Carbs rebuilt

[don62]

Hi
I got my SU HS4 rebuilt. The kit used was SU-13 from MOSS.
The carbs number is AUD 2164(which I cannot find any jetheads for that model), It's and MGB 76. It's not the original carbs. I'm not too sure if the kit used for the rebuilt was the good one. The needle is MB. I got them
done by an expert in my area but it still run bad.
He want's to replace the jet heads again. I already paid too much for the work and every time I go there she runs worst than when I work on it. The jet heads were adjusted to 6 and 4 flats, which I find really weak. She has a hard time to start(got to use the gaz pedal). When taking an exit fron the Highway the rpm just go down to 300 and dies. I have to keep giving her gaz to keep her alive. The RPM would go from 1500 to 600, very unstable.
I just can't wait to go anywhere without touching the carbs or trottle.

Help please!

Don

 

[Bugeye58]

Don, the standard jets for HS4's are all the same. Did the throttle shafts get replaced during the rebuild? Worn shafts will cause all kinds of problems, ranging from hard starting, poor idle, to backfiring and no power. Another thing to check for is a vacuum leak around the intake manifold. Spray carburetor cleaner at the manifold gasket with the engine running, and see if the engine speed changes. If it does, there's a leak. Do the same for the throttle shafts.
Pull the damper from each carburetor, and push the piston up with your finger. When you let it go, it should drop and land with an audible "Click." This means the jets are centered properly, and the piston isn't binding in any way.
Make sure the carbs are properly snynchronised, and you get equal movement of the throttle plates.
Six flats should be close on the jet setting, but 4 is pretty lean.
Hope I've helped some.
Jeff

 

[PAUL161]

Hi
I got my SU HS4 rebuilt. The kit used was SU-13 from MOSS.
The carbs number is AUD 2164(which I cannot find any jetheads for that model), It's and MGB 76. It's not the original carbs. I'm not too sure if the kit used for the rebuilt was the good one. The needle is MB. I got them
done by an expert in my area but it still run bad.
He want's to replace the jet heads again. I already paid too much for the work and every time I go there she runs worst than when I work on it. The jet heads were adjusted to 6 and 4 flats, which I find really weak. She has a hard time to start(got to use the gaz pedal). When taking an exit fron the Highway the rpm just go down to 300 and dies. I have to keep giving her gaz to keep her alive. The RPM would go from 1500 to 600, very unstable.
I just can't wait to go anywhere without touching the carbs or trottle.

Help please!

Don

Sounds like your "EXPERT",

isn't quite the expert he professes.

 

[piman]

Hello Don,

the carburettor model is HS4, the AUD number is the specification, with the variables for the same model carburettor being the needle, damper spring, float chanber configuration etc, i.e. how the carburettor is tailored for a particular engine.

I would suggest doing a full set up which probably takes as long as it takes me to type this. Remove the dashpot assemblies and make sure the piston and chamber inside is clean. If not use ONLY a solvent cleaner to get rid of varnish etc. Check that the needle shoulder is flush with the piston bottom face, if not loosen the grub screw and re-position the needle.

Adjust the jets in the carburettor flush with the internal bridge, i.e the piece that projects into the choke passage. Unscrew the jet 12 flats, which is two full turns. If you have an electric fuel pump, switch on the ignition and see where the fuel level ends up, Typically it will be about an 1\8" bellow the jet tube. (HS4 have non adjustable floats as far as level goes. However if it is too high, which could cause flooding, unscrew the needle valve body and add a washer to it so as to lower it in the lid)

Replace the dashpot\piston assemblies and then lift each piston in turn to check that they fall smoothly and stop with a click (as Jeff has said already). (If one or both do not, screw the jet back up fully, slacken the outer nut that secures the jet tube, push down on the piston then tighten the tube back up. Unscrew the jet back down 12 flats.

