https://www.youtube.com/watch?v=KlJM4UhbQ7A

Dan, I can't thank you enough!  Turned-out to be the air cleaner gasket being upside down, thus blocking the vents.Wow! What an adventure.  I've learned a great deal about my little truck, and SU carburetors.  I checked the timing first, just to make sure it was spot on, and the advance was performing as it should.  When that checked-out, I went for the air cleaner.  Not a lot of room in these little cars.  I've acquired quite a collection of cushions and padding that help my knees, and keep my arms from harms way on top of the engine.  It's been quite a journey. New carburetor. Stuck needle. My fault from sitting too long, and then a stuck needle jet, also probably my fault from the little car sitting for too long a period without running.  In the end, I've had fun, and learned a great deal.  Thanks again Dan, and to all those who offered help, "thank you!"

The way a vacuum-advance distributor works is that as the rpms increase, the mechanical advance moves the spark farther in advance to start the air/fuel burn sooner to get the biggest bang at the right time of the piston stroke - the burn isn't instantaneous, but progressive (in milliseconds). A similar distributor without vacuum advance works the same way. BY disconnecting the vacuum advance, your distributor will function like the non-vac-advance distributor. At idle, you would get only the preset initial timing advance (e.g 8 -12 degrees). That is why you need to set the timing at a specified idle speed for your dizzy so that the mechanical advance does not activate AND disconnect the vacuum advance so it does not activate either.
Remember the vacuum tube needs to be plugged at the carb so you don't have an air leak.

Note your engine does not need any advance to start - both mechanical and vacuum advances are not active at idle. (Even a 123 electronic ignition system provides zero advance until the engine reaches 500 rpm, whereupon the digital advance curve kicks in, with or without the vacuum advance option.)

On a road car, vacuum advance is actually a fuel saving device. Race engines don't need or use it. It allows the engine to run at idle on a leaner mixture than it normally would. On a road car with this functioning properly, when you are stopped at idle, the vacuum advance is working hardest. As you open the throttle (reducing the vacuum after the throttle plate, AKA manifold vacuum), the vacuum advance eases as the mechanical advance takes over with increasing rpms. So, at full throttle, yes the vacuum advance would be off and the mechanical advance would be on according rpms. When you punch the throttle, the effect would be the same as if the vacuum advance was disconnected.
I'm guessing you will still have the stumble.

Stumble is either incorrect air/fuel mixture for the engine speed or incorrect timing for the engine speed, or both. It is never a good idea to mess with both at the same time. Since you are not having  much luck with the carb, verify your timing is correct for your engine AND your dizzy.

Disconnect the vacuum line and plug it at the carb. Set the idle speed to spec, according to your dizzy. spec. (It may be different than normal idle. Set the idle timing to spec using a timing light with all techie features turned off - you want a raw reading. Test the mechanical advance to see if it moves the timing mark appropriately. The timing mark should rise off the scale as you increase rpm. Let it go back to idle. reconnect the vacuum advance - the timing marks should immediately go off-scale again. As you slowly open the throttle, you may see the timing mark drop until the mechanical advance kicks in, though it may not come back onto the scale. 

If either the vacuum or mechanical advance are not working properly find out why and fix it. Fly-weights might be stuck, return springs may be broken or missing or the vacuum unit may be seized.

Once you are sure the timing is functioning properly, it's back to the carb.

FYI, both my old HS4 and my spare HIF44 have the vacuum advance ports just after the throttle plate, so the effect on the operation of the vacuum advance would be the same. They read manifold vacuum as typical for a road car.

I've been out of town for a while, but I wanted to get back to this thread.  Tried changing oil, but to no effect.  I'll try disconnecting the vacuum line to see what that does.  After that, I'm taking the carb back off to check that the air ports aren't blocked.  I should be able to report back by Sunday evening.  I hate leaving this type of thread unfinished, as someone later may have the same problem.  Thank you again for all the replies! 

You're probably losing all vac-advance when you open the throttle which won't help diagnose issues. 

