Ok maybe the idle jet theory is wrong. All I know is that the AFR goes off the scale when going down long hills.
I have been very pleased with it - rapid response and no requirement for fresh air calibration which the Innovate LC-1 needs. I also like the fact you can daisy chain other sensors onto the same display and cycle through them by touching part of the display. The only thing that I had to do with this was to extend the data cable from the PLX AFR computer box in the engine bay to the display on the dash and it worked, but when I daisychained a rev counter computer unit (homebrew not PLX standard box) into the data cable the rev counter computer needed extra buffering to drive the long wire.
Did the PLX come with a Wideband or did you have to source you're own? Will look into it, seeing as you've done the leg work That AFR downhill definitely needs further investigation....so it leans out when going downhill....presumably this is at low load (small throttle opening)....so you're only running on the primary circuits...I can see why you thought it'd be the primary idle...very low load (or just off idle) will translate to bigger ported vac signal from the carb (assuming you're running vac), so the ignition is going to be more advanced...meaning what fuel air mixture is present is getting burned more quickly....but this in itself wouldn't account for a lean AFR figure on the sensor (I wouldn't have thought) Have you logged the AFR relative to RPM when going downhill? As this will give you some idea of which part of the transition you're in carb/timing wise. the other curiousity is that plenty of baja bugs and sand rails use the prog because it's less touchy than twins on the rough stuff, suggesting that going downhill shouldn't be too much concern for this carb design....I'm intrigued (confused and thinking!)
Hi @mikedjames ....just been trolling through some related threads on other forums (general car and motorbike stuff) and there are a fair few references to leaning out going downhill due to running freer flowing silencers (esp. sports bikes) namely reduced back-pressure at low load, exacerbating lean condition....enough to cause backfiring...if yours is leaning out and you're 100% leak free, maybe it's an idiosyncracy of your exhaust system, which has only started to show up once you've got the jetting more or less dialed in....previously over rich setting would have masked this, especially at low load.....just a thought? could be off the mark.... How you'd rectify this without incremental jet changes I don't know....think of a way to rig up an experimental back pressure valve...
The PLX kit came as a complete package with the Bosch LSU4.5 sensor, the little red computer box and multifunction LED/OLED display for $200 from the manufacturer which once I paid duty and VAT etc came to £200 or so. I have since seen a UK supplier offering it for about £170. It annoyed me that it used a LSU4.5 because I already had a LSU4.9 sensor because I was considering building my own wideband setup from the Bosch controller chip outwards. The lean/popping problem comes at about 30mph in third gear down e.g. a 1:10 hill when I am definitely engine braking as well as braking normally (no servo so no vacuum leak there.. ) . As you suggest, the exhaust is not stock - its one of the JK stainless single quiet packs which has a larger bore than the EMPI twin quiet pack - the stainless tubes go over the ends of the heat exchanger outlets without having to be flared like the EMPI version. It definitely promotes occasional moments when you find the rev counter has peaked out at 5500 RPM in second gear when you didnt mean to... Perhaps the Baja guys are always flooring it down the hills, it takes a reasonable distance, maybe half a mile downhill to get to the popping. Of course there is another burn mode which looks 'lean' to the PLX , which is when there is no spark. The O2 percentage of unburnt fuel/air mix at any ratio is still the about 20% of normal air. I dont think its that because tapping the throttle stops the popping and the bus accelerates immediately.
So maybe be it's simply a case of leaning out when decelerating....which is normal....but you're able to see it happening on the guage. Do you get a similar reading when you floor it, then put the clutch in and let your foot of the gas? If you're au fait with circuit boards and electronica, have you never thought about crank timed ignition?
I got fed up with the throttle linkage - I drove 1600 miles round France and the throttle linkage stretched (my version had an additional bodge aluminum bracket in it because the throttle cable was too short) This shows it with the second barrel barely opening (with a brick on the throttle pedal) If I manually pushed the throttle lever open, then it would rotate the throttle plate another 20 degrees and completely bring in the second barrel. If you look at the throttle cable angle you can see why this is impossible to achieve with the movement of the pedal, pulling against the springs in the carb. So the aluminium joining plate would just bend instead of moving the throttle. So I made a bell crank assembly Or another view I used a ball bearing race from an old printer, welding the pulley that the race was pressed into to the crank which is made from 2mm sheet steel, and I cut a strip of Ikea medicine cupboard stainless steel for the connecting plate at the top . I extended the throttle cable with some studding and so was able to make it reach the lever. The lever is also set up to provide a 1.1 times increase in movement of the throttle for the movement from the pedal. The construction is still in the prototype phase. When it is correct, I will paint it all up.
