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Old 03-16-2018, 01:24 AM   #1
smncutler
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Default Why are my SU's doing this?

Hey, I had my Amazon's SU HS6's rebuilt and rebushed, but I'm having trouble setting the mixture. The mixture nuts are both out so far that they can be turned with a finger, but the damper-flicking method causes the idle to dip as if it's too lean. I'm in the process of flushing bad gas out of the car, I filled up the nasty gas tank with premium and two bottles of seafoam, so I wonder if that could be part of the issue. I did flush 2-3 gallons through the fuel line into a gas can with a rag and air compressor, but it's still light yellow and stinky on the other end of three fuel filters. Engine is a 200K+ 69 B20 with Isky VV-71 cam, rebuilt head etc etc.

https://www.youtube.com/watch?v=CFva...ature=youtu.be

Thanks for your thoughts!

-Simon
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Old 03-16-2018, 05:26 AM   #2
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If you have a fuel line running between the 2 SU,s they are known to swell on the inside if very old ...........
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Old 03-16-2018, 08:30 AM   #3
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are those carbs jetted for 2.0L displacement? if not that might be the cause of bad tunability.
if the carbs are jetted for 1.6L or 1.8L you will have lean running issues when you try to use those carbs on a 2.0L
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Old 03-16-2018, 09:09 AM   #4
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First off - that thing is running horribly. Possibly on 2 cylinders.

1) You won't have any luck tuning the carbs without everything else being right.
- So check the valve clearances. And check for flat lobes while you're doing it - anything flat enough to cause any issues, especially at idle, will be VERY obvious to the eye, no need to actually measure valve lift, just look for a glaring lack of lift.
- Check the plugs and wires and cap
- Check the timing
- Check the compression

2) If it still runs like crapola, shut the motor off and pull off the float bowl covers and see if
- there is gas in both bowls
-roughly the same amount of gas (fill level) - there is a procedure to set the proper fill level by bending a tab on the float arm/lever.
-check for crap in the bottom of the bowls - sludge, rust, water, crap, whatever. It's supposed to be filtered before it gets to the carb, small bits of stuff getting to them can end up blocking the inlet to the hose that lead to the jet.

Good news is you don't have gas pouring out of the overflow vents on the carbs. *golf clap*

I've run B18 carbs on a B20 before without rejetting them, it certainly doesn't cause gross running issues like that. There's something else wrong here.

PS: When you lift the pistons to check mixture, how are you doing it? Via the built in pins, or just pulling the pistons up? How much are you moving the pistons up?

With the car off (because there's no point in doing it with the car running) pull both pistons all the way up and then let them fall freely. Do they descend smoothly and somewhat slowly? Do they both go down at the same speed? What sort of oil is in the dampers? Any signs of the needles even faintly touching the jets at any point? Do the pistons both go ALL the way down? Unfortunately, despite being built in an age of interchangeable parts, the pistons and domes on SU carbs aren't very interchangeable. They're usually hand fitted in the SU factory for a very close fit - without any sort of seal between them. Mixing and matching them later on doesn't always work well. And the jets need to be centered on the needle, and the needles can't be bent at all.

There are also some BAD tuning recommendations out and around on the internet which say to lift the piston on carb A and then set carb B based on the results. And some people who will vigorously defend the technique. It's just plain wrong (ask them how they'd tune a Mini with a single SU carb, or a jaguar with 3 SU's, if you are supposed to adjust the 'other' carb? lol).
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Old 03-16-2018, 10:11 PM   #5
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I had Tom Bryant rebuild them, I told him to do whatever was needed for a somewhat cammed B20. If anyone knows what they're doing with these carbs it's him. He installed new jets, needles, new throttle shafts and Delrin bushings, damper springs, you name it, and I used Mobil 1 15w-50 in the dashpots at his recommendation.
I know I have fuel and while it is gross it isn't chunky. All new fuel hoses including the one between the tank and the metal fuel line, and I've gone through multiple sets of three filters at once to get the crap out. Currently running a glass replaceable cartridge filter for chunky goo and a paper for varnish, in addition to the AC fuel pump's screen and integrated sediment trap.
New plugs and wires, timing at 16 degrees, head rebuilt, but my compression probably isn't great. I have an oil leak from an exhaust leak on cylinder #1 (uh huh). Maybe I should start with having the block built.
Valve clearances are ok, though I made the mistake of reusing the old lifters with a new iPd cam. I ran it briefly with the valve cover off and the lift looked consistent.
I did set the idle quite low, but that doesn't explain the stumbling at part throttle. If it was running way rich, could the additional acceleration enrichment cause that?
I did probably lift the pistons too much - I first used the plunger on the side but it barely had any effect so I wrongly assumed I was doing it wrong, and lifted them like 7 mm with a screwdriver. They do go all the way down but not as rapidly as they probably should.

