MGB: SU HS4 idle

Hey quick question:

I have a set of SU HS4's on my 76B and for the first year or so they have been running great and honestly at speed they are still great. Lately though it seems like the idle is a little off. when I just sit in the car running it seems to idle down around 900 which is how they originally idle but over a period of time it drops to 500 then below and eventually dies. This happens over about 10 or 12 minutes.

The first question is: Any tips for this?

The second question is: what is a good reference for SU's?

Thanks,

Josh

Have you tuned it lately? It sounds like carbon load up.

no but then again they are brand new they went in, in late september of last year.

So, how many miles have you put on? Was all the electrical stuff also new? As points wear, the gap shrinks. As plugs wear, the gap increases.

yep all new i am guessing around 2000 miles

sorry im not helping much with your question but i just got the same air filters as you have and i was wondering if you knew off-hand what size(length) bolts you used for mounting your airfilters and if you went in through the filter or behind the carb. IOW how'd u mount them? thanks..

evan

I have the same problem on my 74 Roadster with SU HS4's and haven't been able to figure it out. When it starts to die, I'll pull out the choke a little to keep it from dying at a stop light. I've replaced fuel lines, float gaskets, cleaned the carbs with carb cleaner, taken the dashpots off and cleaned them, tuned the mixture, but it still does it. One thing I'm not sure of is the fast idle screw. Would that have anything to do with this? What's the fast idle used for?

fast idle is the rpm speed your car runs at before you pump the gas to drop to low idle I think.

perhaps it's heat?

Since you are sitting at idle?

The heat shield fitted with HS4's is actually from
earlier SU setups with the offset float. Under the
heat shield is two sections of asbestos. The design
places this shielding directly under the float chamber.

You're HIF's have an integrated float which actually puts
the float chamber inside the asbestos shielding and
directly above the siamese exhaust port.

of course it's no doubt a "loading" issue.

Might consider getting a little stick on shielding
and install some twixt the existing sections.

I have not seen the replacement shields and
do not know if they rivit a whole section of
shielding on the underside or just match
the two section style.

Since you mention "at idle" just a thought...

A piston could be sticking a little. If the carbs have the spring loaded needles then needle centering shouldn't be a problem. If the needles are fixed-type then they may be off-center. The fast idle screw is set so that it is supposed to open the throttle a little before it actually lowers the jet when you use the choke.
When you pull the choke a tinch the fast idle screw opens the throttle; pull it further and it starts lowering the jet for choke action.
At least that's what I gleaned from the SU carb video. I could be, and often am, wrong.
To see if the pistons are sticking stick your finger in there and lift them up a little and let them go. They should drop back with an audible click when they fall. If they don't then they're sticking.

Evan - I am using the conical K&Ns on my HS-4s as well. Don't remember exactly what length I used, but the bolts are just long enough to go through the filter base plate and cork gasket, through the carb, and into the choke fitting / back plate. I went in from the front - on my carbs (AUD-135s, 63-67 vintage) the choke fitting and back plate is threaded already, so you can't go in from the back.

When mine were doing this (AUD-135s), I found the points gap was off, and the timing was not advanced enough.

What kind of HS-4s are you using?

yeah...something is starving it...

Josh: Do your HS-4s have those little poppet valves on the throttle disks? The later HS-4s had them. Over time the springs are known to weaken, and when they do a slowly falling hot idle is a symptom, as they allow too much air to flow, thereby leaning the mixture.

I got it mostly figured out yesterday when I put the SU's in I tuned them in the cold weather and now that things are warming up they needed the idle screwes tweaked a bit. Not much just a little. They were running a little lean in addition. Now I am idleing around 1000 consistently. Thanks for the help