"When my 68 Plus 4 warms up I find that the throttle linkage springs on my SU's don't seem to be doing their job. The engine revs. creep up and it takes a good kick on the gas to get the revs. down to proper idle (under 1000). Sometimes this doesn't even work. Any suggestions?
Thomas Van Zuiden, 69
Plus 4-4 seater (6843)
I dealt with the same problem, after cleaning the linkage, by fitting a separate extra spring operating on the linkage to return it to base.
John Sheally II
Your problem is probably that the throttle shafts in one or both carbs are worm and need new bushings. That is probably reason the reason for the revs. If it is not then check the throttle cable as it may have gotten water in it and has formed rust on the cable wire and is not letting the cable move freely in the housing. FINALLY LOOK AT THE BRAS OR STEEL TIP AT THE END OF ThE CABLE SLEEVE FOR OBLONGED WEAR. All of the above thing contribute to your problem.
Hope that this solves your problem.
Sincerely, John H. Sheally III
This is a common problem with SU carburetors. It is usually caused by wear. There are two wear points, the throttle bearings and the throttle body itself.
Wear in the throttle bearings usually causes an air leak and makes it difficult (if not impossible) to get a decent idle at low RPM.
Wear in the throttle bearings also contributes to the other wear point which is where the butterfly contacts the throttle body. In time a groove is worn in the body and the butterfly will hang up on the edge of the groove causing the high speed idle until the throttle is "kicked." It is not really a matter of return springs.