G'day all. Took the Mini out today. Had to pick up the boy at his new school. Where happily I was complimented on my choice of transport. I'd been worried I'd be branded as the hoon parent at the new school. Unhappily, the Mini was missing at light throttle. Especially when I was travelling next to a Repco Colt. It's always been a bit unhappy at low revs (i.e. 50-60 in 4th, it usually stutters - it's below the cam), but today it even missed a (fair) bit at 70. I'm after ideas on what could be causing this...

To give a bit of history, I had the car tuned by a reputable mechanic maybe 18 months ago. I know that's a while ago, but it wouldn't have done 1500 miles since then. I've fouled quite a few sets of plugs (before and after the tune), mostly through the engine not running for long enough (i.e. using BP6's and not driving far enough, driving with the choke on a touch to compensate for a slightly too low idle screw setting, car sitting in a panel shop, etc). The fouled plugs have all been rather sooty, leading to misfiring under acceleration.

The current set of plugs, well, they were new BP5's, and fine. Until one day I started the car hot but with the choke on. It missed most of the way home, which was only a short run (under power though, so different to now - I'm sure I was shooting flame!). The next run (out and back) it missed all the way through 80 zones until I hit a 100 zone, then ran beautifully the rest of the time (both ways - 100 zone is maybe 4km into the drive each way?). Which made me think that getting the plugs truly up to temperature enabled better spark (i.e. the plugs were dirty, not dead). So I pulled them and, being sick of spending up to $20 every few trips for a new set of plugs, I tried the Doc's trick of heating them to remove excess carbon. I don't have an oxy, but the wife was away, so I got away with using the BBQ. The plugs came up pretty clean - especially after rubbing some alfoil on them (did it while they were hot, the carbon came straight off onto the foil, even with the plugs sitting in the bluest part of the burner's flame, the carbon didn't drop off without encouragement). I am happy to consider that the plugs could be the problem, but... I would've thought they'd have been a problem under load. I can floor it in second gear and rev it out (no idea of revs, no tacho in the car, out probably doesn't mean much above 70 - exiting the school, which is on a highway) without a drama, no missing at all. The current problem's under constant/light load. Enough of the history!

So, specs!
1300, unsure of overbore. Decent cam (no power off idle), far too big 3-into-1 under the body 2" full race exhaust manifold with 2.25" tail pipe.
Twin 1.25" SU's (HS2's). Unsure of needles.
SimonBBC's Red electronic ignition module in a 43D4 distributor - noted because of the info here, though, being a 43, with mechanical advance, that might not be as big an issue..?
BP5's.
Engine is not overheating/pinging/etc. Always runs BP Ultimate 98 (local servo).

And to summarize the miss again:
- Slight throbbing, almost a miss at 50/60 in 4th. Engine below the cam, so not entirely unusual - in fact, it's currently less switch-like (i.e. can apply progressive power, not power on or off) at 60 than normal.
- Definite irregular miss at 70 in 4th. Engine on the cam, static throttle (shouldn't be affected by SU damper spring/oil?). Engine was warm.
- Single (but regular) miss upon depressing clutch (i.e. removing load) when shifting up, mainly when not quite warm.
Could it be I'm suddenly running a little lean..? Any ideas would, as ever, be much appreciated.


EDIT: It appears I've dropped this in General Discussion. Could a friendly mod please move this to Mini Chat..?

Could be a number of things, nothing is ever that staright forward.

One item I'd be checking is yr coil output voltage, if it's too low it will behave as you have described. Also dissy cap, rotor button & HT Leads.

Not easily tested in an accurate way, but a road-side type check would be to see how big a gap the HT will jump at the plug, ideally no less than 10mm.

_________________

I'd have a look at your electronics first - it's the easiest thing to check through for worn parts. I also have a SimonBBC red ignition module on a 43D, with no problems whatsoever.

