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:
|Bing||US bikes incl. EPA (my bike had those installed)|
|Clymer||1981-1982 US models acc. to Clymer|
|Clymer||1983-1986 US models acc. to Clymer|
|Haynes||1981-1982 US models acc. to Haynes|
|Haynes||1983-1986 US models acc. to Haynes|
|Haynes||1981-1982 UK models acc. to Haynes|
|Haynes||1983-1986 UK models acc. to Haynes|
|User 1||05/87 G/S with stock airfilter and exhaust|
|User 2||early G/S|
|User 3||reported improvement over 135/2.68/45/3rd|
|User 4||reported improvement over US stock setting (Emission Control ???)|
|User 5||no EPA, 37mpg fully loaded|
|Test 1||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:
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.