MLM:
Think about the symptoms - they occur more or less consistently above a certain rpm. That pretty much rules out a slipping or loose timing chain or any of the linkage to the valves or dizzy. If the chain slipped,it would stay slipped. If it is slack, it would have the same negative effect (late valve timing) at all rpms. Similarly a worn cam would always be worn. A broken valve spring would stay broken, etc.

A weak valve spring would show up as valve float above a certain rpm - it feels like you suddenly lifted your right foot, but would settle down again once you got below the rpm range.
A sticking exhaust valve would probably stay open, producing valve rattle and likely continue to overheat and get even more stuck. But it would occur once the valve got hot enough, and not at a consistent rpm. You might be able to replicate this situation by causing the engine to get very hot without getting to that rpm - block off the rad or work the engine hard uphill etc., staying out of that rpm range.
I'm still thinking it is a fuelling problem and/or a timing advance problem.

Do not click on the link or the username, it's a corrupted file.

mod please delete his post and account...ty

ps: DM, click the link and tell me i'm WRONG .




tony

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I hear you!  I'm not too disappointed about the money on parts i've spent so far because most have been upgrades.  although i'm still upset the problem persists.  I thought for sure it'd be a simple fix.  but dang!  i'm becoming more convinced it's something internal.  I've tried all the usual suspects.  

I've discovered a few little mistakes made by the previous owner.  the last being the springs put on incorrectly(on the outside) on the rear brakes- making them flopped out- and rendering the handbrake useless and destroying the shoes.  plus the bodgy springs on the carb.  that's next.  
who knows whats going on internally.  I don't mind working on the car. I enjoy it to an extent.  but this isn't fun and i'm ready to enjoy the car.  if the fella with the engines for sale is reading this give me a good deal on that complete engine!  
thanks for all the advice fellas!  we will sort this thing out!

In case i did not make myself clear the first time don't waste your time and money replacing the timing chain. Just throwing parts at the car is not going to help you clear up the problem, well it might but you may never get to change the correct part and just be frustrated and out a lot of money. 

I'm with you on this.  C'mon Powerball!

In lieu of that, you MUST find the gremlin.  No fair buying a replacement 1275.  You have to slay this monster because we all want to hear the outcome.  It's not about you.  It's all about US

But it's not the timing chain. 

all plugs are a nice brown.
I'm not sure exactly how to do the screw driver test.
but i have misted the cap and wires with water and didn't see any arcing.

No news to report.  The engine wouldn't fail in the driveway the other day (but did on the road) so i was unable to check for a loose rocker. 
No time to do anything major until March.  But I believe my next job is to change the timing chain. I also have an extra head off an MG Midget I might try.  
I'm also at the point of keeping my eye out for another 1275cc engine. Then putting this one on a bench to sort and eventually putting it in my ratrod.  
the two listed in the for sale section here are just a little more than i'd like to spend right now, but the powerball is tonight and i bought a ticket so we'll see how that goes. 

Any news to report?

thanks to everyone.  work is busy right now. i'll get to all suggestions asap and will report back.

I don't think the smoke is coming from the filler cap - everything is wet (slam panel, coil, plug wires etc.) as if the engine had recently been sprayed/cleaned.  My guess is that the smoke/vapor/vapour is just water evaporating off of the diff cover.

You mentioned changing the valve springs, so I'm curious to hear which rockers are you using?  Also, what springs did you install and what did you adjust the valves to?

To my ear, it almost sounds as if your rockers are striking the inside of the rocker cover - relatively common when using the minifin-style rocker covers, especially if you use a wrench to tighten the valve cover - hence the benefit using of T-bars and knurled nuts.

I also didn't hear any 'miss' at 4K rpm, so I suspect it's a basic tuning issue, perhaps incorrect needle, perhaps unsuitable dizzy advance curve which is hindering performance at higher rpms under load.

My 2 cents is that you really should try to find a dyno.

I would disagree and say if it was enough smoke for you and me to see on the video a wisp of it coming out of the closed valve cover cap, then
(1) was it really coming out of the cap, or just swirling around from a oil drip? 
(2) if really coming out of the cap, and it was closed, the only way the smoke could get out was if this was a vented cap (small hole) that should be allowing fresh air in, not blowby out. The normal small amount of blowby is taken care of elsewhere.
(3) did you try the suggestion of running it (especially while warm) with the valve cover cap off and observing how much smoke came out? Report back on this.

It is easy to overlook things when you get frustrated.
You said the valve springs were replaced was that done with the head in place and if removed were any valves replaced ?
When you take the valve cover off and hopefully find the stuck open valve (usually an exhaust) keep an eye on it and see how long it takes to re seat itself, by watching this it should confirm a sticking valve or a valve seat problem.

Not a lot of smoke.  really barely detectable. 
I have another 1275 head. It doesn't have the extra 2 bolts but i don't think that's a problem.  I'll clean it up and install it one of the weekends.  hopefully sooner than later. 
Thanks again guys!

you mentioned in your first post ( in this thread) that there was smoke coming out/around the valve cap.
I could not tell on the video very well.

If you remove the cap while engine is running, does smoke pour out? does it increase with rpm?
this means either:

(1) abnormal high crankcase pressure from blowby from bag rings/cylinder or severely worn exhaust valve guide that is pressurizing the space under the valve cover. This I have seen. I suppose even a "slightly" blown headgasket could do that with communication of exhaust into an oil drain hole leading up to the valve cover space. Doubt this latter scenario, but head needs to come off anyway.

(2) plugged up crankcase ventilation system ( but that would not cause all your rpm and noise problems, and would cause leaking oil seals everywhere  and be a mess) . This I have seen.

Malsal.  Thanks for the tip.  I'll try this as soon as I get a chance.  I thought I had mentioned the noise but perhaps not.  It's been a long thread.  Sorry if i've frustrated some of you but believe me i'm frustrated as well.  

I have to say this getting silly, it's been like pulling teeth to get useful info from the poster and now all of a sudden we have a major engine noise that wasn't mentioned at all over the course of this thread until the video pops up! How do people expect help?...........

In learning my way around an A series.. I once did a valve adjustment and forgot to properly nip up one Rocker nut..

It only took 2 miles for that nut to wind loose= and the tapped adjuster to also loosen up.
That then took valve away on that one cyl.
  

Sure it ran = but on 3 cyls and Horribly! 

 
The loose rocker and the loose lifter clacking around sounded ALOT like what you have. here

Malsal describes the prefect way to check this out =  get it to happen. Shut er down =  Look for loose rockers floating around.
 As advised as described it sounds like the valve is binding open then you get the clack clack - but after lower revs the valv closes again returning lower rpm's and Idle.

You may be looking at replacement Cyl Head= or a full rebuild. 

I agree with the 'fuel starvation' theory.  While watching the video that you posted, I couldn't see a clamp on the fuel line at the outlet side of the fuel filter.  Long shot, but is it possible that at higher RPM that you start sucking air through the fuel line to the carb or would a faulty connection be indicated by fuel leak??  Don't know.

Well as they say "a picture is worth a thousand words", we should have asked for a video sooner as i don't remember any mention of an internal engine noise.
I also suspect a valve train issue, there may be other issues but the noise is the one that i would concentrate on right now. Run the car with the valve cover sitting loose with the retaining nuts removed (cuts down on the messiness) then when the stumbling and that racket starts shut it down remove the cover and check the valve clearances asap.