Carb jets for economy.

[Woznaldo]

I hi-jacked Jimmy GTT's thread about a possible accelerator pump problem:

https://www.rtoc.org/boards/showthread.php?t=16756

but I never really got to the bottom of it or whether or not it was accel pump in the first place?

I've been reading the 'changing carb jets' article

https://www.rtoc.org/boards/showthread.php?t=460

and want to know if i'm heading in the right direction? My carb has been running fairly rich everywhere and a mate recently commented on the black smoke coming out the back when I pulled away, so I want to get the car running better and maybe a bit more economical?

What I've done so far to lean things up and get the fuelling back on track:

Main Jet: 110 (soldered and re-drilled 135 main)
1st Stage: 1.0mm (std)
2nd Stage: 1.1mm
a/c jet: 1.1mm (only because the head on the std 1.25mm jet was chewed out)

Using my LM1 wideband i'm getting the following:

Idle: Mid 13's
Light Throttle: High 11's and low 12's
Mid Throttle (off boost): Richens up to mid to low 11's
Mid throttle (as boost moves past zero): Leans to high 13's
WOT Haven't got that far because of the above.

The engine is running the std turbo at 15psi.

I'm thinking open the 2nd Stage to 1.2mm and go again.

There is another problem, when I initially put in a bit of throttle, the car hesitates and the AFR jumps to 23 for a split second and then continues to read as above? That almost sounds like the accel pump not playing the game?

Do I need to think about touching the 1st Stage now that I'm running smaller than std Main Jet?

[Alex]

For 15 psi (is that @ manifold?) arguably a standard carb jet spec will be fine. You could try opening up the 2nd stage if it does get a little on WOT.....

[Woznaldo]

STD carb jets were safe but a little on the rich side and after reading Marts and James comments on wanting smaller than STD main jets for economy in Stu's thread for brand new jets, I thought I'd give it a go.

Since the first post I've opened the 2nd stage to 1.2 and fueling is now pretty good. There is still a bit of hesitation on initial application of throttle?

When I had the top of the carb off, the accel pump tube fell out? Will it being that loose effect anything?

[Alex]

I can't imagine that'll be helping matters. What's the AFR figures now? Try returning the air corrector back to standard if you can.....

[Woznaldo]

I can't imagine that'll be helping matters. What's the AFR figures now? Try returning the air corrector back to standard if you can.....

I'll have to open up one of spare a/c jets if the one in my spare carb is knackered.

I only went out for a quick run but I'm getting the following now:

Idle: Mid 14's
Light Throttle: Initially getting high 11's then returning to low 13's
Mid Throttle: Mid 12's dropping as boost becomes positive to low 12's, high 11's
WOT: Low 12's, high 11's dropping to low 11's at 6000rpm in 3rd and 4th.

Just to confirm, I'm running 15 psi (carb base/manifold) with std T2.

I'm still getting this hesitation and now I'm worried that I may be causing damage to the engine. While at a steady speed and throttle setting, say 3000rpm, when I squeeze the throttle down I get hesitation like the engine is bogging down and then it picks up again a second later. AFRs during this are the worrying part, with vacuum reading 15 Hg/mm the AFRs are low 16s! the engine stutters and then accelerates normally and AFRs drop to low 13's?

[Woznaldo]

With reference to the last para in the last post, will a loose accel tube/pipe cause this and if so, are the low 16 AFR reading likely to cause detonation?

[Alastair]

Woz,

It sounds like you need to set up the carb accellerator pump using Mart's patented 5mm drill bit set up method. It works a treat, the previous rich running will have masked some of the symptoms you now have.

Al

[Brigsy]

110mm main is small & that is probably causing the lean spot before the enrichment cct comes into play imo.

Id personally go for a 115/120 main & lean the c.o off to 15s on idle. You should be able to get the afr/s to 14/15 on cruise assuming this is stock cam.

With stock jetting & 1.2 2nd stage i can get approx 35mpg on a run with mixed driving, easy 350+ miles on a tank if taking it steady.

[Woznaldo]

Woz,

It sounds like you need to set up the carb accellerator pump using Mart's patented 5mm drill bit set up method. It works a treat, the previous rich running will have masked some of the symptoms you now have.

Al

Al, I've done this already and took my time doing it.

[Woznaldo]

110mm main is small & that is probably causing the lean spot before the enrichment cct comes into play imo.

Id personally go for a 115/120 main & lean the c.o off to 15s on idle. You should be able to get the afr/s to 14/15 on cruise assuming this is stock cam.

With stock jetting & 1.2 2nd stage i can get approx 35mpg on a run with mixed driving, easy 350+ miles on a tank if taking it steady.