liking the linkage Mikedjames....I fannied about and came up with a two-pulley set up which worked really well (no pics unfortunately), it achieved the same thing as your prototype....got the cable entry at the right angle to rotate the spindle fully from zero to WOT....after that it was a case of dialing in the cable length to get a nice pedal....I did this by putting a barrel adjuster on the pedal end and playing with the pedal stop (late model), so there was no lash in the cable at all.... I've since gone for the 'Aircooled.net' manifold which has the improved heating and a heim-jointed linkage coming off the manifold casting itself....even that was pretty pants tbh...I've changed out the heims for old italian IDF ones and done a bit of aditional bracketry to improve things.....feels pretty good
you may find that a few other prog users would be interested in buying a ready made....assuming you have the time plus if you made a billet spacer (incorporating a vac take off) to fit between the carb base and manifold from which to mount the pivot linkage, you'd have a really nice bit of kit...i know a certain gentleman on here who's a wizz at billet engineering
Interesting idea about the billet block - my pivot point could have come off a stud mounted in the block rather than a bit of metal I found with some folds already in it (I think there was a loft ladder or a greenhouse in its ancestry) and the geometry would have worked. But I would have to watch the height of a billet block - the airbox I made is pretty tall and the carb only just clears the studs if i press it slightly against the engine bay insulation. I did consider a trick involving an angled arm with an adjustment screw to alter the effective length of the lower arm.
If it hits 12.5 AFR at 4500 RPM and 75 GPS mph, why do I need different carbs The whole project spent the same kind of money but in a different direction. And I seem to have scored a full flow oil cooler installation as well without going much over the cost of the Dells.
True, but its a bit like spending a fortune over clocking your PC.... When you could have spent the same money just getting the right one first time.
Thanks for sharing your experiences. I plan to lose my aftermarket airfilter and get hold of an old oil bath filter. I might shell out for one of these to make it easier/neater: http://www.ebay.co.uk/itm/361076098424?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT A bunch of recommendations at this link, too: http://www.thesamba.com/vw/forum/viewtopic.php?t=386388
Thanks for the feedback ; continuing the saga: I noticed that at full throttle the AFR would go very low and the power did not feel quite 100%. So I changed the secondary main jet to a 140 and this has helped noticeably. As a side effect I knocked the No2 plug lead off by accident, and then drove my bus. The AFR indicated excessively lean and it was quite difficult to go up hills. While I was looking at the linkage following rebuilding the pedal assembly in the cab which decided to fall off complete with a stack of rusty metal plates welded to a hole in the floor, I changed the linkage geometry to align it better with the throttle cable tube. The pivot point bracket on the carb has the pivot 35mm down from the top of the carb base. The throttle cable is connected 45mm below the pivot, the connection to the throttle wheel on the carb is 60mm above the pivot so the gain factor is now 1.33:1 between the cable and the throttle movement. I realise now that the spring in the EMPI kit is pulling on the throttle spindle in a way that means the more tension in the spring, the faster the throttle spindle wears as all the pull is from one side. This showed as an unstable idle as the throttle plate would jam just off closed because of the spring tension. I have moved the return spring so it pulls on the top of the lever in my setup. The result is that it pushes on the top link of my setup, rather than trying to return the throttle to zero by heaving very hard sideways on the spindle. Some sketches I made of the linkage
In the link I posted above john@aircooled.net recommends a 180-190 secondary main jet (with a 60 secondary idle jet).
I did use that information from the experts on the Samba as a start point, but like everything, there are differences in every setup. I do have 170,180 and 190 main jets in my collection but I have never fitted them : I can believe the larger jets for a larger or higher revving engine than a 4500rpm 1641, but in my setup, with power valve still operational, and all the heat sources available, The 140 secondary with an 80 idle does not return to having a lean flat spot when I floor it. The 150 + 80 tends towards 11:1 AFR flat out under load. With the current jetting, it sits around 12-13:1 slight uphill on motorways at 65mph, with slight downhill at 65mph leading to the 14-15:1 ratios wide open throttle.
I made this for mine, gets full throttle, I have extended the pedal end too though. I used the thin steel cable from B&Q as a new throttle cable so it's long enough and use a heat exchanger cable clamp at the pedal end. Has not stretched in a couple of years so seems fine. Also at the right angle so no spindle wear.
ideally, someone with the necessary tooling would make a steel or ali spacer (about 5mm thick) which won't insulate the carb base as much as the standard bakelite one, which normally comes with the kit. This metal spacer could also have a suitably lugged extension* to accommodate the throttle linkage, plus a pre-drilled vac take off... *not too disimilar to the plates that fit under the air filter you get in drla and IDF hex bar set ups, as in the pic like this, but inverted...and thicker...and to fit the manifold...with a vac take off
