Looking for Healey HD8 SU Guru.

Cross posting from the Classic Motorsports board since there is more population on this side:

I am working on a Healey 3000 and having trouble gtting the engine to run smooth. The engine was rebuilt and is mounted on a stand so we can pre-run and debug. Good thing since it has been a long road and it still runs poorly.

Here is what I have checked so far:

Engine has been compression and leak tested. 175 psi across the board and 10% is highest leakdown reading. Distributor is in good shape, new points and timing set to 16 BTDC. Total advance about 35 degrees based on dial-back testing light. Intake centerline 110 degrees verified with degree wheel and dial indicator on lifter. Valve lash set a 0.012" hot (works out to about 0.015" hot". Have searched and not found any vacuum leaks in the intake other than the ones on the bottom corner of the intake manifold; the ones with the long tube and what I think is a calibrated orifice (whats with those anyways?)

This leaves me at the carbs. The engine runs, but the vacuun signal is bouncy around 14 inches of mercury. The exhaust on these cars is split into 3 cylinders, and while the exhaust representing the rear cylinders has a smooth pulse, the one representing the front 3 has a snarly popping sound that makes me think this has to do with the front carb?

All plugs seem a healty mocha brown color.

The carb that is on the engine was rebuilt. Not by me. I am not familiar with the HD8 version of the SU, but I do speak fluent SUese as it pertains to HS4 for the MGBs and H1s for the Bugeyes.

The carbs were rebuilt by the owner with the help of a machinist who did the throttle shafts. Is there anything that commonly goes wrong whith re-building these carbs that I can start looking for? The diaphragm assembly in particular is a deviation that I am not familiar with. Any way the owner could have screwed this up? What to look for?

I checked the float levels because it sure sounded like low floats, but they were the correct 7/16 setting.

Been at this for a few months so am running out of idead. Any and all suggestions are welcome.

Not an SU expert, but I do have a copy of the BMC full mechanical service parts list for a AH-3000 (bj7-bj8) - hey, you never know when you'll need one of these. It's a catalog of every component of the car - with part numbers in exploded view - carbs too. Let me know if it would help.

IIRC the throttle shaft can get worn and allow a vacuum leak there.

NOHOME wrote: 16 BTDC. Total advance about 35 degrees based on dial-back testing light. Have searched and not found any vacuum leaks in the intake other than the ones on the bottom corner of the intake manifold; the ones with the long tube and what I think is a calibrated orifice (whats with those anyways?)

16? Really? I've been tinkering with MGs, Healeys, etc for nearly 30 years and have found the engines most happy at 10. In fact, I'm not sure the timing marks go past 10 or 15.

As for the vacuum leak, are you talking about the tube that runs from the manifold and should connect to the distributor?

I don't know how familiar you are with Lucas distributors, there are two advance mechanisms (assuming you don't have a full mech). As vacuum decreases, advances increases. In other words, if you disconnect the vaccum advance, you will be at full advance at idle, leaving only the mechanical advance. You can check the mechanical advance by removing the points plate and looking for the number on the weights. Add that to your advance on the pulley (with vacuum disconnected) to get the total advance.

Maybe I missed it, but what is it you're trying to fix? If it's just a lumpy idle, that might be normal. Timing gear wear can cause a little miss at idle. If it's missing higher up in the revs, try twisting the distributor as it's running to see if it smooths out.

Numero uno: balance the carbs with a Uni Syn or similar.

Get the airflow right before fiddling with the mixture at all. If you balance the carbs, rev the engine and recheck them at idle they should all be the same still. If they aren't there's a bad throttle shaft in the bunch somewhere. It will never idle right until that is fixed.

Assuming the synch is OK, pull the dashpots. Then run the mixture screw up until the jet is even with the 'bridge'. Mark the 'flat' that's closest to the outside of the engine (so you can easily see it when you are looking down the carb throat).

