throttle body | Moto-Abruzzo

27/09/2025

TPS (AP8124866) failure and replacement

OK, so a quick post before the one about “Fixing the MCCruise control”. Remember the magical fix-itself TPS problem I mentioned last post? Well it mysteriously un-fixed itself in a rather worse fashion while sat in lane three of the A420/A34 roundabout at 06:30 in the morning! Idling beautifully at 1,250rpm one instant, a little shudder or two to get my attention and then silence …… just as the lights turned green. To be fair she fired straight up again on the button, but now the idle was barely 1,000rpm and the Capo really doesn’t like the idle that low. I got to work, went to kill the ignition and she sat sweet as can be back at 1,250rpm ….. now if there’s one thing I sure hate, it’s an intermittent fault. So rather than the Capo and I falling out, I went into work and stewed on the problem while peering out of the canteen window at the bike and sulking like an irritated school boy.

Later, I fired it up to head home and all was lovely with the world. To be honest I was more annoyed than if it was broke – broke I can fix. We got home in an uneventful ride, but again the idle was low …. 1,000~1,100rpm but stable. This was now officially a lovers tiff! I’d gone over so many possibilities in my head, but in the end gut feeling was still saying it was a faulty TPS. So that’s what I went with. Next day I dug out one of the spare TPS’s and tested it – 1.2KΩ ±10% pins A-B and 1.2KΩ ±10% thru to 2.4KΩ ±10% pins A-C with the throttle closed to wide open. The following day the swap took no more than 40 minutes going steady – tank raised and perched on top of the airbox, remove two T30H screws and the electrical connector to release the TPS …………. swap sensors and reverse everything to put it all back together, simples. Then of course the magical and most important bit ….. use TuneECU to reset the TPS. Now I noted the TPS voltage as 0.27v throttle closed and 4.08 throttle fully open – this was constant and repeatable every time – perfect. Finally, I took the bike for a nice 20 mile ride and once back secure in the yard, ticking away in the September sunshine as she sat cooling down, I cracked open a cold beer in celebration of a job that appears to be successful.

Things I learned along the way ….. in TuneECU you can see that the TPS position is sampled for 16 distinct columns in the fuel maps. Of those 16 positions, 7 are for throttle positions of <10% while the other 9 positions account for the remaining 90% of the throttle movement! So it’s fair to say it is used mostly by the ECU for small throttle openings …. primarily the 0-6% range and this when measured afterwards was where the TPS was giving erratic readings. Symptoms on the bike – well primarily erratic idle, but also poor running (mostly rich – smell) in the small throttle opening range. Crack the taps and give the old girl the reins and the Capo rode perfectly normally. Fair to say the symptoms are similar but slightly different to an air leak and of course common to both cylinders, so nothing like an ignition issue. It was more random – cold, hot, time etc. it just didn’t feel like it was a leak. Anyway, the job is done now.

After a few days of the Capo behaving itself, I dug out the Dremel and opened up the sensor to have a look at the tracks inside for a definitive answer, a small Capo autopsy if you will. Once inside, I certainly didn’t expect it to be built the way it is, I expected a simple potentiometer and wiper arm ……… in reality, it uses a plastic strip with several resistors printed onto it and a wiper arm that bridges the two large resistors (left on pic). From the look of it, one of these wipers had lost spring and was no longer making good contact with the resistor it swept over.

As of 26/09/2025 AP8124866 is available from Ultimateparts.net at £224.46 inc. VAT, but only available on back order from Fowlers (Bristol). At that price it makes sourcing one on Ebay most certainly your best option.

