Iain T Posted April 18, 2022 Report Share Posted April 18, 2022 One for the technical gurus. I took the car out for the first time in a few weeks and it ran fine but still (according to the afr gauge) running weak at over 3500 on acceleration. Tickover is ok at 700rpm and around 13.5 afr. I have 6S needles which have much more taper than the standard 6AC to cope with the better air flow of my head etc. I do have a pair of 7B and 6J needles but they are narrower in the first half and would make the mixture richer. Not something I need. A lot more head scratching, tea and alcohol, not necessarily in that order required. Is it worthwhile tapering the 6S needle. If so where? The last 3rd? Lengthening the spring to increase the force pushing the air piston down? Any other tests anyone can think of. A clue might be at a stable higher rev/speed the afr reads it is weak? I use 20/50 engine oil in the dash pots. Looking forward to meeting at Duxford, I'm praying for no rain🙏 Iain Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 18, 2022 Report Share Posted April 18, 2022 a stronger or heavier spring will richen up there are different diaphragm clamp rings of different weights ie a 1600 has no spring but double thick heavy clamp rings Pete Link to comment Share on other sites More sharing options...
Iain T Posted April 18, 2022 Author Report Share Posted April 18, 2022 10 minutes ago, Pete Lewis said: there are different diaphragm clamp rings of different weights ie a 1600 has no spring but double thick heavy clamp rings Pete, I have the standard springs fitted. I do have the lighter springs but then that would make it even weaker. The third spring rate for a CD Stom is 3 times the force of mine (120g vs over 300g) and seems way too strong. But what do I know.....not a lot! Perhaps they would work? Clamp rings? What are they? Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 19, 2022 Report Share Posted April 19, 2022 its the ring that holds the diaphragm to the air piston with the 4 screws there are std and heavy versions i guess you could make up a weight as a trial idea to sit on the top Pete Link to comment Share on other sites More sharing options...
Iain T Posted April 19, 2022 Author Report Share Posted April 19, 2022 1 hour ago, Pete Lewis said: its the ring that holds the diaphragm to the air piston with the 4 screws there are std and heavy versions i guess you could make up a weight as a trial idea to sit on the top Another variant into the pot, where's that bottle of rum! Wouldn't that then make the low rev too rich? I will try the 7B needles to see if the slightly more end taper has the desired effect at constant higher revs. On WOT it's fine until the revs build up. Trying to alter the taper on a needle and matching a pair is an art form I don't really want to try. Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 19, 2022 Report Share Posted April 19, 2022 well the weight idea is a constant the spring is progressive and in the club museum is a needle lathe dont know if it works Pete Link to comment Share on other sites More sharing options...
Iain T Posted April 19, 2022 Author Report Share Posted April 19, 2022 I think I need to check if the air piston is rising to the top before I get to high revs. Difficult to do in my carpark so I'll take it some place I won't annoy the neighbours. If it is then it could be solved by spring or clamp weight. If it isn't rising too early then its needle twiddlin or change. The 6A needle tapers more from the 8th position on down to 0.050" for 11 to 13 positions. BP and Shell will be happy! Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 19, 2022 Report Share Posted April 19, 2022 when we first upped the 1600 to whatever it achived Dave Pearson suggested he thought they used 13/60 needles (6E) in their tuned 1600s i tried them but they didnt suit so we stuck with std airbox and std needles . no springs in the1600's to play with be they Stroms or SU the spring range seems to jump from weady to bed springs with little inbetween never played with 2 litre high tech tuning Pete Link to comment Share on other sites More sharing options...
Iain T Posted April 19, 2022 Author Report Share Posted April 19, 2022 1 hour ago, Pete Lewis said: be they Stroms or SU the spring range seems to jump from weady to bed springs with little inbetween They do indeed. Been for an Italian tune and holding at a constant 4500rpm the afr reads 14-14.5 so actually OK? It seems as though it goes lean on acceleration at higher revs therefore spring or heavier dashpot oil change? Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 19, 2022 Report Share Posted April 19, 2022 yes add some goo to the dash pot see what happens Pete 1 Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 20, 2022 Report Share Posted April 20, 2022 as the dashpot is the enrichment device as would an accelerator pump then slowing the rapid rise will richen up a flat spot on accelerating not sure that ep90 is more viscous than 20/50 but its a simple thing to play with pete Link to comment Share on other sites More sharing options...
Iain T Posted April 20, 2022 Author Report Share Posted April 20, 2022 1 hour ago, Pete Lewis said: not sure that ep90 is more viscous than 20/50 but its a simple thing to play with My thoughts exactly, easy to change and the sun's out a nice day for a drive! Link to comment Share on other sites More sharing options...
