Airbox, K&N air filter, and exhaust changes - tuning a Citycom

[Thoppa]

Hi all,

After commuting on a Citycom for a month, I decided to start modifying it. I've done this a lot on motorcycles.

I've been running the modifications for around 800kms and all is good so I thought I'd share. When I got the scoot, she had under 4,000kms, and she's now almost 6,000 ready for a big service.

I've put all this info and other stuff on my blog if you're curious. https://myl8test.wordpress.com/mbikes/sym-citycom-300i/

I opened up the airbox and checked out the pipe connecting the airbox to the throttle body. It's long and twisty and has a much larger diameter (42mm) than the airbox air inlet tube (33mm). The pipe could do with a better flare into the airbox.



The inlet tube in the front of the airbox cover is removable so out it came. However, the circular hole this left was still much smaller than the 42mm pipe, so I trimmed out all the plastic and added some epoxy putty to smooth the new intake. This gave a similar intake area to the 42mm diameter pipe. Like this :



and the square ridges too



After that, I removed the tab of plastic on the inner half of the airbox, since this another way to restrict the air intake to less than the 42mm pipe.



And then finally, to smooth the intake area as best as I could, I added some foam filler and a layer of epoxy putty to the cavity. Like this :



It's very subjective, but the throttle feels like it has a slightly better response. For sure, there is now some induction noise which makes the scoot sound more like it has an engine. It isn't loud at full throttle, and sounds good to my ears.

When the air filter is due for replacement, I may consider fabbing my own using a K+N filter.

Exhaust

I didn't want to pay big bucks for an exhaust because the engine clearly isn't going to deliver significantly more power and the CVT means that power would only be noticed on top speed runs. The main reasons to change the exhaust were the looks and the weight.

So I got an exhaust for a Honda CBX 250 (?) and an interface from Taobao, figuring that they were so cheap (US$40 plus US$20 shipping to here in Hong Kong) I could give them a go and not be unhappy if they turned out to be trash.

The interface with sizes :
http://item.taobao.com/item.htm?spm=a1z09.2.9.23.7fLR34&id=36930450577&_u=s10d5b0g8171

The exhaust :

And the ad:
http://item.taobao.com/item.htm?spm=a1z09.2.9.64.7fLR34&id=38369933258&_u=s10d5b0g60b7

I was surprised to find they were good. The interface inner diameter was 1mm too tight so that got filed down to fit. The outer diameter was 2mm too small so I added some thin steel pipe split along its length for clamp tightening. The exhaust bolted on without interfering with the original exhaust mounts or sticking out from the side of the machine. It fitted very well in fact. I got very very lucky with that.

The exhaust sound is very deep but much too loud for me without the decibel killer fitted, and sounds kind of farty with it, and the exhaust didn't seem to make any change to the performance. That was really surprising - I'd expect that it would have less back pressure and that would either cock up the fuelling, or knock torque off the bottom end for a small gain at the top. But actually, no obvious fuelling change at all - just more noise.

However, the original with its bolts weighed around 8kgs and the replacement weighs 2.5kgs, so that's significant, especially since the 5.5kgs reduction is unsprung mass at at the back of an already back-heavy machine. It also looks much better to me, so it's a keeper.

I recently checked the spark plug, after about 750kms, to see if it was running too lean. Surprisingly, it's not. So that's great, although I'll wait until I've seen how a new plug colours to be sure. Fuel economy has averaged at a little over 32km per litre, which is a slight improvement, but I wouldn't assume that is only these changes.





Before :



After : 



I've also been slowly re-spraying the silver parts with either black or graphite paint. The first things I did were the grab handles and side panels, and I've also done the rear clamp, rear shocks, front forks and I did the wheels with black spokes and graphite rims. I'm very pleased with the look. 



EDIT Forgot to say - I removed the AISV stuff -it adds air into the exhaust below 3,500rpm when slowing down on a closed throttle. That caused some pops and it affects emissions, not performance, so off it came. And the pops stopped. 



Air comes from inside a low frame tube on the spark plug side of the engine, through a rubber tube into an air filter can, and through another rubber tube into the Air Injection Solenoid Valve. When the AISV is open, the air flows through another rubber tube into the head where there is a reed valve. The output from the valve flows out of the head and through a metal tube to the exhaust port. So I removed all the rubber tubing and filter can, plugged the frame tube, plugged the AISV but left it electrically connected, and plugged the head entry to the reed valve. This came off :



So overall, I'd definitely recommend the airbox mod but the exhaust hasn't made much difference.

