With a week to go before the trackday, and the car MOT’d, I turned my attention to trying to resolve this sticking throttle issue. From searching the internet, I could see that it was the return springs just not being strong enough to hold the plates closed against the vacuum of a car engine. I couldn’t find any uprated springs, but I did find a solution in America that mimicked the return cable on the bike. It basically had a small cable attached to the throttle cam which in turn was attached to a spring, and it all bolted nicely to the cable routing. Unfortunately I would never be able to get it in time, so I decided to try and knock something similar up myself.
I popped into Halfords and picked up a bike brake cable, and some of the electrical connectors that I hate. I used some spare vacuum hose for an outer cable, filed down the end so it would fit in the throttle cam and made a loop at the other end so I could hook it onto a spring. This also made it adjustable, albeit it not easily.
I used an old cam tensioner spring and hooked it onto a bolt mounted in the inlet manifold support bracket position… then hooked the cable over the end.
I fired the engine up and the throttle closed every time after blipping with no delay. It was a bit stiff (that spring is quite strong) and so it wasn’t comfortable but at least it was closing now.
On the drive to the track it worked well, and I could feather the throttle as I pleased. For the first mile… until I put my foot down. That must have made everything settle because I still had the stiff throttle, but now it wasn’t closing just like before. Doh! I had a look at the track and it all looked as before. During the course of the day, it would sometimes work and sometimes not. Right idea it seemed, but needed refining.
The following week I picked up a box of springs from Halfords. I found the one that stretched the most (strangely not the longest one), fitted that and adjusted the cable. Throttle was much lighter now, certainly more ‘normal. When I drove the Westfield to work on the Friday I noticed that on the way home the throttle was closing, but it seemed there was a slight delay in doing so. By the time I got home, the throttle wasn’t closing too well at all. I think I’ve stretched the spring so will try another type.