Now flip the pistons upside down and be certain the taper on the needle ends at the bottom of the brass bushing in the piston. The 'shoulder' is at #1 in these pics.

Now clean and assemble the pistons to the vacuum chambers WITHOUT the springs and install them on the carbs. Lift the piston with a screwdriver and let it drop, the needle should go all the way into the jet. If it doesn't, the jet is not centered in the carb body.

To center the jet, loosen the nut above the mixture nut about 1/2 turn, then raise the piston and let it drop again. Once it drops all the way smoothly, tighten the nut and recheck.

The D in HD stands for diaphragm, so the jet isn't centered that way.

This should fix things, if it is the carbs:HD How-To

Apis_Mellifera wrote: The D in HD stands for diaphragm, so the jet isn't centered that way. This should fix things, if it is the carbs:HD How-To

Fixed the link for you, but I'll repeat it here just in case:

HD How-To

You are correct that they have the diaphagm below the carb bodies. The jets do still need to be centered properly. If they are not, the piston won't drop all the way down and the mixture will never be right. The diaphagms with the attached jets and the jet bearings etc are visible at the top of this picture. The float chamber has to come off to get to the jet bearing for centering.

Be certain the float level is correct, on SU's that is CRITICAL.

Spraying carb cleaner at the throttle shafts is not going to tell you much about shaft wear, BTW. Even new and properly fitting shafts can still suck a small amount of air. That's why Stromberg etc went to rubber seals on the shafts later on.

Curmudgeon wrote: Numero uno: balance the carbs with a Uni Syn or similar. Get the airflow right before fiddling with the mixture at all. If you balance the carbs, rev the engine and recheck them at idle they should all be the same still. If they aren't there's a bad throttle shaft in the bunch somewhere. It will never idle right until that is fixed. Assuming the synch is OK, pull the dashpots. Then run the mixture screw up until the jet is even with the 'bridge'. Mark the 'flat' that's closest to the outside of the engine (so you can easily see it when you are looking down the carb throat). Now flip the pistons upside down and be certain the taper on the needle ends at the bottom of the brass bushing in the piston. The 'shoulder' is at #1 in these pics. Now clean and assemble the pistons to the vacuum chambers WITHOUT the springs and install them on the carbs. Lift the piston with a screwdriver and let it drop, the needle should go all the way into the jet. If it doesn't, the jet is not centered in the carb body. To center the jet, loosen the nut above the mixture nut about 1/2 turn, then raise the piston and let it drop again. Once it drops all the way smoothly, tighten the nut and recheck.

Curmudgeon:

You may be on to something. The front cylinders are behaving as if they are lean. The needle protruding would cause that. I will investigate

HDs don't have a mixture nut. H and HS carbs do. HDs have a screw that raises and lowers jet, adjusting the mixture. The jets are self centering in that they can only go into the carb body one way and they move up and down. The needles on the other hand will need to be centered if they aren't the spring-loaded, self-centering type found on most HS and HIF carbs.

I can't remember if HDs have them, but most of the others have a lifting pin for the piston. The shop manual describes what the resulting engine note means (rich or lean) when they are pressed.

SUs are about as basic as they come. If the fronts are lean, more than likely the jet is up too much. Follow Curmudgeon's method of "zeroing" the jets and then lower them equally. I have two of those Uni-Syns. I don't use them. Just like the one pictured, the center adjustment thing has a flat spot and doesn't seat all the way.

I am not real clear on how the text relates to the picture of the two needles?

My interpretation is that one or the other is "incorrect" and will cause a problem that cant be resolved by adjusting the jet carrier.

If I had to guess, I would say that the jet in the front carb may be extending too far out of the piston and lowering too far into the jet. This would cause a lean condition.

Is this correct or am I miss-reading the image?

