PaulAA Posted October 2, 2017 Report Share Posted October 2, 2017 I know that that this has been discussed repeatedly before, but, rather than asking for advice, I'm keen to find out what people who are running SU HS6s think of their needle choice. I am not sure about mine. I have a US head, skimmed to CR of approx 9.5 and mated to a Wishbone 518v2 cam. Everything else on the engine is standard, with consistent cylinder pressure of roughly 120psi wet across all six cyls. I have twin HS6s, which are regularly adjusted with gas analysis to a CO output of 5%, but I would like to see if different needles will make a difference to power delivery and efficiency. I have BDM needles. What are you SU folk out there running and what do you think of your choice? Paul Quote Link to post Share on other sites
DRD Posted October 2, 2017 Report Share Posted October 2, 2017 I've got BAG needles, they seem fine. My engine is a similar spec but has a gas flowed early CP unleaded head. Your pressures seem rather low, maybe its your gauge? Darren Quote Link to post Share on other sites
PaulAA Posted October 2, 2017 Author Report Share Posted October 2, 2017 Hi Darren It's my neighbour's gauge: probably time to buy my own... Thanks for the feedback. Cheers Paul Quote Link to post Share on other sites
mike3md Posted October 2, 2017 Report Share Posted October 2, 2017 Darren, Have you used this to compare profiles? http://www.mintylamb.co.uk/suneedle/ Mike Quote Link to post Share on other sites
DRD Posted October 2, 2017 Report Share Posted October 2, 2017 Darren, Have you used this to compare profiles? http://www.mintylamb.co.uk/suneedle/ Mike Yes it's a very useful app. Quote Link to post Share on other sites
astontr6 Posted October 4, 2017 Report Share Posted October 4, 2017 Hi Paul, My book gives the following based on the fact that these needle recommendations are going to be checked out on a Rolling Road etc. HS6 HS6 175CD Fixed Biased Fixed Standard BDB 2E +EXH TW 2D +K+N BDQ +HEAD BDM 1AS +CAM SM BAE Rich BAM Hope this helps, Bruce. Quote Link to post Share on other sites
PaulAA Posted October 4, 2017 Author Report Share Posted October 4, 2017 Hi Bruce Thanks - that's interesting. Both BAE and BAM are on my list, but my challenge at the outset of this process is the fact that (i) needles are 20 quid a pop each and (ii) I can't find a rolling road in this neck of the woods, which comes equipped with anything other than a spotty oik young mechanic who has never heard of carburettors. Paul Quote Link to post Share on other sites
Peter Cobbold Posted October 4, 2017 Report Share Posted October 4, 2017 (edited) Paul, Not the young mechanics fault- SUs are a lot more complicated than most of us realise. Tuning an SU needle at full throttle ona rolling road is asking for too-lean mixtures at cruise. DIY on the road with a UEGO for AFR and inspection camera for piston lift (=needle station) Slides 65 onwards of the inaudible Malvern talk: https://supertrarged.wordpress.com/2017/07/10/how-does-an-su-carburettor-work/ Peter Edited October 4, 2017 by Peter Cobbold Quote Link to post Share on other sites
PaulAA Posted October 4, 2017 Author Report Share Posted October 4, 2017 Thanks, Peter I promise that I read and digested the presentation (I think I asked you for clarification on a couple of points at the time). Regrettably, ambition far outstrips both my (limited) skill and my resource and............. I was trying to short-cut by peer group comparison. Caught red-handed! Paul Quote Link to post Share on other sites
Peter Cobbold Posted October 4, 2017 Report Share Posted October 4, 2017 Thanks, Peter I promise that I read and digested the presentation (I think I asked you for clarification on a couple of points at the time). Regrettably, ambition far outstrips both my (limited) skill and my resource and............. I was trying to short-cut by peer group comparison. Caught red-handed! Paul Paul, Whats the problem with the present needles? perhaps we can find a solution. The last slide shows the huge tolerance on mixtures. If the plugs look good it might be more productive and cheaper to focus on the sparks. A PI advance curve wont get the best out of a conversion to carbs. Peter Quote Link to post Share on other sites
Tim D. Posted October 5, 2017 Report Share Posted October 5, 2017 Where are you based? Have used a number of rolling roads that know their SUs might be able to.suggest someone. Tim Quote Link to post Share on other sites
stuart Posted October 5, 2017 Report Share Posted October 5, 2017 Paul, Whats the problem with the present needles? perhaps we can find a solution. The last slide shows the huge tolerance on mixtures. If the plugs look good it might be more productive and cheaper to focus on the sparks. A PI advance curve wont get the best out of a conversion to carbs. Peter Its a US import so never been PI and finding a rolling road in Poland that understands SU`s is improbable to say the least! Stuart. Quote Link to post Share on other sites
