Throttle return / idle

[ethel]

Since I rebuilt my carbs (Strombergs) I've got a problem with the idle. If I blip the throttle it revs and then drops back to idle - just like it should, but if I release the pedal slowly the revs don't drop below about 1500. I can get back to idle either by blipping the thottle or giving the linkage a gentle push on the plate where the link rods are attached, or by closing the lever on the carb spindle up to the idle screw.
I think if the springs on the carb spindles were stronger the problem woudn't exist, so is it possible that I didn't wind them up enough when I fitted the new spindles, on the bench there seemed to be enough spring force to close the butterflies - I'm wondering if it's possible to get another full turn on the return springs?
Any suggestions for other causes or solutions, I guess it could be that the butterflies aren't quite centred on the spindle, or the spindles are bindingwhere they go through the carb body....
Thanks, John.

[v8ken]

I suppose when the engine is running,the airflow through the carb creates more resistance on the butterflies,than when you tested it on the bench.

[Phil S]

I was thinking this sounds like the butterfly is just out of line as I was reading this and you have the same thought. I have had similar and the only way I was able to see it was with the carb off and a strong light behind the butterfly and looking towards the light

[jakesmig]

I cannot recall how the Strombergs were linked (my last dealings with them on a Stag was in 1978), but with twin Su's the trick is to undo the linkage between them, tune each carb separately then reattach the linkage. To achieve the best results an airflow meter had to be used to balance the carbs at idle. It often surprised me how apparently well 'eared in' balanced carbs were found to be way out when a flow meter was used.
Don't know if any of that will be a help, but the principle should be near enough the same.
John

[ethel]

....the trick is to undo the linkage between them, tune each carb separately then reattach the linkage......
John

I did originally set the carbs up individually before I attached the linkage and I've tried lengthening and shortening the link between the carbs by a turn or two, but with no effect on the problem, I think though when I get back in the garage I'll disconnect the linkage and try slowly lowering the revs on each side individually and at least then I might be able to tell if the problem is one sticking carb also keeping the other partially open, or maybe even the linkage itself sticking.

Thanks for the replies,
John.

[jbuckl]

I had this problem. A previous owner had added a tension spring between the r. h. Choke bracket and the lost motion quadrant on the throttle linkage. Seen a few with that. It can make the throttle pedal feel heavier. I removed the spring & fitted 2 new throttle return springs a the spindles. The return to idle was still inconsistent.
Assume the std springs are on the weak side.

[NickA]

Interesting; I had exactly this problem but a full overall of the carbs cured it. So it must be something that's happened during your carb overhaul.

[jbuckl]

I was lucky to get new old stock carbs recently. Still has variable return to idle speed.

[Wisey]

I found the float chamber vent valve return springs stopped my throttle reaching it's idle position when shutting gently, as above a blip overcomes this and gives a true idle.

[ethel]

I found the float chamber vent valve return springs stopped my throttle reaching it's idle position when shutting gently, as above a blip overcomes this and gives a true idle.

I think you're on to something there, I've just had a poke around the vent valve levers and the one on the rh carb seems to be sticking. If I try to close the throttle gently then the throttle lever comes to rest on the vent valve lever without fully actuating it but giving the valve lever a gentle prod with a screwdriver puts it in the correct position and takes the load off the throttle mechanism.

Thanks, John.