1969Mk3Spitfire

Sorry but I didn’t make myself clear. I used the welding rod alongside the pushrod to measure its overall length of 193mm.

That’s the nub of the problem. I’m not as yet understanding the relationship between valves, rings and incomplete combustion. Specifically, how to test without tearing down the engine. I’d hate it to be a wild goose chase. Pete L, you have me worried suggesting that yellow flashes signifying control ring issues. Can you please explain more…….is it possible to investigate to confirm?

The timing is one of the few things that I haven’t touched. I’m confident that it’s idling as rough as a badgers backside because I’ve now completely cocked-up the mixture on the front carb. Hopefully, I’ll be able to address this tomorrow to at least restore a decent idle.

Thank you Joseph and Wagger. Yes, the jets are centred. I went through that pain a few years ago. Likewise, I don’t think that the piston is sticking but I’ll check and clean it. Don’t know if it’s relevant but just looked at the WSM IPL and noticed that there should be a ferrel/bush between the small screw and the choke bracket as it attaches to the jet. Both of my brackets waft around in the breeze. A bush may help to apply more upward force to keep jet hard against the nut. I’ll have another attempt to set the mixture on the front carb tomorrow. More and more thinking that the linkages between the two carbs is having an effect, too. I need to read the manual another hundred times before I understand what it’s saying.

Thanks. I don’t know how it’s measured but I put a welding rod though the head until it rested upon the tappet and it measured approx 193mm to the rockers.

The best thing I’ve done today is to put my tools away and make a cup of tea. Colourtune in No 4 was a nice blue, as expected. No 1 was yellow and blue. No 2 (sooty) was mostly yellow and, subjectively, didn’t look to be firing as frequently/regularly as the other two. Screwing the jet nut appeared to have absolutely no affect upon combustion. I believe that the front carb choke mechanism has a (not exclusive) effect upon the issue. Often, but not always, I can push the jet a fraction upwards until it’s hard against the screw. I’ve played with the linkages and now it’s slightly better but I don’t think it’s right. Also, every few times that I twist the connecting rod (actuate the choke), the jet will stick open. I then reset the jet to exactly the same dimension as the rear carb and took it for a ride. It was approaching commuting hour so some slow moving traffic. Engine idle now sounds as rough as a badgers backside. The exhaust note sounds like a spitting V8. At 40 mph, light road load, it will misfire as though it is momentarily starved of fuel. If, however, I put my foot down it will go like a scalded cat, very brisk acceleration. I did my customary spark plug removal at the end of the 3-4 mile drive. No 4 looks as though it’s starting to clean after my tinkering but No 1 and No 2 both look as though they have done a shift down the coal mine. Is there anyone, commercial or willing privateer, within a reasonable distance of south Manchester with an intimate knowledge of SU carbs?

When it first happened, I changed all of the ignition components but to no avail.

I think I could be persuaded to train to become the elf and safety inspector for that investigation. 😁

A pal has suggested swapping the jet and needle (with or without piston) between the two carbs to see if this moves the sooty plug. What does the forum think?

Does anyone have any experience of using this test, if so, what for and was it helpful and informative? Any recommendation of a Brand? They are plentiful on ebay for around £25......any good for occasional use? I'm trying to track down a sooty plug issue.

I've just observed that one of my pushrods has a bend. Trying to straighten it is not my first preference. Does anyone have a single spare for sale?

Time to go back to basics. The No 2 plug is sooty / dry. Not shiny and wet so I don't think it's oil (but can't be sure of anything with a 50+ year old engine). This is hydrocarbon so either an excess of fuel or insufficient air. Setting the jet at "2 turns below the bridge" and using the pin method to assess mixture, the rear carb is pretty-much correct but the front carb (cylinders 1 and 2) needs at least one full turn lean. I don't recall there being such a difference between the two when I first set-up the carbs 3 years ago, before the sooty plug issue manifested itself. With clean/new plugs, the engine revs freely and pulls well, no flat spots so no evidence of an air leak. What's the likelihood of a blockage within No 2 induction? It's looking more and more likely that I have to start tearing down the top end. Does the Club sell a head gasket set? I've been reading about a cylinder compression leak-down test. I'm going to post within the Tools section but does anyone have any experience and care to comment about the value of such a test?

Point taken, Pete 😃

Thanks, John, worth a try. Do you have a favoured brand and know a equivalent part number to N9Y/BP5ES?

That was an interesting exercise, not so easy to measure. Measured push rod movement at the tappet adjuster bolt. Exhaust valves: 1=6.91mm, 2=6.99mm, 3=7.09mm, 4=7.18mm. Average 7.04mm Inlet valves: 1=7.07mm, 2=6.95mm, 3=7.0mm, 4=7.19mm. Average 7.05mm I suspect a good deal of measurement error but they all look consistent to me? Not spotted before but I noticed No 3 cylinder inlet push rod is bent. If I can find another, presumably it’s just a simple swap and readjust the tappet? Still nothing jumping out at me as being causal to the sooty plug issue. People have mentioned running on a 2-stroke mix, what’s the consensus? Next steps are to think about head overhaul and/or carb overhaul. Thoughts, costs?

According to the European Tyre and Rim Technical Organisation, ETRTO, handbook, 5.5 inch rims are not recommended for 155/80 section tyres.

