throttle or sounds flat, the main jet is rich. Decrease the size of the main jet. The main jetting can be set quickly at a 1/4 mile drag strip. For this test, you can ignore the ET (elapsed time). Making sure the engine is warmed up, make your run down the strip making note of your final MPH. Keep increasing the size of the main jet until MPH begins to drop. Then drop back one jet size. Your jetting should now be correct.

Another source states:
To determine if the bike is lean (to little fuel for the amount of air reaching the engine through the carburetor) at any given throttle opening, partially cover the air-filter intake with a piece of duct tape; if the carburetion improves, it's running lean. If you suspect the bike is running rich (too much fuel in the air-fuel mixture), remove the airbox top or the air cleaner element; if the changes are for the better, the bike was running too rich.

And here's what I did on my R80G/S with 32mm Bings:
Started with the leanest setting 135/45/2.68/2nd and used BP6ES plugs (slighly colder than stock because my W7DC ran way too hot - cracked isolator). I polished the original needles. The bike actually did not run too bad, but backfired badly. In addition it had hesitation round the 3000rpm mark when you opened the throttle. I set the needle one notch higher, so 3rd notch from top. Another test run confirmed that the hesitation had moved up now to around 4000rm. Since the bike was now completely warmed up, I set the idle mixture (using the highest rpm method) and synch'ed the carbs.

Next test run with larger main jet 140/2.68/45/3rd: less backfiring, still hesitation at 4000rpm
Another test (now back to the original main jet) 150/2.68/45/3rd: perfect

The bike pulls strong and rev's easily over 6500pm in 1st, 2nd and 3rd. I am sure the gas mileage will still be higher that I would like it to be, but it runs really nice. The spark plugs are nicely tanned too. All in all it took an hour and a half and about $40 for a couple of jets to get it done. Well worth the effort.


More jetting: R100GS dual plugged engine
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