HIF series carbs are normally manifold vacuum rather than ported, so when you open the throttle it's 99% certain that only the mechanical advance is working.  Try disconnecting the vac advance altogether and going for a test run to prove either way.

No. A lighter weight oil would allow the damping mechanism that Dan described to not damp as much. If trying a different oil, try 30W rather than standard 20W SU oil. You would first need to suction out the current oil with an small syringe.

Are you sure you have enough of any type oil in the carb?
If not sure about the fill level, just add some more. Any inadvertent extra will get harmlessly drawn out into the intake.

If none of the things Dan has suggested or I have suggested work, then why not try another needle? My 1275 did not bog with the stock BFY but it did run better a richer BCE. 

I can't help with the oil viscosity question - I've never had any SU oil or ATF fluid to try.
People have tried everything from motorcycle fork oil  to 20W50 motor oil (early Mini owner's manuals apparently recommended the same weight of oil as used in the engine). I believe some people blend two weights until they think it is right.

I have to also confess I don't know for sure what weight is in my HIF. I think it might have 5W30 oil in it.

This little truck is running a cone K&N.  There is just enough room to install it, but it looks like I'm taking it off again to check for the ports.  There is a thick rubber gasket between the air cleaner mounting plate and carb.  Maybe 1/8" thick. Never even thought it might be blocking a vent.  Maybe I'll try a lighter oil first. Would ATF be a good choice after the SU oil? The bottle isn't marked with any particular weight, but I believe ATF will be a lighter viscosity.

Yaaaaay!

Glad you got it working so much better.

The throttle hesitation you are now getting can be due to the weight of oil in the dashpot damper. The damper slows the rise of the dashpot piston as vacuum tries to lift it, giving you the accelerator pump effect. When the rise is delayed but the engine is sucking on the carb area between the bridge (at the top of the jet) and the bottom of the dashpot (where the needle is) it puts added suction on the jet and you get an extra burst of fuel being sucked in, even though then needle hasn't moved. Then the dashpot begins to rise against the flow of oil in the damper and you get a momentary richness as the engine rpm increases. When the dashpot gets to the level suited to the amount of throttle, the damper keeps it from bouncing up and down, randomly messing of the air/fuel ratio.

Another thing to check:
You did not mention what type of air cleaner you have installed. If it is one of the cone or pancake type bolted directly on, you need to make sure the air ports on the face of the carb in the picture are not blocked. I'm not sure which one does what, but I believe one of them is the relief vent for the bell of the dashpot. If it is blocked, the dashpot piston will have trouble moving. If you are using the SU elbow and a convention air cleaner, the elbow will allow the vents to function, though the gasket also needs to have the appropriate openings.

Jeez!  Learning a great deal about 'SU' carbs.  So, it turns-out that moving the mixture screw in and out had no effect.  Couldn't understand that at all, until I realized the needle jet wasn't moving up and down. Once again I pulled the carb, and removed the float bowl.  The jet was frozen in place, so moving the screw did nothing. Took it apart, cleaned, re-set the float, and voila!!!  The little truck has come to life. I actually ended-up backing the jet down considerably, enriching the mixture. Now I can drive without popping through the intake, and without engaging the enrichment circuit. Incredible!!  The engine still can't handle quick full throttle.  Now that I have the car running way better, it's time to try and dial it in.  It can handle significant throttle, but bogs down, for a while, if I stab the accelerator.  The truck is a totally different little beast.  It runs 100% better, but I'm thinking there's room for improvement. I've got an Innovate Wideband tool, but the cable isn't long enough to reach from the tailpipe to the cab. I think that's my next project. Many thanks for all the replies. These SU's are a totally different animal than anything I've played with before.

manila said "I can't understand why changing the fuel mixture has no effect. "

When you remove the dashpot and check the level of the top of the jet, does it move when you turn the mixture screw? As can be seen in the photo of the carb guts, the bottom of the jet is hooked into the slot in the lever. The lever also has a small but hefty spring under it that its mounting screw passes through. That is the fulcrum for the lever's movement.