Cheers !

[phipsd]

Neither are the air box mods using a K&N filter if you want your bike to last. Testing done years ago by the late Gorden Jennings at Cycle showed that stock paper filters the best by far. One reason why K&N's flow so well is that they do a poor job of filtering dust and grit.

A clean engine is a long lasting engine.

[Thoppa]

Thanks for that. I'd prefer to keep using paper, but they are disposable, and cost US$28 here, and that's supposed to be every 6,000km.

I've never run a machine until it dies before so I have no idea how much an effect the filter type has on engine life. I have used K&N and DNA filters on other machines without problems but then again, the longest was about 30,000kms using a K&N in an XR250.

More air flow might mean the fuelling gets messed up from being too lean? The DNA filter http://www.motorcycle-exhausts.co.uk/DNA_AIR_FILTER_SYM_CITYCOM_300_201014--product--16243.html#.U4NpoiimU1I Audioguy posted about flows three times the volume.

DNA FCD air filter flow: 202.00 CFM (Cubic feet per minute) @1,5”H2O corrected @ 25degrees Celsius
SYM stock paper filter: 62.10 CFM (Cubic feet per minute) @1,5”H2O corrected @ 25degrees Celsius

Well, my guess at the maths is based on this
https://www.donaldson.com/en/engine/support/datalibrary/056798.pdf
.... Airflow (m3/min) = (Engine Size (Liters) x RPM) x VE / 2000

..so the engine is 263cc or .263 litres, let's say rpm is 6,000 , and VE is 0.90 for a 4 stroke,

..so that's a maximum flow of .263 x 6000 x 0.9 / 2000 = 0.71 cubic metres a minute. 0.71 x 35.3 = 25 cubic feet per minute. Pulsation factor is 2.1 so that means 50 CFM @ 6,000rpm.

So does it need a less restrictive filter ? Try it and see.

I've been experimenting with a taper on the port for the connecting pipe inside the airbox. I added some 6mm rubber tube, tied in a loop with fishing line inside, to make a C shape around the port. I filled the area to make a smoother, wider and more tapered port. That changed the induction noise to a deeper note - a purr - and the throttle seems to have more initial response.

cheers

[phipsd]

One of the best performance improvements you can make to your SYM is simply to ride it. It took about 20,000 km for mine to run it's best. It was much quicker off the line and faster on the hills. Considering the relatively limited power; the CVT and the engine tuning work really well together. If I could squeak another couple of HP out of it at the top end but I lose bottom end and midrange that doesn't seem like a good deal to me. So mine stays stock.

[Thoppa]

Yeah, I read Kiwiscoot's long thread and he said the same - engine just keeps loosening up. Good news !

However, I'm an inveterate meddler and I have a degree in mechanical engineering, with decades of experience in tuning carb bikes, so it is somewhat guided meddling. I've modded two FI bikes - an XT660 and an ER6F.

If you look at the intake port, which is effectively the velocity stack, you'll see a very small rounded edge.

To get smooth flow into the velocity stack, the curve of the bellmouth should be 1/4 of the pipe. The pipe is just over 40mm so it should have a curve of radius 10mm. They gave it 2mm. Flow from the airbox into the engine should be much improved by simply adding a correctly sized bellmouth and it's a certainty that using a correct bellmouth will improve throttle response.

cheers

[Thoppa]

And here's the flare... I bodged the first tube - too small a donut - but managed to overcome that error in the end.



Add 15mm tube cut in half at the sides



Smooth and round - tip for using epoxy putty is to keep cleaning your fingers, and then when you want to smooth it, keep them wet.



The difference in throttle response is very very noticeable ! I've never had to add a bellmouth before - every FI bike already had them. For example, the ER6 has bellmouths on the air inlet at the bottom of the pic below. And the throttle bodies have bellmouths that protrude into the airbox through the two holes at the top of the pic.

Why Sym didn't design the airbox with a bellmouth is beyond me. The difference is so noticeable - it makes no sense to throw away a performance gain like that. I guess they don't have experience designing high-performance machines so perhaps they simply don't know?

