Idling AFRs going rich?

[Leloby]

I have a newly refurbed carb with 0.9 air corrector, 120 main and 1.4 2nd stage. It was running pretty much OK at 14psi T25 hybrid.

On startup, it runs nicely at 14.odd with minimal revs.

But now, as soon as it's revved, AFR drops to 10s and won't recover. Goes lumpy and doesn't respond to mixture screw adjustment.

It's as if the choke is on, but there is no choke and butterfly is wide open.

Black soot on exhaust elbow etc.

Also, when it was running right, all AFRs were bang on but after boost on cruise, they would go to 17 and the car would cutout.

I can't get my head round it and can't find a similar issue on the forum.

Any ideas?

[fat roofer]

Might be a stupid suggestion..... but, I'd check the mixture screw first just to make sure the tip hasn't snapped of in the carb.... try and take it out and it should have a plain pin-like end at the bottom of the screw section.

[Leloby]

That was my first idea when it started going weird. But it's fine and I've tried 2 with new o-rings too.
Everything is refurbed in the carb.
I'm worried it's head gasket or something as I don't have a lot of time and need the car race-ready.

[Leloby]

Again this morning, starts on the button and runs perfectly at 14.1 for about 30 seconds then goes to 10s and runs like crap.

Is there a chance the AEI is knackered?

[Alex]

Check the plugs and see if they're sooted up or damp with fuel. I'd be changing that A/C back to standard also.

[Leloby]

Only have an hour later, do you think sooty plugs will make it run 10s after starting at 14?

[Alex]

Are we talking on idle? I remember when I had a 0.9 A/C jet it my plugs sooted up badly and it ran terribly when cold.

[Leloby]

Yes, does it just need a good booting then?

[Alex]

No, it needs the A/C jet changing! definitely change the jet as it'll over fuel for the boost you're running.

[Leloby]

Ok. I'll give it a go. Didn't think that would bother idle.

[Brigsy]

The smaller air corrector is not causing your idle problem, i bet its fuel dripping into the venturi when it shouldnt be.

It will probably be one of these faults - needle jet not sealing/float cracked/carb top gasket leaking/fuel pressure too high. I have also seen faulty carb body too.

[Leloby]

The fuel pressure is a thought. I checked the various carb castings etc and the whole thing is spot on.
It's the same after rebuild as it was before so it's possibly fuel pressure or something external to carb.

The question is: if the fpr is knackered, can it increase pressure?

[Leloby]

Going to answer that myself: Yes.

I remember an AX gt with same problem - my bad memory.

Will order Malpassi tomorrow.

[Brigsy]

Check the pressure before you renew it with a gauge - 4psi on idle. To be honest they rarely fail.

[Alex]

Ok. I'll give it a go. Didn't think that would bother idle.

It won't directly cause an idle problem but it will run rich with that jet in situ.

[Leloby]

It ran ok with it before though. Only leaning out on cruise after a lap or some serious boosting. I put that down to an air leak at the elbow gasket. Now fixed.

It's sooting up at idle. I reckon it's the fuel pressure regulator now coming to think of it.

[Alex]

Hook up a gauge and see what's what. It should be 4psi about boost pressure. I've never had any trouble with my 24 year old FRP but you never know.

What tracks have you been on lately?

[Leloby]

Do you mean fit an in-line gauge so I can monitor pressure on the road?

I'm not far from Knockhill and there's a hot-hatch/French day on the 31st August so it needs to be "bomb-proof" for that. Not much time right now though.

[Alex]

Yes in line. Your issue seems to happen at idle so I'd check it there first.

[Leloby]

Pressure gauge wavering around 4psi. Bought a new Malpassi FPR009 - it's not made any difference.

Car starts on the button.
Idles nicely at 1k.
14.odd on the AFM - steady. All looking good.
As soon as there is any throttle, AFRs drop to 10.0 and stay there.
Revving engine makes no difference. 10s dead.
Idle goes lumpy and it eventually cuts out - doesn't respond to air screw adjustment.

Carb overhauled, everything same as before. New turbo gaskets too.
Plugs clean.

I'm stumped.

[tubbyG]

Hi matey, Did you rebuild the carb yourself?

I have encountered very similar symptoms in the past when I did not build the carb correctly. (it's been a while but...) On the drivers side of the carb - accelerator pump mechanism. There is a spring which needs to be seated 100% in the metal diaphram cup. It's so easy for it to become misaligned when refitting.... worth checking as you have done checked all the usual

[GTphil]

If you have any spare carbs, try the enrichment block off another carb. I had a carb once that was fueling perfectly, then all of a sudden very similar symptoms to yours. Flat 10 afr as soon as any throttle was applied. I tried all sorts before i changed said enrichment block. I couldn't even find anything wrong with it when i took it off, visually it was exactly the same as the one that worked perfectly. Must have been something inside the block it's self.