25/04/202126/04/2021

Fuel Injectors & RST Throttle body

Aprilia Caponord ETV1000 Rally-Raid RST Futura throttle body rebuild Sometime way-back-when, I bought a used set of throttle bodies off an RTS Futura (51mm v Caponord 47mm) with the view of doing some tuning/comparison work on the Raid with the 3D printed hybrid velocity stacks (51mm diameter but Caponord height). What really happened though was far less glamourous and only involved a sturdy box and a shelf! But about a month ago I dug them out again and realised that I pretty much had all the ingredients required to do a full clean and rebuild. I had stainless steel 4x10mm and 5x12mm Torx screws on the shelf as well as all necessary seals. The ultrasonic cleaning bath would do the brunt of the work and I could send the injectors away for professional cleaning at a reasonable cost. Aprilia Caponord ETV1000 Rally-Raid RST Futura throttle body rebuild

And so, with a plan to hand, apart it came. First the injectors were sent away for cleaning. The turn-around was quick and the total cost was about £30 including P&P. Thankfully both injectors got a clean bill of health. Next, the throttle body and associated parts went into the ultrasonic cleaning bath. I had bought the 6L version …. but when you take into account the smaller basket inside, the usable capacity is probably something more like 4 – 5L … just enough luckily, for the throttle bodies. If I was in the market for another one, I’d definitely opt for the 10L or maybe even try to stretch to the 15L for good measure. Aprilia Caponord ETV1000 Rally-Raid RST Futura throttle body rebuild

It was amazing the amount of debris that came out of them … in the end they looked almost brand new again! The rebuild was very straight forward with all new ‘O’-rings and screws used. Pleased? Yes I think so. They are refurbished and ready for action which is the important thing. The only downside is that some of the mild-steel components are still showing a patina of rust. It would have been nice to re-plate them to give them that as-new look as well. For now I’ve cleaned off what I can and coated the parts for protection, but one day I’d like to have a crack at home plating … zinc, nickel or passivate of some sort might be interesting. Aprilia Caponord ETV1000 Rally-Raid RST Futura throttle body rebuild

Parts used:

2x 4x10mm Stainless Steel Torx screws

4x 5x12mm Stainless Steel Torx screws

4x 10mm OD 2mm section Viton O-rings

2x 14.1mm OD 2.4mm section Viton O-rings

2x BS203 O-rings

1x BS008 O-ring

Just throwing it out there ……. If I were to offer these out as a bagged kit, would anyone be interested?

09/06/2019

TuneECU IS your friend … just don’t forget it!

Aprilia Caponord ETV1000 Rally-Raid TuneECU TPS reset I’ve just had a fun few hours preparing the Capo for its MOT (annual inspection) and one of the issues I wanted to get around to sorting was the slightly high tick-over.

For quite a few months now the idle has been 1,500 – 1,550 when warm – not enough to be troublesome, but noticeable. Problem is ….. I’ve plain forgot to do anything about it once the ignition was turned off! So today I dusted off the cable and charged the old ASUS notebook and plugged in.

Thankfully it was just as I hoped, the TPS (Throttle Position Sensor) was not at zero with a closed throttle. It was reading 1%. A tap on the ‘TPS Reset’ had it reading 0% and the idle was fine once again. So, the question is, have I got a TPS on its way out, or was it simply the fact that I hadn’t done a reset in 18 months ( 20K+ miles), something I used to do at every service. I’m hoping it’s the latter and will endevour to make sure I hook up TuneECU during all future services. Funny really that a tool I used to hook up on an almost weekly basis slowly slid to the dark recesses of the workshop over time.

22/10/201728/04/2019

Hot Grips, Grip Puppies or Hot Puppies?

Hot on the heels of the last post, here we go again, with the next little problem to work on. It looks like the bad wrist isn’t going to recover much more now without an operation, and Dog knows when that’ll happen. So I’ve been pondering how to reduce the load (torque) my poor old wrist feels at the twist grip.

It seems to me there are two ways to do this ….. firstly, increase the diameter of the twist grip to get more leverage, the second is to replace the cam at the throttle body to do the same job (more on this later) – or maybe a combination of the two! So let’s look at the first option.

The Aprilia heated grips (with replacement Ariete grip rubbers) are 32.8mm diameter and probably the cheapest and easiest way to increase this would be to simply fit a set of Grip Puppies. But the fact is the Aprilia heated grips are quite poor when it comes to output. The 13Ω elements are really only any good for chilly Spring, Summer and Autumn mornings and certainly don’t do much on a freezing Winters day in the UK. So I started to hunt around for a better heated grip with more power. In the end I opted for a set of Oxford Products Premium Adventure grips (OF690).

