HS4 tuning

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mobylette
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HS4 tuning

Postby mobylette » Tue Jul 24, 2018 12:18 pm

Following on from my "no oil pressure" problem, I am now trying to get the carburation correct on the rebuilt engine. I had a problem with the carb which was somewhat resolved by putting correct grade oil in the dashpot. Things are much improved, but there is still a hesitancy in pick up especially from idle. The mixture has been set correctly at idle using the lifting pin.The car will run much better with the choke (enrichment) out, in fact it pulls like a train, but of course hunts at idle until the choke is pushed back in. I was wondering if there is too much air getting into the inlet via the breather on the rocker box, and whether there should be a restricter introduced into the pipe. I intend to try the car with the pipe blocked off to see what difference this makes. I have attached a picture of the current set up.
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mobylette
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Re: HS4 tuning

Postby mobylette » Tue Jul 24, 2018 12:29 pm

I should add that the carb was a little used item originally fitted to a 1275 mini. I have just tried it with the pipe blocked off, no difference, so I guess I am looking a different needle?

Monty-4
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Re: HS4 tuning

Postby Monty-4 » Tue Jul 24, 2018 12:43 pm

I wonder if it might be worth trying a stronger dashpot spring (yellow I epect) before going through a range of needles and testing. Do you know which needle and spring is in there now? AAA is often quoted as a starting point for mildly upgraded 1275s.
Last edited by Monty-4 on Tue Jul 24, 2018 1:02 pm, edited 2 times in total.
68' 4-door Saloon, another 'Monty'.

pgp001
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Re: HS4 tuning

Postby pgp001 » Tue Jul 24, 2018 1:00 pm

It might be worth getting in touch with Burlen Fuel Systems, they may have some data they could share with regard to needles and springs etc.

https://burlen.co.uk/
http://sucarb.co.uk/cf/vehicle/list/?ma ... icle=Minor

Phil

mobylette
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Re: HS4 tuning

Postby mobylette » Tue Jul 24, 2018 1:30 pm

Thanks, I should add my engine is standard spec 1098 apart from inlet manifold and carb. I don't know the current spring and needle so will check.
Cheers
Martin

philthehill
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Re: HS4 tuning

Postby philthehill » Tue Jul 24, 2018 2:36 pm

Before the advent of the web and easily obtained SU spare parts I used to have to modify my SU carb needles to suit the application.
I would therefore suggest that you lightly run a very fine file down the length of the needle and fit the needle so as the flat (if you could call it that) is facing the air cleaner.
You may have to do it several times to get it right but it worked for me every time.

There should be a tag on the carb which gives the number of the carb i.e. AUD354 which has a AAQ needle & Yellow spring. Interrogating the information on the Burlen site and comparing the number of the carb you should be able to determine the needle and spring colour that is if it has not been messed around with.
Phil
Last edited by philthehill on Tue Jul 24, 2018 2:44 pm, edited 1 time in total.


liammonty
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Re: HS4 tuning

Postby liammonty » Tue Jul 24, 2018 2:40 pm

If you don't know what needle is in there, then as you suggest, find out! I would suggest that until you get the needle in the ballpark, there's no point doing much else. If it's set up right at idle, but lean at speed, then that suggests to me that the needle is likely wrong for your set up - you said it pulls better with a bit of choke at speed which suggests it's lean in the mid-high range. If you change the oil in the dashpot, change the spring etc to improve the flat spot on acceleration, it may help with the flat spot on initial acceleration, but won't cure the general running. When you know the needle is right, you need to look at those things. So deal with it methodically - needle first, then look at dashpot oil if required, and spring if needed too. I can't imagine that you would need a restrictor in the breather - that would be a 'sticking plaster' to mask the symptoms of a carb that isn't set up right.

People will disagree with me on this, but the only way to get the needle right without days of messing around is a rolling road session. If you shop around, you should be able to get this for not much over £100. It really is money well spent - I'm speaking from experience here as I have been through what you're going through now! If you don't do that and try different needles with road testing alone, the danger is that you end up with a needle that is far too rich in the mid range - the symptoms of rich running are less pronounced than lean so you may well not even notice. I used the AAA (in an HIF38) on my 1030 (overbored 948) and it was way too rich, but of course it's a different set up to yours.

Good luck!

philthehill
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Re: HS4 tuning

Postby philthehill » Tue Jul 24, 2018 4:16 pm

liammonty
I do not disagree with you as regards the rolling road but £100 plus is £100 plus and then you will have the needle(s) and spring(s) etc. to pay for.
Also some distance may need to be travelled to find a rolling road that has spare needles, jets and damper springs readily available.

For a little bit of effort and keeping a eye on the plug colour that £100 plus could be well spent elsewhere.
Phil


mobylette
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Re: HS4 tuning

Postby mobylette » Thu Jul 26, 2018 5:03 pm

Right, I have found the spec tab, the carb is an HS4 AUD325AR. A bit of googling shows that this is specified as one of a pair for an MGB 1800cc. I can't find any other information to suggest this was ever specified for a 1275 A series mini engine (as the seller stated).