I was still getting the hesitation with the STD 120 main, just not as bad?

I'm going to try the accel pump tube from my spare carb body first and go from there.

[Woznaldo]

Right, I've changed the accel pump tube and there was no change. Unsurprisingly as, it was just as loose as the original.

Can anyone tell me the effect of changing the jet in the float chamber (not the main jet or the the a/c jet that the screwdriver is pointing at, the one you can see)

[Woznaldo]

What will be the effect of increasing the size of the 1st Stage jet to 1.1mm? i.e. where in the rev range will it make itself evident?

[Brigsy]

Increasing the 1st stage will make the fuelling richer at low boost (under 3psi i think) before the 2nd stage kicks in, will probably chuck black smoke out on initial turbo spoolup.

I doubt it will cure your hesitation woz, try adjusting the acc pump arm a little. Many gtt carbs suffer from this problem, hard to cure sometimes

[James5]

Increasing the 1st stage will make the fuelling richer at low boost (under 3psi i think) before the 2nd stage kicks in, will probably chuck black smoke out on initial turbo spoolup.

I doubt it will cure your hesitation woz, try adjusting the acc pump arm a little. Many gtt carbs suffer from this problem, hard to cure sometimes

As Brigsy says 1st stage enlarging will make fuelling rich on low boost, to be honest this is what many tuners used to do they used to swap the 1st stage jet for the a/c jet making the fuel curve horrible.

Again as Brigsy advices adjust the acc pump arm using the 5mm drill bit method

[Andrew Cooke]

STD carb jets were safe but a little on the rich side and after reading Marts and James comments on wanting smaller than STD main jets for economy in Stu's thread for brand new jets, I thought I'd give it a go.

the brass erodes with running, all these carbs will have oversize jets by now.

[Andrew Cooke]

have you tried looking into the carb with the engine running, you can often see fuel running out of odd places (you shouldn't see any fuel anywhere at idle). Also with the engine off open the throttle whilst peering inside, you may well see what's going wrong.

[Woznaldo]

Increasing the 1st stage will make the fuelling richer at low boost (under 3psi i think) before the 2nd stage kicks in, will probably chuck black smoke out on initial turbo spoolup.

I doubt it will cure your hesitation woz, try adjusting the acc pump arm a little. Many gtt carbs suffer from this problem, hard to cure sometimes

Thanks Brigsy, I'll try adjusting the accel rod to see if it lessons the hesitation effect or not and report back. It will have to wait until tomorrow now.

[Woznaldo]

the brass erodes with running, all these carbs will have oversize jets by now.

Andy, the carb has a new (solder and redrilled) main jet, drilled 2nd Stage and new 1.1mm a/c jet. I've stuck a 1.0mm (well, closer to 0.95 with the digital calipers) drill bit in to the 1st Stage and it's a little loose but not too bad?

I know there are other orifices else where in the carb but, probably not in too bad a place with wear?

I have had the elbow off the top of the carb and it looks leak and dribble free. Might be worth another look though?

[Woznaldo]

I've uploaded a Solex manual with similar info to the stuff that's already there but, there are a few better diagrams:

https://www.rtoc.org/files/?path=Tech...20Information/

[Woznaldo]

I carried out some adjustments on the accel pump rod and it improved things a little but no real difference. I'm going to have a crack at the 1st stage, just out of curiosity really. More than happy for people to say "told you so" when the black clouds come pouring out the back!

[Tony Walker]

good luck, great thread. Desperately need to eek some mpg outta mine.

[Woznaldo]

OK, I've pulled the carb apart again and changed the a/c jet to 1.2mm (as close to std as I could get) and upped the 1st stage to 1.1mm. This did seem to improve things a bit and the AFRs dont get as lean as the last run.

There is still some hesitation but not as bad. I might go up again on the 1st stage to see what happens?

If it all seems to be going backwards, I'm just going to go back to the 120 main and be done with it.

I'll let you know how I get on.

So to confirm the carb specs are:

110 Main
1.2 2nd Stage
1.1 1st Stage
1.2 A/C

[Woznaldo]

After opening the 1st Stage to 1.2 it did improve things but, I think the 2nd Stage would need to be opened up a bit more too. I'm not prepared to try this as I don't have a decent spare carb to replace it if it all goes pear shaped!

To that end I've reverted back to a std spec with just the 2nd Stage being where I left it at 1.2mm. I've road tested it and it's a bit rich in places but, safe everywhere and no more hesitation!

I didn't have the resources to try to make a smaller main jet work but, I'm sure members within the club are more than capable if they wanted to.