These grips are approx. 35mm diameter, so that’s one box ticked, and they have heater elements averaging approx. 7Ω …… that’s almost half the resistance and will mean twice the output. The grips are 132mm long and can be trimmed if necessary down to 122mm, so fit shouldn’t be an issue.

The heat adjustment is in five steps (30%, 40%, 50%, 75% and 100%) using PWM (Pulse Wave Modulation). In other words, a microcontroller turns the power on for a certain percentage of the 1.3 second cycle ….. so 75% output means the grips are on for approx. 0.975 seconds, then turned off for 0.325 seconds – total 1.3 seconds – then the cycle repeats itself. Now this is all well and good but they really missed the mark to my mind. Microcontrollers are great little gadgets and can be programmed for all sorts of things and as far as these grips go, they only scratched the surface. First off, when you turn off the grips they turn back on at the default 30% or 100% if you hold the button down – NEVER at the last setting you may have been using*. Secondly, a rapid warm up would have been nice, say 100% for 2 minutes at start-up, then back to your last used setting ….. but none of this is programmed in on the OF690.

In fact isn’t it about time grips came with temperature sensors in-built? Think about it ….. set the desired heat and ride along happy as larry with toasty fingers on a bitter winters day. Stop for a minute and take your hand off the grip and it now cools rapidly – the cooling temp sensor feeds back to the controller, which now ramps up the output to try and restore the grip temp back to that desired. You then plonk your size 10 winter mitt back on the well heated grip, and now the grip is insulated the system reduces power to maintain the temperature. And it goes without saying that both grips should do this independently!

Anyway I digress, the whole point is that the new grips will give me a welcome increase in diameter, even then I have the option to add Grip Puppies over the top but I think this might be overkill! But who knows ……

Next post I’ll look at the pro’s and con’s of changing the cam at the throttle body to achieve a reduced torque at the twist-grip.

*This function along with 9 heat settings instead of 5 is available on the Hotgrips Advanced Adventure UK SPECIFIC (EL690UK). Why didn’t I buy these? Simply because I got the OF690 set for £30 cheaper! 😀

07/02/201728/04/2019

Airbox crankcase vent – Mk2

Aprilia Caponord ETV1000 Rally-Raid engine crankcase vent pipe to airbox and throttle body

With the velocity stacks and snorkel measured up and squirreled away in CAD, it was time to rebuild the airbox …… except billy-butter-fingers here, went and knocked the airbox base off a chair onto the floor – not far – but enough for it to land awkward and break the spigot for the crankcase vent. Unfortunately the spare I’d made, I gave away to a friend last year! So there I am trying to remember where I’d bought the bits to make Aprilia Caponord ETV1000 Rally-Raid engine crankcase vent flow analysis another one, when I had a “Stop the bus!” moment …… don’t waste time and fuel going into town, sit down, draw something in CAD and print that puppy! 😀

Here’s the finished article …. it has a slightly larger inlet/outlet cross-section, although the previous one worked just fine and this time the fit for the grey pipe is better. internally there is a chamber to help reduce gas flow and (hopefully) convince some of the oil to condense and run back down into the crankcase, not out into the airbox. That’s the theory anyway, but it’ll probably turn out to be complete bollocks! Either way, it looks neater so that’s a positive step forward ……. now, what else can I break today! 😀

Aprilia Caponord ETV1000 Rallyb-Raid engine crankcase airbox vent 3D print

The MK1 vent did well and lasted almost 2 years (37K miles) and in that time I never had a moments issue with oil in the airbox going where it shouldn’t. Let’s hope this one does as well!

02/02/201703/05/2019

Hybrid velocity stacks

If all goes to plan, later this year the second Rally-Raid that’s currently being rebuilt, will finally get a motor installed. The plan is to use a big-bore motor …. an 1,103cc in place of the standard 998cc. The compression and valve timing will remain bog-standard Caponord (10.5/1 Inlet timing – 25°BTDC/37°ABDC and Exhaust timing – 57°BBDC/5°ATDC). However to accommodate the increase in air-flow, I’ve decided to use Futura 51mm throttle bodies instead of the Caponord 47mm items.

