Converting a 990 to Carburetors

Work in progress


Converting a 990 from fuel injection to 950 carburetors is a big job, although it isn’t particularly difficult if one has the patience and certain bits and pieces from a 950.

The obvious 950 bits and pieces needed include the complete Keihin CV carburetor assembly, ignition timing wheel, main wiring harness and ECU. Getting the 950 air box assembly is helpful, but the EFI airbox can be made to work if one from a 950 isn’t available. Not so obvious bits include choke cable, clutch master cylinder cover with choke lever, intake manifold boots, O2 sensor plugs, Safari fuel pump blank off plate and petcock.

Stripping the 990

Removing the 990 main wiring harness proved to be the biggest headache of the conversion. To create enough clearance so the main wiring harness can be removed out through the front of the bike required loosening both the radiator and oil tank. They don’t need to be removed, but rather just loosen by removing all shouldered mounting bolts. I also unbolted the instrument and fairing mounting bracket.

Removing the rear O2 sensor required removing the entire exhaust system from the bike, which in turn, required removing the luggage racks. Nothing like having to get completely naked to go to the toilet….

Figure 1. Chassis Stripped Bare of Main Wire Harness

Figure 2. Instrument Cluster Removed for Easier Access

Figure 3. Main EFI Wire Harness


Because the 950 main wiring harness is much less complicated than that of the 990, installing it was easier than removing the 990 harness. For the most part, it follows the same routing as the 990 harness, but with much less fanfare. After a couple of trial and error fitments, everything pretty much fell into place.

The 990 has the ECU mounted up under the fairing. The 950 ECU mounts behind the steering head and fuse box; however, the 990 frame does not have the 950 ECU mounting tabs. There is a grounding tab on the right side that can be used and I used my caveman skills to fabricate a mounting tab for the left side that is held in place by a hose clamp. In reality, a bunch of tie wraps would have probably sufficed.

Figure 4. 950 ECU Location

Figure 5. 950 ECU Location Top View (Note: wires are NOT routed properly in this photo)

Figure 6. Homemade ECU Mounting Tab

Figure 7. ECU Mounting Tab Top View

Bypassing the side stand switch is different on the 950 and 990. The 990 uses a hall effect switch which requires a side stand switch eliminator module. To bypass the side stand switch on the 950; however, only requires that the green and black wires on the switch side of the connector be soldered together.

I re-used the 990 instrument cluster and fairing bracket. It has some extra relay mounting tabs which aren’t used for the 950 wiring harness, but that doesn’t matter.

The 950 carburetors that I got had the optional heaters, which connect to the accessory connectors behind the fairing. I mounted the temperature sensor where the 990 ECU used to be located.

Some people have reported ignition coils that use 3-pin instead of 2-pin connectors. In my case, the 950 wiring harness connectors plugged right into my existing 990 ignition coils.

Vacuum Lines and Fuel Pump

Since I did away with the manifold cross-over line from the 990 set-up, I drilled out the restricted nipples and used them to attach vacuum lines routed up to the rear of the airbox. These lines are used for synchronizing the carburetors without needing to remove the gas tanks.

I used one of the old manifold pressure sensor nipples to power the flush-mounted Mikuni vacumm operated fuel pump, which I also mounted on the rear of the airbox.

I used a M6x1.0 socket head bolt with a copper washer to plug the remaining hole in the forward intake manifold. I may use this port in the future for a Pingel vacuum operated fuel cut-off valve if I prove incapable of remembering to shut off the tank petcock valve to prevent carburetor flooding.

After reading all the nonsense about the stock 950 electric fuel pump and considering the good luck I’ve had with Mikuni vacuum operated fuel pumps in the past, I opted to go the Mikuni route. Several vendors offer kits, but since I already had the bits and pieces laying around, I rolled my own using a flush mounted pump. Notice that it is slightly offset to right. In addition to leaving room for the carb synch tubes, that’s where the pump fit best on the air box.

Figure 8. Mikuni flush-mount Vacuum Fuel Pump and Carb Synch Tubes

Figure 9. Dry Break Connector on Main Fuel Supply Line

Airbox Mods

Given the restricted access posed by the stock 950 airbox, it is not clear to me how one would adjust the stock Idle Air Mixture Control screws. I had a similar access issue on my 640 Adventure and used the R&D flexible Idle Air Mixture Control Screw with great convenience. So I decided to do the same with the 950 carbs. Doing so, however, requires some drilling in the bottom of the 950 airbox.

Fortunately, I sourced the R&Ds from Ken at and he provided a usable diagram of which holes to enlarge and where new ones needed to be drilled not just for the R&D cables, but also for the separate vent lines for each carb. On later 950s, the carb vent lines were tee’d together before exiting the bottom of the airbox. Several of those in the know claim that having separate vent lines is mo’betta, so this is the way I went. Having the second vent line requires a new hole in the bottom of the airbox. Ken also provided vent tubes of the correct lengths (one is longer than the other).

