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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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  • 1 year later...
On 5/26/2019 at 9:16 PM, TRseks said:

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

Hi, I have an almost identical set up with a US engine, SU’s, probably low compression head and mild cam. Could I ask what needles you decided were best in the end BDQ, BAG or perhaps another? 
Thanks Adrian

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8 hours ago, Adrian Pettitt said:

Thanks John, think that would be ok with a lower compression US engine? 

The volume of mixture per stroke is not affected by comp rat and the mixture strength (AFR) for combustion has a large leeway. Almost any AFR from 15 to 10 will combust fine. The needle profile was determined by experiment and experience by SU, records long lost. The cruise positions on the needle deliver a weaker mixture but the same position on the needle will deliver a richer mixture at >30deg butterfly opening. If you had a higher than normal CR that mixture might need to be richer ( needle thinner) to avoid detonation. But a lower CR than standard does not need richer mixtures, it wont knock even on 95 RON.

Getting the best mpg needs fine-tuning the cruise mixture and optimising the vac advance. Thats where finessing the needle becomes important. But a low comp head is nbg for best mpg !!

Peter

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Adrian,  That spec is for a conversion from USA engine originally with CD Strombergs, to SU's, but I have no idea what compression the USA engines had with Strombergs.

John

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USA CRs were progressively lower; 8.5, 8 and 7,75 in final year. Needed to comply with NOx legislation, maybe in CA

Peter

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My US spec 6 1970 8.50. 72/73 models 7.75. 74 model 7.5.   I fitted a pair of HS6''s from a 2500TC I will try to find which needles I fitted, I seem to recall that they were BDQ.  I also replaced the exhaust manifold with an extractor and also with sports exhaust.  No complaints regarding performance since I changed form the breathless Stromberg and the emissions set up 

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On 5/10/2021 at 5:36 PM, Adrian Pettitt said:

Hi, I have an almost identical set up with a US engine, SU’s, probably low compression head and mild cam. Could I ask what needles you decided were best in the end BDQ, BAG or perhaps another? 
Thanks Adrian

Hi Adrian, I installed the BDQ needles and the engine ran ok with good cold start, choke on at first but quickly off, and the appropriate colour on the plugs. But I noticed (or maybe it was just in my head) that the engine was hesitating on higher RPM compared to the Strombergs. So I had a look at which needles would be richer on higher RPM, and bought a set of BFW needles. With these installed the engine actually misfired at higher RPM so I changed back. But misfire was still there so I suspected the spark plugs, which were old. So with new spark plugs the misfire was gone but I still suspect the high RPM performance is a bit less then what I remembered from the Strombergs, so I will install the BFW needles again to see what happens this time with the new plugs.

 

My engine is as far as I found from 1970, but with a newer head from 1974, not sure if a previous owner skimmed a bit off the head or even changed the cam, probably impossible to know without taking it apart. I got SS headers from Racetorations and a big bore exhaust. Original distributor with disconnected retard. All the emmission system is long gone. I also removed the waxstat`s from the SU`s, replaced with penny coins.

 

Magnus

Edited by TRseks
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Thanks for the help everyone, particularly Magnus for the jet advice. My car has the crankcase emissions modified and this canister fitted which I don’t think is particularly effective. I’m a little mystified that one pipe is vented to atmosphere down near the block and the other connected to the rocker cover. I’d assumed that the only vent to atmosphere was via the filter? From experience what have others done with a twin SU set up with a manual fuel pump? 
1, just vent the rocker cover to atmosphere underneath?
2, vent the rocker cover to the inlet manifold somewhere?
3, fit an electric fuel pump and vent the crankcase via the mechanical fuel pump aperture and the rocker cover somewhere. 
 

I just get the feeling that this tank arrangement whilst pretty is over pressurising the crankcase somewhat? 
 

Any help would be great. 
P.S. Also is an 88 degree thermostat too hot for this engine? 

D5BE4BF2-2945-407D-B150-9B8B883C2A27.jpeg

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I vented the rocker cover to the carbs, this might not be possible on all SU carbs? I did this as I was adviced it would be good for decreasing crankcase pressure, seems to work ok. Oil leaks from the engine is acceptable, whatever that means...!

 

I also made a heat shield out of two layers of 1mm aluminium with heat mats between and underneath, and around the starter motor as the SS manifold radiate more heat than the cast iron original, and modern fuels are more prone to struggle with hot days.

 

OJiH6sX.jpg

 

kGQlrmE.jpg

 

Magnus

Edited by TRseks
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1 hour ago, TRseks said:

I vented the rocker cover to the carbs, this might not be possible on all SU carbs? I did this as I was adviced it would be good for decreasing crankcase pressure, seems to work ok. Oil leaks from the engine is acceptable, whatever that means...!

 

I also made a heat shield out of two layers of 1mm aluminium with heat mats between and underneath, and around the starter motor as the SS manifold radiate more heat than the cast iron original, and modern fuels are more prone to struggle with hot days.

 

OJiH6sX.jpg

 

kGQlrmE.jpg

 

Magnus

Magnus, you are fast becoming my favourite poster! See photo of my carbs below, I will be able to modify mine in the same way! Does your t piece have a non return valve in? Thanks Adrian 

193DE202-6EC8-4D9F-A2C1-7FD221431DED.jpeg

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