I imagine that we are all guilty of tinkering with more than one thing, never truly knowing if/how/why we fixed an issue. I don’t understand why it is one cylinder only. The two carbs differ in the position of the jet nut and the choke leverage mechanism to lift the jet (the latter could be a consequence of the former). Messing about today, I managed to flip the inner choke spring and it took a lot of swearing and cursing to get it back in place. I still don’t fully understand the full working of the mechanism and the linkages between the two. Not sure about an air leak. Subjectively, acceleration is smooth and my past experience of carb wear and air leaks involves rough running. Float chamber is a new thought. I’d need to get spare gaskets before going down this route and also to try to understand the relationship between float setting, one cylinder sooting and a misfire. Does anyone have the number for “Triumph Anonymous” as I need help😭

Indeed, it was the first thing I did, alas to no avail. Thanks for the suggestion, though.

Just had a great week meeting up with ex-work colleagues, rdv in Belfast. Rollercoaster downwind sleigh ride return to the Mull of Kintyre in 8 1/2 hours, a lot of fun surfing down the waves …..and so back to my Spitfire. I took it for a ride today and this included a 60 mph stretch of the A556. As I approached home the engine had an occasional hiccup. Removed the plugs and, no surprises, 1 and 4 looked good, 2 was sooty (I rarely remove No 3 as access is a pig). I’ve fairly closely examined the choke linkages and at rest, the lever position of the rear carb is exerting more upward force to hold the jet in place. The forward carb choke lever exerts little upward force to hold the jet in place, albeit this is probably because the forward carb jet screw is set at least one full turn further “in” than its oppo to get the same pin-lift response. The forward carb choke return action is not as positive as the rear but this could be another red herring. Currently, it’s taking about 10 miles to soot the plug sufficiently to cause a misfire and I’ve run out of ideas 😭

The curmudgeon in me usually disapproves of people using mobile phones when at a motorway service station but I’m now on the M74 with a a revised opinion 🙂 Thanks to you all for your valued input. I’ve had a single cylinder sooting issue for a while which now seems to have manifested itself into a misfire. I’m 110% sure that it is not a plug issue. I’m not at all sure about pretty much everything else. I’ve measured dizzie play and it is tiny, tiny. I’ve replaced all of the ignition service items except condenser (club shop condenser was rubbish and went into the bin). There is at least one full turn difference in jet position of the two carbs and this intuitively doesn’t seem right to me albeit both now exhibit the correct mixture setting when using the lift pin. Otherwise, the car runs and accelerates smoothly, no hint of air entering the system through excessive wear. Car has 54,000 miles only. There could be mileage in the earlier comment from Mr Badwolf to further investigate the choke linkage. It does seem incredibly crude that the link swivels/articulates based upon a slack screw at the base of the jet. I guess that’s part of the appeal of these cars.

I'm absolutely delighted, George, that you were able to strike a deal with the trader and buy those seats. I see it as a tangible benefit of TSSC membership that by keeping an eye on the forum I was aware of your need and was able to make you aware of them. Great outcome.

That's an interesting comment and one which I shall investigate upon my return. Many thanks. In general, is is "normal" to have the jet position / screw set fairly different between the two carbs to seemingly achieve the same mixture, as measured by the lift pin method? Fearing the wrath from some, is it worth employing my 45 year old colourtune?

Really appreciate input from all. I removed the carb tops this morning to measure jet drop. I remember a couple of years ago setting them both to flush - 2 turns. The front carb appeared now to be set much leaner than the rear. I also disconnected the choke cable and there’s no doubt that there is more friction and an occasional binding in the front carb. I’ve further slackened the screw holding the lever to the bottom of the jet. I’ve also given all components a good clean. With the front jet reset to flush - 2 turns I started the engine. By the time I’d set mixture with the pin, air filter off, I had leaned one full turn, pretty much from where I started. I refitted the air filter for final check. Sadly, it’s not possible to have two consecutive days of sunshine in Manchester and today it’s raining so the car is staying in the garage. Free revving, it sounds very willing. It doesn’t seem right that the jet position on the two carbs are so different but appear to be set against the pin technique? Any views? Unlikely to be able to do more and/or road test for a week or so as I have a sailing break with a couple of pals next week, hoping to get to Belfast, weather permitting.

Thank you, both. I said in an earlier post that I had just leaned the front carb by one flat. Lifting the do-dah causes the engine to slightly increase in speed, then slow down. the rear carb exhibits pretty much the same characteristic but, from looking at the plugs, it’s obvious that 3 and 4 pots have better combustion than 1 and 2. I will lean the front carb by another flat and try again. How did you have in mind to configure a vacuum gauge in the manifold? What should I be looking for? The exhaust is not smokey. It hasn’t done a lot of miles since preseason service so no measurable oil consumption. 1/2 turn sounds a lot, Pete? I’ll change No2 plug if you think there’s a possibility of it breaking down, albeit it’s only done a couple of hundred miles. Air filters are new (ish) and are OE. The car is unmolested, original, albeit restored. Noted that a decent run is at the speed of sound (there’s certainly a sonic boom in the cabin) for a good few miles, then coast into a lay-by to pull the plugs. I appreciate your help in trying to resolve the mystery. It’s frustrating and annoying in equal measure.

We went to Tatton Park today, exceptionally well attended by both exhibitors and visitors. Uncharacteristically glorious weather for south Manchester. It’s less than 10 miles from home but includes some dual carriageway so tried to give the engine an Italian tune-up. Any good this may have done was subsequently lost by queuing at idle for half an hour to park at the event. Car performed very well. On the journey home we took an alternative route, single carriageway country lanes. Almost home and the pesky misfire raised its ugly head. Same symptoms as “first” time, that is “instant” miss and “instant” recovery. This is a picture of plug 1, 2 and 4 (no 3 is an absolute pig to remove as it hides behind the dizzie). Mileage on 1 and 2 is around 25 since last brushed clean. No 4 combustion looks good. No 1 is still probably a bit rich. No 2 makes me want to thrash the car with a big stick. Still can’t rationalise why it gets so sooty, so quickly. More ideas most welcome.