The needle is the metering mechanism in an SU. There are three jet sizes for an HD8 and three needles spec'ed for a 3000. I don't know what size jet you need, but I'd guess .090 or .100. The size will be on it. The base mixture is adjusted in an HD and HIF via a screw. The mixture nut described above is found in H and HS carbs. Both of these modes raise or lower the jet, which adjusts the mixture across the rev rage. The needle is one of two types: fixed and floating (self-centering). The picture shows the two types; self-centering on the right. The needles come in many different profiles noted by the diameter at specific locations down the length of the needles (the numbers 1-13 and 1-16). This is how the mixture is regulated throughout the throttle position. The needles generally have a two or three character designation (ABC and AAC in Spridgets, for example). There are hundreds of needle profiles. Take them out and post what it is. A 3000 should have a UN, UH, or UL. UN being rich, UH being standard, and UL being lean. It should be a fixed needle.

The needles can be installed improperly, but like the jets, only go in one way. If the needle is installed properly, the jet would be up too far rather than the needle being down too far if there is a lean condition. The needles are "set it and forget it".

Remove the dashpot and piston, then have a look (post a picture) of the round brass thing (jet) that the needle was in. If it is flush or higher that the surround surface, it is too high and thus lean.

I have a NOS HD8 on the shelf. I'll take a look to see if the needle is self-centering or not. If the needle moves, it's self centering.

I copied a section of an SU shop manual relating to HD carbs and the needle/jet section on 3000s. I can email it to you if you want. That's probably better than trying to describe things.

SU's and Strombergs had fixed and spring loaded needles, the spring loaded ones were more of a later HIF emissions type thing and that's the difference in the two in the picture. The spring loaded ones are not meant to 'center', they bias toward the rear of the jet orifice. They still have to be installed correctly in the piston.

If there are no pins to raise the pistons, you can do the same thing with an O ring pick or similar, with the engine idling raise the piston ~1/4". If the splashy sound goes away that carb is too lean.

Those HD8's should have fixed needles IIRC. Also IIRC they were not temperature compensated like HIF's. HIF's had a bimetallic spring which raised/lowered the jet in response to temperature changes. I think those HD's have a similar adjustment screw setup only without the temperature compensator. The last time I fiddled with them was on my dad's 1970 E type and that was many moons ago.

OK, making progress here!

The fuel level in the front float was much lower than in the rear one.

Now, the mistery would be WHY since both were set to the same using the 7/16" drill bit.

Only thing I could find was that the float lever hinge on the bad carb was a loose fit on the hinge pin and allowed a lot of side-to-side movement.

I did a bit of tightening with a pair of needle nose pliers and stuck it all back together. Fuel levels seem pretty close now.

Engine runds much better also. Still not perfect, but not bad for an initial tune. Still a touch of burble from the front three while the rear three are wisper smooth!

So, here is a question for the gallery: The HD8 carbs have an idle circuit. The manual describes closing this adjustment needle and then backing out 3.5 turns to set the idle and carb balance.

Another manual I have says that if you have throttle idle screws, then you leave the idle circuit mixture screws closed and do all your carb balancing and idle adjustments with the throttle idle screws.

I did not really need this kind of confusion at this point.

Anyone know what the difference is when setting low speed running with one method or the other?

Been a while since I fiddled with the ones on my dad's, but are you thinking of the 'trim screw'? That's like a fine adjustment done after the 'gross' adjustment is done with the throttle stop screw and balancing the carbs. IIRC the trim screw adjusts air, not fuel.

Curmudgeon wrote: Been a while since I fiddled with the ones on my dad's, but are you thinking of the 'trim screw'? That's like a fine adjustment done after the 'gross' adjustment is done with the throttle stop screw and balancing the carbs. IIRC the trim screw adjusts air, not fuel.

Funny how no two manuals reffer to this screw with the same name!

If they are just air bleeds, then I will give them a tweek and see if I can perfect the idle.

Car certainly runs better now.

This entire saga has been done with the engine running on a test stand; if it had been installed in the car, we would have crushed it a few months back since everything on a Healye is designed to be impossible to work on!