The V8 Owners Forum -

southernd wrote:So say 67/56 is leaner tab 62/52?

I may be getting confused here but I sent somebody my 'Eddy' spreadsheet which allows you to compare any jet and rod combo with any other, I thought that it was you, maybe not.

Anyway, it is not quite as simple as just looking at the rods on their own, you need to look at the jets as well. The numbers on the rods are just thou of an inch diameters, so a 67-55 rod has two diameters the cruise is 67 thou thick, the power circuit is 55 thou thick. (You get the idea for other rods 65-52 etc. By the way there isn't a rod that is "67-56")

The numbers on the jets are just diameters, a 386 jet is a "3" series jet that is 86 thou in diameter. A 413 jet is a "4" series jet, in other words a jet that is over 100 thou in diameter, a 413 jet is in fact 113 thou in diameter. a 410 jet is 110 thou in diameter, a 407 jet is..oh you get the idea.

The fuel flow though the primary side of the carb is determined by the size of the primary jets BUT remember that the rods are poking into the jets which makes the flow area smaller. When the rod is in the down (cruise) position it blocks off more of the jet than when the thin part of the rod is in the jet.

So when looking at a set of rods that are smaller than the set you have currently fitted to the carb just changing the rods will make a big percentage change to the setup, it will be a lot richer but if you fit a smaller jet at the same time as changing the rods then even though you have fitted thinner rods the setup could in fact be a leaner setup.

Ah now I get you

So my last set up was 80 jet and 62/52 for rods. The new one is set up as 86 with 65/52. It runs nice enough but is missing a bit of power off the top end. I don't fancy pulling the jet out, I'll put the 62/52 back in and see how it runs

I only used the rods as example, I couldn't think of one off the top of my head

Just seen your post, I'll try that combo first

http://www.v8forum.co.uk/forum/viewtopi ... 62b60401cd

The primary jets and the rods really are for setting up the cruise circuit and the acceleration at half throttle. Above half throttle and at WOT the secondaries start to come into play. At WOT (in other words what people call the top end) the secondary jets are what you need to be changing. The smaller Rover engines tend to need larger secondaries than the larger Rover engines, this is all down to the vacuum that the engine pulls at high revs. The larger the vacuum at WOT the smaller the secondary jets need to be. The standard secondaries (95 from memory) are not a bad size for a 3.5 engine, a 3.9 might only want the jets dropping to 92 or something like that. All this does depend a lot on the cam fitted. I have probably said this before....You need to monitor the exhaust gases! The AFR ratios for the varrious modes of operation of a run-of-the-mill 4 stroke lump are well known, at WOT you want to be running 12.5-12.8:1 AFR.

I've ordered a wideband AFR so I can get the carb just right. I tried swapping the jets about.

Firstly I did a couple of upgrades.

NGK7 standard plugs replaced with irridium.

Standard leads with magnecor.

I left the 65/52 in and went for a long run, around 100 miles. Sparks plugs were bleach white and performance wasn't great. Changed to 62/52 this morning before Goodwood and running very well. I haven't checked the spark plugs yet, I'll do that in the morning.