Broad as it is long

Mine flies up to about an indicated 60mph or so. There are however quite a lot of variables in head castings etc. so some will be quicker than others.

The most likely causes might be incorrect valve timing or poor sealing (valve seats or piston rings). It will be difficult to investigate further without at least taking the head off).

You can check the valve timing roughly as the overlap is equal on these engines (at TDC, both valves should be open the same amount).

It's worth reading up on Mike Pemberton's valve timing articles in 'Roadholder' a year or so ago. It would seem to be quite common to find engines set incorrectly. (Well worth getting in touch with Mike if you're struggling).

Have you swapped spark plugs to see if there is any improvement?

Sounds exactly like a sticky or tight valve guide,these always react to load and hills.

Another vote for valve timing here. My 16H had similarly poor performance and I found the inlet cam wheel had to be setone toothoff the pin mark for the valves to be close to equally off their seatsat TDC. Big improvement.

Another one to check is the plug reach, I have a head on which the electrode on a standard 1/2" reach plug hides up in the plug hole - which cant help.

Hi Paul, I've just finished my 16H restoration and have had similar results in performance. I didn't change the piston and rings in my engine as I could see it had recently been done. At first my bike was slowing up as 30mph and I had to ride locally for 100 miles or so. The engine was loosening up a bit and I did a few rides out of town but I could still feel it holding back at about 40. It has slowly got better the more I've run it in. I had done 250 mile before I took it to Normandy for D-Day and it started to hold back after a long ride out. After that it was fine and I had no problem keeping up with the other bikes usually riding between 45-55 mph. I did another 430 miles there, I did give it a blast with an indicated 65mph but everyone says my speedo is wrong, although newly reconditioned. They may be right though as a few miles later the needle started spinning and flicking uncontrollably then packed in. So I'm going to strap the sat-nav to the handlebars to get a propper reading.

I thought the piston and rings would have been run in by the previous owner but obviously not. The slowing down or holding back I have been told is the piston and rings nipping up in the bore. I didn't push the bike if this happened and it seems to have bedded in ok now. Maybe your bike needs running in unless you know the engine has performed better before.

I feel the book ignition advance figure is very safe-sided for modern fuel. Different /values appear in different lists - I'm using 7/16 BTDC at full advance. I'm going to set it further advanced - I can always push the lever back a bit.

My pipes are bright blue already - only 45 miles on them - looks (and feels) too far retarted to me. Plug is OK so not obviously too lean and it runs on full advance almost as soon as it is moving.

It's all a question of time - off to get wet at Banbury tomorrow. Don't know why I do these things...

I've set mine about 7/16ths or a shade more as well David, as I had the same thoughts.

Why not optimise the ignition timing by a bit of on-road analysis? Pick a quiet stretch of level road and accelerate with full throttle in top gear from 30mph between two points (street lights, fence posts, whatever) and keep advancing the timing by tiny amounts to achieve the quickest time between the two points. If it starts to pink you've overdone it so retard it just enough to stop the pinking. Job done.