Odd problem.

I think I just found the problem.

My idea with the long intake was something akin to the Sonoramic Commando engine on Chrysler products.. And I just looked up the torque curve on the 361SC V8 and noticed something interesting.
The torque curve compared to the normal v8 is completely different - the SC curve climbs steeply to 2900rpm, levels, and drops like a stone at 3600rpm or so where the normal curve ends lower but just continues to a much lower maximum as engine speed increases.
The interesting part is all the way up to 68mph the car is noticeably quicker - but at that point it hits the mother of all flatspots. Which happens to be 3800rpm - just when the SC engine is beginning to nosedive.
I think what is happening is to do with the resonator - at idle it has zero effect (so rich mixture is not required. At slower speeds it's just the resonator effect (which has been dealt with by adjusting the mixture) but at just the right speed the ram air effect & the resonator combination causes a wobbly to be thrown and a temporary weak mixture - which the SU can't compensate for because it doesn't have an acceleration jet like a Solex or Zenith - but the slight choke can because it reduces air slightly. The resonator can work like that and it picks up again.

This causes problems. I can't run any richer at idle because 4 flats you can smell unburnt petrol at idle - so three is about right. I'm not sure what to do at this point. One thing I noticed was that it always misfires on the late side - it sounds like fire-fire-miss-fire or fire-fire-miss-miss - I seem to remember that the second of each pair runs leaner (unequal scavenging) - and a firing order of 1342 would be first-first-second-second.

BUT

It does create an interesting possibility for the EFI car. That will have fly by wire fuel metering - so running a 30" intake pipe as on my car could have very interesting effects. There shouldn't be a flat spot because the engine is actively metering its fuel and the resonator might give a noticeable performance improvement without too much expense. It might not help fuel consumption though.

Have you checked the voltage at the coil low tension supply with the engine running both at idle and at higher rpm. Given your headlamp, flasher and gauge issues there may not be enough current in the coil at higher rpm if the voltage is low. It could also be a defective coil, it is claimed many apparent SU carb issues are actually the fault of the Prince of Darkness (ie Lucas).

David

It's probably worth getting that checked but I'm not sure I can replicate it parked up. I'll have to look in the manual if there is a method to do it, a recommended value, and then find someone who can. I have my suspicions that the spitfire cable might have something to do with it too but I'm loath to put the original back on because the engine sounds so miserable running that - but it might be worth a short test.
It'll do 60mph perfectly happily now and for 99% of what I do that's adequate. I drive to the speed limit as my instructor taught me (advanced police drivers driving instructor). So being wheezy at 50—55mph really wasn't making for a nice experience - especially with BMW driving idiots rammed up my boot constantly.

As you have repeatedly droned on about. And as I have repeatedly droned on about in response - that's *exactly* how it was until I replaced the air filter that was clogged. NOTHING else has been touched.
*if* I pull the long intake tract and do the same run as I did yesterday I am 95% sure it will have gone right back to performing as it did beforehand - with a nice flat low torque curve and the right fuel mixture.
As I said - I will get it looked at - but since I haven't mucked about with the timing and no one else has either since the thing practically has a prod-nose security guard watching it 24/7 - it's not an issue.
The 800-1000 slightly rich idle 3700-3800 drop out fit the way a helmholtz resonator intake alone would affect this car perfectly - it's why with the modern ones the short tract is used at low engine speeds and high. The gain in torque the car shows in mid range is noticeable & expected.
I'll say it again just to be clear. I drove this back 300 miles on knackered tyres, with half deflated suspension with fuel piddling out the fuel line & it sat happily at 70-75mph for 75% of that and would have sat there all day. I will have the timing checked out and as I said before you are probably right and it has been set up weird. But I am *not* going to be playing with it when I don't know what I'm doing and it is emphatically not the cause of these issues which are all related to the following

previously blocked air filter
damaged air filter can
carburettor tuning with blocked air filter causing lean mixture.

The multispark cable may be affecting this slightly (I think as the engine speed increases and there's less time for flame propagation it may start to cause issues with the resonator, where it didn't before) but as it's protecting the already rather knackered engine I'm only going to do one high speed test with the short one before it goes back on and stays there.

There is a tuner locally that works with classics including SU cars and when my health is in a sufficient state and I've some spare cash it will be looked at, but please try to understand whoever set that engine up isn't me, I haven't interfered with it, and even first engineering principles state, if you have a problem look at what has changed, not what hasn't. As for the 70mph power issue it's *exactly* what's expected of a one point adjustable carburettor engine with a resonance system fitted. The 361/383 Chrysler engines did the same & the variable length manifold was designed to solve that problem which it does admirably but since they aren't an off the shelf item for the ADO17...
I think however this system will work well with the 1800DPFI car because it's got on the fly fuel metering. It'd work better also with a Solex or Zenith type carb because they have more than one tuneable jet (I'm not sure the spec of the SC carbs but they definitely aren't of a SU type, I think they're Holley from memory).

Hi Rosie.

Tony Wood has the air filter cover retaining bolt. Contact on spareathought[AT]landcrab[DOT]net.

Also remember that the inlet pipe should be behind the engine, pointing at 8 o'clock or 6 o'clock - see the marks on the front face of the air filter box. There should be no "ram" effcet into the air filter.

Chris.

It was the fuel.

Filled up and it immediately sounded different - it's still not right but swapped the coil cable over to the spitfire one and it only misfired once at 65mph with my foot nailed to the carpet - before I could barely use 1/4 throttle @ 50mph before it did that. Accelerates up hill like before and although it's still not liking motorway speeds, it's starting to complain and balk at 65 as opposed to spitting and misfiring, I'm pretty sure that clearing this tank out & then refilling with V Power it'll be back to normal. The engine temperature was up to its usual position too.

In short, don't use Tesco Momentum fuel - you'll probably have problems with it. Shame I had to spend £50 and several days of agony for a non problem but there you go. Anything with ethanol is a *bad* idea.

Come on Mr P. Don’t try telling us you spent on premium petrol.
You must have picked the wrong pump by mistake
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