The carbs I have for my car came with the engine, they were in a sorry state but as I had only paid 150 quid for the whole lot I didn't expect too much. I stripped and cleaned them over a year ago so was pretty surprised when the engine fired straight up and ran pretty well. I knew they were were not bang on but they were close enough for initial testing and I didn't have a carb balancer for finishing the job. That got rectified the other day when I purchased a new old stock Gunsons balancer - it's primitive but it does the job and it was very cheap. Turns out the carbs were way out, I thought that may have been the case as they exhaust gas stung the eyes a bit, always a sign of an incomplete burn. I had already checked the timing, it was spot on.
I had made my first attempt a few days back but half way through I ran out of petrol. This time half way through the engine stopped but I couldn't figure out why until I checked for spark - there wasn't one. I was getting 12 volts to the coil but got nothing at the plugs. The problem quickly showed itself when I took the distributor cap off - the points were closed right up. I noticed some melted plastic on the contacts. I removed the points, cleaned and reinstalled but I don't know why that happened, I suspect the coil is pulling too much current and the points are over heating. A new set is on order but I will use the old ones for now, I reset the gap and it's running again so at least I can finish setting the carbs.
I had noticed that when I tried to adjust the left carb the screw wasn't making any difference, this was because the length of the rod that joins the two throttle mechanisms together was too long and was stopping the adjuster from working. Easy fix, now I could set the screws and get my balance right. I am pleased to say it is now spot on and the engine is running smoother than ever, I do need to reset the timing though, it's a couple of degrees out from where I altered the points.
Next job on the SU's is to adjust the mixture, they are a bit unique in that on most carbs the mixture screw only affects idle but on the SU it sets the mixture across the entire throttle range. It does that by adjusting the height of the jet relative to the needle. There is only one jet in each carb, rather than the more usual 2. They are basically a cv carb so they have sliders in them that are actuated by vacuum - they lift as vacuum increases and adjust the size of the venturi, very much like the Mikunis or Kei Hins on motorcycles before they all went injection. The way to adjust the mixture is to lift the slider very slightly and see if the revs rise or drop, if they rise the mixture is rich, if they drop the mixture is weak. Screwing the adjuster in richens the mixture by lowering the jet, screwing it out leans it up by raising the jet - it's simple when you know the trick. Some models of carb have a little pin so you can raise the piston, mine don't so a small screw driver is needed. With everything set correctly the car should easily pass the emissions requirements, which vary depending on the age of the engine. Here's the problem - dating these engines is not easy, I know mine is from 1972 but the published list gives really vague dates that just say something like between 1963 and 1976. The IVA rules change with age so if you have a pre 1973 engine it's only a visible smoke test. Up until 1976 it's 3.5 per cent co and 1200ppm hydrocarbons. My engine is a 10.5 to 1 compression ratio from a Rover P6 and it has a very low serial number but DVSA require written proof. I think my engine will meet the 1976 standard anyway but I would prefer to get a dating certificate anyway. I need to look in to that.
Anyway the problems really started when I went to dial in the left hand carb, it just wasn't responding to the adjustment screw and I couldn't get it rich enough to run properly, even with the mixture screw right in the engine would still fade if I lifted the venturi piston. Rather confused I opened up the bad carb and noted that the jet assembly was of such a length that if I dropped the jet any further it would hit the bottom of the carb float bowl - this was getting weird. I took the other carb apart so I could compare the two, while I was at it I verified that both carbs had correct needles - they did, BBV'S. I spotted the problem when I compared the length of the jet assemblies - the carb I could not dial in was over 3mm longer than the good one. I must apologise for the lack of photographs here, I wanted to get the job done, my phone was flat so I got no photos. Anyway the assembly is made up of a plastic top piece in to which slides an aluminium tube that holds the main jet. The jet is then retained by a metal collar that slides over the plastic part. The photo below is of a similar one in the hope it helps my garbled words make sense.
I carefully slid the collar off and squeezed the part in my vice until it was exactly the same length as the one in the other carb, I confirmed it with my digital caliper - 68.68mm. The two needles are different, one has a black top the other is white, the white one being for the left hand carb when viewed from the front of the engine. The entry pipe is handed, hence the need for 2 different parts, to save you looking it up the black part number is wzx1452, the white one is wzx1453. They are quite expensive so I would check the length before condemning one.
With it all back together I started the car and let it warm up then started my tests, although it was already obvious that it was running better - smoother and more crisp in it's pick up. Slight tweaks were required, a bit of a tweak to the balance was needed from where I had them apart, I still need to double check the ignition timing but I think it's pretty close.
It really is running as close to perfect as I think I am likely to get it, it will tick over at 500 RPM, pick up is clean, there is no visible smoke from either exhaust. I needs a road test before final tweaking, I need to make sure it doesn't pink under heavy load, if it does I may need to retard the ignition a degree or two.
Link to the video here - hope somebody finds it useful. I just had a thought, I forgot to mention that while the carbs were in bits I checked the float height, they were a fair way out. It's important to do this before you spend any time tuning or you could well end up having to start again, float height is critical to the mixture.
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