Meanwhile on top of this chunky aluminium marvel sits a pair of velocity stacks. These stacks vary in height (and diameter) depending on the intended tune of the engine. The RSV Mille of course is designed as a race-rep and as such wants high horsepower at high RPM – hence 57mm throttle bodies and very short velocity stacks. On the other hand the Caponord was tuned for improved low-end grunt and so has small throttle bodies (47mm) and tall velocity stacks to maintain good gas flow speed at low RPM. The Futura seems to sit firmly between the two!

Now of course I could simply use the medium height velocity stacks straight off a Futura, but I decided to go a different route and print a new pair of hybrid stacks – Caponord height BUT 51mm diameter to fit the Futura throttle bodies. Unlike ABS, Colorfabb Ngen (Co-Polyester) can’t be vapour polished with Acetone, so I’ll have to sand the venturi down with a variety of grades of wet-and-dry up to 2,000 grit and maybe finish it off with something like Quixx plastic polish – if it works on this stuff! Here’s a couple of pics comparing the original and new version – straight out of the printer!

Unfortunately 3D parts (unless made on high-end machines) don’t typically have the same strength as injection molded or machined parts – but they do make great ‘proof-of-concept’ parts! If these stacks prove to be a positive step forward, but not durable enough for the working environment, I can at least get the drawings to the machine shop and have them made in aluminium … but that’ll be a tad more expensive than 85p each off the printer! 🙁

UPDATE

Just had a spare half-hour to rub some 100/400 & 1200 grit paper down one of the stacks and all I can say is – WOW! This material rubs up lovely and probably a couple more sessions will see it through. All the print-ridges have gone and I can’t feel anything but a nice smooth surface that retains a print pattern that makes it look quite distinctive. 🙂

Aprilia Caponord ETV1000 RST1000 Futura Rally-Raid hybrid velocity stack

25/05/201603/05/2019

Airbox breather update – (almost) a year on

Just gave the Capo motor a service. No drama, everything went according to plan. I just thought I’d share a couple of pics of the inside of the airbox – with the modified breather – straight after the air-filter assembly was lifted. No cleaning, no Photoshop ….. just as it looks after 5,354 miles. The throttle butterflies, velocity stacks and idle air control valve (IACV) are all perfectly clean.

The extended drain tube from the airbox (down below the oil filter casing) held about a spoon full of oil when drained and what can be seen in the photo was only a light coating in the bottom of the airbox. One sheet of kitchen towel had that as clean as a whistle in a few seconds. Since doing this breather modification I’ve ALWAYS run the oil level to the HIGH mark on the tank. So without the tedious job of removing and cleaning out the complete airbox, it was a pleasure to simply replace the Athena air filter and button everything back together.

Aprilia Caponord ETV1000 Rally-Raid airbox breather drain

26/09/201526/09/2015

MCCruise control revisited

While the Caponord was stripped for the vacuum check-valve replacement, I decided to get my act in gear and finally, once and for all, unequivocally ….. Sort out the MCCruise control/throttle cables. Last year I relocated the CIU (Cable Interface Unit) to the left hand side of the throttle body. This had pros and cons ……….. The upside was a lighter feel on the throttle grip, the downside, the Servo/CIU and CIU/Throttle body cables were overly long and the Servo cable fed into the CIU from the wrong side. However, the cruise was working perfectly and so it became one of those ‘mañana’ jobs!

Well today is that tomorrow! First I jotted down a list of what had to be done:

1. Make a spacer to fit onto the side of the CIU. This is to move the throttle cable adjuster further out and make the inner cable a better fit on the spool.
2. Drill new cable holes in the CIU so it can be rotated 180° so the Servo cable enters from the rear not the front as is the current arrangement.
3. Shorten the CIU/Throttle cable while allowing for the new spacer at the CIU – 350mm down to 190mm.
4. Shorten the 1,150mm Servo cable to a more reasonable 750mm!

