In the beginning…

When I first got the Aprilia, starting the bike with a cold engine would go something like this:

1. Kickstand up.
2. Ignition key on.
3. Shift gearbox to neutral.
4. Kill switch set to Run.
5. Snake right hand between plastic bodywork and aluminum frame. Fumble around blindly as I awkwardly reach for the Mikuni TM24 carburetor plunger mechanism and pull up to activate the starter enrichment circuit (choke).
6. Press starter button.
7. Gently rev engine for a minute or so while stationary. Put on helmet and gloves.
8. Set off down the road gently, upshifting before 7,000 rpm, keeping an eye on water temp gauge.
9. After about 3/4 mile, just when engine starts to break up due to running excessively rich but before the water temp gauge has begun to move, pull over to side of road.
10. Remove gloves just as engine stalls out. Snake right hand between plastic bodywork and aluminum frame. Fumble around blindly as I awkwardly reach for the Mikuni TM24 carburetor plunger mechanism and push down to turn off the starter enrichment circuit (choke).
11. Shift gearbox to neutral. Press starter button. Put gloves back on. Set off once again.

As you can imagine, this got old real quick. I figured there had to be a better way. And there was. It would just require buying a few parts and installing them. So in 2013 I embarked on a mission to convert my Aprilia’s carburetor to a cable actuated starter enrichment circuit (choke).

It’s not a choke per se since it doesn’t lessen the amount of air entering the engine. Rather, it exposes an additional passageway inside the carburetor for air to travel over the fuel reservoir, thus sucking more fuel into the engine. It enrichens the mixture when in the plunger is in the up position, and blocks the passageway when in the down position. But saying and typing choke is a lot easier than starter enrichment circuit and most people understand what you mean.

The Parts

I picked up a brand new Aprilia LH light switch with integrated choke lever (p/n AP8224413) from an eBay seller based in the UK. This setup was used on certain Aprilia RS50 SE-TS models and some RXV / SXV 450 bikes. This control is nearly identical to the original LH switch fitted to AM-USA models (p/n AP8212986) with the following differences:

1. The flash to pass button is blue instead of gray.
2. It uses two different length Phillips head screws to clamp the two halves together. These screws are larger in diameter than the equal length socket head cap screws used on AP8212986.
3. Choke lever and choke graphic.

Interestingly, the original switch (AP8212986) has the threaded hole to accept a choke cable but uses a dummy choke lever inside the switch and lacks the choke graphic. If only the little lever part was available separately I could have saved a bunch of money! That being said, it was worth the wait to get the parts from England to save over 50% on the same part offered by US retailers. I also purchased a brand new Genuine Aprilia RS50 SE TS Choke cable (p/n AP8214219) from the same UK seller on eBay. Thankfully, this cable worked on my bike without any modifications despite the tiny cable ball end fitting meant for a Dell’Orto carb and not a Mikuni.

Next I purchased a Mikuni Genuine Choke Kit MK-412 cable type starter system, advertised by the seller and the official US Mikuni distributor (Sudco) as fitting all Mikuni VM and TM carbs. Turns out this is not the case. MK-412 only fits the larger Mikuni 30mm+ carbs, not the little 24mm on my bike. I didn’t know this at the time, so I installed the MK-412 parts in my carburetor and they didn’t function worth a damn. The starter motor would nearly drain the battery before the cold engine would start. Something obviously wasn’t right. But the fix would have to wait until later.

Modifications

Fast forward nearly 4 years and I decide that enough is enough, it’s finally come time to fix this stupid thing once and for all. I had let the bike sit in the garage most of the summer because the weather was too darn hot to ride. When I started it up one day the throttle was sticky. I figured it was my fault for letting it sit too long without being started and ridden. So I decided to take the carb apart to clean it. Turns out it had nothing to do with the carburetor. The rubber hand grip on the throttle tube was getting hung up on the metal bar end. So I spaced it out with a few washers and a longer bolt and now the throttle snaps shut with authority.

But since I had disassembled the carburetor, I decided now is the ideal time to get the starter enrichment circuit working. So I studied the parts and figured out what needed to be done. Here you can see the root of the problem. This is the Mikuni TM24 starter enrichment mechanism. The hand pull-up style on left is what my carb was originally equipped with. The cable actuated style on the right is what I believed would work but doesn’t. When the cable actuated plunger is all the way up, it is in the same position as the pull-up style in the down position. Thus, the cable actuated plunger never exposed the passageway for the enrichment circuit to function properly. I need to find some way to get the cable actuated plunger to go up higher when I pull on the cable lever.

Sudco is the Mikuni distributor in the USA. They offer the entire cable start mechanism as a kit of parts using a single part number MK-412 (Sudco pn 002-351). Come to find out this kit is meant for VM30-VM44 spigot mount carbs, TM32/34/36/38 and 39mm and 41mm Pro-Series carbs. No wonder it didn’t fit my TM24 despite the retailer saying it fits all TM, TMX and VM carbs. Sudco updated their product catalog to reflect this also, but it takes a while for their dealers to catch on. Apparently Sudco have another kit, pn 002-353 that fits VM28/418 (TM28), the main difference being a different plunger (VM15SC4/86 for the TM28 vs VM15SC4/85 for the others). That doesn’t really help me as I have a TM24. I’ve tried both plungers and neither plunger fits the TM24 correctly. The VM15SC4/85 plunger is actually closer in shape to the OEM pull-up style plunger. But it is just too darn long. Turns out there is no readily accessible kit of parts to convert my TM24 to a cable actuated choke. So I’d have to make or modify my own.

 

When I first did this modification I didn’t understand everything that was going on. Now that I better understand, I can forge a plan to fix it. This means I have to buy more parts or modify what I’ve got. Modifying is free, so here’s what to do:

1. Turn the upper portion of the starter plunger to a smaller diameter so it can fit up inside the body of the starter plunger fitting. There simply isn’t enough meat on that brass fitting to bore the inside larger without compromising the threads. So it is the plunger must be made smaller.
2. Shorten the plunger length by removing material from the bottom portion of the plunger.
3. Drill a deeper hole in the starter plunger fitting so that the spring and the upper portion of the plunger can all fit up inside the starter plunger fitting when you actuate the starter enrichment circuit.

I chucked the VM15SC4/85 plunger in the drill press and turned down the diameter of the plunger with some fine sand paper. I think I also used a very fine file at one point. The brass is very soft and machines very easily so go slowly and stop to measure often. It’s easy to overshoot the mark as I did. Thankfully, you don’t need 0.01mm accuracy here.

Results

Happily, I can report that the modified cable actuated starter plunger parts are doing their job. The choke lever has a positive action in both directions. When the starter enrichment circuit is activated the bike starts almost immediately when cold. So much better than before when I was wearing out my battery and my starter motor to get the thing started. Once warmed up if I activate the starter enrichment circuit again the engine runs rich for a few seconds and eventually stalls out. Flipping the lever off the bike starts back up instantly with just a little bit of throttle to clear the flood. So it seems to be working as expected both on/off and hot/cold.