[nissvit]

I know this isn't perfect tech but was an interesting day and am just hoping to share some new info

I borrowed a cool toy from work on the weekend - a five gas exhaust analyser - to try and sort out Steve's carby problem(s). Engine specs: Vitara 1.6 block, 8 valve vitara head, 1.3 sierra intake manifold and 1.3 sierra carby.

Before checking the exhaust gas we serviced the car to make sure it was close to running ok. service specs - new leads, 0.8mm plug gaps - timed to 8 degrees.

Mixtures were checked at idle (easily adjusted via idle mixture screw on side of carby) and at cruising revs (approx 2800 for the sierras) - not so easily adjusted as this mixture is dependant on the primary fuel jet which feeds from inside the fuel bowl of the carby.

What we found: with the standard jets (0.99mm in primary & 1.05mm in secondary) the engine was way lean at cruising rpm.

geek bit
Nominally perfect air/fuel ratio is 14.7 parts air to one part fuel which is otherwise known as a lambda ratio of one. Steve's car was at about 15.3 parts air to one part fuel which had a lamba ratio of 1.08.. so it was lean.
end geek bit

We found a reference vehicle - (its great having neighbours with totally stock cars) and checked the mixtures at similiar revs - with the results that the stock sierra had fairly consistent lambda ratios of about 0.96 (just a little bit rich).

To achieve these exhaust mixture ratios in Steve's car we used a 1.05mm jet in the primary and then a 1.42mm jet in the secondary.
With a carby clean and these jet sizes we acheived a lambda ratio of approx 1 at idle, and 0.93 at 2800.. The reference books books I was using (bosch handbook) reckons that max torque is acheived at .9 lambda exhaust mixture so I was happy with the 0.93.

As this was effectively a no-load test we didn't get to test the secondary circuit reading but will see how this goes for now. The 1.42mm jetted secondary now allows you to feel the car accelerate at 80k when you open the second throat, which it didn't want to do before.

So in short form - if you are running a 1.6 bottom end with sierra carby, and it feels a bit doughy, try a 1.05mm jet in the primary and (if everything else is basically the same as Steve's) you will have the primary throat mixture about right.


cheers,
Chris

[want33s]

Well done guys.... A tiny mod like that could save a small fortune in fuel or.............. burnt valves...
Jas.

[Damo]

Very interesting, thanks for sharing!

Just for the sake of having a complete picture, what exhaust is on it?

[Bruce]

Yep, well done to Chris for working things out for Steve. Chris is great with carbs.

[royce]

I am going to have a fiddle with a mild 1.3 soon to set it up correctly, where did you get the bigger secondary jet from?

[nissvit]

the 142 was actually a primary jet from a TB42 carby gq patrol- that I wanted again, just after I put it in steves carby .

Most jap cars seem to use jets with the same size threads, I have a heap of old corolla carbs ( the k series engines 12-1500). they used various jets from about 106-112 in primaries, and 119,130,145 & as large as 162 in the secondaries.

I have number drills to make exactly the size I want, and you can solder the jets closed if it is too big, and then re-drill them.

[Rockhopper]

nissvit wrote:

I have number drills to make exactly the size I want, and you can solder the jets closed if it is too big, and then re-drill them.

Just on that, many many moons ago when I worked in a workshop, we had a set of carby jet drill bits. Are they still available and where do you get them from??