I have to say here and now that I do like learning new skills or adapting old ones to suit a new task. I also have to admit that in a good few years on this Earth, I’ve never made up or adjusted the length of clutch/choke/throttle cables before! So time to adapt the electronics soldering skills and brush up on required technique courtesy of the Internet. Now I’m not going to bore you to tears telling you how to do it – there is tons of advice on various websites/YouTube. Suffice to say, sort out the duff advice from the good, have the right equipment to hand and practice, practice, practice …… do the job once and do it right! All the cable bits and pieces I needed came from Venhill in the UK.

By late afternoon everything was buttoned up and the cruise control recalibrated after the throttle bodies were rebalanced, fault codes cleared and TPS reset via TuneECU. As the sun faded behind the hills I cracked open a cold beer happy with how the day played out …. Time then to put the tools away and get back to revising for the next Amateur Radio Exam!

The MCCruise has been installed for approx. 25,000 miles and worked perfectly throughout. The CIU was relocated because the initial location proved to be problematic – causing slight binding of the throttle cable. This meant that the cables supplied in the kit (as specified by me) were now the wrong length and the orientation of cable entry into the CIU was wrong.This is not the fault of MCCruise, what they sent is what I asked for! Should anyone else wish to go down this route I will of course be glad to provide notes/measurements etc so a kit will be a simple bolt-on job.

04/09/201514/10/2015

Pucker up and suck or blow …. your choice!

Here’s todays little quiz ……

What’s this?
Where does it go?
How many are on YOUR bike?

20150830_142552

Of course the smarty-pants among you will scream out the answers to 1 & 2 in double quick time – it’s a check-valve or none-return valve and it fits between the throttle body and clutch in the vacuum line. Excellent stuff, spot on …… but question 3 …… one valve or more than one? Hmmm.

Here’s what Aprilia show in the parts manual (click for photo of matching engine) …..

The front cylinder throttle body port has nothing but a piece of clear hose and a bung – the manometer/VAC gauge test point. All the slipper-clutch hose/valve assembly is ONLY attached to the rear cylinder throttle body port and TPS port. This may well be fine and dandy for an 01-03 bike.

But if you have a Rally-Raid or an 04-onward bike, next time you’re under the fuel tank/airbox take a good look at the front hose …… my guess is you’ll have a ‘T’-piece and more black hose disappearing downward below the mire of cables and wires running alongside the frame. Now this might be no surprise to you, but it had me scratching my head for a while I can tell you! 😕

And this is what I’ve got on the Rally-Raid (click to enlarge)……

A second complete set of pipes AND a second check valve connecting to another ‘Y’-piece. Why do this? Well this is my hunch, guesstimation, call it what you will ….. the 01-03 Capo has I believe a clutch pack that is 43.6mm thick, the 04-08 is 44.9mm thick. A difference of 1.3mm – in other words the valve springs will be compressed an extra 1.3mm on later bikes and that means a little more spring preload for the vacuum system to overcome when getting the clutch to slip, hence the second set of connections to improve the vacuum – 2 cylinder are better than 1! Also, two sources of vacuum from cylinders running out of phase (60° remember) may well help smooth out the pulses you can sometimes feel through the clutch lever. It simply looks like Aprilia modified our blessed Caponords and just didn’t bother to update the parts manual.

So what’s all the hullabaloo, why rip this poor little thing from the Capo’s delicate innards? Well the long and short of it is that one of them is pooped, shot, completely knackered ….. where air should only flow one way, it flows happily in both. Looking at the defective duckbill valve reminds me of Daffy-Duck cartoons, the ones where Elmer Fudd rearranges his bill with a shotgun! No worries, simply order a replacement …….

……. Aprilia part no AP8104251 Just make sure you’re sitting down when you look at the price! 🙁

I’ll shortly open up a new page with some more 3D images to try to better illustrate how the system is operating at different stages and how to go about testing the valves and what to expect.

13/04/201501/11/2015

All things being equal

gauge It’s been a while since the Capo was serviced and one job has still remained outstanding – in fact it has been ‘outstandingly’ outstanding for the past few services since I lost my old Davida vacuum gauge set! Yes, the perplexing throttle-body synchronisation*. Truth be told, the Capo has been running just fine for ages, but it never hurts to check it once in a blue moon!