How well does it idle? I tuned my 998 to idle fine, but it was running lean at higher revs. I took it to a mini specialist for a good tune - the cheapest jump in power I've ever had. It sounds like it could be running rich rather than lean, judging by the sooting of your plugs. The engine will misfire when running rich, though usually only at low revs or when taking the throttle off.

Also, check your dizzy advance. With a 43D you don't have a vac advance, but it's a good bet that the advance curve is off, perhaps further off for the lower revs than high.

If your plugs are carbon black or sooty black then it's too rich. To me it sounds like entirely isolated to carb needle. When you are part throttle or high RPM it's smooth? It sounds like a missfire at low rpm because it's choking on too much fuel. Is there any backfiring if you stomp on the pedal?

_________________
Lillee - 1969 Morris Mini K

Thanks for the replies guys. Much appreciated. I've just pulled the plugs to inspect them. #4 needed re-gapping (it was a little wide, all are now confirmed at 0.025"). #4 was a little sootier than the others, but even it had a clean electrode. The electrodes/insulators were (generally) tan. I haven't been out today (not sure when it'll get its next run, poor thing!), but I'll do the same again after the next run.

GT, I'll definitely check the HT spark jump - one night. Easier to see then.

Idle is generally pretty good. The only issue I have with it is how low it is without the choke - I've upped it, but it's still low enough to be lopey and feel like it's going to stall without a blip of the throttle - especially if it's not really thoroughly warm.

In terms of dissy curve... The tune up 18 months ago was immediately after fitting the Red Ignition Module. The purpose of the tune was to confirm timing and needles - I know the tuner tried 3 different types of needle. He has been tuning and racing Minis since the 60's and has a good name (including on here - rightly much love for him), so I'd have thought he had it pretty right.

The missing yesterday was only on part throttle. It only missed one time when I had it flat, I shifted out of second (which had been flat - pulling out of the school back onto the highway) and it missed through third. The rest of the time that it missed it was little throttle, light load, constant speed, low to middle revs, flat ground. If it'd been missing under load, I'd have immediately suspected the electrics (yet another set of fouled plugs). But given it wasn't really under load... I just wasn't sure.

Oh, and just for some perspective, the longish run I gave it yesterday? 6.2km out, the same back. First run in 3 weeks. 6.2km? That's a really long run for my Mini. The previous 'run' was 3 or maybe 4km total. I know, it's not really a long run, but I really don't drive that much. The benefits of working from home. I guess there's got to be some payoff for working all hours of the night...

I might go for a run in the morning and see how it behaves... Try stomping on the pedal, etc.

Maybe it just needs a bush mechanics tune up.....
Flat chat for 100Ks

You may be right, MG... Car hasn't left the garage since this was posted.

I did a spark test, it was jumping about 8mm, but... It was a dodgy test. Car had been sitting nearly a week, it was without the car running (i.e. all plugs were disconnected, engine not running, cranking over on the battery)... And, given I've got 90º leads (at both ends!), I had to stick a short piece of 6mm wire (50A) in the end of the lead to give me something to short across.

So, no real news. But I'll post some when I've got some. Even if it's a massively outdated thread by then. There's nothing worse than finding a problem without ever learning of the solution...

Throttle spindle worn? Leak in the vacuum advance somewhere?

_________________

Well... It's been a week since I last posted in here. Which means that I finally took it out yesterday.

So, since the problem run, I'd had the plugs out and cleaned them. I haven't inspected them since the last run, but the car was definitely different this time. This time was another school run, so 6.2km out, 6.2km back. Car got up to (water) temp about 5km into the run out. Parked for about 10 minutes to pick up the boy, then drove home. Water temp was still indicating it was up to temp when I started the car to drive home. School's on a highway, so it's straight out into a 100 zone. Thought details of the type of trip might be helpful to understand when things happened.

