9 replies to this topic

[Cerberus]

1993 Italian Job, 1275 Carb, stage one kit.

What should the Air Fuel ratio be?


The car has had its CAT removed, but apparently, it was fine going through its MOT, the AFR is currently about 11.23 IIRC.

This seems lean to us, but we're not sure on mini engines. But it would make sense as it would need to be leaned off to get through the MOT wouldn't it?

Also, the plugs were still white, not very biscuit brown at all, which suggested lean mixture again to us.

Its apparently producing 46bhp at the wheels though, with the 11.24 AFR.


So is that about the right sort of AFR?

Also, what should the compression be on the 1275? From looking around here about 160-190 is all right, correct?

Thanks

[pikey7]

14.7:1 is the perfect afr in that at that level it is the most efficient burn... i.e. the air and fuel ration will burn completely.

However, while under load, the engine requires more fuel than that, so 11-ish is fine.

I think you're overanalysing it. If it runs fine, and passes the MOT, then there's no problem.

[Carlzilla]

What pikey7 said, but mini engines like to run a little rich anyhow.

[Turbo Nick]

waaaaay too rich imo.

Mine runs 14:1 @ idle, 14.5:1 ish at cruise and 12:1 on boost.

[Cerberus]

waaaaay too rich imo.

Mine runs 14:1 @ idle, 14.5:1 ish at cruise and 12:1 on boost.

Well there are symptoms that would suggest a rich mixture too. Like seemingly poor fuel consumption, (I'm currently measuring this properly so will know what its actually doing, but I'm sure just recently it only did 100 miles to a tank).

It does shift when you give it the beans, and temps have been fine, usually just below the white normal mark in normal driving, only goes up to normal mark when in traffic.


Just trying to get as much info on what's normal, I've got a few issues with the car, one of which is, when in 3rd gear, at 30mph, with light throttle, it feels as though the engine is missing, sort of a little jerky, its not massive jerks, you can just feel it, I'm positive its not quite running right.
Also, the vacuum system seems pants, most of the pipe/connectors are broken, and the system didn't seem plumbed up correctly, as the dizzy would only have been getting vacuum when the thermostatic valve opened, which to us seemed wrong.

And the car is very 'boggy' when cold. I am aware that this might just be the nature of a classic and that it needs to warm up. But when its cold, no matter where the choke is, you have to rev it to about 3k when pulling away otherwise it'll just splutter and drop the revs and have no power, then pull up as the revs increase.

The main issue, if you've seen my other post, is a very bad vibration at 2500rpm, and we wanted to try and eliminate any problems with the engine.

Thanks for the info though, makes me feel a bit more confident in it.

[BLUE ITALIAN JOB]

if your vac system is pants so will the A/F ratio , the italian jos is based on the sprite - 50hp 1275 , and will norm be good for many miles with no fuss , get the leaks sorted before setting or checking any settings on the engine ,
the vac hose goes from carb to the thermo switch T then to dissy , from the other side of the thermo switch goes to the charcole canister - when the egine has reached the right temp the swich opens and draws vapour back from the canister into the engine thru /via the vac tube to be burned by the engine , most cars i have seen have done away with that part and run the vac hose direct from carb to dissy ,
BUT whatever type of system you use all parts will need to be air tight , any air getting WILL affect any readings at the tail pipe and will affect the timing of the dissy ,

[MRA]

Most of the faults point towards a too rich mixture.

[Cerberus]

We've just realised, that its actually rich.

11.26 parts air to 1 part fuel.

It pops alot on over-run etc so its got to be rich to do that.


Thanks for the info on the vac hoses. Ours was setup wrong.

It was connected as follows,

single pip from carb to themostatic. Then the other side of the thermostatic had the T piece, which went to charcoal canister and to dizzy.

[Turbo Nick]

popping on the over run can also be down to air leaks and the wrong ignition timing.

i'd suggest checking all vac pipes and connections, manifold and carb gaskets etc.

[Cerberus]

We know the vac pipes are rubbish. So we're going to sort that out properly.

The manifold seal seems fine, we've sprayed wd40 around it and there was no increase in engine speed.

We think the ignition is all right.