[Thoppa]

Another update - found a K&N that was suitable to fabricate into a re-usable air filter for the Citycom. It's from a Silverwing 600. Some tests specs :

and some images

and my new filter

other side


I fitted a new CR8EIX to see how she runs but I can hear she is leaner - the purr has a dry rasp now. So next up is to modify the FI to suit. I put the air temp sensor in the freezer for an hours and it was reading 5.5K ohms. It reads 2K at room temp, and the maximum is 18K according to the manual. So I'll try adding resistance in series and see how that goes. The FI also has a CO adjustment but I need the factory diagnostic tool for that - maybe a trip to the dealer will also be necessary.

Oh, and she takes off on small throttle openings now. 

cheers

[Kiwiscoot]

http://scootdawg.proboards.com/thread/16772/sym-citycom-300i-review-maintenance?q=air
http://scootdawg.proboards.com/thread/16772/sym-citycom-300i-review-maintenance?page=2&q=air

Mods I did when I had my Citycom...did 48000kms with no problems ...this is for the nay-sayers.

[Thoppa]

Great stuff - I loved reading that thread - thank you ! And I missed that there is another bell mouth fan, Globs, on that thread too. Great minds... haha. 



Yesterday I did 80kms on the new plug and air filter - the plug did not colour at all - that's way too lean. Thankfully it was 80% humidity yesterday so not much chance of detonation.

Today, the last job was to add some resistance in series with the air temperature sensor to make her run richer. I used a Bourns 10K ohm 20-turn potentiometer and some 2 core mains cable. The cable was soldered onto the 10K pot so that turning the adjuster clockwise increased the resistance.



The cable was routed down through an existing hole in the underseat storage so the pot sits next to the immobiliser switch. It’s temporary until I’ve established what resistance is needed in summer and winter. Removing the storage bucket should still be okay because the battery cover comes out first and this offers a bigger hole for the pot to get though.



Then the cable is routed to meet up with the air temp sensor. I cut the green-brown wire (cut either green-brown or green-red, but not both) and soldered in the pot to add the resistance in series with the sensor. So G_BR got cut into G_BR|  |G_BR and the pot is added between the two cut ends, so G_BR+Pot+G_BR.



I checked the connections were all good and then soldered the connections, and covered them in insulation tape. The pot was set to 2.2K, as a guesstimate based on experience of doing the same thing to an XT660.



Checking the resistance at the air temp connector the DMM read 4.5K, showing the ECU has an internal resistance of around 2.2K ohms. I ran the bike for an hour and checked the plug several times, increasing the resistance until I got dark grey deposits on the spark plug electrode. That was at 6.2K ohms, which means the pot is around 4K ohms.

The rasp has gone and she is purring so I'm confident this is now set up for some long-term testing and fine adjustment.

Cheers !

[Thoppa]

Another update... modified the MAP sensor.

I took her for a service at the SYM dealer so I could plug in the diagnostic computer and see what the readings are. The temperature that day was 25 Celsius and the computer said 1 deg – a drop of 24, or 8.4% extra air density for the extra 4.9K resistance.

There was one old warning which was from the AISV, and every reading was normal except for the MAP (manifold absolute pressure read 32, the lower limit)  and Baro (barometer reading 97kpa with 98kpa as minimum). This makes sense – the less restrictive air filter means less pressure for the same volume of air = more volume at the same pressure.  So it would be better to modify the reading from the MAP sensor than the temperature sensor. Both would have the same effect –  a higher voltage from the MAP sensor means more pressure, which means more fuel is needed – same as a colder temperature by adding resistance. However, colder temps might retard the ignition, reducing the power unnecessarily, whereas modifying the MAP voltage would return the pressure to normal.

So I made an op amp circuit using an OPA2277 op amp – non-inverting with negative feedback to set the gain to 1.07, or 7%. It’s a very simple, standard circuit. I can post it if anyone is interested, or you can just Google "non-inverting negative feedback op amp circuit". I installed it into the MAP sensor circuit and turned down the resistance of the temp sensor circuit so that was back to normal, and extra fuel would be added thanks to the increase in the pressure readings.

I chose 7% to leave her a little lean for the very humid days in summer, and then I can use the temp sensor resistance to add fuel on the drier days in winter if needs be.

