S3 V12: The car doesn't respond to throttle when cold

Hello,

my car stopped responding to the throttle when it’s cold. Once the car warms up, it gets progressively better. When it’s fully warm, the problem disappears. When cold, even at full throttle, the RPM stops at 1500 -then after a few seconds, it jumps to 2500 and than in bursts further upwards…

Any ideas?

Thanks a lot!

Greets,
Stefan

That could be caused by any one of a number of things wrong with your carburetors, (the manual lists nine), but one thing they don’t mention is the cold start valves which are actuated by the Choke control. They have a bunch of tiny holes in a disk and if they get plugged the mixture will not be enriched, which is necessary for cold running.

oh sorry. I forgot to mention some things.

carburetors are fine. They were redone last year and i checked again - fuel level and choke

Choke is correctly setuped and all 4 carbs get the same amount of air.

Also the car only runs with choke pulled when cold - otherwise it dies at idle.

How is the throttle response itself, is it as it should be or does it lag or stumble, etc.? Is it just the rpm ‘limiter’? (I say just but have no idea why that happens).
I see that the choke sems to be working correctly.

9/10 times carburetter problems are ignition related. They say, and they have a point.

The throttle itself is fine when warm. No delay, no lag, no stumble.

I can make a video tomorrow.

I was thinking that it could be a mixture issue (when cold). I had hit reply just as you had sent yours. Video is always helpful.

David makes a point - what type of ignition are you using?
original OPUS?
upgraded OPUS?
Lumenition?
Pertronix?
123?

The mechanical distributor advance can be sluggish when cold and free up as the engine warms up. This gets worse until it does not advance when warm. Easy to check and fix. Maintenance calls for a drop of oil under the rotor but it is hardly ever done.

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My jag has the original Opus.

Video can be seen here:

The OPUS doesn’t know how to adjust the spark - it just fires when it’s told to, so if you are not misfiring, then the OPUS is a red herring. It suggests that you aren’t delivering the required amount of advance.

On a similar note, if you can deliver enough fuel to idle, it says nothing about being able to deliver enough fuel to sustain high rpm immediately from cold.

You can differentiate these from each other by revving the engine under a strobe light and a start from cold.

kind regards
Marek

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Hello Marek,

I‘m not quite sure what you mean. Regarding fuel I think pump should be ok because car goes up to 6500 rpm without issues when warm but sticks for 2-4s at 2000rpm when could. It also goes further after those small pauses

It’s a good idea, choke needs lots of fuel, it might go very lean when accelerating cold, and as it warms up it can run leaner so it is no longer noticeable.

Makes sense but wouldn’t this result in a max rpm of X instead of just pausing at 2000rpm for some seconds? Any easy way to check fuel pump? Fuel filter is new.

I have no idea if this is realistic, but would imagine that if it can happen it accelerates well until a float bowl is empty, then it stumbles, the float bowl fills up again, then it gets a bit more fuel,…
The easiest and most complete test should be one for fuel pressure.

If it isn’t misfiring as badly, just runs into a wall, have you checked your advance weights?

Normally there is no misfiring. Ignition is set to 12° OT.

Don’t underestimate how much fuel the choke is being asked to supply.

Using a fuel injected car and looking at the answer backwards, the Zytek ECU’d XJR-S has a fuel pulsewidth which goes from 2.5mSec at low load to nearly 20mSec at full load. My fuel injected car has acceleration fuel adjustments which cap out at about 20% and a tapered warmup enrichment of 15% from cold. If the choke doesn’t fully deliver the enrichment plus the dashpot enrichment under acceleration doesn’t quite deliver that 35% extra in a timely manner, then you may well stutter.

Those are optimal numbers - you are not adjusting the choke every few milliseconds and you are supplying the fuel a long way away from where it is used. In between, there is plenty of scope for much of that fuel to condense on the cold inlet manifold walls and not reach its destination.

Added to that, the amount of fuel needed to start the car from ice cold is about sixfold that needed to restart it from fully warm. Fuel injected cars always do this right, but a carburettored car will very easily to deliver slightly the wrong amount of fuel, perhaps too rich or perhaps too lean, for every possible scenario.

Aside from the strobe test for the ignition when revving after a cold start, try also to alter the choke setting to see whether that has any bearing on your being able to accelerate better.

kind regards
Marek

Sorry for the delay but it seems i was able to solve it.

There were two issues:

1.) one of my brand new spark plugs didn’t fire - i couldn’t find any visible issue but it did only work when warm

2,) oil level in carbs was to low - so the pistons went up to fast (seems to be only an issue when cold)

Greets,
Stefan

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I suggest you now click on the “Check Mark” icon on your post; it places a check mark in front of the title of this posting.
It indicates the problem / issue has been solved.
It’ll help follow-on adventurers if trouble-shooting the same/similar issue to know this is a (potential) answer

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