3.5 SU Fast Idle Setup

[Pos]

Hi,

I’m looking for some advice setting the fast idle on my 3.5 running twin SU HIF6s.

My engine will start with the choke out after a few turns but it is lumpy and poppy and does not have a smooth fast idle.

With the choke pulled out, I can confirm that the fast idle quadrants in both carbs are maxed out (I.e as rich as they can get) but I am not sure what my idle should be when fast.

I have tried to match the butterfly openings on both carbs with the choke partially open, but it goes off the scale on my meter with the choke fully out. I think the carbs need to be drawing a little more air with the choke out.

Is it assumed that the choke should always be out as far as possible for a cold start? Is there a nifty way to set this?

Thanks,
-Tom

[stevieturbo]

Choke needs to be out as far as it needs. Neither full, middle, or anywhere, just whatever is needed at any particular time.

That's why cables of old were fairly rigid, or could be locked at any position.

Really, balance and tune the carbs when warm and get that bit right and then start to address any cold start concerns

[Pos]

Thank you.

I've got them well balanced when warm, it runs really well.

Just trying to establish the balance for cold start in my head. I'd assume the fast idle on full choke should be somewhere around 1500 - 1700rpm?

I might see if I can source a synchrometer that is less sensitive - Ensuring that the fast idle is matched on both carbs at higher rpm is difficult with the gauge I have.

[stevieturbo]

If the main throttle blades/airflow are balanced at warm idle, and driving and tuning are good when warm, I wouldn't get too worried about cold start setup

As long as it starts, runs well and idles somewhere that feels right for you for driving off etc....never worry too much. You'll only be on the choke really for a couple of minutes at most
There is no "perfect" or specific numbers it must be, just make it so it works, and works well.