Float Valve Sizing. HS6 SU

[ianh1968]

My 1480 is pulling about 100+Bhp worth of fuel through a single HS6...
... and I suspect that I have a fuel starvation issue.

I don't think it's the pump as I've just replaced this, it is an STC505.

I've checked the size of the current orrifice in the float bowl jet and this
appears to be 96 thou diameter as a No41 wire gauge drill will go in, but
nothing larger.

Research has revealed that a 0.125" jet is available as SU part No. WZX1090...
MiniSpares says this is used on the turbos and these were "only" 93Bhp.

Is it likely that the 0.096" jet in the float bowl is too small?

I have a dyno session booked for next Wednesday and want to make best use
of the time there - It's only 10 minutes away from a decent carb shop, but I'd
like to get the engine running reasonably well before I go.

Ian

[wantafaster1]

For what it's worth I'm at 110 bhp with an HIF44 and I did ask the rolling road man if it was leaning off due to supply but he said no it was fine. I have a facet pump.

[wantafaster1]

Incidentally why do you think there is a problem now?

[LuisM]

I've seen people drilling the float bowl valve hole, as cure for fuel starvation... apparently it solved the problem.

[ianh1968]

Incidentally why do you think there is a problem now?

Brisk accelaration up to 6500RPM in first is OK, but I then get a stutter when revving it in second. (and third).
It feels the same as when I "forget" to turn on the fuel pump and after a second or so, it recovers.

I've tried various permutations of needles and springs and can't get rid of it.
Ditto the timing which has been varied between 30 and 36 degrees full advance.

<SNIP>I'm at 110 bhp with an HIF44 and I did ask the rolling road man if it was leaning off due to supply but he said no it was fine.<SNIP>

When the same engine was only 1380cc and producing about the same BHP as yours,
my latest dyno op said mine was OK as well. The engine is now 100cc larger...

I've seen people drilling the float bowl valve hole, as cure for fuel starvation...
apparently it solved the problem.

My Dad did it on his Marcos 1380 engine and this also "cured" the problem,
but as he never dyno'd the engine, I have never been sure if that was pure luck or not...
(It was 30 years ago and the pump may not have been very good either...).

I am going to try a larger float jet and will report back.

Ian

[mk1]

Could be worth you PM-ing su_mark.

M

[carbon]

Ian,

If it's an electric pump an alternative check would be to measure fuel flow through your complete tank-pump-pipe system to carb inlet with and without a 'dummy' restrictor made up with 0.096 bore before going into jerrycan or similar. Weigh can, measure time for couple of gallons, weigh can again to get flow rates.

Then see how measured fuel flow compares with requirement for your engine, and effect of the 0.096 bore float jet.

[ianh1968]

I bought a couple of the 0.125" float bowl jet assemblies today...
... at a total (bargain) price of £13.67 each. (Part No WZX1090).

For the small amount of work involved, I might as well just fit one
and see how it goes. It will either help, or it won't - So I don't think
that a technical analysis will really be necessary in this instance.

The second one will either end up being a spare, of will go on the other engine.

For reference, I understand that there are at least 3 sizes of float jet
for the HS type carbs, 0.070", 0.096" and 0.125". If you are swapping
float bowl lids around, it's certainly something to be aware of.

There is another even larger jet listed as WZX1091 at 0.156" (5/32"),
but I don't think it's the same basic pattern.

Ian

[Vegard]

All my engines have ran better on twins rather than a single Hs6. Throttle response is especially improved. I was long a fan of a single hs6/hif44.
No more!

[Spider]

Ian, it can't hurt to front up to the Dyno with two float lid assemblies, ready to go.

None the less, I'd be checking if fuel delivery is an issue here, there could be something else which may not be apparent at present - who knows?

Fit a T Piece right at the fuel inlet to the float bowl and then plumb in, temporarily a fuel pressure gauge, there's loads of them about, and not too expensive, then go for a good drive (read: thrashing). The pressure gauge should be rock solid, if you notice it dip what so ever, especially at full noise, then you have your answer but also another question;- why? But, at least you'll know where to start looking.

Can I suggest that just because you are running a top quality pump, it doesn't mean there's an issue here. Best not to assume anything.

[su_mark]

I would be surprised if you need anything larger than the .096. I forget the physics behind it and it's not really worth going into here, but a simple comparison with other larger engined vehicles would give you some idea as to where the different sizes are used. The SU catalogue would of course be handy here...

I've seen people drilling the float bowl valve hole, as cure for fuel starvation...
apparently it solved the problem.

My Dad did it on his Marcos 1380 engine and this also "cured" the problem,
but as he never dyno'd the engine, I have never been sure if that was pure luck or not...
(It was 30 years ago and the pump may not have been very good either...).

I am going to try a larger float jet and will report back.

Ian

The above is not a good idea. All good quality needle valves have the seatings 'staked' to match the angle of the needle tip. If you drill them out you not only increase the risk of ovaling the hole creating a poor seal but also the sealing action of the needle tip is reduced to the now tiny upper edge of the straight drilled hole (if you see what I mean). Admittedly it is less of a problem with the latest Viton tips as they are a little more compliant, but they will wear quicker against a 'straight edged' hole.

[Spider]

I would be surprised if you need anything larger than the .096. I forget the physics behind it and it's not really worth going into here, but a simple comparison with other larger engined vehicles would give you some idea as to where the different sizes are used.

What about comparing the Jet dia. to the Float Seat? I would say you could get away with a Float Seat a tad smaller than the Jet, especially for street use, but on the track, it might be a different story?

