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Thread: SU hs4 carbs

  1. #1
    Jedi Hopeful
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    SU hs4 carbs

    just figured i'd ask if anyone has a tip for tuning the hs4 carb for winter. ever since the weather got real cold, the b is running like crap.
    a little backfiring here and there, with the engine stalling sometimes from idle/acceleration. seems sluggish then all of a sudden will regain its power. i tried to do the mixture adjustment myself, but i kinda screwed up. i tried to turn them both the same amount, but got carried away.
    (brain freeze) how do i reset them to start all over again? also on the idle adjusting nut on the bottom of the suction chamber(dashpot), is the adjustment right for rich and left for lean or vice versa?
    what about dashpot oil? i'm using 20/50 castrol.
    is it too heavy for this weather?
    thanks guys and gals.

  2. #2
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    Re: SU hs4 carbs

    Your dash pot oil is WAY too heavy. Try auto transmission fluid, it helps keep down the deposits on the valves. And don't overfill.

  3. #3
    Jedi Hopeful Lynn Kirkpatrick's Avatar
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    Re: SU hs4 carbs

    <blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by XJRpilot:
    just figured i'd ask if anyone has a tip for tuning the hs4 carb for winter. ever since the weather got real cold, the b is running like crap.
    a little backfiring here and there, with the engine stalling sometimes from idle/acceleration. seems sluggish then all of a sudden will regain its power. i tried to do the mixture adjustment myself, but i kinda screwed up. i tried to turn them both the same amount, but got carried away.
    (brain freeze) how do i reset them to start all over again? also on the idle adjusting nut on the bottom of the suction chamber(dashpot), is the adjustment right for rich and left for lean or vice versa?
    what about dashpot oil? i'm using 20/50 castrol.
    is it too heavy for this weather?
    thanks guys and gals.
    <hr></blockquote>

    If you haven't done it before, adjusting the SU's is too complicated to describe here. The directins may be in the forum someplace (Basil?), or go to Google.com and search on SU carbs. I bought the doodad that used to balance the carbs at idle & it works. It was worth the $'s.

    ABOUT THE DASHPOT OIL: everything printed says 20w-50, which I have been running for the 3+ years with no fouling, should AT oil be used in the winter & 20w-50 in summer? Does it make a difference? Is AT OK all year?

  4. #4
    Jedi Knight
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    Re: SU hs4 carbs

    I would always say checkeverything else before your carbs!! There may have been a little water in the tank or anything!! First thing, add gasline antifreeze, second thing check points, timing and plugs!! Last thing carb settings!! GOOD LUCK BUDDY!!!

  5. #5

    Re: SU hs4 carbs

    Lynn's right re the SU's - you need a fair amount of info. The best source to start with is a shop manual.

    Re the SU damper oil - I spent a lot of time on this years ago (on a Jag) experimenting with different weights. I tried one, drove the car, sucked it out and tried another. I tried light machine oil through 40 weight and settled on 30 weight as the best (it wasn't winter, though). There wasn't much difference between them except for the very light oil. I think 20/50 is fine.

  6. #6
    Jedi Hopeful
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    Re: SU hs4 carbs

    i have the repair manual but the directions are somewhat confusing. looking for the laymens version. as for the carb air flow meter, i can use the hose in the ear method for now. i am planning to get one after the new year. what about marvel mystery oil? the 20/50 looks awful thick in the dashpots in this cold weather.

  7. #7
    Obi Wan
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    Re: SU hs4 carbs

    Hello XJRpilot,

    Mixture adjustment is very easy, screw the nut down for a richer mixture, this pulls down the jet. As a ball park starting point, remove both
    dashpots and pistons\springs and screw the jets all the way up until they are flush with the bridge. Undo the adjusting nut twelve flats, that is two full turns down, you should now be near correct. (Also make sure your choke linkage is free and returns the jet full home).
    (It doesn't matter what needle you have fitted, they all have the same idle dimension, it won't run well over the rest of the range if the needles' are wrong though.)
    Check that your piston and dashpot are clean inside, and if not use a solvent cleaner to clean them. (DO NOT USE ANY SORT OF ABRASIVE, EVEN METAL POLISH). Also check that the needle is fitted with the shoulder flush with the piston lower face. Relace and carry out your balancing, (the tube method is quite acceptable), and idle speed adjustment.
    S.U.'s are really quite easy to adjust as long as a couple of basics are correct, and in my experience do not go off tune quickly.

    Alec [img]graemlins/thirsty.gif[/img]

  8. #8

    Re: SU hs4 carbs

    I agree with Alec's explaination above for basic tuning of SUs (and he's right, they are pretty easy to deal with).

    Gary's comment about checking *everything else* is also a good idea...at least check plugs (and points, if you have them).

    If the car is slow to warm up (is it?), check thermostat....could be a *low* thermostat (160 F.), which will cause poor cold weather running.

    Re: Marvel Mystery Oil. It's mostly kerosene (a light oil). I've been runing 10W-30 or 30 weight for years with success. I went to a carb forum at the AH Encounter event in Princeton this summer; the speaker said he swears by 3-in-One oil (which is like 5 weight oil and similar to Marvel Mystery Oil or even ATF). He got a lot of grief about this, but I think this might be OK for a car that has the carb dampers checked frequently. I'll probably continue with 10W-30, since it's always one hand.

  9. #9
    Jedi Hopeful
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    Re: SU hs4 carbs

    thanks alec and nial. i thought there should be a starting point for setting the jets. twelve flats = two full turns. its funny because my better half said a flat is probably the flat part of a bolt. not a complete turn. then she said if a bolt
    has six sides than it has six flats. i was speechless. maybe i ought to have her tune my su's. [img]images/icons/blush.gif[/img]
    as for the 30wgt oil in the dashpots, sure it isnt too thick? its awful cold around here.
    the oil looks like molasses in siberia. couldnt i go lighter for winter, than switch as the weather gets warm?

  10. #10
    Obi Wan
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    Re: SU hs4 carbs

    Hello again XJR,

    Sorry, forgot about the oil. The correct grade is S.A.E. 20, and in practice I used engine oil, 20\50 which works fine in our temperatures. I think in very cold weather a thinner oil will work fine. The oil is to dampen the (too) rapid rise of the piston when you open the throttle and stops the mixture going weak. Just play around until you feel that it runs well. Bear in mind that the underbonnet temperature will warm everything up.
    One basic I did not mention, make sure the needles are centred in their jets. This should only need doing after a rebuild, but if the jet nut has been slackened by mistake your needle can bind on the jet preventing it from going to the idle position. Take the air cleaner off and lift the piston by hand and release, it should drop down with a click onto the bridge. If it does not, then (assuming the piston is free in the dashpot) screw the adjusting nut fully up, slacken the jet clamping nut, (directly above the adjusting nut) and push down on the piston to centre the jet. Tighten the clamp nut the back off the adjusting nut two full turns and readjust the mixture. It is harder to write about than do.

    Alec
    [img]graemlins/thirsty.gif[/img]

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