Hello
A65T usually has a 928 but I was wondering if 932 can be jetted to work? I have one in good condition and rather buy a premier I thought I might try?
Thanks
Matthew
Yes it can, and it may not be necessary to smooth the port, you can always do that later. The step from 32 to 28 can break up fuel droplets so it can be beneficial. If you have the head off you can always open it up with a Dremel then. The A65 L had a pair of 32 mm so you have plenty of leeway.
My 830cc Thunderbolt uses a 32mm. I made a jig to hold the manifold in the chuck,
Put a 5° taper on it and used a 1/4" thick spacer as well, machined them all together.
Just bolting straight on will work, but the taper worked best. Also removed some steps from the inside of the manifold.
Doubt you'd gain much though on a 650.
Hi,
At lower revs the velocity won't the incoming mixture be relatively slow and not mixed as well as it could be? At higher engine speeds it will be better.
It is a bit like drawing a liquid through a straw. Too small a dimeter and it takes a lot of effort but if the diameter is too big then you cannot draw much up the straw.
Dave.
But don't forget the A65 Lighting ran well, idled and picked up fine with a 32 mm Concentric on each cylinder so there's a lot of margin for increased breathing. I ran an OIL Lighting for many years and I took the air filters off and the increase in flow was enough to go up one jet size so even as 2 x 32mm there was scope for more flow.
Quote from: limeyrob on 26 Sep, 2025, 08:12 But don't forget the A65 Lighting ran well, idled and picked up fine with a 32 mm Concentric on each cylinder so there's a lot of margin for increased breathing. I ran an OIL Lighting for many years and I took the air filters off and the increase in flow was enough to go up one jet size so even as 2 x 32mm there was scope for more flow.
Having used and owned at least one of every version of cylinder head that came out of the factory, there is a great difference between late lightning heads, early (c64/65) lightning heads and thunderbolt heads. There is a great difference between an OIF lightning head and a thunderbolt head. You could also add here that the 750 A70 still used a 30mm concentric. The Lightning clubman as fast as a spitfire and there was 4mm of carb diameter difference.
The Thunderbolt head is closer in design (port diameters) to the Early lightning head - as was used on the clubman. The port near the valve is much smaller than a 66-on Lightning head.
There is also air speed through the carb which Dave mentions, a bigger carb and air speed is reduced. That is why you needed to go up a jet size, because there isn't enough draw on the jet to flow the same amount of fuel. Removing the filters may improve flow, but it ALSO reduces vacuum.
(flow rates for jets are calculated against a constant, you can't just raise or lower the pressure and get the same flow rate).
Dave is correct, the bigger you go the more you will loose at the bottom end. On a 650' you'll gain very little at the top, least nothing that can't also be hindered by other inadequacies within the engine or ignition system. What you loose at the bottom will be more noticeable. Having had 28mm and 32mm on my Lightning with early twin carb head. The 928's are the best all around performance, If I wanted to do 110mph on the motorway then I would find the advantage with 932's, but as an all round performer your loosing more than you'll gain.