Just before heading off to Goodwood FOS….

[BartdeBruijn]

A few days before heading of to the great Goodwood FOS 911T 2.0 with weber 40IDA started running tough. Also dull popping in exhaust. Still running on all cilinders.
Today started her again, same story, rough through whole rev range.
To see if there was one cilinder misbehaving I measured the temp of separate headers.
1-2-3 were significant cooler then 4-5-6.
So carbs, right!?
Strange though if it is fuel(delivery) why whole rev range. Going from idle jet to main jet…..
Will pull carbs however all input is welcome!

Thanks,
Bart

[Scott A]

I would just pull out the jets, one by one and blow compress air. See where that gets you.

[BartdeBruijn]

thanks, will pull those(and main) first, was bit desperate to remove carbs already.....

[BartdeBruijn]

Follow-up:
Pulled all 6 idle jets and as clean as possible, also all 4 fuel screens on bajonet bolts are fully clean!

As said, bank 1-2-3 was while warmed up on exhaust tubes +/- 50 C cooler than side 4-5-6. Engine was running rough and dull popping. Started within 20 minutes drive..
Could it be shifted cam timing as it is a whole bank ��
Any other suggestions?

Regards,
Bart

[1QuickS]

Check for blockage of the idle air correction jets. They are the brass items pressed into top of carb body, very close to the stud in the picture. Dirt can get past air cleaner and block these little orifices which are 1.10 mm in diameter.

However, for all three header primaries to be cool at once suggests something other than a blocked idle air correction jet.

Also, pull emulsion tubes and check for blockage of the holes in them.

[edmayo]

Pull all the spark plugs, see what 1-2-3 looks like compared to 4-5-6, might give you a hint as to whether one side is rich or lean. Before taking carbs off to do whatever you were going to do, check float levels since engine must be running for that. Since you say this just happened overnight it likely won't be an adjustment since adjustments don't go bad while just sitting there. More likely piece of dirt somewhere, maybe in needle valves, that's why you check float level.

[1QuickS]

Something else to consider is the mounting brackets for the throttle cross bar may have come loose which would allow a change in side-to-side air flow balance. Check this balance at 3000 RPM to verify.

[BartdeBruijn]

update:

- checked float levels, all same but bit on low side (not with running car -> will do tomorrow)
- did air flow measurement left side and right side all same and also between left and right side perfectly same

started car again and purring like a kitten, also through the whole rev range
went for short spin around the block and same happens again.....rough running through whole rev range and dull popping in exhaust! dull popping when coasting down and when slowly building up revs from idle upwards (progression area)
both idling as higher up, only when short burst (accelerator pump kicks in) it sounds ok’ish

so I will do the float level measurement on running engine and hope to see if the rough running is related to a needle valve or something.

best,
Bart

Something else to consider is the mounting brackets for the throttle cross bar may have come loose which would allow a change in side-to-side air flow balance. Check this balance at 3000 RPM to verify.

[Scott A]

Depending on the popping location.
It could be an exhaust leak.

The cars popped when new.
My car pops downhill all the time, especially if my gas pedal is somewhat neutral…and I’m not accelerating.
But you know your car…and you would sense a change.

[edmayo]

When was the last time you checked and cleaned the filter sock in the tank ?