You`d honestly be better off ringing someone like Pendles to order a basic Parma hand throttle.
You can`t be robbed at just over £20, & the difference in performance will be like chalk & cheese.
There have been discussions on this forum about upgrading the digital controllers with replacement springs, INOX, etc. All low cost improvements.
If you are talking analogue, some of those tips might work as well.
I am sure someone else will chime in with recommendations for some DIY improvements.
......... whilst waiting for the comprehensive answer, I would suggest that you give the slider (rheostat) a clean with a cotton bud dipped in meths. Repeat this with a fresh bud until there is no more gunk coming off. A very light dab of vaseline or a tiny drop of Inox will help with continuity.
Make sure that the spring is not binding on the posts nor fouling the wires. Stretching the end loop and the spring slightly can be benficial.
The next thing to check is the quality of the solder joints on all the wires, don't forget the ones on the buttons as well. The solder should look like a "volcano" and have a nice shiny appearance. If it looks to be grey and pitted then the joint might well be dry; a dry joint can cause all sort of miseries.
Cleanliness is next to godliness. Not sure who said this but.
Give the windings a good clean with lighter fluid or some other solvent as has been said on a Q-tip. Then take a piece of paper with with some solvent on it and slide it between the winding and the wiper. No more black stuff put a tiny bit of sewing machine oil or other light lubricant and that bit is good to go.
OR If you are feeling flush buy a can of switch cleaner and give them a light spray. While moving the wiper.
Are all the controllers from sets and are they all the same vintage. We have found controller that are not all the same resistance so there may be some variance there also. If they are clean they should all work.
It is possible that the crimped joints have some galvanic corrosion. Have a look if there is any evidence of something not looking clean a tiny drop of vinegar on a pin in the joint should help that. If you bought switch cleaner fire some of that in the joint.
Give the 3mm stereo plug a good clean also. You can use dish washing ScotchBrite for that.
Before lubricating the pivots and where the trigger slides with Vaseline or similar check the sliding part for wear, you can smooth out dips with fine sandpaper.
Those stock, 60 ohm (if I remember correctly) controllers are really not very good. I know you mentioned not having the funds available for replacements but do not spend any additional funds trying to fix those things.
If one or all of the easy fixes manages to help, congratulations. Save any additional funds for better controllers, 45 ohm Parmas are not too expensive and would be a big improvement. Sometimes in our quest to save money we can be "penny-wise and pound foolish." I know I have been in that position too many times.
When I finally decided I would start "getting into" this hobby in a big way, I bought a pair of PM controllers and an adjustable power supply. Cost me the equivalent of 7 cars I figure, but it improved the performance of every car I had then and have purchased since.
If you've so many that are duff then this is probably due to their poor quality and totally useless ability to cope with modern high strength magnets and ranges of hi-spec motors. Fixing the duff ones will therefore lead to them breaking down again, or at best endless hours of dull unresponsive racing; so what's the point of fixing them?
You can buy replacements on Ebay if you are short of £££ but you'd be better off saving up and getting a couple of 25ohm Parma 'Economy' controllers that will last for years.
Simon, or anyone else, I have 4 Sport controllers that I no longer need they are free if you want them, just cover the postage. Just checked them and all 4 are working. If you would like them let me know, I'd rather see them go to a good home than the recycle bin.
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