Clive Posted April 20, 2022 Report Share Posted April 20, 2022 16 hours ago, Iain T said: They do indeed. Been for an Italian tune and holding at a constant 4500rpm the afr reads 14-14.5 so actually OK? It seems as though it goes lean on acceleration at higher revs therefore spring or heavier dashpot oil change? Stronger springs/heavy dashpot will/may mean a possible change in needles too. But just acceleration enrichment looks like heavier oil. There are heavier (25-60, 40-75) etc oils out there. Not sure how hot the dashpots get, but I have some straight 50 weight oil here that is like treacle when cold. Or are there different piston dampers available? Link to comment Share on other sites More sharing options...
Iain T Posted April 20, 2022 Author Report Share Posted April 20, 2022 28 minutes ago, clive said: Stronger springs/heavy dashpot will/may mean a possible change in needles too. But just acceleration enrichment looks like heavier oil. There are heavier (25-60, 40-75) etc oils out there. Not sure how hot the dashpots get, but I have some straight 50 weight oil here that is like treacle when cold. Heavier oil is a quick and simple solution but the engine/carbs must be up to temp to fully trial. Various oil viscosity/temperature is a nightmare. I'll report back. Iain Link to comment Share on other sites More sharing options...
Clive Posted April 20, 2022 Report Share Posted April 20, 2022 Muligrade oil should help, but will still thin when hot. I reckon carbs get "warm" but not too hot to touch. Probably 70degrees? But that will thin even a muligrade. Too thick and the pistons will be stuck on cold days. Maybe summer and winter weight oils? Link to comment Share on other sites More sharing options...
Iain T Posted April 20, 2022 Author Report Share Posted April 20, 2022 1 hour ago, clive said: Muligrade oil should help, but will still thin when hot. I reckon carbs get "warm" but not too hot to touch. Probably 70degrees? But that will thin even a muligrade. Too thick and the pistons will be stuck on cold days. Maybe summer and winter weight oils? I believe a 20W50 behaves as a 20 grade when cold, for easy starting, then 50 (thicker) when hot. It's going to be trial and error but at least it gives the car a blast! On another note I changed the oil, the Heritage(?) one from the club bought at Duxford, and noticed the pressure is higher. From cold pressure is 90psi! I now seem to have developed an old car dribble of nice clean oil in the catch tray🥺. More bloomin' digging around to find where its coming from....... Iain Link to comment Share on other sites More sharing options...
Iain T Posted June 27, 2022 Author Report Share Posted June 27, 2022 Update, I spoke to a an oil supplier who suggested using SAE50 single grade oil in the dash pots. He said this would remain more viscous at normal running temperatures. I tried it and after a good run on Sunday it certainly did make a difference. Acceleration at higher revs it didn't lean out. Early days but fingers crossed as its an easy fix. Iain Link to comment Share on other sites More sharing options...
Iain T Posted August 2, 2022 Author Report Share Posted August 2, 2022 Another update for those of us who are foolish or bored enough to twiddle from standard. The 6J needles are OK low end but go lean over 3000rpm, the 6S needles are lean low end and ok top end. I've tried both with 20/50 and 50 single grade. So I installed the 7B needles and 20/50 dashpot oil. Bogged down and kangaroo'd at low revs and I could see the AFR meter jumping up indicating too lean. So I tried the heavier SAE50 oil and so far its much better. Ten mile run today and at over 3000rpm maintains around 14-14.5 so that's ok and so is tickover at around 13.5. Plus it now pulls like a train with loads more grunt with good numbers at WOT. So after pfaffing around with loads of needles, springs and oils and listening to the rolling road guy saying the 7B were too rich, admittedly I have had the carbs reconditioned since then, I have potentially ended up with 7B as the best needles with a heavy grade dashpot oil. I had almost resigned myself to modifying needles myself, in fact I'd already marked the 1/8th inch registers on the 6S needles, or trying out fuel injection from the guy at Chelmsford! I hope the bloomin' works as it's now more annoying than a challenge! I can hear Pete saying don't deviate from standard or you get in trouble....... Iain Link to comment Share on other sites More sharing options...
Pete Lewis Posted August 2, 2022 Report Share Posted August 2, 2022 who said that ???? these sort of modifications are fine if you can measure the results , the trial and error can take for ages and cost a fortune in needles you dont want in the club museum is a needle lathe you need to sneak in and get it working you could make moonlight funds from this Pete Link to comment Share on other sites More sharing options...
Iain T Posted August 2, 2022 Author Report Share Posted August 2, 2022 8 minutes ago, Pete Lewis said: these sort of modifications are fine if you can measure the results , the trial and error can take for ages and cost a fortune in needles you dont want I agree, without having a Lambda sensor and guage it would be impossible for me to get it correct and would have to spend fortunes in rolling road costs. I'm keeping the 6AC, 6S, 6J needles and low pressure springs. You never know what's going to happen next! I've posted this as I found the amount of actual info regarding needles for modified engines is woefully limited. I hope it helps someone! Iain Link to comment Share on other sites More sharing options...
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