Figure 10. Stock 950 Airbox

Figure 11. 950 Airbox modified for R&D flexible Idle Air Mixture Control Screw’s and Separate Carb Vents


Even though I have confidence in my jetting ability, I realized that accessing the carbs on a 950 multiple times to swap jets would be a royal pain in the ass. So I took the easy way out and got a jetting kit from prescribed for my riding elevation and 2-1 exhaust with single Akrapovic silencer and insert.

Turns out I had the correct pilot jets already, but I ended up changing the idle air jet and main jets to Ken’s recommendations. Unfortunately, I had assumed that I had a stock jet needle, but as it turns out, the carbs had some aftermarket needle, probably a FactoryPro needle. I decided to proceed anyway while the stock OE needles were ordered. I thought about trying to find used needles, but I wanted to make sure I had known good needles, so I ordered new at the staggering price of $53 per needle (these should be $15).

I used the float height gauge to set the floats to 3.5 mm per Ken’s recommendation. As shown in the photos, the floats had been set quite a bit higher than 3.5 mm by the previous owner.

Figure 12. 950 Keihin CV Carbs

Figure 13. Idle Air Jet

Figure 14. Main Jet

Figure 15. Pilot Jet

Figure 16. Previous Float Height

Figure 17. Final Float Height

Figure 18. Stock Carb Vent Y-connector. Replaced with separate vents for each carb.

Figure 19. R&D Flexible Idle Air Mixture Control Screws Installed


The 950 ignition timing wheel is different from that used by the 990. Getting to the ignition timing wheel requires removing all the clutch disks and loosening the clutch basket so it can be slid outward.

Figure 20. Clutch Cover Removed to access the Ignition Timing Wheel

Figure 21. Ignition Timing Wheel and Sensor. Big nut is left hand thread.

Figure 22. Carbs installed in the Air Box

Figure 23. Air Filter and Velocity Stacks Installed

Figure 24. Airbox all buttoned up

Because the 990 in-tank fuel pump is no longer needed, it is removed and a Safari tanks blank off plate and manual Acerbis universal petcock is installed over the hole. The mounting holes in the blank-off plate didn’t line up with the tank threaded inserts, so two holes in the blank-off plate needed to be elongated.

Figure 25. Fuel Pump Blank-off Plate

Figure 26. Misaligned Mounting Holes

Perhaps not to surprising, the outward appearance of the bike has changed little after converting to carbs. Telltale signs included the choke cable, R&D Idle Air Mixture Control Screws and idle speed adjustment knob. Otherwise, the bike looks like a bone stock 990 Adventure S with the cool mummy graphics.

Figure 27. Petcock and Idle Speed Adjustment Knob

Figure 28. R&D Idle Air Mixture Control Screw Knobs

Figure 29. Choke Lever Mounted on Clutch Master Cylinder


After a few cranks with the starter to prime the Mikuni fuel pump and fill carb float bowls, the bike started right up. Fiddling with the R&D Idle Air Mixture Control Screws and idle speed screw got the thing to idle right at 1400-1500 RPM at 4-bars temperature without any popping on deceleration.

Gone was the snatchy trailing throttle and hunting/jerking at steady state cruising in the 3,500-4,500 RPM band. Mid-corner roll-on’s are smooth as silk and, if anything has been given up on the top-end performance, I can’t tell. If I started to notice any such decrease in top-end performance, I could certainly go the extra step and install Keihin FCRs, but at this time I see no such need for my type of riding and style.

These carburetors provide drivability superior to any injected 990 I have ridden to date.

Special thanks goes out to Head2Wind (aka Ken ) for his continued assistance throughout this conversion process.

So what did all this cost?

If you start with a 990 and only purchased those items necessary to covert to CV carbs, retail list would be over US$3,200. For that amount, I would have gone straight to Keihin 39mm FCRs.

Fortunately, however, I bought a lot of the 950 bits used from fellow inmates, which dropped the pricing down to about US$1,300. (Updated: Be sure to add a set of 950 cams as detailed below)

Here’s a costed bill of material for those of you interested:
Cost Sheet

UPDATE:The transformation is complete and to say that I’m pleased with the results would be an understatement.

In the end, swapping out the stock 990 cams with those from a 950 was what made the final difference for the carbs. Unbelievably, I now have absolutely no complaints about the throttle response, overall drivability or outright performance. All are absolutely superb.

I will likely tinker a bit more with needle clip position and, possibly, float level to see if any improvement in gas mileage can be had. When pushed hard on the back country roads, I have been consistently getting 31 mpg (vs 35 mpg on the R1200GS) and as high as 37 mpg (vs 39 mpg on the R1200GS) if I ride like a tourist. (add 5% if you use the KTM odometer mileage instead of GPS).

I am currently running a Facet pump with a fuel pressure regulator, set at 3 PSIG maximum (…1#post15245091), in lieu of the Mikuni vacuum pump described in my earlier post. I put a gauge on the Mikuni pump and watched it while riding. I saw pressure variations that I couldn’t rationalize. To eliminate as many unknowns as possible during the tuning phase, I simply installed the Facet and regulator to ensure a steady supply of gas to the carbs.