I have recently had a fail on the MOT due to emissions. Limits and actual figures obtained; CO: 3.5% (5.6%) HC: 1200ppm (5782ppm) What do I need to adjust on the carbs to bring these levels down? The engine and box were pulled out of the car complete around 18months ago, so the engine hasn't ran for a while until recently. Only change from when the engine was dry stored has been new plugs, leads, cap and rotor arm. Also is the emissions test taken at a fast idle of about 2500rpm? Thanks in advance! James.
Unless your engine is in bad shape it's generally poor ignition and unburnt fuel for high HC. Check the timing static and with a strobe. No idea about carb averages, but after a new cambelt, tensioner and timing setup on my digi it's done 2.5%, 142ppm on this last test.
check timing and maybe a rolling road tune is needed? You could adjust mixture screws just remember where they where to start with!
5000+ ppm is a lot of unburnt fuel, was it misfiring?? Yes the CO is high, but its the higher HC that needs more attention. You won't really fix much by adjusting the mixture, it has a more serious issue such as a misfire/carb flooding/something equally **** Depending on the year of car, the emission test will be done at idle. It's the later (post 92 registered) cat-equipped vehicles that have a fast idle and natural idle test.
I'll check the timing and see what that's saying. The engine does have a cough and splutter around 2100rpm which clears above this engine speed which is down to a not so brilliant design/inlet manifold too short for fuel and air to mix better. The last MOT results I have when the carbs when on (new carbs) was 2.34% and 414ppm, the engine when removed a couple of years ago was sat in a lockup so nothing has moved or changed - other than the ignition items mentioned above a couple of months ago. Car is 1983 and engine from a 1990 Corrado. When I was at the MOT place I'm sure I heard him hold it at a fast idle - at the point where it coughs and splutters...
Even though the test itself on something this age will be done at idle, you always do a 'purge' of holding it at fast idle for 30 secs or so instructed on the gas analyser screen. If it's running really bad during the 'purge' it may not 'settle down' enough before the analyser goes into the 'testing' screen. As you can see the original results are miles away where you are now, so something is 'wrong'
Thank you for that Gary re: procedure and the purging. I took the plugs out and all show signs of black sooty dry deposits (only on no.4 could you see where the electrode and centre prong has been firing) The leads were changed but these were good secondhand items. I'll try and look in to the ignition side further.
I had similar results when I took my mk1 on carbs for an mot. New plugs later and I had an mot Could be as simple as that.
I would look at the idle circuit. Are the mixture volume screws about the right setting? After that there is a range of about 10 idle jets weak to rich. A new set of 4 is not that dear Jon
High HC and CO ppm feed gas concentration, is an indication of a rich mixture. If the carbs are balanced, you may want to reduce idle fuel flow via the mixture screw and idle jets. At part load however you may need to look at emulsifiers.
Surely if the car has passed MOT previously with the same carbs/jetting Then there must be a running issue elsewhere? 5000+ ppm is a lot of unburnt fuel.. right?
Start by looking for basic things like air leaks (usually the CO will be much lower and HC high though, but worth checking) Spray WD40/similar around the intake/carb sealing washers etc see if engine note improves. Make sure servo/pipework isn't leaking. Was car hot for the test?
Yep car was hot for test, heard the radiator fan kicking in a few times. (On a similar subject, how much difference would the fan being on during the test effect things? I couldn't personally see the car being tested but I'm sure the procedure would have been carried out correctly)