Start the engine, and let it warm up.
Remove the air cleaners if they are still on and listen to the intake 'hiss' with a bit of tube, if there is a variation you will need to slacken the clamp on the common throttle spindle and adjust the idle until both are balanced giving an equal 'hiss' Tighten the clamp and adjust the idle speed.

Note that the idle mixture on S.U.'s is independent of what needle is fitted so you should see a reasonably correct mixture at idle with the above setting.Try a test run and see how she runs. You get an idea of the mixture just by blipping the throttle, if it picks up quickly, you aren't far away. If you have to adjust the jets a long way from the 12 flats there is something else wrong.

I personally disagree with Jeff regarding the throttle spindles, worn spindles do upset the idle but not much else and the car can still run well. The amount of air leaking into the spindle is relatively small once the throttle is open so it doesn't cause too much of a problem.

Alec

 

[Bugeye58]

I personally disagree with Jeff regarding the throttle spindles, worn spindles do upset the idle but not much else and the car can still run well. The amount of air leaking into the spindle is relatively small once the throttle is open so it doesn't cause too much of a problem.

Alec

You're right, of course, Alec. I was thinking major vacuum leak as I was typing, and put things out of sequence. Thanks for the clarification.
Jeff /bcforum/images/%%GRAEMLIN_URL%%/blush.gif

 

[don62]

When I turn the fuel pump on it's flooding the carbs.
Even if the jet heads are fully up. I gues this is why I'm having a hard time to adjust them.
You're saying that the level should be about an 1\8" bellow the top of the jet head when the pump is on if I'm not mistaking? By bending the plate that is holding the float, would it change the fuel level in the chamber?

Thanks Don

 

[DrEntropy]

Start from the place the fuel goes into the carbs. i.e. float chambers, needle and seat, float height. Set it according to the Bentley or other manual. If the thing floods the carb or carbs, you need to find out why. Bad needle & seat, or maybe something on the seat keeps it from closing. Cleaning the seat with a cotton swab and some carb cleaner may take care of it. In any case the float should be stopping fuel flow when raised by fuel level and be adjusted correctly for rise/fall. You will never be able to "adjust" anything "downstream" to compensate for it.

The jet should be down the 12 flats for an initial setting, if all else is right the thing should run passably well that way.

 

[piman]

Hello Don,

that is the problem that needs sorting first, as the Doctor says you cannot compensate by other adjustments.

It is probable that the needle valve(s) are in need of attention. The HS4 is not designed so as to adjust the level by bending, as the earlier H types were. If the needles valve assemblies are OK, the add a washer between it and the float chamber so lowering the fuel level. Another possibility is an after market fuel pump which delivers too high a pressure, about 2 psi is adequate. Too much will force the needle valves open and cause flooding.

Alec

 

[don62]

I replaced the needle valves. The old ones was controlled by a ball the news ones have a needle instead and a slightly longer. The float setting should be at 1/8" according to the manual, I have 1/4" at least.
Even if the pump is off, there is some fuel going into the carbs and when it's on then it's flooding. That's why too that I have fuel leaking underneath the jet heads were the adj nut is. I'm wondering if the fuel lines could be wrongly installed, it seems that the seats valves haven't too much effect to stop the fuel. I'm open to any suggestions and I'm sure like you mentionned that this is my real problem from the beginning. I trust this forum much more than my "mechanic"

Thanks

Don

 

[DrEntropy]

1/4 inch is waay too low *ordinarily*. Sounds as if the pump is the wrong one. Too much pressure. If it's over 3 or 4 PSI it'll push the needles off their seats no matter where the float level is.

The lids on the float bowls... the fuel supply lines should go into the passage where the needle valve is. It'll be the higher of the two pipes on each lid... If you have overflow pipes on the lids and the supply goes into those you would have the same problem you describe, too.

 

[jlaird]

As usual Docs got it. Too much fuel pressure.

 

[piman]

Hello Jack,

you obviously missed my earlier post?
However, I believe the doctor may well have a more viable option, that is the fuel inlets being connected to the overflow outlets.
Don's reference to fuel flowing without the fuel pump being on gives a clue to that.

