The carbs are a bit of a long story Joe

The timeline is something like this:
1) Dellorto DHLA40 with 32mm chokes. This ran perfectly at the time and set the benchmark
2) Twin HS2 on a Cooper S manifold. The performance was close enough I couldn't tell the difference
3) Dellorto DHLA40 with 34mm chokes. This has had the best performance so far
4) Dual HS2 on the Peter Manton manifold. I chose these as they were used on the sports 850 and was available as an upgrade kit in the 60's. The manifold appears to have been designed for an 850 engine and the ports are absolutely tiny, even with some clean up with a sanding tool in the dremel it's still not 1" diameter all the way through. I'm a bit hesitant to bore it right out for better performance and ruin it

Generally speaking, the Dellorto is much louder, sounds better, sharper throttle response. The SU carbs are much smoother and quieter.

Next/future options are:
5) HIF44 single carb (which is 1.75" diameter). These are the go-to carb for anything 1275 in the UK and a more modern alternative to the HS6. The single carb definitely appeals and it should be more than sufficient for my engine
6) Twin H4 carbs (which are 1.5" diameter). These were used in the 60's on racing minis as they bolt straight up to the standard manifold. These just need a clean and reassembly
7) Dellorto DHLA40 with 34mm chokes after a rebuild

Good point about the double head gasket too. It is the cheapest option and easily reversible so worth a shot

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Fingers crossed my 1098 and 4 synchro box goes in this weekend maintaining the magic wand. Cant wait for the extra horses to be around!

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Your silliness has been noted.

You’ll definitely notice a big difference from an 850! Did you get a stock 1098 or has it had a few mods?

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Hi Tim, Engine went in over the last two days courtesy of Rob Diamante and holy crap what a difference! The only downside was travelling home in the rain and then having to go back to work. I cant wait to get it out for a run in the dry weather.

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Your silliness has been noted.

A little update for 2024 - I started the year with the twin HS2 carbs on the Peter Manton manifold. The engine had been running decently, but it had been getting more noticeable that the carbs weren't in optimum condition. I'd previously bought a rebuild kit to refurb the pair, but I was supplied some defective spindles so I ended up assembling them with the old worn spindles. When driving it's a decent mixture, but when coming back to idle it's super lean at 16-17:1 on the gauge. If I stop and let it idle it'll just keep getting richer and richer, which indicates one or both of the float bowls aren't maintaining the right fuel level. They had also been a bit tricky to start, as only one carb has a choke and it was a regular occurrence to foul a plug on the choke side. Ideally if I keep the dual HS2 carbs they'll need a proper rebuild and another kit..


I'd had the HIF44 carb at this point for about 12 months and I'd bought a new seal kit, some extra needles and an extra carb spring. It was already very clean inside and out and it had no play on the spindle at all.

Before



After


It only took a few hours to remove the HS2 carbs and install the HIF. It was an easy bolt on swap and it started right up and idled smoothly. I managed a few laps around the block, and after a few needle changes & picking the right damper oil and spring it drives really well. It's also nice to know there's only one carb and I don't need to worry about syncing multiple carbs or barrels on the Dellorto


The next job I'd planned was to lower the compression ratio. As per earlier thoughts in this thread the most straightforward way to test was to run with two head gaskets. I'd done some calculations and this would lower the CR from 11.5:1 to 10.5:1


I cleaned up the head and block gasket faces and bolted it all together with the double gaskets. The only thing that was any different was the rocker arm adjustment needed to be a bit further in than before.



So an update on the double head gasket experiment:

For most of the time I've been struggling to get the timing under control I was having pinging at around 3000rpm. It was only at part throttle, the plugs were changed to cooler ones, the timing was reduced to 15 degrees, the fuel was on the rich side, I used 98 octane fuel and octane booster, removed the vacuum advance etc but it never cured the problem. I ended up with a strange inverted timing curve. (The base timing was set at 10 degrees, so all the advance numbers here have 10 extra on top)


Now I've got a proper shaped timing curve and the vacuum advance is hooked up too. At 3000rpm the advance is set to 25 degrees and it's been running really well. Idle is smoother than before, starting is good, no run-on, no pinging. In terms of performance it seems much the same, so possibly the extra timing has offset the lower compression


Overall the double head gasket has been a huge success, it's not a long term solution but it's given me an idea of how to get it running better in the future. I will need to investigate the best way forward:
Dish the pistons and/or open up the head to lower compression
Go to a longer duration camshaft to reduce effective compression
Change to a different head like a 12G295 which has bigger chambers
Drop back to a 998 crank

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