Twin H4 tickover

[johnv]

I have a set of twin H4s that I've rebuilt with all new parts, including re-bushed throttle spindles (thanks Mark ). The car goes well, no real problems, but I can't get the tickover to drop to nothing with the butterflies closed, the baseline seems to be about 750-800 ish. Which is fine, but it surely shouldn't run at all? I've checked for air leaks with a detector spray, and checked that the butterflies are closing fully. What else to check? Thanks

[Herbert]

You may need to re-bush the carb body where the spindles pass thru. Air is getting into the engine from somewhere. Have you got a flow meter so you can check ?

http://sucarb.co.uk/air-flow-meter.html

[GraemeC]

As with most things, parts aren't what they used to be. I've found that butterflies now are just stamped out, whereas 'old' ones had a ground edge with the angle stamped into the disc - these fit to the bore FAR better.

[johnv]

You may need to re-bush the carb body where the spindles pass thru. Air is getting into the engine from somewhere. Have you got a flow meter so you can check ?

http://sucarb.co.uk/air-flow-meter.html

Been done Roland

[johnv]

As with most things, parts aren't what they used to be. I've found that butterflies now are just stamped out, whereas 'old' ones had a ground edge with the angle stamped into the disc - these fit to the bore FAR better.

Worth a try.. I'll try putting the old ones back on

[GraemeC]

I found mine would drop 'naturally' below around 1000, but if I pushed the linkage further closed by hand I could get that a bit lower.
Like you, I expected to be able to wind the tickover down so far that car wouldn't run, but this wasn't the case - I could back the screws of completely with no change.
I managed to get hold of some NOS 1.5" discs and it was MUCH better.

[Herbert]

There is also the problem that the spindles are "tied" together, so adjusting one side affects the other. I have (somewhere) the job sheet to fit spindle ends to make them the same as HS2 and HS4 carbs. I have found the sheet and post it. Most of the bits are still available from SU Burlens, but get a Mortgage first....so Ive attached a parts list diagram

H4 Fork & Peg info Sheet.pdf

C-AJJ3301K.pdf

[Exminiman]

could it be the needle profile or needle recessed ?
To run it must be getting fuel still
This seems quite good, https://classicminidiy.com/technical/needles nyou can put your needle in and then search for one that has a leaner starting point
EDIT believe this uses the data from Mintylamb http://www.mintylamb.co.uk/suneedle/

[Peter Laidler]

Fiddly, I know but you've got to start from afresh if you have already put new spindles, bushes and butterflies in. DO NOT ASSUME anything. Loosen the bolts that allows the clamps on the left -to-right link rods. This separates the carbs. Now listen to the intake of each carb separately and then adjust the idle screws to give you the lowest tick-over speed that you can reasonably get away with, while each carb is separated from the other. When you've got that tighten up the clamp so that each opens the throttle at EXACTLY the same time and the same amount.

As you can understand from this, carb set-ups are a practical, seeing and doing thing, on the bench first. You can really over complicate SU carbs even though SU made 'em simple!

To be honest, I'm not a big advocate of re-bushing the carb butterfly spindle. If the spindle or body is slightly worn then I simply machine the spindle at a point half way into the carb body at each end and slip a neoprene O ring into the groove. New butterfly screws inserted loosely, close butterfly to suit the throttle body where it will self-centre. Done that, self centred butterflies, air flow closed right down....., lock screws up. End of.

[Thorp]

Good advice Peter. 750/800 sounds ok. As has already been alluded to, I suspect it’s most likely a slight leak around the butterflies.

The bodies of carbs do wear and obviously if they get vapour blasted a few times that takes a few thou off.

I had some Jag triple HD8 carbs rebushed by Burlens; at the same time they fit the spindles and butterflies and then do a seal test. One of them wouldn’t seal; they had to machine a special butterfly approx 30 thou oversize to pass their test. Visually it looked fine, but the air test said otherwise.

Prob not worth worrying too much about as most keen mini drivers will have their foot down...

[mk1]

I usually re use butterflies. the old ones certainly fit better. But the new ones aren't terrible.

TBH, I wouldn't consider this as a problem. You can bring the tickover down to below what you are looking for so why worry.