This is definitely a common issue with the MK3’s…
Has anyone managed to find a solution that actually sorts the problem, short of replacing them?
hi Chris
if you have an early MK3 then there is a replacement seatbelt guide available for this issue from MX5 Parts
https://www.mx5parts.co.uk/seat-belt-guide-mk3-2005-2008-p-2351.html
this may help 
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Is that literally the cure to the problem??
I was sure it was something to do with the inertia reels!?
I have the slow retracting issue and plan to start with replacing these, as this is a very easy option. I have seen others washing the seatbelts but with little success, there will be others here shortly to help I am sure have you tried seeing what they retract like out of the guides ?
Yeah I’ve just been and tried it and it makes absolutely no difference… They get half way and then stop!
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Another one is hitting somewhere with a big hammer and shaving a little from the guide.
Probably sounds brutal the above, I tried but gave up, mine are still slow to retract after trying the wash treatment.
MOT tester has been asked about them (for my curiosity nothing else) he said they are ok do the job.
with mine it makes a differance but certainly not the whole issue
It sounds like there may be a problem specific to the Mk3 but I had the issue with our Mk2.5. Washing the belts helped but didn’t solve the problem so I used Silicone spray the whole length of the belt, both sides, which has sorted the problem. I also used some Silicone spray on the belt guide. The trick now is to get your fingers out of the way before you let go.
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Yeah I’ve heard a few people mention silicone spray… For what that costs I might as well give it a go.
I’ll report back with my findings!
Believe it or not, a good clean sorts the problem. I’ve done it and it worked. Pull the strap out as far as possible, plunge it in a bucket of warm soapy water and scrub gently with a medium stiff brush. Rinse the strap thoroughly and the dry with a towel. Leave the strap on a dry towel over night to dry properly, Bob’s your Uncle!! Honestly it worked on my 2011 3.5 
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Ill do that before giving it a dose of silicone spray too
The updated guides help but the real issue is dirt. I soaked my belt in a bucket of vanish for a few hours, scrubbed it with a brush, rinsed well and left it to dry. With the new guides the problem has gone.
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On a Mk1, I did a deep clean with one of those foaming Vanish laundry cleaners. The junk that came out was amazing, and the belts worked perfectly after. However, during this I discovered a little nick in the webbing. I found a company down in Surrey who could economically reweb seatbelts, and who sent me swatches for me to choose (you could have any colour, and I had a choice of 5 different red webbings for my S-Limited). Cost was less than £70, and they were returned to me looking like brand new belts. Red mk1 belts are unobtanium from Mazda anyhow, but the price of a new Mk1 belt is £100, so rewebbing was a bargain.
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Inspired I’ll give mine another go at cleaning.
At least another lockdown job and it remains fine looking at the forecast, drying weather.
cleaning silcone spray and new guides for me now thanks all
So, gave them a good clean with detergent earlier… No noticeable difference but I’ll know more tomorrow when they’re dry, ill then apply the silicone spray and report back
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definitelty need a scrub after a soak in washing powder mixture!
Then a rinse after 
Silicon is an added extra which helps too
I had this problem, but have cured it by wiping the full length of the seat belt with leather polish, which is the polish I use on the seats. At the moment I am using McGuire’s, but there is probably no reason why any other leather polish will not do the same thing…?
I had same issue, left them soaking and agitated them every now and then over an afternoon. Massive improvement. I tried washing up liquid first - didn’t make a lot of difference. When I tried with washing detergent it worked a treat 
I used washing powder like MX-Pilot and worked perfectly. It went from never returning to always even with old guide. I used teflon fabric spray afterwards to stop any dirt sticking in future. Wont silicon attract the dirt ? as the silicon I have is oily.
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