Thanks for the advice so far, it's been very well received.

[Brigsy]

At least you got it sorted mate

I think on a totally stock engine a smaller main would work. It all depends how lean the engine will go before it hesitates at low rpm/off boost situations.

[Woznaldo]

At least you got it sorted mate

I think on a totally stock engine a smaller main would work. It all depends how lean the engine will go before it hesitates at low rpm/off boost situations.

You're probably right. I went straight for the 110 Main Jet when a 115 may have given me a better chance of making it work?

I want to convert to EFI soon but thought having a play with the carb would be a good learning experience before the change.

[Woznaldo]

Increasing the 1st stage will make the fuelling richer at low boost (under 3psi i think) before the 2nd stage kicks in, will probably chuck black smoke out on initial turbo spoolup.

I doubt it will cure your hesitation woz, try adjusting the acc pump arm a little. Many gtt carbs suffer from this problem, hard to cure sometimes

OK, I haven't made any changes but the carb is still running very rich. Would reverting to a smaller than std 1st stage help get rid of some of the richness down low?

[Woznaldo]

OK, I haven't made any changes but the carb is still running very rich. Would reverting to a smaller than std 1st stage help get rid of some of the richness down low?

Having re-read Scoff's notes in this article:

https://www.rtoc.org/boards/showthread.php?t=4734

It would seem my suggestion would indeed lean it out a bit so, i'll give it a try and report back.

[Brigsy]

Reducing 1st stage will only lean off cruising on low boost or spoolup mate

Suck it and see, it may work for you? on bigger turbos decreasing the 1st made a massive difference on spoolup for me.

Standard carbs are fairly rich at some points in the rev range so dont be suprised if you never achieve perfect fuelling.

[Woznaldo]

Reducing 1st stage will only lean off cruising on low boost or spoolup mate

Suck it and see, it may work for you? on bigger turbos decreasing the 1st made a massive difference on spoolup for me.

Standard carbs are fairly rich at some points in the rev range so dont be suprised if you never achieve perfect fuelling.

That's what i'm after Brigsy. It's the cruise and low boost that's rich. High boost is low 12s high 11s, which i'm happy with.

Forgot to say that I've soldered and redrilled a jet to 0.8mm. It's now in the carb but won't be able to run until tomorrow morning.

[Woznaldo]

I tried the 0.8mm 1st stage and there was indeed an improvement. The engine now runs a bit leaner in the light throttle low boost range. mid 12s to low 13s. I might go even smaller to a 0.6 then re-drill to 0.7 if iv'e gone too far?

[Woznaldo]

I'm not sure if this is the cause but I think it may have something to do with it? Could the very rich (9-10s) area during low load feathered throttle have something to do with the loose fitting accel pump tube? It's the only thing that I haven't been able to change and the problem has been consistent. The only change coming from changing the main jet (which adds more or less fuel everywhere)

Here's a cross section pic:



I think the only ways I change the fit of the accel tube are to either use a different carb body or use a low strength thread lock to hold it in place?

[Tony Walker]

mine has a weird symptom like this, part throttle ok but move the pedal even 1mm further and it goes off the scale rich??

[Woznaldo]

mine has a weird symptom like this, part throttle ok but move the pedal even 1mm further and it goes off the scale rich??

I think this is quite common on a few cars/carbs. I'm going to try to seal the Accel Pump brass tube with some low strength thread loc and see what happens.

I also added an extra digram and some text, for reference, about the Accel Pump.

[Woznaldo]

Looking at the picture in post #31, I'm now not sure if the accel pump brass tube is supposed to move up and down?

Cancel the above, you can see by looking at the underside of the gasket that the tube does NOT move. I've now applied the low strength thread loc and will let it set, then go for a test drive later tonight.

[Woznaldo]

OK, I sealed up the outside of the accel pump tube and carb body and have been running the car and I can report absolutely no change what so ever!

[Woznaldo]

I've dug up an old thread as my car has had this problem for a year now and the back of the car is getting covered with black spot deposits of unburnt fuel and I don't want to keep cleaning it off every week.

I'm going to change the 1st stage housing complete and go from there. I do have a slight problem. While trying to remove the 1st stage jet, the jet was seized in place and my screwdriver just chewed the jet head to bits. The actual jet orifice is not effected so it probably won't matter at this stage.