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Old 03-17-2018, 01:30 AM   #6
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Have you tried running on only good gas? I suggest using the highest octane you can get.

When gas get old, it also loses its volatility.

And sometimes old gas has gunk in liquid suspension that can get past the fuel filter and cause problems.

Over 20 years ago, we rebuilt a Jeep engine for a customer who brought us just the engine. He installed the rebuilt engine and within a short time the engine seized. And when we disassembled the engine to investigate, we found it was gummed up with what we determined to be gunk that came from his gas tank.

It turned out, his gas tank had old dried up gas inside from sitting in the desert for over 10 years and all he did to bring the tank back into service was to fill and drain the tank which probably flushed out some solids. But when he filled the tank a 2nd time, the new fuel dissolved some of the built up gunk inside the tank which made its way into the engine.

After a proper gas tank service along with a complete cleaning, inspection and re-assembling of the engine, it ran fine.
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Old 03-17-2018, 01:53 AM   #7
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Good point. I'll drain the float bowls and rig up a redneck gas can with some 91 and report back. When I first ran it a few months ago I didn't have enough filtration and I stuck an intake valve open, that sucked. At the moment the gas itself is fresh premium, but dissolved in it is the crap that encrusted the empty tank for 30 years.
Two cans of seafoam (32 gallons worth) in 10 gallons of gas oughta loosen up some crud. The big rust chunks are through but it looks and stanks like decade old piss.

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Old 03-19-2018, 12:25 AM   #8
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Flushed lots of gas out, ran it a bunch, and did some tuning. Sprayed in a bunch of B12 chemtool with some revs, helped a little, has marginally more power but it's still slower than a watered-down chevette. Fuel is nearly clear, some new filters for a total of four. Turning the idle way down helped with matching the carbs, I could hear and feel the difference in every other exhaust pulse, got them pretty even then set it to a fast idle. It hardly sputters at all at part throttle (out of gear), it gives a nice harmonized whoosh, will test drive asap.
With the mixture a couple flats leaner it no longer smokes like a diesel when I lift off the throttle, though I'm sure the compression isn't great, I know it's burning some crude. I tried but failed to do a compression test using the *exact same* garbage Evertough gauge that gave my 240 a zero psi reading last week. I made sure it was all hooked up right, lubed the O-rings and all that and it just pissed air. Never again renting one from oreilly auto farts, and I even work there. A B20 with zero compression won't run well enough even to weakly putter around at 22 mph, so the tester must be junk.

The weird part is that it positively hauled ass when I first drove it on chunky unfiltered whiskey lookin' gas, awful rich tune and all that. The varnish stuck a valve open after the maiden voyage but good lordy it went like a hot damn at first.
The past few days and past few test drives have resulted in awfully inconsistent power and willingness to rev up, and a few loud pops/bangs/dying. Only since I did the aforementioned cleaning and adjustment has it seemed to clear up. I'll report back after the next drive, any thoughts in the meantime?

