Club 80-90 Forums

Carb used to work fine, but something has gone wrong, looked at James Akers excellent you tuber, but before I do that process, wondering what has happened. Before engine would start fine, revved high then when warmed up, a couple of taps on the accelerator pedal reduced revs to a lower idle rate, this no longer happens and engine revs remain high. So something has changed. Even broke down completely and RAC rescued me, cleaned jets and float chamber and got me going. Engine now idles as mentioned above but not reduces automatically. Auto choke flap moves closes and opens. Wondering what area or part needs investigating before rebuild, any ideas please.

I read this as revs remain high at all times still?

If yes, maybe the step down mechanism is bunged up with dry grease? ( Easy to check without dismantling anything )

A spare carb I got as part of a bundle of parts did that too, from memory it was the flaps on the bottom not closing - a long time ago, and something says in my head it was a plastic cam broken so the flap just moved on its own accord.

 

Thanks, revs still remain high, step down mechanism, heard of it but unsure of location or adjustment.

TONYT25T25 wrote: ↑07 May 2025, 22:03 Thanks, revs still remain high, step down mechanism, heard of it but unsure of location or adjustment.

It's to the right of the carb body. Give it a good clean up as mentioned above and go from there.

Thanks, I vaguely remember spraying that item with oil or grease in the past which may have contributed to my problems.

Check the little return spring on the step down too, it can become disloged and cause the high reves as the unit sticks.

The cam for the secondary has probably broken which will mean the vaccum will always keep the secondary butterly open thus your revs high. That or your choke is sticking as others have said.

Gave the step down unit a blast with some brake cleaner spray, started engine and fired into life immediately. Revved far to high (much higher than before) so switched engine off. Adjusted fast idle screw (the one that points out of the carb towards the back) fiddled with this before to increase revs as engine kept stalling when coming to a stop. Unsure what it alters on carb. Bit more adjusting and engine starting and idling nicely. Leaving it over night so I can start from cold to see if it starts, idles at speed then when warm reduces revs by tapping accelerator as mentioned in first post.

Weird that it started revving high out of nowhere though.

You don't have a sticky throttle cable, do you, Tony?

Maybe just unhook it if you still get issues, see if idle return to normal, and make sure the cam motion is good.

My money is still on the cam return spring, the symptoms sound exactly the same. 

Smiffo wrote: ↑08 May 2025, 21:13 Weird that it started revving high out of nowhere though.

You don't have a sticky throttle cable, do you, Tony?

Maybe just unhook it if you still get issues, see if idle return to normal, and make sure the cam motion is good.

Working on theory that spraying brake cleaner on it, released some mechanism, making the adjustment of the fast idle screw no longer needed to the extent it was altered to prior to cleaning the area.

TwinTurbo wrote: ↑09 May 2025, 05:37 My money is still on the cam return spring, the symptoms sound exactly the same. 

thanks, need to locate that item?

Stesaw wrote: ↑08 May 2025, 17:16 The cam for the secondary has probably broken which will mean the vaccum will always keep the secondary butterly open thus your revs high. That or your choke is sticking as others have said.

Will look at that also, when i locate it.

Its attached to the rod from the rear vaccum unit on the carb.

Stesaw wrote: ↑09 May 2025, 08:26 Its attached to the rod from the rear vaccum unit on the carb.

Thanks