I wasn’t about to lay out a fair-sized wad of cash for another (excellent) Davida set or buy a Carbtune II again in a hurry, so I thought it was about time to go the Poundland route and build my own manometer for a few pennies and with a bit of scrap kicking around the barn. The nice thing is that there’s a mountain of info on the internet about how to do this yourself, just pick what suits your needs best and modify for your own bike.

So what do you need? Well just a sturdy board, door or wall on which to mount the kit, a length of 6mmID clear tube, a suitable liquid and a way to connect it to the bike. That’s the nut’s and bolts of it, but a simple addition will make the setup ‘user friendly’ as you’ll see later. Although the tube is straight forward, the liquid is a bit more controversial …. Some say coloured water, some 2 stroke oil, some EP90 gearbox oil …… you get the idea! I chose some good old Scottoil Blue. Why? Because it was on hand, because it is basically ATF and has a fairly low viscosity, because IF it gets ingested by the motor it won’t cause any damage and because it turns out (purely by chance!) to work REALLY well!

*Workshop manual page 4-18-00

Now for a bit of physics ……

OU-1970's-style First off, what kind of vacuum are we looking at from the Capo motor? From measurements, it looks to be somewhere in the range of 22-24cmHg (based on an erratic Carbtune II) per cylinder measured against atmospheric pressure …. Now that’s not much for a mercury manometer – barely the length of a sheet of A4 paper. A nice compact manometer then, except that unfortunately mercury is almost impossible to get hold of because it’s deemed way too dangerous for us potato-heads to use safely. So what does this mean in terms of manometer height if we use liquids of a lower density? Well…..

Mercury 22-24cm (Ideal!)
Water 299 – 326cm (free-ish and known density but hard to see at a distance)
Light oil 345 – 376cm (coloured – easy to read, density varies on type of oil)

So here’s our first problem …… measuring each cylinder individually will require a water manometer at least 3.5m tall and an oil one even taller, clearly not exactly practical or compact! The solution? Well the Capo comes to the rescue …..

Being a twin, the Capo simply needs a differential setup – that is, measure both cylinders against each other, not against atmospheric pressure. In theory they should cancel each other out if perfectly balanced and so the manometer would read zero. Any imbalance will have the liquid slightly higher in one tube and lower in the other …… so by measuring differential pressure we don’t need a manometer anywhere near as tall, but remember, even small differences in pressure will make big changes in liquid level, so the manometer still needs to be quite tall. In the end I built mine on an old wardrobe door – a total height of 155cm, with about 60cc of Scottoil filling about 40% of this. With hindsight, it’s about twice as tall as it needs to be, but hey you live and learn!

Putting it to use …..

With the Capo nicely warmed up after a little ride, the tank lifted and the manometer plugged into the Capo’s vacuum ports, she was fired up again. The oil level in the tubes drifted apart and settled at approx. 9.5cm (equivalent to approx. 0.6cmHg), a gentle nudge of the screw for the front cylinder on the throttle body saw the level drop to just below 2-2.5cm (approx. 0.15cmHg) – comfortably within the 0.5cmHg accuracy quoted by Synchromate and oodles better than the 2cmHg per division of the Davida gauges. So that’s a £2 rig versus the commercial £70/£170 rigs ……..

….. and one other thing, this was without ANY damping (valve/jet/cotton wool etc.) in the line because the Scottoil works perfectly well as its own damper. It pulses gently by no more than about 2-3mm in the tube but is viscous enough to respond reasonably quickly to changes in vacuum. Oh and that addition I mentioned …… simply two small sealed containers greater than the volume of oil in the manometer, one placed in each line. Now if either vacuum line should come adrift the container on that line acts as a trap to capture the oil before it can get swallowed up by the motor!

So ultimately it’s cheap, self calibrating with excellent resolution around the balance point ….. but not exactly portable! And so in typical MA fashion, the mind wanders off to thoughts of a compact electronic version. Powered by the bike, self calibrating – kind of like this one!