Carb Gets Very Lean on Initial Throttle Application?

[Woznaldo]

As per the title, when I start the car on a cold morning on the choke it's fine, but once it starts to get warm and, more importantly, the choke is off, initial application of the Throttle sees the AFRs go very lean (16-17) for a moment and the engine bogs for a moment. AFRs then fairly rich (11.5-12) and the engine pulls cleanly.

The symptom is also present on the motorway. The engine gets a bit rough when I am trying to cruise at a constant light throttle (AFRs 16-17 again). If I push the throttle in a little further, the car accelerates normally (AFRs 11.5-12).

Carb has been rebuilt recently and blown through with an airline before assembly. The setup is as follows:

Main: 120
1st Stage: 1.0
2nd Stage: 1.2

I did setup the Throttle Pump with the 5mm drill bit and fuel was pumping out just fine when I set up the Throttle Cable during fitment?

It's worth pointing out that I'm running a Wasted Spark EDIS4 Ignition setup through a MS1 Extra ECU (EFI not connected, hence the Carb question). The MAP is standard late Renix 209 from Scoff apart from some tweeks around the Idle range (500-1000rpm).

Woz

[Andrew Cooke]

I don't know the answer, but it might also be going very rich for a moment, if you wet the lamda it will show lean, and the only clue that it's rich rather than lean is that it shows rich before returning to normal. If that's the case it might be excessive fuel pressure, or an iffy float valve? Are both of those standard? Maybe float level is high?

Another possibility is that you're losing sparks for a moment, you'll still have fuel, wet lambda, shows lean, then shows rich as the sparks return and the excess fuel gets burnt off?

[Woznaldo]

The thing that makes me think that it is going lean is that it doesn't happen when the choke is on, effectively making it richer?

Everything on the Carb is standard apart from what's stated above.

[GTphil]

I just got to the bottom of a similar issue with the help of other members on here.

My issue was the emulsion tube/accelerator jet was blocked all be it slightly, on initial Throttle it would go off the scale lean then settle out slightly richer than normal. It was mentioned in my thread that the emulsion tube is only .4mm so blocks up spectacularly easily. They are also difficult to clean out properly.

So often after carbs are re-built/cleaned this sort of issue appears I think it must be impossible to completely clean it out and inadvertently something ends up blocked or disturbed. This may not be your issue but worth a try.

I could feel a noticeable difference when I blew down said tube, I had three and the clearest one solved my issue.

[Woznaldo]

Thanks Phil, I'll pull the top off the Carb and take a look. I do have another carb that I might be able to use. If not, I'll have to buy one of those hobby Untrasound Cleaners?

[michael tierney]

another issue with the pump nozzle ...theres a steel ball inside which keeps the fuel from returning to the float chamber.if it doesent seal correctly u can reface it with a plain endmill cutter on a lathe or mill/drill ,

[Woznaldo]

I replaced my emulsion tube with the one from my other carb, but despite it 'looking' a lot better, there was no difference. :(

Maybe I need to try and improve the seal? Michael, can you explain a bit further please?

[michael tierney]

when u shake the pump nozzle u should be able to hear it rattle,if ur pump nozzle base seal isn't leaking when u press the throttle and the squirt is a bit weak or late then the little ball inside is,nt sealing properly..u can pull the assembly apart to inspect it ,and test it. or if u have another carb handy try it instead in ur good carb!
another snag with ur cruising leaness could be if the float isn't matched to the carb(mixed up)there's two markings on them i.e. J2 on the top and J4 on the bottom and I hav'nt figured them out yet.....maybe MART has had a bit of experience with the floats!!??:(

[Woznaldo]

I haven't done anything else to the carb, but I do want to get some clarification on what and when the Air Corrector (AC) does? i.e. the std AC is 1.25mm, what is the effect of reducing the jet to 1.1mm and where in the boost range?

[JP Racing]

The air corrector affects fueling top end ;) its alot further up the rev range it affects not low down. Ive got to look at a carb tomorrow with similar problems I will let you know what I find out tomorrow just incase its the same priblem you have.

[JP Racing]

Also forgot to put the smaller the ac is the better the fueling top end will be. I used to run a 0.8 and fuel was good all the way through 5th gear to redline ;)

[Woznaldo]

JP, seems like GTphil sorted his carb with a similar issue by changing the Emulsion Tube (third or fourth time a treat). I tried changing mine, but only have one spare to try from another ropey carb.

http://www.rtoc.org/boards/showthread.php?t=36228

Has anyone got a spare one or two they can send me? I'll pay for them and cover the shipping.

Woz

[Woznaldo]

I refitted my original Emulsion Tube after having had it soaking in some carb cleaner and then pulling it apart and cleaning some more. It didn't seem to be blocked in the first place, but never say never.

I've been on a short drive and it does seem to have improved, but no completely cured. Maybe I need to blow the circuit through?

I'll see how things are over the next day or two ad report back....

[GTphil]

I know you have probably tried this but keep on repeatedly cleaning the idle jet out, by taking it out pulling it apart and blowing through it with carb cleaner, once the carbs been apart i have always had to clean mine a million times (maybe a slight exaggeration:laugh:) before it would run right. Seems to carry on blocking up for a while.

Also i once cleaned it out so many times and it made no difference at all, not sure why but i checked the idle jet with a magnifying glass and there was a tiny bit of the threaded part in between the silicone hoses trapped in the end that the carb cleaner wasn't shifting.

Again just another few simple things to try.

Darn carbs! :mad:

[LiamR]

I know you have probably tried this but keep on repeatedly cleaning the idle jet out, by taking it out pulling it apart and blowing through it with carb cleaner, once the carbs been apart i have always had to clean mine a million times (maybe a slight exaggeration:laugh:) before it would run right. Seems to carry on blocking up for a while.

Also i once cleaned it out so many times and it made no difference at all, not sure why but i checked the idle jet with a magnifying glass and there was a tiny bit of the threaded part in between the silicone hoses trapped in the end that the carb cleaner wasn't shifting.

Again just another few simple things to try.

Darn carbs! :mad:

Top advice....clean, clean and clean again! :)

[Woznaldo]

Thanks guys, I'll check the idle jet too.

[Woznaldo]

Ok, on my 10 min drive to work this morning the car appeared to be a little better, but ultimately not much improvement. On the drive home it started behaving a bit weirdly, and then started to stall at junctions and even stalled as I pulled up to the house? When I got home it was raining quite badly so I didn't check anything until it had stopped.

I pulled the Idle Jet and gave it a good clean with some carb cleaner threw it back in and the car idled cleanly. Went on a quick test drive and again there was an improvement on the original problem and no sign of stalling!

I've also noticed that once it's past the initial hesitation, it runs quite rich (high 11s) until a bit of bost kicks in. Could this be a 1st stage issue as in maybe not standard?

[GTphil]

I currently have a .9 first stage, smaller than standard, and I get high 11's on initial throttle application then it rises to high 12's until it comes on boost then it's flat 12.

[Woznaldo]

I have the hesitation for a split second then it goes straight to mid 11s and starts to lean a little as boost climbs, to around the low 13s and then as boost starts to come on strong it richans back to low 12s.

I think that sounds more like the 1st stage area?

[Red October]

Woz did you figure this out mate?

[Red October]

Well if anyone is curious of how to solve this issue I managed to clean my carb about 6 times before it cleared. The leaning is because of crap stuck in jets. If it happens whilst driving to remedy the issue just raise the choke to half until you have time to clean it out.