Guide: Tuning the BMW F650
- hospersengineering
- 11 mei
- 10 minuten om te lezen
Bijgewerkt op: 19 mei
This guide is written for BMW F650 Funduro & Strada owners, who are looking for performance increase, lower fuel-consumption and more reliability/longevity, without the need for high lift cams, big bores or cilinderhead porting. The guide is based on years of tuning clients F650's and feedback from thousands of BMW F650 owners in the international BMW F650 Facebook community. The experience gained from testing many different (30+) setups, on a dyno with an exhaust gas analyzer, allows me to write this guide with confidence.
Tuning is usually a long and slow process, with this guide I aim to make that process easier. I can supply a custom jetkit to set you in the right direction. See the products page of my website.
Guide content
Preliminary note about model differences
Exhausts explained
Preparing the F650 for tuning
Common problems (+solutions)
Electrical problems
Sparkplugcaps and sparkplugleads
Fuelfilter
Carburetor condition
Floatheight
False air
Mixture screw O-ring
Camshaft postioning
Easy tuning options
Iridium sparkplugs
Oil
Inlet Manifold
Exhaustheaders
Airfilter
Rejetting the carburetors
Carburetor basics
Plug color interpretation
Adjusting the carburetor settings
Example of a rejetting procedure
Preliminary note about model differences
There are many small differences, but there is one essential difference between the F650’s made between 1993-1996 and the 1997-2000 models. The older models have #140 mainjets installed from the factory and 2 holes in the top of the airbox. The newer models (1997-2000) have these holes closed off. To compensate for that restriction, they installed #132,5 mainjets.
If you own the newer model, and want to follow this tuning guide, you are required to open up these 2 holes. They are exactly 22mm in diameter. This can be done with a soldering iron, a heated flatblade with the airbox still in the frame. You could get a better result with a 22mm spade drill bit or a step drill bit, but then you’ll have to remove the airbox from the frame.
Exhausts explained
The stock exhaust is the main limit for making power on the F650. It makes good low end torque, but it runs out of breath very quickly. It also acts as a huge heat sink, which has melted many side-covers and airbox-housings. Other problems such as, disintegrating graphite exhaust gaskets, cracking mounting points and the overall weight, make it a terrific candidate to replace. So many people do.
Unfortunately many people mount an aftermarket exhaust and expect the carburation to sort itself out. It results in a bike that backfires on engine braking (closed throttle), indicating a lean idle circuit. Terrible mileage and excessive engine temperatures follow. Most motorcycles backfire with aftermarket exhaust systems, therefore it is considered normal.
Only very rarely, the aftermarket exhaust is specifically meant for the F650. Exemplary is the EGU exhaust system, which is no longer in production. More often, it is a link pipe made for the F650, but supplied with a standard type muffler, used for many different bikes in the range of 600-1000cc’s, such as the Dominator/GPR/MSR exhausts.
There are 2 main types of muffler-design; perforated tube or chambered exhaust. The perforated tube type is great for making horsepower, the downside is that the muffler packing slowly disappears. Sometimes within 15.000 kilometers. The chambered exhaust is maintenance free, but more restrictive, so it makes less horsepower. For offroad and enduro use, a chambered exhaust might be the way to go, because it makes less noise and a little more torque in the low rpm range.
All currently available aftermarket exhausts for the F650 are the perforated tube type, so you don’t have a lot of options. Unless you can fabricate/buy a linkpipe and find a separate chambered muffler to go with it.
At this moment I strongly suggest a GPR Satinox, it is pricey, but a good match for the F650 Rotax engine. Dominator linkpipe design will melt the airbox and the overall buildquality is lacking.
All types and models of muffler can be used to work with the original carburetors, using idle jets between 45 - 55 and mainjets 140 - 155.
Preparing the F650 for tuning
If your F650 is not running perfectly, tuning it will be impossible.
Common problems (+solutions) are:
Electrical problems can feel like fuel-related problems. When the voltage rectifier fails, the bike will not go over 3500 rpm. It sounds like the carburetors are plugged. The reality is that the rectifier failed, and may have cooked the battery dry. The solution is to replace the rectifier, fill the battery with demineralized water. Relocate the rectifier to a cooler place to avoid overheating. Search for corroded plug connections in the 3 yellow wires.