Alec

 

[DrEntropy]

/bcforum/images/%%GRAEMLIN_URL%%/wink.gif

...just one more possible answer to what *could* be happening. Remember: this car has been in the hands of "experts"! /bcforum/images/%%GRAEMLIN_URL%%/devilgrin.gif

 

[don62]

Thank you all once again!

You were right, I replaced The needle valves in the fuel chambers, resync and retuned, wow, unbeleivable.
It's just not the same car anymore, much more powerful and sounds like music to my ears.
The idle is always stable now, you made my day.
My "expert" had said that he reajusted the floats and rebuilt the carbs but the real problem was thoses needles.
Like you said, there wasn]t any possibility to adjust the carbs until that problem was solved.

Thanks Again.

Don

 

[piman]

Hello Don,

that's excellent, once set up they should require very little maintenance. Keep the dashpots topped up with oil and change air filters.

Alec

 

[DrEntropy]

/bcforum/images/%%GRAEMLIN_URL%%/thumbsup.gif

 

[Rob_Edwards]

I personally disagree with Jeff regarding the throttle spindles, worn spindles do upset the idle but not much else and the car can still run well. The amount of air leaking into the spindle is relatively small once the throttle is open so it doesn't cause too much of a problem.

Alec

You're right, of course, Alec. I was thinking major vacuum leak as I was typing, and put things out of sequence. Thanks for the clarification.
Jeff /bcforum/images/%%GRAEMLIN_URL%%/blush.gif

I agree with Jeff's initial statement -- worn throttle shafts WILL cause problems. Here's why: At idle, there is very little flow through the carbs so a little leak past the shaft will be a significant part of the total airflow. To get a decent idle mixture, you'll have to richen it up (maybe a lot) to offset the leak past the shafts. At road speed, the air past the shafts is an insignificant part of the airflow, but your jets are set as if there was a big leak, so the mixture at speed is going to be too rich -- perhaps much too rich. And at idle, the leak is not going to constant so your idle will be unstable -- sometime too rich, sometime too lean, sometimes fine....

If you've got worn throttle shafts, it's well worth it to fix them. Often you can get by just replacing the shafts. Remove the throttle discs (make sure to have new screws; don't reuse the old ones) and slide the shaft one way or the other to get an unworn portion of the shaft into the throttle bushes. If you can't feel any play, all you need is shafts. If you can feel play, you need shafts and bushes (or have the existing bushes reamed for oversized shafts).

Glad the needle valves fixed your problem!!!

HTH!

 

[Hap Waldrop]

Sounds like the wrong needle for starters, and yes the throttle shaft play is always a issue on old HS4s, for the most part in my rebuilds ( I rebuild 50 sets of SU carbs a year)with HS4s I go with the oversized throttle shaft to get back to spec. If the carb has a spring or biased needle then stock would be a ABD, if fixed needle, it would be #5.
Standard flat setting is seven down from the top, however I set mine with a dial caliper at about .070 down from the bridge.

Paul, on a sidenote, we (FBCC) are having a SU carb building tech session in Clemson, SC on Sunday July 15th, if you would like to attend, give me a call, we'd love to have you. I'll be conducting the tech session, so we're going to have fun with it /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

 

[piman]

Hello Rob, Hap,

we have to disagree somewhat. Ideally worn shafts should be repaired but in Dom's case it seemed there was far more wrong. The other point is, although the idle mixture is what the carburettor is adjusted to, it is sensible to check the mixture over the whole range of throttle openings. However if this upsets the idle it is liveable with.

Hap, why do you find that the jet setting you use is so far from what S.U. recommend?

Alec

 

[Nunyas]

Hap, what did you use to polish those dash pots? I must have spent 12+ hours on my HS-6 dash pot with liquid Brasso and Never Dull wadding just to get it to a cloudy haze...

I'm currently considering using a very fine grade sand paper (say 320 git or finer) to get through the crud before going at it with Brasso again.