Scarabeo 150 Carb Settings

Cagiva

2001 Aprilia Scarabeo 150 - Carburetor settings info from November 2010 installation of new pilot jet to alleviate bog / stalling issue on cold startup. FYI - this is what I did and what worked for me - proceed with appropriate caution and at your own risk.

Verified all carb vacuum hoses had already been rerouted with a prior owner's service at authorized dealer too. I also replaced the carb slide while all was apart as well and noted very slightly visible possible hairline cracking across one corner (1700 miles on odo at that point) on the slide removed.

I'm at sea level southern Oregon coast, and air temp ranged from 60 during day down to 45 at night during setup noted below.
Pilot Jet Part Number:

Part numbers are: N424-21038 (size 38) and the N424-21040 (size 40) pilot jets.

I got both a size 38 and a size 40 froma local Honda moto dealer. They don't carry or sell scooters, but sell MX and off-road bikes by the boat load. Lots of MX bikes have Keihin carbs that use that same N424-21 Pilot Jet. They do a lot of tuning and setup there so I thought they would have parts on-hand (and they did).

I called them up before giving AF1 Racing or CarbParts.com a call (CarbParts now out of business? Link is invalid now and searches can't locate), and in 30 seconds on the phone they said they had them, confirmed they had them in stock, and confirmed the part number. 15 minutes later and $5.25 each I had them in my hand.

They look just like the diagram on CarbPart's website and the exact same as in the photo of part WP142138 on AF1's website. They are official Keihin jets with the Keihin symbol and size number stamped on them. The dealer's source for the jets was WPS (Western Power Sports Distributor), and they had 4-packs of jets of all kinds.

The factory service manual available for download from AFI has the procedures for airbox and carb removal- the full sequence is in various parts of the manual, all referenced to each other. Remove cowl, top box and rack, air filter, seat pan, undo hose clamps to airbox for all lines etc.

Pilot size that was finally installed: Size 40. I found a 38S in the carb upon disassembly (not sure what the "S" stands for), and the 38S appeared to be slightly plugged. I compared it to the size 38 I bought at the Honda dealer. Service Manual size listed is a size 38, not a 38S. The size 38 from the dealer had visibly larger emulsion holes(but not by a huge amount)in the tube compared to the 38S that came out of the carb. The holes were in the exact same location though. So, I put in the size 38 to try it out, and long story short it didn't make much of an improvement compared to the 38S that came out of the carb - both being a 38 the effect seemed to be the same despite slight difference in emulsion hole size. I'm guessing that the prior owner tried to have some poor running fixed at a shop somewhere, based on evidence I found of prior airbox/carb removal and the setting I found on the fuel screw awhile back, and perhaps they only could source a 38S at the time. I don't have any evidence that a size 35 was installed for delivery to the USA (others have noted a size 35 spec'd for the USA). Our premium grade fuel has ethanol 10% (and ethanol in other grades of fuel) and the premium is 91 octane. Note that this may be the reason (other than being right down at sea level) that a size 40 is needed for my 01' 150 and a 38 just won't quite run properly in all conditions for cracked throttle.

Fuel mixture screw setting: I ended up leaving it at 2 1/2 turns out for now with the size 40 pilot jet. Manual setting is 2 1/4 turns out for the stock carb with the stock 38. With both 38's I was going up in number of turns out to get highest idle speed to 3 - 3 1/2 (about where I found it upon first inspection awhile back). Despite higher fuel at idle with the fuel screw turned way out, the bog on cracked throttle was still there. Now (so far) there's no bog, and there's a nice smooth idle with no lean hunting or hesitation.

Idle RPM: Range is now 1620-1680 after fully warmed up for a minimum of 15-20 minutes. Idle rpm soon after the Bystarter extended (1-2 minutes) was a range of 1540-1580, which slowly rose to the mid 1600's when fully warm. Service Manual range is 1600 +- 100. No fall off in idle after blipping, and no slow return to idle RPM after blipping. Nice snap/close to throttle and smooth throttle - no hesitation or lumpiness. I use a small hand held portable tach made for single cyl units that attaches to the spark plug wire - available from almost every MC shop out there. Made as an hour/tach meter for power sports equipment and generators/lawnmowers etc.

Bystarter ohms (resistance) 4.9-5.0 ohms when cold (55-60 degrees). This is the acceptable resistance for a good unit, per guidance I've read.