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Old 03-19-2018, 04:34 AM   #9
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popping and banging on decel usually point to a lean mixture.
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Old 03-19-2018, 06:31 AM   #10
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Hi smncutler, you are running four filters inline?
If you add more than one filter behind the pump the pressure will go down and the barrels won't have the right level. If you buy one from filter king, the one with the glas bowl, you will have no problems. The better ones are adjustable too.

you have to drive the car, no engine with SU likes it to run on idle or without resistance
good luck, Kay
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Old 03-19-2018, 06:33 AM   #11
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Quote:
Originally Posted by Janspeed View Post
popping and banging on decel out of the carbs usually point to a lean mixture.
popping and banging on decel out of the exhaust usually point to a rich mixture

regards, Kay
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Old 03-19-2018, 11:23 AM   #12
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So does the lift stumble. If it’s overly rich, it’ll stumble on part throttle then pick up. Go back to 3 flats from the jet being flush with the bridge.
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Old 03-19-2018, 12:16 PM   #13
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It was stumbling awfully on part throttle and then picking up before I leaned the mixture 3 flats. It died once or twice with a terrifying bang while putting around in first gear but started right up again.
The pops and all that were only on accel in first/second gear, acted like being bogged but wasn't.
Never had any pops or anything weird on decel except clouds of smoke.
Thanks for all the tips guys!

I have the AC mechanical pump with integrated filter, a glass filter with replaceable mesh screen for big chunks, and two big plastic ones with pleated paper elements. It appears that plenty of fuel is flowing through, and the bowls seem to be filling up properly, but I could be wrong.

Kay are you saying that the fuel pump can't pump enough gas at idle and cause it to run lean, so I have to drive the car to get enough fuel to the carbs?

Should I put an AFR sensor+gauge on exhaust runners 1 and 4 so I can balance my mixture more easily? Or would that make me a bitchass little bitch who's afraid of a little carburetor?

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Old 03-19-2018, 01:17 PM   #14
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Go grab 2 sets of spark plugs. Run the car to operating temp how it is. Then swap plugs, start it, rev it once, shut it off and check the plugs. Adjust from there, wipe down those plugs with some carb cleaner, reinstall, repeat, if needed swap to the other new plugs. Just remember that since these act on airflow, get the airflow the same before and after any adjustment to fuel.
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Old 03-19-2018, 01:24 PM   #15
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Overly rich can do just what overly lean can. Too rich and the exhaust fills with gas, red manifold and when you shut down, bang! Now you need to check for leaks too. Since the intake and exhaust are on the same gasket, you’ve likely got a leak from exhaust to intake runner.
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Old 03-19-2018, 01:47 PM   #16
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Also, make sure the fiddly choke mechanisn that version of the SU had isn't hanging open.
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Old 03-20-2018, 01:23 PM   #17
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Choke disconnected so no worries there
Fairly clean intake mating surfaces and new gaskets but I'll check for leaks, throw on another gasket
Spark plugs 1 and 3 are covered in oily crap, 2 and 4 have bubbly yellow brown deposits with tiny flecks of what I hope isn't melted piston.

my homework is to check compression, new plugs, carb tune as you described, anything else?
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Old 03-20-2018, 01:45 PM   #18
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The choke mechanism can still hang open. Just visually verify that the jets are up against the adjusters, I guess, and not positioned even slightly down. The choke mechanism just alters the mixture tuning by shoving the jets downward.

Beware of shocking the crap out of yourself and all, but also maybe try to isolate which cylinders are running the worst - the ones that make little/no difference at idle.

I'm guessing you don't have an air balancer gauge? Wihtout one of those it can be a very iterative process to balance the airflow between the crabs, and adjust the mixture as well. Because slight differences in mixture and airflow can hide each other back and forth.

But, generally speaking:
1) Adjust the valves and make sure there are no flat cam lobes - you've done this
2) Check compression and make sure there isn't some sort of severe mechanical issue with the piston/block/head/valves
3) Clean properly gapped plugs, good wires, good cap, good rotor, so you're not listening to ignition issues while you tweedle with the carbs
4) Air leaks past the carbs *can* screw with your tuning - since you do the tuning at idle where the air leaks have (by far) the most effect.
5) Ensure that the carbs have no fuel supply issues - both carb bowls fill with gas to an appropriate level, and both jets and their tubes are free of obstructions and gas cna freely flow from the bowl to the jet.