Sparkplugcaps and sparkplugleads are often corroded and have high resistance, resulting in a weak spark. Especially BERU sparkplugcaps with metal housing should be replaced. Measure the resistance with a multimeter set to the range of 20 kOhms, one side inside the plugcap, the other on the frame or engine-bolt. The perfect number is 14 kOhms. If the number is higher than 15 kOhm, you can unscrew the plugcap and snip 10mm of the lead. Best is to replace the leads and caps all together.
Using a secondary fuelfilter can cause problems. Some people ride years without any problems, but it is an unnecessary addition. There is already one filter inside the gastank, and one filter in the T-junction between the carburetors. Another inline fuelfilter is not needed and should be removed.
The carburetors have to be in perfect condition. Torn diaphragms, worn slidecarriers, leaking floatbowl-gaskets and bad float-needles all should to be replaced before tuning. You can have the carburators ultrasonically cleaned, to make sure all tiny passages are cleared of dirt.
If the floatheight is too far off it can cause the bike to run lean or rich. It will seem like the jets might be too small or too big, since the plugs are white/black. The floatheight should be adjusted to exactly 14,6mm from the floatbowl gasketsurface to the top of the float. Verifying the floatheight is difficult on the Mikuni BST33 carburetors, because the floatbowl pushes the floatbracket in place when it is mounted. But when the floatbowl is on, you cannot measure or adjust the floats. You’ll have to simulate the pressure on the bracket by hand, while measuring and adjusting. See the video on the Jetkit product page for more detail.
False air can enter the system between the airfilter-carburetor connection, carburetor-cilinderhead connection or cylinderhead-exhaustheaders and various other points in the exhaust system. All false air that enters before the cilinderhead results in lean running. All the false air that enters after the cilinderhead results in backfires.
The mixture screw O-ring is often leaking. This creates another possibility for false air entering the system. The new thicker o-ring, included in the jetkit, is best installed first, push it down in to the carburetor with a toothpick, then drop in the washer, then the spring and finally the mixture screw. Use some WD40 or silicone spray so the mixture screw has no effort to push itself through the o-ring.
Camshaft postioning: the exhaust camshafts timing can be off from the Rotax factory. This makes a difference of 4hp, so check if your exhaustcam is correct. The upper situation is bad for power. Rotate it one thooth clockwise to get the lower situation. The circles represent the camshaft gears, the lines represent the timing marks when the piston is at TDC.
Easy tuning options
Iridium sparkplugs NGK DR8EIX are a nice option. But note that the ‘R’ stands for Resistor, which means the sparkplug itself has ~3-5 kOhm of extra resistance. The coils have 9 kOhm in good condition. 14 kOhms is the total limit. The standard sparkplugs D8EA do not have any resistance build-in. To use Iridium sparkplugs you need to remove the resistor in the sparkplugcap, which is about 5 kOhm, and replace it with a piece of M5 bolt of equal length as the resistor. A big flathead screwdriver is all that’s needed to unscrew the resistor from inside the sparkplugcap.
Oil. A dangerous topic, so I will give my opinion, based on my experience with many different F650’s. I use Motorex 15W50 Top Speed. It is a full synthetic, which I prefer due to its many superior properties. However, if the clutch is used to mineral of semi-synthetic, it can cause the clutch to slip. Replacing the clutchplates and using full synthetic from the start solves that problem. I have over 60.000km on my current clutchplates using full synthetic.
The inlet manifold has some rubber protruding around the inlet, hindering the airflow. It can be cut away with a razorblade.
Some years have exhaustheaders with a thick weldbead on the inside, hindering exhaustgasflow. They can be ground away fairly quickly using a dremel, set to a high rpm, and a Tungsten carbide burr.
Hiflo foam airfilter has a less dense foam-structure than the original BMW filter element, allowing higher airflow. (included in the Jetkit, pre-oiled)
Rejetting the carburetors
Carburetor basics
Some basic knowledge about carburetors should be known.
Carburetors are just fancy mixers to combine fuel and air in a specific ratio. To make it easier to understand, this graph has been simplified by omitting other parts that also influence the mixture.
The idle jet & mixture screw work together.
The idle jet is the main fuel supply for the lower rpm-range.
The mixture screw regulates the air-fuel ratio at idle.
Turning the mixture screw out, adds more fuel at idle.
The mixture screw functions optimally between 0,5 and 4,5 turns out (from seated)
The mixture screw is very sensitive; ½ turn can make a lot of difference.
