9 replies to this topic

[tj80]

Hi,

The fast idle cam on my wife's 1990 Mayfair is sticking when you put the choke in - the revs sit high until blipping the throttle. I can see the throttle screw isn't sliding smoothly along the fast idle cam, there's decent spring pressure on the choke return so I think the surface of the fast idle cam is probably just rough and/or gummed up.

Would you lubricate the cam surface and if so what with? Grease will just gum up and attract dirt, would silicone spray be a sensible choice? Or just something like 3-in-1 oil?

Cheers,
Tim

[Ethel]

HS4? I can't remember the specifics, but there's a sort of 2 stage mechanism. The fast idle cam moves another bit that moves the jet only after it's reached the fast idle setting point.

[tj80]

HIF, but you're right it's still a 2 stage mechanism. See photo - basically when the choke is almost all the way in the jet is in the normal position but the cam acts on the throttle screw to nudge the idle speed up a bit. Problem is that the screw isn't sliding smoothly along the cam and is holding it in a fast idle position until you blip the throttle. Doing that moves the screw away from the cam, the cam springs to its fully closed position as it's now not being held in position by the screw, and when you release the throttle it can return to the proper idle position.

You can see the cam looks a bit rusty so I'm guessing it's rough and that's the problem. Ideas for cleaning and/or lubing?

Cheers,
Tim

Attached Files

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Edited by tj80, 21 October 2020 - 03:57 PM.

[GraemeC]

That's an HS not a HIF

 

I'd start by cleaning up the edge of the cam plate with some 400 grit emery or wet and dry.  Get rid of the rust to a clean, shiny metal surface then just a wipe of oil to stop it rusting again quickly. 

Edited by GraemeC, 21 October 2020 - 04:00 PM.

[tj80]

Thanks - have to admit I haven't dug that deep but didn't notice a separate float so assumed HIF! I must just be unobservent! :)

[Maccmike8]

The return spring may be worn, choke cable could be fouling.

Undo the cable screw at the carb and push and pull the choke, see if it moves freely.

[Spider]

Looking at that, you may need to replace it (or re-plate it). Cleaning it up as Graeme suggests will work as a temporary fix, but with it's plating now gone, it won't take long before it does it again.

[Ethel]

Ooo that's crusty!

 

Not sure you got my poin,t it's not just the cam rubbing against the fast idle screw. There's another bit of stamped steel behind it that's turned by a lug on the cam which in turn pushes the rod to the jet. I'm pretty sure they both have return springs - or that could  easily be in the past tense if the photo is anything to go by.

 

If you're moved to investigate, take photos and note where the ends of the springs sit first, I think it's ptsd from trying to reassemble one that's clouded my memory.

[tj80]

Hmm. Now you all mention it that really does look manky doesn't it! Thing is, the car drives really nicely so I'm a bit loath to take the carb to bits for what's basically a minor irritation. If it were me I'd just get used to it, but it's the wife's car not mine (not that she's even noticed this problem!).

Think I'll try cleaning it up to see if that helps. The second stage bit is working fine to lower the jet and increase the mixture but fair point on the return spring - didn't realise there were two.

[Maccmike8]

The cable nut looks good at least