After a bit more fiddling and test drives I think I have come to a reasonable compromise. I have richened the idle mixture as much as possible without causing hunting and now have a clean pick up from idle with the engine pulling well. I will go for a longer drive soon, and check the plug colour.

philthehill
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Re: HS4 tuning

Postby philthehill » Thu Jul 26, 2018 9:10 pm

The damper spring is the same as the 1098cc Minor 1000...……….AUC4387 which is yellow.
The needle for a AUD325 is FX.

The carb for the Minor 1000 is AUD354.
The damper spring for a AUD354 is AUC4387 which is yellow.
The needle for a AUD354 is AN.

I would suggest that you are not far out with your current settings so just keep an eye on the colour of the plugs.
If still after a reasonable run/period the plugs are showing rich you could always try weaker needle GZ.


mobylette
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Re: HS4 tuning

Postby mobylette » Fri Jul 27, 2018 8:21 am

Thanks for that information Phil. I may well order the AN needle and see what difference it makes. I will have a bit of driving about and plug checking first. Cheers . Martin.

philthehill
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Re: HS4 tuning

Postby philthehill » Fri Jul 27, 2018 8:47 am

If I was going to purchase a needle it would be the GZ because the taper of the FX needle appears to be about right so the weaker GZ needle would give a slightly weaker mixture throughout the rev range.
I see that needles are on special offer at Burlen Fuels.
Phil


liammonty
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Re: HS4 tuning

Postby liammonty » Fri Jul 27, 2018 3:44 pm

I thought the OP suspected weak running currently, as the car runs better through the Rev range with the choke out?!

philthehill
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Re: HS4 tuning

Postby philthehill » Fri Jul 27, 2018 6:31 pm

The FX needle (standard for the MGB carb) whilst screwed down sufficiently to attain better running of the Minor engine is possibly too rich for the engine. When splitting the airflow by 2 (the MGB has 2 x 1.5" carbs for the 1800cc engine) the airflow through each carb will be greater than when the same single 1.5" carb is fitted to the Minor. Therefore as the air flow requirements into the Minor engine will be less with the single 1.5" carb I would suggest that the carb would require the weaker needle.
Only road testing and checking the colour of the plugs (should be biscuit brown) will determine if the needle is right.
Any other way of checking that the needle is right outweighs the cost of procuring one or more needles.
Phil


mobylette
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Re: HS4 tuning

Postby mobylette » Fri Oct 12, 2018 3:43 pm

To update this problem, I have been driving the car with the idle far too rich to give reasonable pick up, but decided today to take the HS4 carb top off and have a look see.

I should have done this at the beginning, but the carb looked so new and untouched that I assumed (never assume) all was well. On dismantling I found that the spring under the biased needle was missing. I would not have thought that this would have made much difference, but I made one up out of a biro spring. I set up the needle holder as per the Burlen site. It may not have been set correctly before, but I didn't check (I have never met a spring loaded SU needle before). Before starting the engine I weakened the mixture by 8 flats and set off. Once warm the engine was idling too fast but smoothly and the pick up from idle was fine. Reset idle speed and all is good. I am very surprised at the difference this tiny spring has made. Thinking about it now, I suppose that the needle flapping about in the jet could have been the cause of the surging when coming off idle. Richening the mixture would have masked this.

The needle fitted is marked ABP, I looked at the spring, but the paint had faded , there was some red but I didn't see the other colour.

On another matter, if any one needs welding in the Surrey area I have someone I can highly recommend. He recently did some work for me which I am very happy with. He also offers a mobile welding service. I have no connection etc etc..

geoberni
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Re: HS4 tuning

Postby geoberni » Fri Oct 12, 2018 10:34 pm

On dismantling I found that the spring under the biased needle was missing. I would not have thought that this would have made much difference, but I made one up out of a biro spring. I set up the needle holder as per the Burlen site.
OK, you lost me at this point.
Which spring?
This little one behind the needle?
SU4 Spring.JPG
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Basil the 1955 series II

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mobylette
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Re: HS4 tuning

Postby mobylette » Sat Oct 13, 2018 7:39 am

Yes, that's the one. Without it the needle can just flap about. When fitted the needle is pushed to one side.

geoberni
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Re: HS4 tuning

Postby geoberni » Sat Oct 13, 2018 9:49 am

Thanks
I watched the Carb strip demo at the Derby Offices Open Day last weekend and that aspect wasn't mentioned when the needle came out.
Basil the 1955 series II

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philthehill
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Re: HS4 tuning

Postby philthehill » Sat Oct 13, 2018 10:16 am

Not all HS4 carbs have the same construction/needle/jet arrangement.
The type being discussed is a HS4C which has the floating needle.
See Burlen Fuel's technical section on SU carbs for further details.

http://sucarb.co.uk/technical-hs-type-c ... djustments


mobylette
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Re: HS4 tuning

Postby mobylette » Wed Oct 31, 2018 2:40 pm

Final update hopefully.

The manifold I fitted has the facility to be plumbed in to the heater circuit. I have read conflicting views on this. Some say you want to keep the incoming charge as cool as possible, others say by warming the manifold it prevents the mixture condensing. Anyhow I found that the engine needed to be kept on choke much longer than when fitted with the original cast iron "hot spot" manifold, and this exacerbated my perceived weak running problems. Today i plumbed in the manifold and the engine is now running & pulling better than ever. :D

I have noticed that the heater only blows look warm, so I will be checking the thermostat next, but thing are definitely going in the right direction.
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