[Roeland]

Roeland - Who did all this work? What's the cranking compression? Are the tyres stock? In good shape & rated for 100+ MPH? Are they aligned & balanced? Steering stem bearings properly lubed & adjusted? Rear shocks OK? Stock? Modified? Front fork oil clean, proper level & viscosity? Any bags, fairings or luggage bolted on, that could affect stability at speed. What's your skill level? Things happen really fast past 85 or so, it's best to approach speed incrementally. How do you really know your bike runs 100 MPH if you can't ride it? Does it have a compression release? Didn't the builder show you the starting technique?. These 612's can require 3mm or more of compression plates to reduce cranking compression to levels that won't harm the engine. Has that been done? If the cranking compression is 125 -- 130 PSI it should be as kickstartable as it was before. Talk to the builder, you need answers.

I did the work myself, except for the porting of the head and the beehive valve springs; that was done by ACE. The compression is standard. The back tire is a larger size and together with the sprockets should give a theoretical speed of just over 100mph. I dismantled and re-installed the front fork, changed the oil, etc.  The back shocks are still standard at their hardest setting to avoid the tire rubbing against the fender. The tires are knobbly….that could be the problem. I also fitted the Carberry vibration reduction plate and changed all the bearings, oil pump, hydraulic lifters etc. The previous engine was a 535 cc high compression configuration which was able to go about 93 mph (GPS tracked) with a pillion - the extra weight gave it more stability. I do have saddle bags fitted but I noticed not much change with them or without them. One thing I did note is that the actual frame alignment is not very accurate - the engine bolts did not line up exactly - I loosened everything and tightened the bolts with the engine running. In actual fact with the removal of the engine block one of the engine bolts (nearest to the kick starter) was seized to the engine - I had to cut the engine out of the frame and drill out the entire bolt. The engine is a 2012 C5 UCE engine; it does not have a dedicated compression lever; only the little lever next to the left-hand handle bar grip which I believe acts a bit like a choke? I'm actually not sure how to lower the kick start compression on this engine - I may have to look into this? The engine got now about 2500 km on it and to be honest I'm not that much into high speeds anymore. If I want to go a little faster I take the Harley.

[AzCal Retred]

Roeland - to be clear, what frame is this? B5? C5? Continental GT?
This is a standard 500cc 2012 C5 with a ported head, factory piston?
What is the measured cranking compression?
I believe that the electric start has a decompressor mechanism built in. If the cams aren't stock, this feature is likely gone.
( https://accessories.hitchcocksmotorcycles.com/20040qty=1&continue_url=https%3A%2F%2Fwww.hitchcocksmotorcycles.com%2Fpartsbook-pages%2F4985 )
PART No. 570309 ; CAM GEAR ASSEMBLY-EXHAUST WITH DECOMPRESSOR ASSEMBLY ; £147.79
As far as I know from this forum, people install a "button" decompressor or Bullet decompressor unit in the head if that function is desired. I don't see a "exhaust valve lifter" unit offered for other than the Bullet.
What kind of knobbies are these tyres, trials universals? Motard? If you are on the street at more than maybe 65 MPH on trials universals you're rolling the dice.
Wheel alignment is checked with a string, front to back, on the centerstand. You are verifying that the front & rear tyre contact patches are absolutely in line, not offset. Spokes can be tuned to move the rim a bit, otherwise it's machine work & spacers. The rear sprocket needs to stay centered on the front sprocket while you are doing this as well.
When you oversized the rear tyre you reduced fork rake geometry. More rake generally translates to stability, less for turning ability. All of this interacts in complex ways.
I would put S (112 MPH) or H (130 MPH) rated street tires of the OEM size back on it. The cams should go back to stock, then you should be able to use the e-start. These are 27-30 HP machines, it takes about 35 HP to "reliably" run 100+ MPH, rider blended into the paint. Then you'd have something you could ride. If it easily runs freeway speeds, that's good enough. That ACE head will buy you some there!
If you want to go fast, it's too easy to acquire a 600 class 80 HP+ Japanese asphalt scratcher that'll run 130+ for $2500 - $4000, less than the cost of any hop up you could ever do to the Enfield, and safer too. You are buying race-proven geometry & suspension and a current-tech motor that will move it down the road at terrifying velocities. I've even seen a few rideable Hayabusas & ZX12's for under $5000 with their 170+ top end speeds. Enjoy a rideable Enfield, leave real speed to the "clean sheet of paper" designs. Or get a Royal Enfield 650 Interceptor or Continental, they'll broach the "ton" right out of the box and don't weigh much more than the singles.