Float Height: I checked in order to supply to others as a reference point. Height was Exactly 15.0 mm (not 14.9 or 15.1, but exactly 15.0) checked with digital verneer calipers several times, exactly 1/2 way up the slope nearly next to the brass tube extending from the carb. Float tab was just resting/touching the needle pin with no pressure. I leaned the carb over from the side until the tab just made contact with the pin. Measured from the carb body to bottom of the float with float tab just touching the pin - very easy to check/measure. The service manual and other sources I've tried simply list a slash (" / ") in lieu of a measurement.

Additional observations and info:
Initially, the new size 38 ran a touch better. That was in the middle of the day. So, later that night, I tried it out when air temp was much cooler and immediatly had the bog issue, with some hesitation and lean surging. If it had run much better then I would have known the pilot jet was too rich, but since it ran worse in cold weather I knew it was too lean.

So, this morning I put in the 40, and immediatly it started and ran better. It settled right in at a nice smooth idle, it was easy to check idle fuel mixture and to adjust the idle speed compared to the size 38, which hunted and hesitated a bit and was inconsistent in rpms and idle smoothness. I'm still at sea level, temp was about 55 during setup this morning.

I only got a chance to run around the block a awhile due to heavy rain moving in. Didn't try it out later that night in cooler weather or able to do extended running (only about 40 minutes). But, I did go back out to the garage several hours later (mid day) the next day to start from cold (temp gauge needle was back in resting state on bottom pin) though heavy rain kept me from going out to ride, and it started up perfectly and Bystarter induced RPM was properly higher as noted earlier, then the RPM slowly went down over 1-2 minutes as the Bystarter signed off. I let it totally warm up and again noted the RPM's the same as above that were determined this morning after installing the size 40 pilot jet.

I also installed some new fuel line from the pump to the vacuum petcock, and from the vacuum petcock to the carb. It appeared the line from pump to petcock was a bit tight and at risk of bending over at the petcock where the fuel line emerges from under the air box. I installed an in-line fuel filter between the petcock and the carb - that was the reason for the new fuel line I installed there. I put on Parts Unlimited 1/4 inch ID blue urethane fuel line (which had the same OD and ID as the stock fuel line. The inline filter was a generic aftermarket cone shaped one about 2 inches long available from every MC shop out there - they're usually on the cards behind the counter for $4.

Drained fuel out of tank and used fresh fuel (premium 91 octane with 10% ethanol is the only premium they sell here). Cleaned carb (wasn't visibly dirty inside, although the 38S pilot appeared a bit tighter inside as noted above), cleaned carb slide again and checked action, blew out airbox, airfilter was already fine, used electric contact cleaner/lube on electrical connections to fuel pump and heater/bystarter, checked vacuum connections and fuel line connections.

The first time took a bit of time due to changing fuel line and getting the sequence of tear-down figured out. The second time it took me only 35 minutes to get the carb onto the bench. I did remove the carb completely from the bike both times. It was a cinch removing the throttle cables and connectors anyway. Much easier to have it on the bench for inspection, cleaning, and changing jet.

I verified that all carb vacuum hoses had already been rerouted to avoid pinching under the front cowl. I also replaced the carb slide while all was apart as well and noted a very slightly visible possible hairline cracking across one corner (1700 miles on odo at that point) on the slide removed.

For additional pilot jet diagnostic guidance see www.factorypro.com, click on Product Support at the top, then Motorcycle Tuning Tech, then click on CV Carb Tuning, then click on either Low or High RPM Engines (I generally followed the High Rpm Engine guidance in this case as the Scarabeo 150 redline is 9000 rpm), then scroll down to #4-Idle and Low RPM Cruise (and in the 2nd bullet point click on "pj tuning information" for pilot jet detail inquiry).

Please note that Factory Pro requests permission to copy or link to their information as it is proprietary.

The size 40 works perfect. A "38S" was found installed. I tried installing a regular "38" and got the exact same behavior as the original "38S" The "40" allows quick warmup and while it may bog if I try to ride off before the temp needle moves (per factory owners manual - says to wait for temp to move) it may bog at the end of my driveway as I turn onto the street, but within 2-3 blocks I can twist and go perfectly. That only happens first thing in the morning so I suspect the "bog" is related more to the carbon canister having vapors that are affecting optimal mixture just a bit. Later in the day after sitting a bit (at work) I can go out and within a minute or so "warmup" I can ride away with no bog at all. I did reset the air/fuel mixture screw and bought a little electric tach to enable setting the proper idle speed too. I also checked the float height and made sure I had a new slide etc. The scooter just flat hauls now. Throttle response is great in my opinion.