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Adjusting SU HS6

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I just replaced Stromberg carbs with some SU HS6 1 3/4, the Strombergs has worn shafts so it was needed.

I got these SU’s with the car from the previous owner but I don’t know what car they came from. They got the wax stat jets so I took the stats out and replaced with two 1 penny coins. They got .100 jets with BDQ needles, after some research I found that they might work on a US TR6, I believe standard, but with removed emission equipment and with tubular manifold and big bore single exhaust.

 

As a starting point I adjusted both jets all the way up flush with the carb body and then down 1.25mm on both. Checked the float levels which were within spec, float needles close and nothing leaks, air vent on float chambers open. Both pistons fall down as they should, both carbs looks like they haven’t been used much, everything looks as it should, very little movement on the shafts. Oil on the dampers is 15-50 engine oil.

 

I hooked everything up yesterday and started the engine, struggled a bit as it’s no choke but after a while it ran fine with an even tickover, no air filters mounted. I balanced the carbs and both pistons rise at the same time, after a quick run up and down the seems to have good power, felt at least as powerful as with the Strombergs, no hesitation, misfire or other problems.

 

When I came to adjust the mixture I lifted the pistons 1mm using the lifting pins. When the mixture is correct this should lead to a slight increase in the revs and then settle down, in this case the revs fell immediately indicating a lean mixture.

So I adjusted the jets down little by little, but I had to adjust down a lot before the revs stopped falling, now the revs stay the same, so it indicated at least not a rich mixture? 

 

I removed the top and the pistons and the jets are now about 2.5-3mm down, a lot from the expected 1.25mm. Is this normal? 

 

When I look down on the jets I can see petrol on the front carburettor maybe 1-2mm down from the top, on the rear carb it’s 3-4mm down, indicating a lower level in the float chamber, strange thing is that the carb that has the lower level has has according to the float chamber level a higher level of petrol as the float had a closer tolerance, strange. I tested for air leaks with spraying easy start around the shafts etc but all seems good, for now I plugged the vacuum connections but will hook these up on the valve cover outlet later.

 

The question is: can the lifting pin method can be trusted for adjusting the mixture? 

 

I started the car this morning and it started right up without choke so I will assume it’s at least not lean but the opposite? I will get the air filter hooked up and take it for a drive and look at the plugs. Since I don’t know anything about the history of these carbs it might be that the needles need to be changed.

 

 

 

Magnus

 

 

Edited by TRseks

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Magnus,

The constant depression above the jet, ca 0.25psi,  will lift fuel 8 inches so its level in the jet is not crucial, as long as it doesnt overflow on slopes or acceleration.

The mixture is set by - approximately - the area between the needle circumferemnce and the jet bore. Tickover mixture is usualy set very rich indeed so the top of needle area varies little between needles, whatever the engine. In other words setting tickover mix with the piston lifter will only work if you know the rest of the needle is correct for your engine. 

Peter

Edited by Peter Cobbold

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Ok thank you for that, I just thought it is strange that it indicates a lean mixture when turning the jet down that much in theory it should be rich. That it starts fine without choke indicate the opposite... 

Well I will get sorted with air filter, take it for a drive and see what the plugs look like.

 

 

Magnus

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2 hours ago, Peter Cobbold said:

Magnus,

The constant depression above the jet, ca 0.25psi,  will lift fuel 8 inches so its level in the jet is not crucial, as long as it doesnt overflow on slopes or acceleration.

The mixture is set by - approximately - the area between the needle circumferemnce and the jet bore. Tickover mixture is usualy set very rich indeed so the top of needle area varies little between needles, whatever the engine. In other words setting tickover mix with the piston lifter will only work if you know the rest of the needle is correct for your engine. 

Peter

Hi Peter,

Glad to see you here again!

And this post is just an example why the forum needs people like you, amongst your other thoughts.

Best regards,

Waldi

 

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19 hours ago, Waldi said:

Hi Peter,

Glad to see you here again!

