HS4 needle mystery

[bigjones]

I've got a single HS4 on my 1500 Midget.
It's running a bit lean on the stock needle ("5") so I picked up a richer needle ("AAA") from my local LBC shop.

When I got home I realised that this new needle was a "fixed" type and as such will not apparently fit in the carb.

The mystery is that Moss UK lists this "AAA" as a "biased" (or, spring loaded) needle.

Not quite sure what to do about it. Any ideas?

Cheers!

 

[DNK]

Take it back

 

[Hap Waldrop]

A #5 should be plenty rich enough for a twin HS4 1500 engine, #5 is the stock needle for the early MGB 1800 twin HS4 engines, #6 is the next richest needle in the solid needle selection, but I have a hard time believing you need a richer needle than the #5 in the 1500 w/ twin HS4s. How far down did you have the jet from the bridge with the #5?

 

[PeterC]

As a rule, 3 letter needles are biased spring loaded needles.

 

[bigjones]

Take it back

Yeah, but...
I'll try emailing Moss UK and see what they say about the needles. Why do they say AAA is a spring-loaded needle when what I got from the shop is a fixed type?

Hap,
Not sure if it makes a difference, but I have only one HS4 on the 1500 (replacing the ZS). Right now it is at "12 flats down". I tried screwing the jet all the way down but still couldn't get the plugs to blacken up.

In this car, you spend most of your time with the gas pedal mashed to the floor to get any speed up. I've noticed that if I ease up slightly on the pedal it runs a bit better. That must indicate something, not sure what. The SU is a simple carb but sometimes I have a hard time understanding it's workings, ha!

Cheers!

 

[Mickey Richaud]

In this car, you spend most of your time with the gas pedal mashed to the floor to get any speed up. I've noticed that if I ease up slightly on the pedal it runs a bit better. That must indicate something, not sure what. The SU is a simple carb but sometimes I have a hard time understanding it's workings, ha!

Cheers!

THIS may help...

 

[bigjones]

Mickey,

Thanks for the link - nice graphic.

But why would easing of the gas ever so slightly improve combustion?

Cheers!

 

[Deleted member 8987]

plugged exhaust?

 

[DrEntropy]

BJ, does this HS4 by chance have the throttle plate with the "poppet valve"?

 

[Hap Waldrop]

They are labeled wrong above, I don't remember seeing a picture before, the #5 is a solid needle and the AAA is a spring biased needle.

 

[Billm]

What type of oil do you have in the dashpot (if any)?
BillM

 

[bigjones]

TOC,
Well, that's a good one. Maybe a field mouse crawled in there.

Doc,
No, thankfully, doesn't have the poppet valve thingee.

Hap,
The plot thickens - the needle the shop sold me is definitely marked AAA and is the fixed type.

And the one in the carb at the moment is spring loaded and marked "5".

Bill,
I always keep it topped up with 20w-50 Castrol (I'm on my last bottle of that, btw - switching over to Valvoline Racing for the zinc)

Cheers!

 

[Deleted member 8987]

Often you will find a backing off of the throttle giving more power than WOT is a restriced exhaust.
Smashed pipe, folded oulet, muffler come apart.
Not always, but if you're about as rich as you can go and it still acts that way, I'd be uncorking it and see how that works.

 

[bigjones]

TOC,
Actually, I think what I meant to say was that slightly backing off the throttle from WOT gave a smoothing out of the engine. I'm not 100% sure but I don't think it gave an increase in speed.
When you say "uncork" does that mean run w/o the muffler?
Cheers!

 

[Gundy]

Go figure. I'm using AAA needles in my HS4s and they are
definitely spring loaded jobs, as were the AAM and AAE
needles I tried. Could your AAA be mis-marked?

Minty lamb needle compare-o-rama chart link:
https://www.mintylamb.co.uk/suneedle/

 

[Deleted member 8987]

I'd drop the muffler, or even the pipe off the manifold if I suspected an issue.
However, one trick we used to use to confirm plugged exhaust (muffler, cat, or a double-wall pipe with inner pipe collapsed) was to drill a small hole, and use a vacuum gauge that had a portion on the other side of vacuum for fule pressure readings, put a pointed tip on the hose, have an assistant rev up the engine while under the car pressing the cone into the hole.
Best is zero pressure, but you will have a bit.
If a lot, check the innards of the exhaust, if okay, put a sheet metal screw or pop rivet into the hole.

But, I did read that as power dropping off at WOT. If just not as smooth, probably not it.