I’m happy to say this is working well and I’ll be testing this over the next week or two. I can dial in some temperature sensor resistance – about 1K ohms will set her at the same mix as previously, if I think she could do with a little richer mix, but so far, she sounds and feels right. I’ll check the plug again after some more kms.



I cut the output wire red/black and soldered the copper. I removed the insulation from the 5V supply yellow/black and the ground green/red.



The 5V supply is used to power the op amp (red wire), which as a very low quiescent current so doesn’t add any significant extra power demands, and this is grounded through the black wire. The blue wire takes the output of the sensor and passes it directly to the positive input of the op amp. The op amp is difet (a kind of fet or field effect transistor) so it has a very high input impedance and thus doesn’t place any strain on the sensor's output. The output of the op amp is the white wire and is 1.067 times the input. The gain is adjustable by the blue 10K ohm negative feedback potentiometer (a Bourns 20-turn part). The only other components are power supply capacitors between the 5V input and ground – a Philips 4.7uF 35V electrolytic and a 0.1uF X7R ceramic. This helps stabilise the supply so the op amp performs reliably. The OPA2277 will work on a 4V supply, has a very low output offset and offset drift, and is unity-gain stable so it stays accurate – all 4 of these features are essential in this circuit. The input impedance of the ECU is over 100K ohms so the op amp doesn't get strained driving the ECU either. The whole thing uses around 1.2mA. Nothing really and it is very reliable, accurate and stable - and not expensive. The OPA2277 is US$2, the capacitors and potentiometer add another $2. OPA735 is another suitable part but it is harder to work with cos it's very small.



I'll tidy up the installation and properly waterproof it in due course.

I also fitted an LED to the under seat storage - make it easier to see stuff at night. Fed power from the lights - brown wire - and used the ground connection on the immobiliser switch. Added a switch so it isn't on all the time the lights are.

[sc00ter]

Very impressive! Im used to plug and play, the lazy mans tuning. The last complicated scoot I tuned was a Zuma 125, but it has tons of support so fine tuning is way easier than what you are doing. Like I say, VERY impressive!

[Thoppa]

Thanks for the kind words. It's fun tuning engines eh ? 

If I could get the connectors SYM uses then it would be possible to make a plug and play adjustable module for the MAP. But I have no idea how I could get the male/female ones needed, so DIY it is.

After a few more kms, I checked the plug again - even with 6.7% at the MAP and 1KR at the temp sens, she's still running a little lean, but not far off. Time for another adjustment ! It only takes 30 mins to get the bucket/seat out, disconnect the three wires (resistor, bucket light, immobiliser), take off the insulation tape,  adjust the MAP voltage, and then get it all back together again. She's now set at 8.5% MAP voltage gain and no extra temp resistance. Time for more riding !

[sc00ter]

I know Honda and Yamaha use MODEX connectors, since a friend and I made a wire harness for his Honda Ruckus with a 2t Yamaha Zuma motor swap. MODEX supplies a lot of the OEM's, and they (MODEX) have cool little tools to remove the spade ends without stabbing your fingers! Got our MODEX connectors (MODEX makes ALOT of different ends and styles) at a electric supply store down the street (Priest Cain Electrical Supply). I know squat about electrical work and I still love going in there and look around and learn something. But look into MODEX since they supply a lot of OEM's.

[Thoppa]

Thanks - the suppliers I can buy from locally in Hong Kong are RS and Farnell but neither list any Modex parts. Other component suppliers are US based so the shipping/minimum order is too much. However, Molex is available but I dunno if this is the right part and the prices are silly, for example: http://www.molexkits.com/76650-0151

The next time I get the bucket out, I'll take some pics and measure the connectors.

I'm very happy with the way she's running so I've ordered some OPA735 op amps to do a better MAP voltage adjuster. I would prefer their rail-to-rail performance if I have to go higher than 8.5% gain.

I adjusted it yesterday when the air pressure was about 102 - voltage was 2.91 and I boosted it to 3.16. HK gets up to 103.5 and the highest recorded anywhere on the planet is around 108 IIRC. So 8.5% is already over the design envelope, but I assume the engineers would have designed for a very large margin over that so I'm happy it can go a bit higher and still work fine.

[sc00ter]

Maybe it was MOLEX. I was tired last night. I looked in the garage for the spare connector box but its buried behind bicycles and other stuff (spring cleaning). All I know is it's what the OEM's use, and the tools make custom wiring almost fun.