You really need to install a wideband o2 sensor & gauge. Tuning the carb without one is just guessing. Is it going rich or lean when you open the throttle and it has no power. With the sensor you can tell what you need to do and really dial in the carb.

I thought vacuum leak for sure, but I can't find one anywhere. My new carb came with a BFY needle. the guys in England said it was the one for my set-up. The mixture screw phenomena bothers me.  I can't understand why changing the fuel mixture has no effect.  This carb also came with a red spring, which I understand has the lightest pressure @ 4.5 oz's.  Maybe I should try a Yellow @ 8oz's.  I have 3 additional needles.  A BBC, BFF, and BFT.  My SU carb books says the HIF44 is correct with a BFY needle and the 'red' spring.  Arrrggg!

The OPs symptoms sound a lot like a vacuum leak.

Kelley

I don't know which spring would be correct - it was a long time ago for me and it is -15C in the garage... too cold to go digging! I'm thinking I used the shorter, larger diameter spring,

Idle mixture on a HIF 44 is counter-clockwise (out) to lean and clockwise to richen. That is because it pushes against an L-shaped lever, When you turn the screw in, the lever pulls the jet down, making it richer. If you have screwed in the screw all the way, you'd be running very rich. Remove the dashpot and re-adjust the main jet flush with the bridge, then turn the screw in to lower the jet 2mm-3mm. Reassemble and try to adjust mixture.

I added a pic to show the lever connected to the bottom of the jet. (Saves you having to take the bowl apart.)

The second aggravation is that the lever has a bi-metallic component, that flexes as it warms to lean out the mixture as the engine warms up. When you try to adjust the mixture with your car idling, the carb continues to get warmer and warmer, and the lever continues to flex more and more, leaning out the mixture more and more. You end up chasing a moving target. (I could never win that game.) Maybe set it up, get it running and do little tweaks between test drives.

The second picture, by somebody else years ago, shows an original lever on the right. On the left is a modified lever  that eliminated the bi-metallic bits. He (I think) cut away the bi-metallic parts. Other people have replaced the bi-metallic bits with a new piece as shown in the drawing.

I took a different approach and made a new lever entirely out of stainless steel shaped and folded to match the shape. (Stainless steel is HARD to work with hand tools!)

What needle? A 1275 should use a BDL or maybe a BDK.

Checked the damper and spring. I have (1) spring, and it's red.  Also this HIF44 came with an LZX 1509 damper.  I have no idea if these parts are compatible with tuning a 1275 at sea level. I'm also using SU oil, which seems fairly light.  Perhaps not as light as ATF, but I don't know if that's a problem for me.  I'm not at all familiar with these carbs   Is the red spring the heaviest? Given the symptoms I've explained; does it sound like I might have the wrong spring, oil, or damper?  I'm still a little bewildered that adjusting the mixture screw seems to have effect. Thanks again!

Dan, Thanks for the reply.  When I got the new carb, I disassembled it to check the float height.  I never thought to remove the top and check for spring(s).  I'll do that today.  It was alarming that I was able to move the mixture screw, in and out, without effect. I"ll get to work later today, and repot back.

You say it is a new HIF 44.
When I bought a new HIF-44 years ago, I had trouble with too. I eventually discovered that it had been shipped with two dashpot springs in it. Once I figured out which one to use (it was the "other" one), things went much better for me.

Engine stumble as you describe is likely inappropriate "accellerator pump" effect from the dashpot reacting too slowly or too quickly.

Wondering if anyone can help....My 79' p/up, with 1275 motor falls on its' face when the accelerator is pushed hard.  Backing off slowly, the car starts to accelerate.  It sounds like it's sucking a lot of air.  I've re-checked the timing, and tried to adjust the fuel mixture on my new carb. HIF44  Moving the screw in and out does little to nothing.  I've checked that the needle jet is actually moving when I turn the screw in and out.  Car feels very low on power.  Pops sometimes through the carb under acceleration.  Looked all over for vacuum leaks.  Frustrating... Thanks in advance for any ideas.