And this post is just an example why the forum needs people like you, amongst your other thoughts.

Best regards,

Waldi

 

Yes good to hear from you Peter.

Magnus, you may find those needles a bit weak. I used to run BAG on my engine before I had it rebuilt. 

With a Stage 2 tune its now running on BBA needles which were almost half the width of the other needles. I had it tuned on a Rolling Road by Tom Airey who did a great job.

Cheers Darren 

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Good News

 

Mintylamb is back on air so play with it and check out the graphs to see which needle in theory suits your needs. I did this with my 3A years ago after advice on the forum and it helped me choose the correct needles for my car

http://www.mintylamb.co.uk/suneedle/

 

Also Burlen fuel systems sell a reprint  of the excellent SU Carburetters tuning tips fand techniques or around £15  which is indespensible for helping understand and tune SU's

isbn 9781855202559

 

cheers

Alan

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That is good news re mintylamb web site being back up - often came across links to it which wouldn't work 

 

Magnus - would be interested in any pics you can put up ? - I have a acquired a pair of TR7 HS6's for future fitting 

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I had a look at mintylamb, very useful thanks. Can see that BAG needles are a bit richer than BDQ needles.

 

The thing is that the engine is a US model and probably has the low compression head with a milder cam, so would need a leaner needle than a UK PI model, unless the head is ported, skimmed and with a different cam, since I don't know the history of the engine it's hard to say. Will drive the car with the BDQ needles and see how it turns out, if too lean I will probably try f.ex BAG needles.

 

I bought 4 "lost motion brackets" from Moss, and a 5/16 bar from eBay, combined this with the existing bracket from the Stromberg setup, welded on a homemade bracket to the 5/16 bar which linked to the old bracket and it now works fine.

Need to fit air filters, hook up the valve cover vent hose to the carbs and will also fabricate a heat shield for under the carbs as the tubular SS manifold creates a lot of heat.

 

Some pics:

 

https://www.moss-europe.co.uk/catalogsearch/result/?search_group=&q=aue587

 

YKtob15.png

 

GEzR0KR.jpg

 

Ns67XYR.jpg

Edited by TRseks

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The intake manifold shown in the pic. is from a 72 and later engine. It has to be used with a head of the same vintage as the intake port spacing is narrower on the earlier engines. I noticed that your car is a 1970. Do you know if the head or engine has been changed from the original?

Berry

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28 minutes ago, dingle said:

The intake manifold shown in the pic. is from a 72 and later engine. It has to be used with a head of the same vintage as the intake port spacing is narrower on the earlier engines. I noticed that your car is a 1970. Do you know if the head or engine has been changed from the original?

Berry

 

Yes the head is what I remember from 1974, the block I believe is 1970 so it should be ok. The Strombergs carbs which was used until recently I believe was 1970 as well so a bit of a mix.

 

Magnus

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Thanks Magnus 

 

Pics and links are really helpful 

 

cheers

 

Matt

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I fitted twin 13/4” su carbs from a saloon car to a US cc engine without any dramas, I set the nuts underneath to 2 full turns open disconnected the linkages and started the car. With a gunson air balancer, balanced the air flow into the carbs and refitted the linkages with a gunsun co2 meter I adjusted the mixture on both carbs a flat at a time until I had a reading of 4.5% then reduced the tick over speed on one idle screw to 850 rpm. She ran fantastic afterwards

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6 hours ago, elclem1 said:

I fitted twin 13/4” su carbs from a saloon car to a US cc engine without any dramas, I set the nuts underneath to 2 full turns open disconnected the linkages and started the car. With a gunson air balancer, balanced the air flow into the carbs and refitted the linkages with a gunsun co2 meter I adjusted the mixture on both carbs a flat at a time until I had a reading of 4.5% then reduced the tick over speed on one idle screw to 850 rpm. She ran fantastic afterwards

 

Do you know what needles that was fitted?

 

 

Magnus

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I will see if I can find the info but it was some time ago and the car is no longer with me. Clem

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