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MG MGB Technical - Float bowl fuel level (HS 4).

Reams of stuff in the archives, I know - but as a current tuning exercise has highlighted : '73 BGT with standard HS4 carbs and K&N filters; she was running "quite well", so I had left well alone until this week when I had to clear a bit of dirt from the float bowls. This led me to check the basics - viton needle good, floats ok ( but the horrible all-plastic jobs), needles and jets clean and centred ... so why the hesitation on initial acceleration, only clearing when fully warm after a few minutes spirited driving? The only thing I hadn't been happy with was the fuel level in the float bowl and hence the jets - way down low. On checking, I found someone had inserted the 12 thou shims under the needle valve assembly. When you remove the shim, up comes the fuel level - in my case to a level about 120 thou below the top of the jet - and a dramatic difference to fuel availability for atomisation. At cold start, the choke is beneficial for maybe 30 seconds, but then it's not required, and the car accelerates smoothly all the way to 160kph, which will do me. Just thought this might be useful to anyone fiddling this weekend. Cheers from Queensland. 33 celsius today. John.
J P Hall

Interesting-I thought the shims were a means of setting the level with plastic floats, whereas the original metal ones had the ability to bend the "arm".

i.e. only fitted if needed!
Hmm 1 celsius here.....
Michael Beswick

John
With the shims removed as you have done--what was the end result for the measurement from the top of the inverted float to the bowl cap-----compared to spec.

willy
William Revit

Yeah, interesting, Willy o'the south - my 5.5mm drill bit just kisses the float, as against, if I remember, a recommended range of 3.2 to 4.8mm in my favourite SU book. Even though the car is running sweet as a nut, I'm still sorely tempted to invest in a pair of the black floats with metal bar, currently showing on the Aussie Moss supplier's list, to enable any further fine adjustment of fuel level. I just don't like the plastic float preventing any further experimentation. Interested in your thoughts. John.
J P Hall

I quite like the full plastic floats, yes you get the occasional sinker but not very often really and easy to see the fuel in there if that happens. I like the fit on the swivel pins, much better fit than the metal ones.
As far as the black ones--yeah ok BUT you only have to get a scratch on the body of one and down he goes, the fuel soaks in and can't be seen.

Have you got any washers at all left under your needle/seat--- I usually set them to 1/8"(3.2mm) and then pull the chambers off the carbs, pull the choke right on and turn the key on- The fuel level should be 3/8" below the bridge(SU Spec) or 'roughly' level with the top of the jet-tube with the choke fully on.

With the floats set at 1/8"--If the levels at the jet aren't equal you can adjust the highest one down to match the other by adding a shim or two under the float needle and still be in the 3.2 - 4.8 mm spec.range at the float
William Revit

"you only have to get a scratch on the body of one and down he goes"

The so-called StayUp floats! On a complaint to SU Burlen about one having sunk they replied "You must have damaged it".
paulh4

Thanks Willy. How the b.h. do you get your all-plastic floats (no metal lever to bend) down to a clearance of 1/8 inch? Mine won't go any nearer than the 5.5mm, and that's by removing the po's 12thou washers from under the base of the needle seat. I would like to be able to achieve your 3.2mm, and then to measure again the fuel level below the bridge.
In the meantime, further road testing today (any excuse) with carbs set 2 flats leaner, again gave smooth acceleration all the way to a healthy but illegal top speed. I will definitely pull the choke tomorrow and measure the current fuel level in the jet tube accurately. John.
J P Hall

In theory they shouldn't need it. You can space the valve down with washers but if the float is too low without any you are reduced to 'adjusting' the plastic arm.

If it's running well leave it alone!
paulh4

John
If your fuel level below the bridge is close to spec(3/8") and importantly equal ,your job is done--
While you're in there set both jets down 1/16" -.0625" down from the bridge and check the both your needles are set with the step exactly level with the bottom of the piston---Then you know they're both equal and if they need adjusting from there which I doubt--do them both equally so you know where they're set compared to each other
On the 1/8" thing-- just check that you haven't got an old alloy washer jammed in the cap, if not then yeah, the supplied needle/seat sets seem to be getting longer.
If I have a problem getting the levels high enough i spin a tiddle off the gasket face of the seat in the lathe---The ratio is fairly high, you only need a light cut to make a big difference

With the 3/8" fuel level below the bridge, I've also heard quoted .160" for this but my SU book quotes 3/8"
Take your pick------------
William Revit

Those numbers look close to mine, Willy
- life got in the way today, but I'm keen to check tomorrow. I had wondered whether my newish needle/seat might be a tad longer than old ones - I'll measure some for comparison. Thanks for your thoughts as ever. Paul is also right - after this exercise, back to enjoying the motoring.
John.
J P Hall

Rightio Willy :
After adjusting carbs for best a/f ratio, road blasting and readjusting 'til the plugs are a nice clean light-brown burn, I've measured as follows. Keep in mind it's 35 celsius here in the daytime.
Jets now sit at 40 thou below the bridge (up from the initial 65thou setting.).
Fuel level: front carb - 0.485" below bridge.
: rear carb - 0.600" below bridge.
So I still have that rear carb with the lower fuel level, but as I said, the car is running well. I plan to compare the lengths of various needle seats in case I have one a that's a bee's dick shorter - that would level out the fuel levels. Or do your trick on a mate's lathe. Reading some of the woes some MGB owners have with their timing, carb settings etc, I'm mighty glad I have mine running sweetly. The sad thing is that I have to thin the flock next year in order to give more time to the old TC. So the BGT will have to find a new home. 'nother story! Thanks always.
John
J P Hall