The mainjet & needle work together.
The mainjet supplies the fuel for the middle and high rpm-range.
The needle limits the amount of fuel the mainjet can supply, within its working area. Setting the needle clip lower means you are raising the needle, and thus adding fuel.
At wide open throttle, the mainjet is not limited by the needle, and determines soley how much fuel is added to the mix.
1 needle clip position change can make a lot of difference.
The rpm-range, where the idle jet & mixture screw supply fuel, overlap with the area where the mainjet & needle supply fuel. This area of overlap is between 3500-4500 rpm. Adjusting the mixture screw to make the idle richer, while also lifting the needle one clip, means a double fuel increase in this overlap area.
The float regulates how much fuel enters the float bowl. The idle and mainjet drink from the floatbowl. When they consume more fuel, the lower the fuel level in the bowl, the lower the float drops, the more the floatneedle opens, the more fuel is added. And vice versa.
Plug color interpretation
Sparkplug color can help you adjust the air-fuel mixture.
The most important thing to understand is that the plugcolor can only indicate you the air-fuel mixture within a certain range. It doesn’t say how the engine is running at 2000 or at 6000rpm, if you have ridden it at an average of 4500 rpm. The plugcolor is also dependend on spark strength, engine mileage, oil consumption, fuel type, ethanol-percentage and elevation.
Ride the motorcycle for no less than 10 kilometers at 4-5000 rpm to make the sparkplugs change color. Hit the kill switch while riding and stop the bike at the side of the road. Pull the plugs.
The electrode tip gives an indication of the mixture screw setting (and therefore the idle jet size). Grey or brown tip means a good mixture screw setting.
The base ring gives an indication of the needle setting (and therefore the mainjet size). A black ring means a good needle setting.
Adjusting the carburetor settings
Always write down your new settings AND previous settings! Preferably with the reason you changed it. Your future self will thank you.
Needle clip and mix screw adjustments are possible without removing the carburetors from the bike. The needle clips can be adjusted by removing the gastank + bracket and taking off the diaphragm caps. Be sure the needles are seated all the way down in the slides when you drop them back in.
The mixture screws have standard threads, righty tighty, lefty loosy. Turning them in (clockwise) leans out the mixture at idle. Turning them out (anti clockwise) enriches the mixture at idle. When the carburetors are on the motorcycle, it is can be confusing to remember which way to turn in or out, because the screws are now upside-down.
Turn the screws all the way in, until they bottom out. Be careful because they can strip out the threads. Now count the number of turns. It’s best to count each half turn out from seated.
When the engine is hot, you will burn your hands adjusting the mixture screws, so wear gloves.
You need a very short (homemade) flathead screwdriver. Or a ¼ flathead bit works too. Remove the shock preload reservoir to gain easier access.
Example of a rejetting procedure
The best way to explain the process of rejetting is by giving an example:
You have installed a GPR exhaust without DB-killer on your F650. You ride the bike at sea level. You might have all or some of the following symptoms:
It backfires on engine braking (closed throttle)
It has bad fuel consumption (less than 250 kilometers or 150 miles)
The engine gets hot fast when idling.
The sparkplugs can be very pale. Most likely white electrode tip and a grey base circle.
You tried turning the mixture screws out to 4,5 turns, and it helps a little to reduce backfires. Setting needles higher to the 5th clip from the top also helps. But the performance and fuel consumption is still worse than it was with a stock exhaust.
You have hit the limit of both the mixture screws and the needles. So you need bigger jets.
For a GPR exhaust with a straight through muffler and no DB killer, you need:
The base setting for this setup is:
mixture screw at 2,5 turns out from fully seated
needle clip at position 2 from the top
After adjustments and installing the carburetors, you go for a ride. Then hit the kill switch and check the sparkplugs.
If you think the sparkplug color is good, you fill the tank and set the tripcounter to zero. Then ride for another 200km or more, preferably some 60-110kmh roads. Fill the tank again and divide the kilometers by the amount of spent fuel. An average fuel consumption of 1L/21km should be easily achievable. If it is less (<1/19), you should re-evaluate the plugs.
Test small changes, do not change the needle and mixture screw at once.
PS: Adding or removing a DB-killer, changes the flow. I estimate ¼ turn change on both mixture screws, and a ½ needle clip on the needle. (use small washers under the needle clip to lift the needle by a half clip)
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