[ianh1968]

Then see how measured fuel flow compares with requirement for your engine,
and effect of the 0.096 bore float jet.

OK, no "bucket test", but I put in a 0.125" float jet
- Result = No Change...

Could be worth you PM-ing su_mark.

... too late, Mark's already noticed the thread and made a comment - Thanks!

I've seen people drilling the float bowl valve hole,
as cure for fuel starvation...
apparently it solved the problem.

My Dad did it on his Marcos 1380 engine and this also "cured" the problem,

The above is not a good idea.

Yes, agreed, for the reasons specified...
It also occurs to me that 30 years ago, my Dad probably didn't know that there
was more than one size of float bowl jet:
Maybe the one that he "drilled" was only 0.070" to start with?
Maybe the ones LuisM was talking about were only "small-bore" as well?
Maybe 0.096" is plenty big enough - The carb shop said that the 0.125" would probably
be OK if I was burning ethanol (or was it methanol, or methylated spirit?)...
MiniSpares list the 0.125" as being from a "Turdo Metro",
Maybe this was overkill?

Fit a T Piece right at the fuel inlet to the float bowl and then plumb in,
temporarily a fuel pressure gauge,

Yes, I do have a nice liquid filled 0-15PSI gauge and selection of T-pieces etc,
so I could check this out, as per Spider's suggestion. How I would route a pipe
sensibly into the car is another matter - I might just wait until Wednesday's
dyno session.

Other stuff:
The fuel lines are 5/16".
I have a solenoid shutoff valve at the engine end, just prior to a fuel pressure regulator.
The solenoid was fitted after one of those "ball" type float valves caused about a gallon
of petrol to be dumped into the sump overnight one time... I would never use the "ball"
type again because of this, but never got round to removing the solenoid.

It plays up worse when the weather is cold.

The entire system was OK with the same engine as 1380cc with a 286 scatter.
It's now 1480cc with a 276...

I was thinking that the fuelling would be broadly similar from one version of the engine
to the other, but am now having second thoughts. I am no expert on this subject and
an seriously wondering if the original needle and spring have become totally inappropriate.

Maybe the engine will "suck" harder, and will require a leaner needle despite being larger
in capacity?

I will go to the dyno armed with my pressure gauge and enough adapters that I can
install this and/or bypass the solenoid and/or pressure regualtor. I will be taking a laptop
with me as I will be re-mapping the ignition and my dyno op wants me to help him
to install "Needles.ods" and show him how to use it...

Anyone got a spring recommendation?
Previosly with 1380cc it was "Yellow", but I got a surge at 5000rpm which according
to Des Hamill suggested that this was too stiff.

For what it's worth I'm at 110 bhp with an HIF44

What needle and spring are you using?

I think that we'll just have to "suck-it-and-see" on Wednesday.
Another challenge is to try to work out something better for Dean's car as well.
We are both using the same dyno, and are both going to Blyton..

"Mercy's sakes alive, looks likes we've got us a con-voy..."
https://www.youtube.com/watch?v=Sd5ZLJWQmss

Ian

[wantafaster1]

I'm on a yellow spring but a hand profiled needle which started out as a bbw. The yellow spring eliminated the hesitation I used to get when really using the whole rev counter and firing another gear into it. I just use 20/50 in the dashpot but make sure the little damper is in perfect order, it makes a difference.

[ianh1968]

Well, a trip to the dyno soon sorted things out...

First job was to take the filter off and literally look down the carb inlet
with a mirror when a power run was being done. The engine went in
with "SZ" and "blue", but it was quite clear that this was nowhere near
correct. I had to stop the first run at only 4000RPM as the piston had
already topped out.

A Yellow HIF spring was installed, giving us the option of sticking another
non-HIF one up the middle if necessary. This wasn't necessary as the
Yellow on its own was sufficient for the rev band that we were using.

The next needle tried was BCA, with the knurling removed, and this was
an improvement on SZ. Next up was KK, which improved things further.
After that, the air filter was replaced, which again improved things throughout
the rev band.

Needles.Chart.png

Timing was next - I'd set it to 33 degrees centrifugal at 5000RPM as a
starting point. Going up in stages, we ended up with 37 degrees max
at 4500RPM being the best. Going higher gave less power throughout
the whole rev band.

DISCLAIMER HERE: I am not sure if the dizzy is "zeroed" correctly...
I need to check that the max advance of 37 degrees matches up to
a dial-back strobe reading before I take the dizzy out next time.

Next up was to add some "vacuum" - Firstly "too much", then after that,
an improvement at the bottom end where you can't give it "wide-open-throttle".

I've still got the "too big" float jet, but at least I know now that there is
no problem due to lack of fuel.

For the record:
The HS6 has been "Vizarded" in that it has been squared off and has had half
of the butterfly shaft removed and the actual butterfly "knife-edged".
As per the actual instructions for "Vizarding" a carb, the bridge area is still
totally unmodified.

My conclusion here is that with my spec of engine, i.e. "about" 100BHP, with a
sensible spring, I have not had to resort to making a custom needle. (Although
I did take an electric drill and selection of "wet-n-dry" with me today, just in case).

My feelings are that if you take a decent enough selection of needles with
you when setting the engine up, the chances of having to "DIY" will be much
reduced. I had quite a few more needles to try, but they were "intermediates"
so it was not worth trying most of them. For example, once we'd deduced that
"SZ" was not appropriate and that "BCA" was much better, there was liitle
point in trying more needles that were quite similar to "SZ".

Using a dyno with a decent operator certainly beats guessing...

Ian