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Indeed, you can swap the entire inlet manifold and carbs, but probably not a great idea. Unless the HS2's are well past their best and the bigger carbs are really good. Not sure what needles you would need though, but the 1500 ones are unlikely to be ideal (but will work, may be too rich or too lean. Too lean is quite likely.....)

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If your 1300 engine is otherwise standard then in my opinion you'll do better to overhaul the 1.25" SUs you already have.  The 1.5" ones are really too big.

To put it in perspective, I used to have a tuned 1300 in my Herald with gas-flowed, big-valve head, TH3 cam and 4-2-1 tubular exhaust.  This could and did rev to 7000 rpm (or more if I was feeling lucky). I tried a single 1.5" SU on it (on Dolomite log manifold), 2 x 1.25" HS2 SUs (as you have now) and 2 x 1.5" HS4 SUs.

Surprisingly enough, the single gave best results to about 4000 rpm, especially lower down.  The twins did better at higher rpms, but the 2 x 1.5" only made a detectable difference over 6000 rpm (beyond the red line for a standard engine) and definitely hurt the low down torque and driveability.  The 1.25" HS2s were the best compromise and will also be for your engine, which will flow less air than mine did.

The SU is a simple beast and straightforward to overhaul with all parts available new.  The only non-DIY prospect is if the body is worn where the throttle spindles pass through it leading to air leaks.  At that point, getting them professionally overhauled by someone like Andrew Turner is probably your best bet.


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Just read your reply Nick. I was hoping to do the switch to the 1.5" carbs cos I can get hold of a pair in better condition than my 1.25" units. Th jet assembly sticks on both of them, one worse than the other. Maybe an overhaul is the answer, I guess the original spec is best.

From... David.

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Sticking jets after the choke has been operated is down to some simple look and see
the levers and pivots get misaligned with use and some simple lubrication can work wonders
some have found  the jet tube is a littleToo long and trim 4mm off its length
to fit the hs4 you need a replacement manifold and a replacement air filter box Or a new pair of nasty pancakes


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