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...

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

Reply #1 – August 03, 2026, 03:14:29 PM

I changed the manifold to a GTi one, and fitted a corrado downpipe and GTi exhaust on my 1600 Golf Driver and didn't even have to retune the weber carb. It ran fine, and power has improved above 4k revs. So this suggests changing the exhaust manifold and exhaust makes little difference in itself. So the problem is there is either an air leak in the intake somewhere, or some passages in the carb are blocked.