He later guided John Walker's legendary career, but 40 years after Walker's Montreal gold, Jelley has helped Carson to the cusp of qualifying for track and field's most celebrated event.
Jelley retired from coaching in 2000, content to put more time into bridge ("I'm addicted to it") and lawn bowls. But five years later, he received a call from Carson's mother, Julie, wanting advice.
"She was looking for someone to coach Hamish who wasn't too authoritative and could explain why we were doing things," recalls Jelley. "She picked me for some reason, and I agreed."
"We put it to Arch," says Carson. "He asked about me, asked about my history. He called back a few days later and agreed. I was thrilled."
Carson says the 66-year age gap has never been a problem.
"Arch is very sharp, with a quick wit. He keeps things fairly light when the pressure comes on and always has a few wise words when you need them. He's got such a vast knowledge."
Carson, who is based on the Kapiti Coast, even stayed with Jelley and his wife Jean for extended periods in their spare room during training blocks in Auckland.
"It was a great place," says Carson. "Everyone was so relaxed. There was a pool and bowling green just outside your door and the meals were so cheap in the cafe. I can't wait to retire."
Carson and Jelley trained at the famed Lovelock track in Owairaka, and Jelley also sent his pupil on punishing runs around the Waitakere Ranges. When they are in separate cities - Carson is currently training with Nick Willis in the United States - the pair communicate every day via Skype and phone.
"It's so much easier these days," says Jelley. "I can email training plans, feedback, watch race videos. It's very different to when I first started out."
Jelley started competitive running in 1946, after returning from World War II. He describes his wartime experiences as "uneventful . . . a bit lucky" but it needn't have been, as he served on the HMS Bermuda escorting Russian cruisers through the Arctic before becoming a navigator on a British submarine.
He was good enough to finish fourth at the New Zealand Cross Country Championships but was always fascinated by coaching and a meeting with Arthur Lydiard sparked the flame.
"I liked his principles, they were very good," says Jelley. "They were different to a lot of ideas in those days but he got results. I thought, 'if he can do it, why can't I?'"
Jelley took Neville Scott to the 5000m final at the 1964 Olympics and helped Ian Studd win bronze in the mile at the 1966 Commonwealth Games before linking up with John Walker in 1971, a partnership that changed both their lives.
"Arch had an ability to recognise talent from a young age and he made me believe in myself," says Walker. "He used to write me long letters on pads of school paper - 'You could be an Olympic champion, you could be a world champion'. You wanted to work so hard for him the whole time. He trained me to be a 3.50-minute miler, not just a sub-four man, and his belief never wavered."
Walker's breakthrough came at the 1974 Commonwealth Games, when he won silver behind Filbert Bayi.
"I'm usually a fairly quiet guy but apparently after that race, I looked at my watch and then I stood up in my seat and shouted, 'it's a world record, it's a world record,'" laughs Jelley.
After Walker broke the 3m 50s barrier for the mile a year later, the peak of Walker and Jelley's union came at the 1976 Montreal Olympics.
"I expected him to win but you can never be quite sure in the Olympics," says Jelley. "We had three different plans [for the final] but it was left to John to select the right plan, depending on how the race was run. In those days, he wasn't the fastest man in the field over 800m but John felt in a 1500m, he could finish better than they could, and he did."
"Arch kept things relaxed," says Walker. "He suggested we have a sleep on the day of the final, so we did, for about two hours. At that Olympics, he was also helping Rod Dixon and Dick Quax - he had this wonderful ability to share his talent."
New Zealand could have three runners lining up in next month's Olympic 1500m event for the first time.
Nick Willis and Julian Matthews have qualified and Carson is close. He has bettered the B qualifying standard five times and in May ran 3m 36.25s, a toe away from the A qualifying mark of 3m 36.20s. Carson is ranked 40th of the 45 athletes who have qualified for the Games, with the IAAF likely to invite the top 45 to enter, limited to three per country.
"It would mean so much," says Carson, "to be there, and in a pinnacle event. When I was at primary school, I wrote down that I wanted to go to the Olympics and represent my country. It would be great to get there for Arch, too, after all he has achieved and all the years he has put into me."
Carson will continue his preparations with Willis and Matthews in the US, before the final announcement is made later this month.
"[Hamish] had problems last year [a severe wisdom tooth infection, then a major fall when he broke both wrists] and was de-carded," says Jelley. "He lost all his funding, although his club in Wellington [Scottish Harriers] helped him a lot. This year, he has come back as a new man and is running really well. I would be so proud of him."
Jelley is undecided if he would make the trip to Rio - "I'm not sure, it's a long way" - but is keenly plotting Carson's training schedule until then, in between his duties as president of the Mt Albert Bridge Club.
"Arch is one in a million," says Walker. "If anyone deserves a knighthood, it's him. He has coached at so many levels, from grassroots to the top, and always gets people to fulfil their potential."
#
Oldest boxing coach
In July 2014, Guinness World Records certified Abe Pervin as the world's oldest boxing coach. The then 94-year-old was still coaching in Montreal. He coached the Canadian teams at the 1975 Pan American Games and 1976 Montreal Olympics, and helped five professional boxers reach world champion status.
Oldest World Cup coach
The oldest coach at a football World Cup was Otto Rehhagel, who was 71 years 317 days when he managed Greece in their final group game against Argentina in South Africa in 2010.
Youngest FA Cup coach
James Phillips was 22 in 2014 when he coached Romsey Town against Fareham Town to become the youngest manager in the FA Cup's 145-year history. He began his coaching career at 16.
Oldest English football coach
Ivor Powell was one of the finest players of his generation - he set a record transfer fee of £17,500 in 1946 and was capped 14 times by Wales - before turning to coaching in the 1950s. He filled a number of positions, most notably alongside Don Revie at Leeds in the 1960s and '70s. In 2010, Powell retired, aged 93.
Oldest American professional sports coach
Trader Jack, as Connie Mack was affectionately known, managed the Philadelphia Athletics for the baseball club's first 50 seasons from 1901 before retiring aged 87. He was also part-owner of the club and became the first manager to win the World Series three times (five in total) and only manager to twice win consecutive titles.