Enter the endless (nearly) tuning loop:
With the engine fully warmed up:

6) Try to balance the airflow between carbs by opening and closing the idle adjust screws by 1/2 turn. Back and forth. Back and forth on each carb. Until each of the two adjuster screws has roughly the same effect on idle speed when turned up 1/2 turn and then back. Stock manifolds can slightly confuse carb tuning if you don't do this first because they have a balance tube between both sides and one carb can feed all 4 cylinders at idle somewhat decently - if the other isn't flowing enough air. After they are balanced (if it is running decently enough to attempt) move both of them to get close to a good idle speed.
- Make sure that the carb linkages are slightly slack at idle - and that the idle adjuster screw is actually controlling the throttle position and it's not just hanging on the carb body (worn carb throat, uncentered butterfyl valve) of being held open by the throttle linkage.

7) Use the lift pins to lift the pistons. The lift pins should be used because they lift the piston a specific amount and lean it by a specific percentage. If you use some other method to lift them you might lift it too much or too little, and throw off the results. When you lift the piston:
- if the idle speeds up and stays there - it was too rich, screw the adjuster in a flat (raising the jet slightly).
- if the idle briefly speeds up and then settles back - it's about right
- if the idle drops - it was too lean, screw the adjuster out a flat (lowering the jet slightly)
- after each 'test' and potential adjustment, give the engine a little rev to clear it out and then repeat step 6 on each carb
- when both carbs seem to be in their sweet spot - repeat step 6 to ensure that the better-tuned carbs haven't thrown off the apparent airflow balance. If you find you've made non-trivial adjustments to the idle adjusters, do step 7 again. And if you make changes there do step 5 again. Repeat until you aren't making any changes to either idle or mixture.

If it's still running like crap after all that, there's some other sort of problem.
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Old 03-20-2018, 04:33 PM   #19
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OP do first: Verify that all primary and secondary ignition is working right. Many-most of what are assumed to be 'carburator issues' are ignition problems.

Too many times I've seen pushrod engines with Joe bloe's cam and heavy valve springs (because race car... Obviously!) that run poorly due to flat cam/cam ground wrong.

Much good advice above for dealing with your SU's. Scare up a Uni-Syn to quickly and easily balance airflow to carbs. More often then not 'hard to solve' issues are due to basics.... Clean reliable fuel supply at correct pressure with float level where it belongs.
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Old 03-20-2018, 04:45 PM   #20
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SU's really are amazingly simple devices. Very few moving parts, no multiple jets that all flow fuel in overlapping ranges, no pumps. Just a rather elegant and simple solution with one moving part - and a deceivingly simple looking needle. That needle is actually a very preciesely made piece of metal, it is a physical representation of the VE curve of the motor, the maths all made far simpler (and the fuel atomization) by the constant air flow velocity across the jet as the piston moves up and down. The dampers are also there for a reason, when you blip the throttle, they resist going up and the temporarily increased velocity of air across the jet pulls more fuel in than normal - acceleration enrichment.
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Old 03-20-2018, 11:56 PM   #21
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Thanks for all your suggestions! I'll keep futzing with the carbs, do a comp test and lower my base timing from 16 degrees to 10-12.
My rebuilt head has stock everything so I'm more concerned about valve float at redline than a flat cam lobe, and it's the iPd Isky cam so not a huge amount of lift.
I may also just have to keep running it until the gas is at 100%
The jets slide smoothly and are spring loaded properly in place

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Old 03-22-2018, 10:14 PM   #22
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Timing helped a teeny bit. Spark is strong. Comp test between 130-160 all cylinders - the gauge spiked but didn't hold, so my rings are toast or all my valves are stuck open a little. Or the gauge is crap, again. Gas flushing continues. So much rusty oily crap that the liquid in the fuel bowls has separated into layers, I keep soaking that up and crank to refill. I refuse to get a new gas tank ☺

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Old 03-22-2018, 10:29 PM   #23
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Pull the gas tank and have the radiator shop clean it up.
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Old 03-22-2018, 10:44 PM   #24
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At the very least, dump whatever crap is in it now out and start over with fresh clean gas.
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Old 03-22-2018, 11:14 PM   #25
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^^^This.

The layers you're seeing in the float bowls is water in the fuel. Clean the bowls thoroughly making sure to blow out the jet tubes while lifting the pistons. Then connect an external tank with fresh fuel until you have the tank professionally cleaned. Pump fresh fuel through the lines and pump before connecting them back to the carbs.
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