Mk1 VR6 won't Rev past 4000rpm

So it's been fine for the last 500 miles but now the engine won't Rev past 4000.
So you'll hit say 3800 and it will feel odd and restricted and by 4200 your foot can be planted and nothing occurs.
The car is a mk1 and now with a VR6 2.8.
Any help would be great. 
Cheers

I would imagine that you are having a Fuel Flow Limiting.  

So I would look at the Fuel Pump relay, most vw's have a rev limiter in them,  as well as doing a Fuel flow test that is outlined in the Bentley for your engine.

Possibly the Filter is clogged.

I have seen issues where the Fuel pressure is correct, but the Quantity of flow was off cause a similar issue.

Relays have been know to cause this as well as BAD grounds Battery to Frame - Frame to engine, as well as iffy ICM's and ECU's.  

Start with the Fuel tests and possibly the relay filter, then grounds….

diagnostics is the place to start here, check for codes and read thru all the ecu measuring blocks to see all the sensor inputs in real time.

some of the vr6 ecu dont like it if you dont hook up the VSS wire from the clocks, this one is tricky to add in a mk1 as you need gti clocks then add a pin into the 6 pin plug round the back. some mk1 clocks have the ability to add the speed sender and 6 pin plug but not all.

what fuel setup did you put on the car as well, and is the engine obd1 or obd2

Cheers guys
It's got an Emerald ECU with an elec fuel pump 
It's also got OBD1
Any other things you reckon I can try?

what type of fuel pump is it, and have you gone for gti tank plus external, or carb tank with lifter and swirl pot etc

It's a carb tank as you described, not sure on type of pump but it's the standard mk3 pump and filter.
It's running worse now as in popping and stuttering loads, I wonder if it's pump related because the pump has been really loud lately and that's when it cuts out 

mk3 only use 1 pump inside the tank, and it wont fit in the mk1 tank?

have anotgher look, if the tank sender has 3 pins then you have the post-84 mk1 cabrio/scirocco/polo lift pump in there. look under the car and check what type of main pump it is, and if you have a swirl pot between the lift and main pumps.

post pics if not sure!

Sorry it's a while ago since I did the swap, it's actually k jet inline pump and lift pump lol only remembered today

right did you fit a swirl pot between the lifter and main pump?

That's correct yeah

right thats all good then, next check the lifter pump is actually working. pop the feed off and point it into a bucket. pull the coilpack plug off to prevent it starting and crank it for 10 seconds, you should have at least 300cc of fuel in your container

Ok I'll give that a go, I wonder if it's a blocked fuel filter because the tank used to bring rubbish up every now and then, I'll swap that too I think. Anything else you can think of trying? Cheers by the way  

probably worth pulling the lift pump out to have a look for rust in the tank, the lifters sometimes have a mesh filter so check that isnt blocked. if it doesnt have the filter on and it has had rust in the tank the pump probably wont be in the best of condition!

Seems solved!
Took out filter and fuel coming out was black! Swapped filter and then checked the rest of the line inc. tank. It's something I've avoided even when it had the old engine but think it's time to man up and get a new tank :/ lol 
Any further issues I'll let you know but looks like it just couldn't get fuel through the blocked clogged filter lol 
Many thanks!!