High Tickover

[RogerRust]

I've been trying to sort Sphies engine up today.
7 degrees of advance with the stobe at tick over with vacuum disconected.
At first it wouldn't run without the choke so I put wound the jet well down.

Ok, at a fast tickover the colour tune is bunsen blue - goes yellow briefly when I open the throttle then back to blue.

BUT I can't get the tick over down very far. Its ok, but a bit fast. There is free play in the cable and I checked that the linkage isn't sticking. I can't find any air leaks.
If I take the advance and retard pipe off it stalls. If I disconect the breather it stalls. I've tried blocking both of these, but no joy the tickover stays high.

Just to remind you it is 948 with a 12G940 head, 1098 cam and HS4 1.5 inch carb.

Any sugestions welcome.

[Willie]

I'm sure that you know that the' briefly orange' is correct as it is the
enrichment of the mixture when the throttle is opened. I wonder if that carb still has the interconnection between the choke mechanism and the throttle screw?
i.e. the choke has a spring loaded screw adjustment which has to be set
so that the first 1/4" pull on the choke opens the throttle before it actually starts to apply the choke. If this screw is too far in then it is always touching the throttle mechanism and it is impossible to lower the tickover. Have a look.

[RogerRust]

Not that unfortunatley Willie. The good thing about HS4 is all the linkages are on the side where you can see them. By the way I have checked that the float chamber is vertical.

Can you get a leak around the butterfly? There does seem to be a bit of end float on the butterfly throttle spindle.

[chickenjohn]

have you got a strong enough throttle return spring? Weak spring can affect idle speed.

[RogerRust]

Yep, tried that. Even if I manually push the linkage shut still races a bit. I thought it should be possible to close the butterfly completely and therefore stop the engine. Is this not the case?

[Packedup]

You could try dabbing a gloop of grease over both spindle ends to see if air's leaking in there - Though with the cable off and return spring disconnected if you've got any up/ down slop then yes, the spindle holes are somewhat worn!

I've never had any SU fed engine that'll tick over with the throttle screw wound fully out when everything else has been right/ working - The only time I've had an engine that just wouldn't tick over at a proper speed no matter what I did turned out to be a warped inlet (alloy twin HS2 Triumph). Other times it's been a poorly sealed inlet manifold (mismatch between exhaust and inlet flanges on aftermarket bits for example) or other air leak.

Can you get it right if you blank off the breather inlet (bit of hose with a large bolt or 1/2" socket extension shoved down it usually works)? Maybe you've got a split breather or very heavily breathnig engine maybe?

[RogerRust]

I've sorted it.
As usual aftera cup of tea and a think.
I took the carb off. Sure enough even with the throttle held shut there was a gap at the top and bottom of the disc. Now there are 4 ways round that this disc can be fitted. When I turned it round the other way it shuts tight!!

Moral of the story; just because the ebay seller said he'd rebuilt it don't forget to check every thing before fitting the item.

So tomorrow evening I'll wheel her out and have another go at setting it up.

[Packedup]

Well, yes, there is that possibility I suppose!

Not one I've heard of before, sounds like the sort of thing that would leave me a dribbling wreck for weeks until a flash of inspiration...

[Willie]

the butterfly valve is chamfered so that it only fits correctly one way round, as you have discovered!

[Kevin]

Amazing why is it that a cup of tea is such a good problem solver.
Is it because it makes you go back to basics and think simple

[Peetee]

Tea = 42

[RogerRust]

"Twinings the meaning of the universe" I wonder if they know.