1600cc rebuild - flywheel question

[70empi]

Greetings AirSpeed forum, sorry to make my first post a technical question, but I'm trying to narrow down a problem and I'd be grateful for some guidance.

My father and I just finished a rebuild on a 1600cc engine to go in the Buggy pictured here: imgur.com/a/S18SE 

New pistons/cylinders/heads/gaskets/carb-kit, moved the oil cooler, changed clutch and pressure plate.  Everything went smoothly on start-up, no knocking, good oil pressure - I'd call it a success, except for a pronounced vibration between 1500-3000 RPM.  Damn! My gut tells me that it's the flywheel, we didn't notice any markings when taking it off or putting it on, but I think we missed something and I hope it's that and not an unbalanced piston.

Does anyone know firstly if the crank is balanced with the flywheel on, and if so, what markings (if any) does VW put on the flywheel? (yea, we should have marked it before taking it off!)
Secondly, before we take it out and bring it to John's Bug Shop (In Welland, Canada - they've been a great help), can you think of anything else worth considering? 

I learned a lot about different approaches to engine building through this project.  My father put the car together when he was 16 (with some assistance) and this is the first time we've had to take it apart.  Looking forward to passing along some of what we learned here, but in the meantime I appreciate any insight!

Cheers,

-Greg

[Cameron]

AFAIK, VW never balanced crankshafts and flywheels together. The parts were balanced separately, and were never meant to be revved over 4500 RPM.

I can't see your vibration problem being balance related.
Two things I can think of immediately;

1) I think you should double check your ignition system. Check firing order. Make sure you have a spark to each plug. Maybe you have one cylinder missing.

2) Check your valve lash. One cylinder may be loosing compression from lash set too tight.

It's hard to diagnose these kinds of things over the internet. Let us know how you make out.

What do mean by "moved the oil cooler"?

[BUSDADDY]

I agree with Cam, pull each plug wire out of the cap one at a time while it's running at the suspect speed and see if it makes a difference (insulated pliers or thick rubber gloves), if there's no change on one wire you found the missing cylinder. If all 4 wires cause an RPM drop start suspecting the pressure plate, Batan recently had a bad one right out of the box that did the same thing you describe, remove the plate (and clutch disc) and run the engine, if the shake is gone you know what to do.