Saturday, June 20, 2015

Carburetor woes, or How to not kill yourself on a Mikuni Part 1

So once upon a time...not so long ago, I told myself I wasn't going to buy another carbed bike. After constantly fiddling with Leann's GS500 I was just sick of how much work it takes to get them running well.
So here I am with this Aprilia Pegaso: One cylinder with two carburetors, what the hell were they thinking!? Right after I acquired the bike I stripped it down to the carbs to have a look. This isn't the worst job in the world, contrary to what a lot of people on the forums claim.

What I found when I first pulled them off is that they were very clean inside and somebody had already installed the "euro" 110 main jets. Unfortunately the bike feels very "chug-y" at the lower RPMs, which is exactly where I like to keep the revs when slowly navigating a tough section of rutted gravel or dirt road. So I need to play with carbs again!

The up-side to this situation is that the Aprilia Pegaso uses a pair of BST-33 Mikuni carbs.
These are nearly identical to the pair of carbs that were used on the old Suzuki GS500. This means I am fairly comfortable opening them up and how they operate. I also know that replacement parts are readily available, I even have a tray of jets and parts from my time with the GS. Now my browsing of the forums revealed a few major issues with the "social wisdom" contained: Factory float height, factory jet sizes, and factory air screw settings.
I browsed the Aprilia forums and found very little useful information.
I then did an internet search which resulted in a few good items, namely the BST Bible thread on ADV Forums. This is a great source of photos, but all of the information is specific to the KTM LC4 engine platform.
 I also had a few good sources from the GS days like: Float height

After running the bike a few days and having poor luck with the lower RPM I decided to get my baseline straight and figure things out a little better! As with all carb work it is critical that you establish that the engine is running as well as it should for the mileage: This means tune-up time!
-Replace spark plug with new, also buy a handful of the same plug for tuning work later. NGK's are usually $2/ea for basic copper plugs. Dont spend money on fancy plugs until you know the bike is tuned to a decent range.
-Check compression! Ideally you will do a leak-down test, but a simple compression test works in a pinch. You should really test it with the engine both cold, and hot. Sadly testing hot is not very friendly to aluminum heads so use your best judgment there! Cylinder Leak-down Tester
-Check timing if possible. Some newer bike you really cant, but most you should be able to at least put a timing light on it.
-Adjust valves if necessary.
-Use fresh premium fuel.

Okay, now pull the carbs off the bike and disassemble.
Make sure the float bowls are clear of gunk and debris.
Spray carb cleaner through the jets: If no fluid comes out of the throat of the carb somewhere the jet is probably clogged. Get out the fine wires and start unclogging! Dont stop till you get good squirts coming through.
Now comes an area where we need some technical information.
I happen to get a CD of the factory manual for the Pegaso when I bought it. If you don't have a service manual for your bike; buy one. Even if it is the PDF scan for 99 cents off ebay its still better than nothing as it will have the correct torques and such for other work!
Here is what the Aprilia manual tells me:
Carb type - Mikuni BST33
Idle speed - 1400rpm
Main jet - 107.5 (USA) 110 (Austria and Switzerland)
Needle - 5E 76 -2
Spray nozzle - 0 - 2
Idle mix jet - 17.5
By-pass - Dia 0.8mm
Idle mix outlet - Dia 0.8mm
Valve seating - Dia 2.3mm
Starting jet - 65
Idle air jet - Dia 1.5mm
Air screw - 2 turns
Accel cabel adjustment - 2mm
Okay, so that is a lot of numbers and things. Lets break them down a little.
The carbs are Mikuni brand, model BST, with a throat diameter of approximately 33mm.
Correct idle speed of the bike is 1400RPM when everything is happy. If it doesn't run at this speed then something is wrong.
Main jet is the orifice at the bottom of the Needle that controls fueling at most accels and higher load. Needle controls how the fuel flows from the main jet and the spray nozzle/emulsion tube into the engine. It basically constricts flow the further engaged it is.
Spray nozzle is the factories name for an emulsion tube; a term that fat American's might more readily recognize.
Idle mix jet is the jet that controls idle air/fuel mixture, also known as a pilot jet or slow jet. Unfortunately this number does not seem to jive with reality and definitely not with the "social wisdom."
The next 5 lines I will skip here.
Air screw adjustment is critical for a good idle. The approach is to carefully turn the screw all the way until it seats, then turn this number of complete revolutions out.
Accel cable adjustment is simply that you want a certain amount of slack in the throttle cables so that when everything gets hot it does not pull the throttle plate open unintentionally. SO, lets talk quickly about carburetor theory of operation:
This is a generic chart published, i think, by Mikuni a long time ago. This gives you a very general idea of how these different jets and needles affect what areas in the engines operation. The caveat for the BST carb is that you need to replace the X-axis label of "throttle position" with "Engine Load" or "intake vacuum" for it to be correct to our application.
Also important is float height. Float height will affect overall richness of mixture, to the point that if the floats are too high you can get fuel just flowing out of the inlet of the carb.

Here is the stock BMW funduro information sourced from the ChainGang:
  • 1996 Onward Models:
  • Main Fuel Jets #132.5
  • Pilot Jets #41.3 (Idle Air Jets)
  • Needles 5E94, 4th Clip from Top (Blunt End)
  • Carb Venturi (Emulsion Tube) 584□□-2. The first "□" stands for  a rectangle with a small square inside, the second "□" stands for a small rectangle. Thanks to Marty #436)
  • Float Height 14.6mm from the Gasket Surface
  • Idle Mix Screws backed out 3.5 turns from LIGHTLY seated (all the way in).
Sadly these specs are quite a ways off from what I am working with.

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