This is VERY helpful information! My J-H is a later '74 JH5, so I'm assuming it came with the larger port head. However, the engine was replaced, so I'm not 100% sure the current engine is identical to the original. I'll definitely need to verify everything.
Were all the stock tappets cast iron? If so, I'll likely need to replace them, as the engine is reportedly an un-rebuilt unit with 30K miles.
The timing changes are allowed. In fact, I can even add adjustable cam pulleys. I noticed Lotusbits has some vernier units on the shelf - might they be worth having?
By the way, do you have any recommendations for head/balance work? I've had good luck with Headwerks in the past, but I have no idea if they have experience with these engines. Is there anyone semi-local you recommend for 907 work?
The very early 907 Mk I intake ports were 24.5 mm tall. Later, they were increased to 25.5 mm as a running change. At the time of the change, the production of the small port head ended and the large port head became the official standard service replacement. Sorry, but no, I don't know the date or engine number of the port change, but I do believe it was within the life span of the Jensen-Healey's 907 Mk II engine.
If the Update-Backdate rule would apply to that, and if your engine's ports are the small ones (minority of all 907s built), then you could have a small but real increase in port size available. The 907 responds very well to intake improvements, and the 1mm larger ports along with just port matching could pay noticeable dividends.
The J-H 907's stock C-cams were designed to run at 110 MOP, both intake and exhaust. The first test engines met then-current emissions standards, but Lotus was looking down the road at the already announced 1974 standards. The engine met those coming requirements as well, but by such a narrow margin that Lotus wasn't comfortable in terms of warranty costs relative to the engine staying in compliance with age and wear.
The cam timing was changed to 115 MOP for both intake & exhaust so the first production engines could pass the coming 1974 standards by a wider margin. However, that lowered the power by 10 Hp, and the engine became lethargic by comparison to the original 110 timing.
Later J-H engines did go to 110/110 from engine #4030, and North American engines went to 100 IN/110 EX from engine 10480. That last config, along with really lame ignition timing and lean mixture, was the weakest of the lot and lead to the "torqueless wonder" nick name.
Both 110 & 115 MOP are on the same pulleys, whether both values are marked or not. No new parts are required, it's just a matter of how the pulleys are installed. Flip the pulleys over and rotate them a few teeth, and there's the other MOP. Would that violate the rules for your class?
Set both stock cams to 110 MOP, set the static ignition timing to 12 BTDC, BALANCE the carbs, set the idle mixture for peak manifold vacuum at slow idle, and use the 25.5 mm intake ports with a port-matched manifold. That won't unleash a monster, but the engine will at least run like it's interested.
The original tappets were chilled cast iron, and 'old' ones are prone to cracking. I often find cracks the full height of the skirt, and I've had two crack in half in my hands during inspection. A tappet failure is as effective as a broken timing belt at wiping out valves. If you're going to use the engine in competition, bouncing off the rev-limiter, then I strongly recommend that you install a set of the later steel tappets.
When the steel tappets were introduced, Lotus discontinued manufacture of the cast tappets, and the steel ones became the standard service replacement. So, going steel shouldn't be a breach of the rules since you're using factory spec parts.
Cast tappets were the #1 limiter in setting the 907's rev limit. Rods were #2, but at a higher rpm.
Lotus steel tappets are expensive. Both JAE in California and Garry Kemp in the UK sell aftermarket steel tappets that are less expensive while arguably being better. Their tappets also come with optional crown thicknesses that can be used to compensate for re-ground cams, or to move the range of 'required' valve shims further up into the 'available' range. Valve recession and/or grinding the valves forces the use of thinner and thinner shims. When you get to the bottom of the range of available shims, thinner tappet crowns can move the required shim thickness back up higher into the available range.
Factory balance specs were:
Rods to within 2 grams
Crank to within 15 grams
Pistons to within 3.5 grams.
Doing better than that is pretty easy, and still meets the 'within' wording. Shot-peen the stock rods, balance, balance, balance everything from piston crowns to clutch, and you could be running at 8000 rpm with otherwise stock parts.