Also did you set the accelerator pump arm using the 5mm drill bit method?

[Leloby]

Thanks boys. I'll need to get it off again and check the enrichment block and accellerator pump setup. I did have an issue withy the spring so will double check that. The insert on the refurb diaphragm is different so could be that.

It's peeing down here and I need to do work outside right now.....

5mm drill method: just saw the pic on that - will do this ASAP.

I need another carb now for spares. If anyone's got one, I've got cash and I'm getting desperate

[GTphil]

If the accelerator pump arm isn't set correctly it will cause this type of issue. The accelerator arm is the first point of the carb that's used when you hit the gas pedal. Every other setting/jet comes afterwards. So if this isn't set correctly it doesn't really matter what jets are in because it just won't run correctly at all.

They are funny old bits of kit these carbs, sometimes everything can be set correctly and they are either old/worn or damaged somewhere that you can't see. Hope you sort it.

Always worth having a few other carbs around for spares/reference.

[Leloby]

Right, I've taken carb off and setup the 5mm gap. Everything seems to be hunky-dory.

The plugs were fouled so I swapped them out.

On startup, it holds 14.1 - 14.7 but after 30secs of revving up to 3k, AFRs drop to 10-11 and car goes lumpy/cuts out as before.

Is there any chance it's a distributor cap/leads issue?

I don't have spares but I reckon they need replacing anyway?

The other thing I've noticed is the new idle screw o-ring makes it unscrew slowly. It's a right rubbery thing - doesn't create a "hard" seal. I don't like it.

[GTphil]

Try cleaning the idle jet out a few times and see if it improves it.

If the idle screw doesn't seal try getting it to the correct setting then bung the top of the hole up with some sealant. When you say unscrew slowly, do you mean of it's own accord, or when you do it?

I doubt its the plugs and cap, but replacing them if they are worn certainly won't do any harm.

[Leloby]

It's like the screw doesn't tighten properly, just tightens against the rubbery o-ring then when the engine is running it slackens off. Seems to seal ok, just changes the position of it. I'm not actually sure the o-ring is the right size, too thick maybe.

I reckon that could be the culprit. It's something simple, I'm sure if it.

Maybe it's me being a bit simple?

[GTphil]

The sealant will sort that out, you can actually get a cap for the mixture screw I think that slots in upto the screw and stops it working loose.

Your not the first and certainly won't be the last to suffer the curse of the solex 32 dis.....

The mixture screw shouldn't affect the higher rpms unless it's missing, even then it should still run smoothly.

Those idle jets are pesky sods, when ever I even just take the carb off, I have to clean the little blighter 3/4times before it runs right.

[Leloby]

Fixed! Kinda anyway.

Stretched a smaller o-ring over idle jet - it now idles and revs around the 14 mark.

But when on a run, they do go to 10s with a bit of throttle. Light throttle cruising at 13s. Full boost is ok in 12s.

There are spikes of 16s and 17 after boosting. Thank goodness it's a "wideband" AFM!

Basically, AFRs are all over the place.

I'll work on it. Will take ac jet out to 1mm for a start.

[B18ftMOJO5]

getting there derek almost ready to thrash the old girl round the track now.

[Leloby]

It's been ready, been broken, now ready again.

Just going to live with the bouncy AFRs.

Knockhill again soon. New Minilites coming this week after 10week delay!

[GTphil]

If it feels ok and isn't uber rich at to many points then as long at full boost is ok you can play with the rest later, when you say full boost in the 12's how much boost and from what turbo?

Only reason i ask is if it's 20psi plus then you probably want full boost flat 12 at the most and even richer still for prolonged track use.

Again it's all over the place because its a carb. You can lean off spool up by either reducing the 1st stage jet ever so slightly or by using two cup washers behind the first stage spring/cup. Also at the end of the tube on the enrichment block (on the outside). if you remove the metal bump you will reveal a screw this can be used to adjust when the first stage fuel comes in. It's all mostly trial and error as no two carbs or for that matter even engines are the same. So you just have to keep trying different things until your happy. Won't ever be 100% unless you go EFI.

[Leloby]

Now, I've never heard of THAT mod before.

Excellent thanks. Room for hours of tweaking fun.

Its a T25 hybrid at about 14-15psi. 20psi should be no bother once reliability is sorted.

Cheers!