Yes you were.  I had total tunnel vision. Checking the valves was something I should have done right after buying the car.  I did discover it has dual valve springs.

Now thinking about it.  If i were actually running out of gas it seems like it would have been more jerky while missing.

Hopefullly a long test drive this afternoon to confirm. If I have the time. 

OK... I was close!

Valve lash is important, as I found out.
Back when I was a kid I had a 1926 Chev 4 paddock basher. Dad bought 2 for 8 quid.

Mine developed a problem where it would run hard until engine was up to temp, then die. DEAD, no starty. Let it cool, restart, no problem. Rinse & repeat.
Finally traced the problem, it had near zero exhaust valve lash when cold. When it got hot, the ex valves all opened; = no compression, fire goes out.

[edit] typo

I agree. I took a test drive with a spitfire guy today.  He was surprised to see how strong it pulled just to go to pieces at 55.  Then he asked me if i'd adjusted the valves.  When I said no he looked at me like I was crazy and told me to check them.  After adjusting them I took it out and it was the first time i'd been able to cruise highway speed for any distance.

Hopefully I'm not jumping the gun here.  I'll be able to go for a longer test tomorrow. 

hmmmm....

in first post, you said it ran like a bat out of hell until 55

if you had two valves out of adjustment that much to rob power, I would have expected lower rpm symptoms as well, incl idle

Okay.  I think i got it fixed.  I adjusted the valves and the problem vanished. two were way too tight and the others needed minor adjustment.

"Why didn't you check that along time ago dummy?" you ask?  I guess I was just convinced it was running out of gas.

Thanks for all the input fellas!

Now time to replace my clutch and main seal.

Well I got as far as installing a tach, but the Moke is sort of trapped in the garage so not had a chance to try the new coil.  I installed the 4-7 PSI Mr. Gasket "Mini electronic" fuel pump after converting the Moke to negative ground.  Note that I have a "gross jet" needle and seat in my HIF44, as that much fuel pressure can push right past the standard items and inundate your sump (trans) with gasoline.

Before installing the pump, The Moke would "run out of gas" in certain conditions like full power uphill passing lanes.  Now I just get the 'flutter' that feels like a governor kicking in to spoil the fun.

This is the 1345cc high compression engine I built with the 4-synchro remote box running A+ gears and adapted to the original magic wand shifter.  With a 3.44 final, this Moke is "stupid fast", and blasts up to 60 like very few non Honda Minis!

The tachometer will give a lot of clues if it is electrical as you will see the needle fluctuate.

tends to make me think this is a fueling problem, not electrical,

but of course still could be either

It revs no problem in neutral. pulls hard until I reach a high rpm cruising speed in 3rd or 4th.

I'll see about hooking up a tach.

It's got a centrifugal advance.

I'll try the aeresol trick.

Also pulling the choke. 

And I'll double check the dashpot and guide, but I've had it apart more than a few times and it moves freely.

It's strange because this is a 100% consistent problem. 

Have you checked for vacuum leaks?

Spray some brake cleaner or some other aerosol at the manifold mounting face while idling and listen to what the engine does. I didn't read every word, but see if the vacuum advance diaphram is possibly no good (take vacuum line off of carb and suck on it. If you can continuously suck air, the diaphram is bad).

Another check: Remove the dashpot and check the little screwed-down metal guide on the carb body. It's #13 (Screw) and #14 (metal tab) in this pic

Also check the groove in the dashpot slider.

I had that screw come loose on a car. The tab became loose and would start to cock in the groove on the dashpot and act like a horrible flat spot at the same spot high up in the rev range. It took me a long time to find that as the problem. Tightening the screw only helped a little. There ended up being some rough spots created in the channel on the dashpot that would still sometimes catch. A combination of filing, sanding and then steel wool got it working great again.

Good luck.

[edit: Also, if your choke cable is hooked up, go ahead and pull it when your are driving (assuming you don't have a leak in your choke system). If it pulls when you pull the choke, you may be running lean. 

Do you have a tach in the car? If so see if it suddenly drops when you have the issue at 60mph.. if not maybe add one temporarily to see if you are loosing ignition..

I have a fuel filter just like that. Fuel level looks like that. No problem

PS: if you rev engine in neutral, does same problem occur, or only under load in gear?

ARGH! New pump & new Needle. Still a misfire at 60mph.

I just stuck a borrowed electrnic 4psi pump under the hood for testing.  No luck.

I did notice this when I put a clear fuel filter in.  Lots of air in the fuel filter? See how low the fuel is below the out side.

Could this be my issue? Could I be sucking air somewhere?  There are NO gas puddles underneath the car.

Grasping at straws here!

You said "Pump seems to pump fine." The word "seems" could well be prophetic. In my former Cooper S I had running problems twice. One was the fuel pump which had a pin hole in the diaphragm where the motor would cut out after higher speed driving. At street speeds enough fuel was being delivered to not cause a problem. The other time was with an HIF44 with too lean needle where the motor would just not rev beyond 5K rpm. I could run all day long at 4K rpm.

my plan now is to borrow an airtex 2.5-4.5 psi fuel pump from a friend and replace all rubber hoses and try the BDK needle and see if that does the trick.

Is it ok to mount the new fuel pump under the hood just before the carb and leave the su pump in place for testing?

Thanks and I'll let ya know how it works out.

I had a similar sounding experience that kept my '76 Mini 1000 Canadian LHD in the garage and frustrated me for months.

My mini had been significantly/impressively restored by previous owner(s) and repowered with a 1275 perhaps of MG heritage. It has the HIF44 carb. The engine could pull hard but would not operate at sustained freeway speeds or climb reliably without an erratic unpredictable lean misfire. Just backing off or manipluating the choke could effect the problem but on occasion the engine would just quit and I'd coast helplessly to the shoulder.

Put the car on jackstands and started at the fuel tanks and worked my way forward. Found a number of issues along the way but the one that I believe effected the high speed misfire was found with the carb completely disassembled. 

The jet bearing/bushing in my carb was white plastic with a metal bushing. That bushing was loose and it was my thought that during operation air flow could unseat it and effectively lean out and choke the engine.  

Mine has the BDK needle.

Replacement may have come in a carb kit, may have been ordered individually but it was also of plastic/metal construction.

Didn't intend to write a novel but that's my experience. I did leave out the chapters about replacing the wiring harness, ignition switch, fuel filter and pump.

Good luck sorting it out.

Jemal which Mr Gasket pump did you use?  The 4-7 psi or the 2-3.5 psi.

Im not sure if this could be a solution to the problem but this was my experience with a similar issue.  I originally outfitted my 45D4 full centrifugal advance distributor with a pertronix 2 ignitor with matching coil and my engine performed just as you described.  I returned it thinking it was a defective unit but the new one did the same.  I then went with an ignitor 1 and hadn't had a problem since.  It might be worth looking in to.

Jamal.  I'll be following your progress. Please post if you find a solution.  I'll do the same. I think a BDK needle and possible fuel pump is my next try.

 good luck!