Sounding good from here John---600 sounds a bit far down, so yeah, if you can get him up to the same as the other one it would be better, probably not noticeable driving it but you'll feel better knowing it's right and that'll make it feel good anyway--
Just out of interest, are you using any equipment to measure your a/f ratio and if so what are you setting it to/at--------and what fuel-

willy
ps. why sell the B, if you're stuck for room i could find a spot for the TC if you're needing cash---lol
William Revit

Well I thought I'd add a little addendum, Willy. Today Out of curiosity I measured the lengths of several seats, mostly older useable ones I've kept, but also a brand new one - measuring from the flat base (above the thread) to the top surface. No two were the same length. The range was 610 to 628, and everything in between. That begs the question - is the MGB seat a specific length, internal diameter etc for the HS4 carb, and different specs than for (say) the H2? I don't want to mix up mgb parts with TD or Y parts.
Air fuel ratio - at this stage just the "twist the screwdriver" test - borrowing a meter soon.
The TC - as you know, it's an old Melbourne (Templestowe In the 50s/60s I believe) hillclimber, subsequently kept for many years by Chris Gannon. I've been dreaming of running a TC for 50 years, and as I'm now 70, I'm running out of time to get stuck in to it, refurbish a few things and then just use it!
John.
J P Hall

You young blokes"---- get stuck into it and have some fun--I like a good T type--Just get it going and rego'd and then a rolling semi resto. keeping it driveable as much as you can so you can enjoy it-

Interesting with the needle/seat lengths
William Revit

"Young blokes" - sheesh!
So Willy, as far as I can tell from a bit of net surfing, the seat "should" be the same for the H2, H4 and HS4 - I can't find a specified length anywhere,but it looks like a standard internal bore of 96thou, or a fast road/racing version of something like 110. Nothing I can find yet about the length of the needle, whether it be old school brass, newer viton or the grose jet thing. I'm surprised, because if I'm right, the installed length of seat body and/or needle would have as much or more influence on fuel levels (float and jet) as would putting one or more shims under the base of the seat. SO - I'm happy to use whatever needle and seat combinations give me fuel levels the same in both carbs, and as near to spec as possible. Should finish that later today, and in the hope that it's of interest to others, I'll post the result. We never stop learning - in my case I seem to learn more as I get older. John.
J P Hall

To close this off:
Found the best combination of my various float needle seats (given that I now realise they can vary slightly in length) to give the same level of fuel in floats and jets, without needing any shims. Made sure both seats are the 96 thou bore. The gap between float and edge of (inverted) lid is now 4mm, so about mid-range.
Jets both at 62 thou below the bridge.
When the choke lever is fully engaged, the fuel in both bearings is 400 thou below the bridge.
Road test : perfectly fine - gonna take Paul and Willy's advice now, and just enjoy the drive.
Thanks all for your input.
John.

J P Hall

Just as a matter of interest, where exactly is the best place to bend the metal arm on these new-fangled "adjustable" floats? Moss document counsels against bending by pushing the float end (understandable), but nowhere can I find where is the recommended "bending point".
Logic says to me make an adjustment up at the hinge-pin end, to preserve the more or less horizontal plane of the float?
Grateful for others'experience.
John.


J P Hall

Seems to me it should be between where it turns to go into the float and the float valve. I'd use pointed pliers on the wire. I can see why pushing on the float isn't recommended, possibly damaging where the wire goes into the float, or bending right under the valve pin which might affect how it moves. Between valve pin and hinge is a very small distance so could also affect the valve. If you want to keep it level, and it's cocked at the right height, there should be space to put a second kink in to level it up. Although given the curved face to measure against I doubt it matters.

All mine are all-plastic though.
paulh4

Thanks Paul - I'll let you know how I go. I just have to remember which way to bend it to bring the fuel level up about 50thou.
John.
J P Hall

Well, to finish off - and because there must be other B owners like me determined to become masters of their own the SUs - I found the best result by (instead of bending and adjusting new Moss floats) :
-Putting the all-plastic floats back in and using the best combination of seats and viton needles to keep the 4mm clearance in the lid test.
-Setting the jets back to 65thou below the bridge and the fuel (in my case anyway) about 65 below that.
So I wasted a few bucks on a new pair of the Moss Unsinkables - probably give them away when the car goes.
Thanks all for your input.
John.






J P Hall

They look like the SU StayUp floats. Someone wrote on a forum that they had one sink, sent it back and SU said "You must have damaged it when fitting it" ... A bit like the Titanic then.

Incidentally as I said I have no experience of this type of float and from your picture it looked like a wire from the float to the hinge, but it seems now that was an optical illusion as they seem to be a flat metal strip which would take some bending where I suggested.
paulh4

No question you were right, Paul - just that I decided having eyeballed the Nitrophyl floats, that I'd have one more go at getting acceptable heights on the plastic floats - and managed to achieve what I wanted! So the Unsinkables can stay in reserve. John.
J P Hall

This thread was discussed between 01/12/2023 and 18/01/2024

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