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1962 88ss poor running

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My fairly recently acquired bike gets to 50 in third and 60 in fourth. Opening the throttle any further at these speeds has no effect. It also can judder a bit when the throttle is shut down. It is fitted with newish concentric carbs, 30mm, with 220 main jet. The needle jet is set in the highest position.  I am not certain but did the original bikes have a 27mm carb with 250 main jet ? Are my problems due to the wrong carburettor size and main jet or do I need to look elsewhere. I do have some 28mm carb bodies I could fit. Thanking everybody with any suggestions offered. Jon.
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There may be other issues with this bike but my gut feeling says the carb is way to big and wrongly jetterd for this Norton. The original Carb was a 1 /1/16" Monobloc with a 250 main jet. With two of these on the 88SS. Twin 626 or a single 928 Concentric would possibly be better. I would also suggest 240 main jets and do plug chop checks to get the mixture correct.

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Hi Jonathan,

While the issue may well be carb related (sounds like it is running weak), are you sure that the gearing is correct?

There is every chance it has been raised over the last 60 years and the engine simply can't pull it?

Regards,

George.

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I can't believe that too large carb size will big it down at high revs. Might make it less flexible low down. You have lifted the air slides? The correct size was 1 1/16 monoblocs, but mine runs fine on a pair of 28 concentric

I'd check air leaks in manifold, air slides properly lifted, both carbs are opening together, and of course timing. Fuel tap filter blocked,  fuel hose blocked. Needles sound high, but there's no standard to go by so you need to go through the Amal setting up list.

And is the camshaft OK? A rough check might be possible by looking at the movement of the tops of the push rods.

If it was over geared it should get to max in 3rd gear instead of top.

 


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