It didn't miss on the way out. It was happier than the last trip. No missing on partial throttle, well, not true missing. The hesitation was similar to the last drive (first summary point of first post - slight throbbing at 60), but there was nothing worse than that. There was none of the missing at 70 that I had on the last drive. I've realized that the throttle's only down a couple of millimetres (literally!) when running along at 60 or 70, so there's definitely a lot of vacuum in there. Generally, it felt really strong.

The car did miss on the way back. Not a true, hard miss, but a half miss. After starting the car up, it was still up to temp. We got out of the driveway to the highway, and it missed through 1st, 2nd and 3rd. It only missed with wide-open throttle, but it wasn't a hard miss. There was no backfiring, it felt like it was only burning half the fuel, rather than all of it or none of it. That was the only time it happened - I found a few opportunities on the way home to open it up (took a slightly longer way home) through first and second, and it didn't miss again. The only time it missed was when it'd been started hot (no choke, just managing the idle with the throttle) and it was given heaps. Which is needed, because visibility's not great, and it's the highway...

GT, I think the throttle spindle's pretty good. I could easily be wrong (not that experienced with carbs), but it hasn't had that much use since it was tuned, and I'd expect that an experienced guy playing with needles would've noticed any noticeable wear there. Last I checked it, I couldn't feel any slop. But I'll check it again. And I'm running a 43D4 - no vacuum advance. There's a vacuum hose to the brake booster, but I don't imagine that'd be a problem..?

I'm starting to think that a combination of fouled plugs and decent vacuum when running a constant (low) speed may well have been the problem on the trip that started this thread. This trip? I don't know - maybe the plugs cooled off too much, I don't know... I'll monitor it. And maybe drive it a little more - and a little longer.

Well, problem still not resolved. But I've been driving more regularly - using the Mini to do the school run, so it's doing 25km a day now. So I've got more opportunities to ponder it and try things. Such as new fuel.

Today, I cleaned my Ramflos, adjusted up the idle, and also leaned off the needle (I tightened the nut) by 1 flat. It's not a huge miss, I figure that, if it's that the car's too rich, it shouldn't take much adjusting to fix it.

So why have I leaned the car off, given I originally thought I might've been a bit lean..? Well... It was suggested that I pull the choke out when it started missing as a test. This gave a definite result - the miss very quickly became very much worse. If I was running lean, richening the mix with the choke should've cleared the miss. Why might this be so? Well, when it was tuned, much attention was given to getting this part of the rev range right. But even then, it was still a very fine tuning exercise - there wasn't much margin for error. Since it was tuned, I've moved 2,000'. Vertically. Which changes atmospheric pressure by 1.05psi. That might just be enough to make the mix slightly too rich. SU's website states that the needle should be replaced with a weaker needle if you are above 6,000' (or in high heat or humidity - it's been rather humid up here lately), but standard needles on standard engines have much more margin for error than my current needle and engine. Thus I've leaned out - just slightly.

I'll post the results.

What is the timing like Tadhg? Do you know how much mechanical advance you are getting with your dizzy?

_________________
"In two years time your car will be like a lady's clothes, out of date, my car will still be in fashion when I am dead" - Sir Alec speaking to Pininfarina

No idea on the timing. But I wouldn't have thought it would've moved too much after it was set up..? If I don't get rid of the miss (which really is usually quite light) within a couple of flats, I'll look at the timing, too.

Ah, yes, now we see a bit more of the picuture here. Yes, you will need a slightly leaner mixture. For a stock say 8.8:1 compression ratio, generally they are less sensitive to altitude until you get up around 1600 - 2000 metre mark, but - in particular - one the compression ratio have been raised it becomes more sensitive. You maybe able to adjust it with the mixture nut, or it may need a bit more serious work with the needle itself.

_________________

I've done the school run this morning, and it missed less. Much less. Not quite right, but much better. So, another flat leaner (when it's cool), and we'll see how it runs this afternoon.

Thanks to all who helped - especially GT Mowog. Much appreciated.

Your welcome Sounds like your heading in the right direction

_________________