silly rich afr at part throttle

[GTphil]

Well, car passed it's mot yesterday, so thought I would pop down to scoffs for an evening of ear bashing with cars on his rollers! Plus wanted to see how my bro's car performed.

Car ran great afr wise, 13.9 idle/13/14 on cruise and 11.4@wot with 22psi

Then just as I was nearing home have it some beans through 3rd/4th/5th then noticed that my afr's suddenly changed drastically

Cruise, as soon as you touched the accelerator it dropped to flat 10 on the afr guage, then when I put my foot down the would creep upto 12's then slowly drop back down into the 10's.

I haven't had chance to look at the carb yet but my first thoughts are an issue with the flote maybe, then after I have checked that have a look at both enrichment circuts.

It would seem that something has either come loose or a gasket/diaphragm has let go as it happened so suddenly from fueling perfectly.

Any thoughts on what else to check?

[Dave Reed]

Check the mixture screw is still present.. Then I'd be checking the diaphram in the acc pump.

[Alex]

It's reasons like this that people go EFI! I guess the first thing to do is give the idle circuit a clean as it's the easiest thing to start with. Failing that take the carb off, clean it out and check for any splits in the diaprams/gaskets etc.

It wouldn't surprise me if the next time you take it out it's fine again!

[Jonny5]

Afr Lambda sensor failed? Does the car feel/behave any different apart from afr readings?

[Brigsy]

Carb freeze?? If not fuel must be leaking down the carb throat. Check fuel pressure if u have an adjustable fpr fitted.

[GTphil]

It wasn't all that cold last night, this was around 1am this morning, when I pulled over I do remember thinking when I touched the carb top how cold it was, infact it was very very cold, compared to the boost circut pre-intercooler.

It did however stay like this for the rest of the 25mile or so journy home, appeared to run ok, and the idle was fine all the while high 13's low 14's.

I'm itching to have a proper look but stuck in with the twins on super dad duty

[Brigsy]

If you were cruising on a motorway no/low boost with your new fmic it could be carb freeze mate.

[GTphil]

Had no experience with carb freeze before, so it's definatly a possibility, when I'm on the m-way I do tend to drive at 65/70 completly off boost

I will know when I get it out again as I'm assuming it will more than likely be back to it's usual self if it was carb freeze I would have thought.

[GTphil]

Not a very accurate example I know but my ally carb top was like a cold can of beer from the fridge

[s3irios]

I have exactly the same problem. Slight throttle low 10s on gauge. Mid throttle up to high 12s then full boost mid 11s.
I serviced the carb, cleaned thoroughly.New o-rings new idle jet (as i missed the rocket thingy inside) new smaller main jet. Nothing changed.

[GT Josh]

Have you looked at this yet sausage??

[GTphil]

Just been out for a quick look as I had a spare hour, so took the car for a quick drive up the road, still the same, idle afr was 13.9 so spot on as soon as you touch the throttle afr straight down to flat 10 as low as the aem wideband goes then it stays flat 10 till it comes on boost then I rises up to mid 12's then drops down to mid 11's afr.

Took the carb top off and all seemed to be in order with the flote/needle valve.

put the carb top back on, started it up and looked down the carb whilst it was running, idle was fine, as soon as I touched the throttle I could see fuel dribbling down the carb throte.

I'm pressuning this isn't normal and I should get a fine mist of fuel rather than drips/dribbles?

[GT Josh]

Yeah with throttle it should spray. Haz tought me that one, don't know what would cause it to dribble/trickle. Might need to get a rebuild kit bro.

[GTphil]

I have a malpassi non ajustable fuel pressure regulator, just seems strange that it happened all of a sudden after a pull.

Going to take the carb off tonight and give it a check over, this carb was only re-built last year, I built it up with new gaskets and jets as a spare. Has been fine for 1200 miles then suddenly this.

I hope my fuel reg hasn't given up the ghost, again that was new last year also.

[GTphil]

Took the carb off and tryed my spare carb, car was fine with spare carb, so that means it definatly had to be the carb at fault.

I have found two things wrong so far with the carb that came off the car, firstly the carb top gasket was split where the little brass tube comes out from the carb body, secondly the carb base was loose from the carb body, the screws seemed to have worked loose, somehow, I can't see any of the above causing massive over fueling. I would have thought that the carb base being loose would have not been an issue given the two 12mm nuts clamp all that pot down anyway.

[GTphil]

Well, checked the accelerator pump diaphragm and thats fine, had a good look over the whole carb and the only other thing thats suspect to me was how easy the small brass tube was to pull out of the carb base, is this normally really easy to pull out? It isn't on my other carb.

Incidently just been for a drive and I now remember why I changed carbs in the first place, the one that fuels well, has really bad bogging down between gears.

Anybody got any more suggestions?

[s3irios]

I have to say that i'm watching over this thread eagerly as i stated before have the EXACT same issue.
Help people!

[Nottswoody]

Haven't read the whole thread but has no one had the micro drills out and checked the jet sizes?

[GTphil]

Jet sizes are

120 Main
0.9 A/c
1.4Mm second stage
2mm needle valve

The first stage enrichment is standard

From what I can gather the issue has to be something to do with the first stage enrichment circuit, the pump diaphram appears to be in good condition as is the large spring that sits behind it.

When you touch the trottle their appears to be to much fuel dripping down from the accelerator pump nozzle, when I tryed my spare carb when I looked down that whist touching the throttle there was markedly less fuel coming out and much better part throttle afr's.

So I'm assuming the the problem lies not with the diaphragm or spring set up but with either the pick up jet in the bowl or the accelerator pump nozzle is either not sealing and letting fuel pass or has a problem inside.

I'm going to swap the pick up jet and accelerator nozzle from my spare carb and see if that makes any difference.

Gotta love the old carb

[GTphil]

Sorted! The problem lay in the enrichment circut that bolts onto the passenger side of the carb,

First of all I tryed my spare carb, the fueling was instantly better, my spare carb needs a rebuild and made the car run like a bag of spanners hence not wanting to use it.

Then i tryed the carb throttle spindle section and accelerator pump on the carb that overfueled and that made no difference, was still the same, flat 10 afr with the slightest touch on the pedal.

Then i tryed the accelerotor pump nozzle from the carb that fueld well in the carb that overfueld and that made no difference.

Then i tryed the same but with the enrichment circuts and hey presto problem solved.

I have taken the faulty enrichment circut apart and i really can't see a problem with it, all the gaskets and springs all look to be ok. Strange one really

[GT Josh]

[s3irios]

As i said 2 times already i have the same issue, so after reading the above went out took the enrichment chamber out and the only 2 parts i can see that may are faulty in some way are the 2 springs and the round brass thingy with the small metal ball inside.

The springs may are faulty in their tolerances after all those years, being more stiff or more loose.
The brass thing maybe the metal ball isn't sealing properly inside when it's presing towards or something like that.

There isn't anything else inside there able to fault as it's actually a metal mass with holes in it.
Cleaned it again, can't change it since i don't have a spare atm.

Ty GTphil for the very useful info about this irritating and money consuming problem!

[GTphil]

Glad the thread has been some use for you and at least it gives you something else to try, my moneys on it being the same issue as mine.

Hopefully a different enrichment circut with do the trick