PaulAA Posted October 5, 2017 Author Report Share Posted October 5, 2017 Paul, Whats the problem with the present needles? perhaps we can find a solution. The last slide shows the huge tolerance on mixtures. If the plugs look good it might be more productive and cheaper to focus on the sparks. A PI advance curve wont get the best out of a conversion to carbs. Peter Peter No problem as such, but, other than the mpg average, I don't have a point of reference to determine whether the choice of needles is optimal. Plugs are a reasonable shade of pale grey, no sign of deterioration after about 3,000km. Engine is stock US model, except for the skimmed head and wishbone cam. Dizzy is original US D22 with advance only, but disconnected. Appreciable acceleration is in a relatively narrow band (1,800 to 3,500rpm). Power peters out by 4,000rpm, but the engine can be persuaded to spin up to 5,000. More not. Having established (and now re-checked) CO at tickover at around 5%, with timing at 12deg BTDC, I'm assuming that the principal variable is fuel metering. It may be that the BDM is the optimal profile (which Chris Witor has suggested is more likely to be for the Dolly Sprint), but I was wondering what others' experience of other needle profiles is. Paul Quote Link to post Share on other sites
Peter Cobbold Posted October 5, 2017 Report Share Posted October 5, 2017 (edited) Paul, Peter No problem as such, but, other than the mpg average, I don't have a point of reference to determine whether the choice of needles is optimal. Plugs are a reasonable shade of pale grey, no sign of deterioration after about 3,000km. Engine is stock US model, except for the skimmed head and wishbone cam. Dizzy is original US D22 with advance only, but disconnected. Appreciable acceleration is in a relatively narrow band (1,800 to 3,500rpm). Power peters out by 4,000rpm, but the engine can be persuaded to spin up to 5,000. More not. Having established (and now re-checked) CO at tickover at around 5%, with timing at 12deg BTDC, I'm assuming that the principal variable is fuel metering. It may be that the BDM is the optimal profile (which Chris Witor has suggested is more likely to be for the Dolly Sprint), but I was wondering what others' experience of other needle profiles is. Paul Paul, A single capsule is for vacuum retard. I think it retards the spark about 6 crank degrees at tickover and overurn. It can be ignored. The lack of go above 4000 might be due to a lack of advance. As a test set the static to say 18 BTDC and see if it pulls better at higher rpm. TR250 centrifugal advance might be closest to what you want as it had 9.5 compression. Do you know your comp rat ? Probably that's line F or G in the table here: http://www.tr-register.co.uk/forums/index.php?/topic/24915-tr6-ignition-timing/page-2 Good idea to check with a strobe lamp what adv curve you actually have. Is the TDC mark on the pulley true ? Peter Edited October 5, 2017 by Peter Cobbold Quote Link to post Share on other sites
PaulAA Posted October 6, 2017 Author Report Share Posted October 6, 2017 Is the TDC mark on the pulley true ? Peter Peter Key question... and I can't vouch for the accuracy of the TDC mark. I'm sure that if I ask the mechanic who rebuilt the engine two years ago, it will be a resounding 'yes', but I suppose that my only opportunity for confirmation is the next time I have reason to fetch the head off. Compression ratio is approx. 9.5, so yes, the TR250 profile is probably the one I should aim for. Whether relevant or not, there is also a persistent single 'miss' at 1,900rpm during acceleration. The dizzy cover is good and clean, as are all connections - no cracks or leaks. The dizzy itself was stripped and cleaned along with the engine two years ago. Wear, maybe? Paul Quote Link to post Share on other sites
Peter Cobbold Posted October 6, 2017 Report Share Posted October 6, 2017 (edited) Peter Key question... and I can't vouch for the accuracy of the TDC mark. I'm sure that if I ask the mechanic who rebuilt the engine two years ago, it will be a resounding 'yes', but I suppose that my only opportunity for confirmation is the next time I have reason to fetch the head off. Compression ratio is approx. 9.5, so yes, the TR250 profile is probably the one I should aim for. Whether relevant or not, there is also a persistent single 'miss' at 1,900rpm during acceleration. The dizzy cover is good and clean, as are all connections - no cracks or leaks. The dizzy itself was stripped and cleaned along with the engine two years ago. Wear, maybe? Paul Paul A 'piston stop' made from an old sparkplug will allow TDC to be checked - no need to remove head. The outer rim of the pulley carrying the TDC mark can move in relation to the crank keyway because the rubber torsion-damper annulus distorts with age. That 1900 rpm miss is a useful clue. If the miss at 1900rpm happens at full and part throttles that eliminates carbs/mixture. You then need a simple strobe lamp to measure the advance curve over the rpm range, car stationary. Caution: fan hitting hands. The previous owners may have 'improved' the disy weights/springs. (Its all too easy for them to fit replacement springs that are the same fit end to end, but the wire diameter is critical to the stiffness.) Once you know what you've got it should be possible to get closer to the TR250 curve. Peter Edited October 6, 2017 by Peter Cobbold Quote Link to post Share on other sites
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