8v GU with Weber 32/34 DMTL AND 4-2-1 exhaust Super lean

June 02, 2026, 09:06:05 PM

I have a 8v GU that's been running happily on a Weber 32/34 DMTL. I have just swapped out the 4-1 exhaust for a Corrado 4-2 manifold and a Frankenstein 2-1 downpipe.

As it's a Syncro the original down pipe was narrow and squared off due to lack of room. Now it's obviously breathing much better...so well, in fact, I can only get it to run if I fix the choke 80% closed.

It massively leans out with any throttle. I have checked:

Vacuum tubes - no leaks
Carb/adapter/flange/manifold gaskets - no leaks
Intake manifold - no leaks
Exhaust manifold - no leaks
Throttle plate on primary - centred and full closes
Jets: increased the idle from 55 to 60 - no change in idle
Cleaned out carb primary circuit - clean
New fuel filter
Timing checked - 8°BTDC
Vacuum Guage 19.5 in HG
Fuel pump - pumping
Accelerator pump - pumping

I haven't had a chance to pull the plugs but I'd bet money on them looking lean.

I will check the float but it's only 6 months old and was set properly.

Maybe dirty needle valve?

I am at a loss as to what else it may be. Help me MK2 owners. You're my only hope...