[fishead]

mr woz, i apologise in advance if im talkin rubbish ive not really properly read in full.


if your afrs are still as in opening post, rich light throttle cruise, you have a fault.
my 1st bet would be the throttle housing part of the carb
you need to sort this before any thing. light throttle cruise no load, lean as poss without missing. mine starts to get upset around 16.5 mark


a bar on a t2 std cam, maybe 1.2 1.3 on boost enrich, maybe close the a/c down a sniff if needed, that i recon would be it. if you really wanted to lean cruise off maybe close the idle jet, main to maybe 117 an open the enrich to maybe 1.1 1.2, this may work. as andy said carb top off look down vent an see what happens when open the throttle, if the squirt fron the pump jet isnt squirting in the right place, hesitation, the fuel needs to be picked up in the air.

the carb in the pic, is that the one you are using ?

[Jimmy_GTT]

I've dug up an old thread as my car has had this problem for a year now and the back of the car is getting covered with black spot deposits of unburnt fuel and I don't want to keep cleaning it off every week.

I'm going to change the 1st stage housing complete and go from there. I do have a slight problem. While trying to remove the 1st stage jet, the jet was seized in place and my screwdriver just chewed the jet head to bits. The actual jet orifice is not effected so it probably won't matter at this stage.

Have you tried with another carb? Or at least with another enricher housing?
Somehow the 1st stage membrane is not held open by the vacuum. It could be that it is full of deposit. If you take the cover of the 1st stage membrane there should be a small hole in a copper pin. Check if that hole is good. If you can suck air on that hole.
And also take out the membrane and check if the ball valve is closing well.

If you do not have any spare part I can send you some. Just let me know.

[LYNCHSTAR]

Woz,

It sounds like you need to set up the carb accellerator pump using Mart's patented 5mm drill bit set up method. It works a treat, the previous rich running will have masked some of the symptoms you now have.

Al

can you explain the 5mm drill bit to me please im having trouble setting a carb up aswell.

[Woznaldo]

Jimmy, if the enricher housing is the part that the 1st stage jet screws into, then yes I have one. If not, I'm screwed.

The spare carb I have was seized and corroded but some bits cleaned up ok.

[Jimmy_GTT]

Jimmy, if the enricher housing is the part that the 1st stage jet screws into, then yes I have one. If not, I'm screwed.

The spare carb I have was seized and corroded but some buts cleaned up ok.

Yes mate, that's what I'm talking about.

[Woznaldo]

Just changed the Enricher Housing for one from my other carb and there is no change at all?

[Mart]

Woz, a 110 main jet doesn't work on a standard 32dis. Period. It can be made to work, but you have to reduce the main venturi diameter for starters - It's a restriction as it is though; you don't want to be going smaller!

Try a 115 main jet, and see how you get on. There's always a 117.5 oem main jet available if you need a touch more fuel.

Likewise for fitting an oem 125 a/c. You'll be surprised how much difference .05mm makes.

As for the jet at the base of the float chamber, that's the regulation jet for the acc' pump circuit. Wouldn't worry too much about that one.

Acc' pump jet has to be sealed (body) otherwise it'll leak excessive fuel through everytime you touch the throttle. Make sure the little spring is on top of the jet, and that said spring sits in its little hole/home when you put the carb top back on.

As mentioned above, with the engine running (@ hot idle; ie, <1000 rpm) & carb elbow off, there shouldn't be any dribbling of fuel whatsoever into the venturi. If there is, something's not right.

[Woznaldo]

Thanks Mart. I've given up on the economy tuning for now. I just want to getting running right during light throttle.

I'll pull the elbow off later and check for absence of dribble again.

I've got big EFI plans but still getting all the kit together and don't want to be defeated by the carb before the change.

[Dave Reed]

"I've got big EFI plans but still getting all the kit together and don't want to be defeated by the carb before the change."

That's what I always said... Then I changed to EFI! and soon realised how stupid I'd been not doing it earlier

[Woznaldo]

"I've got big EFI plans but still getting all the kit together and don't want to be defeated by the carb before the change."

That's what I always said... Then I changed to EFI! and soon realised how stupid I'd been not doing it earlier

I know if I had the time/cash I'd have changed over to EFI about 4 years ago but I didn't/don't and the carb is teasing me!

[Woznaldo]

An interesting side effect of fitting the new Enricher Housing, other than not fixing the fault, was that the car now picks up from initial application of throttle a lot better. The obvious reason for this is, I was running a smaller the std 1st Stage jet (0.8mm) and changing the Enricher housing put it back to std (1.0mm).

[Mart]

In an otherwise standard spec carb', a 0.8mm first stage will cause lean running.

No need touch that jet, usually.

If your light/progressive throttle is still too rich, try lengthening the throw of the acc' pump lever.

[Woznaldo]

In an otherwise standard spec carb', a 0.8mm first stage will cause lean running.

No need touch that jet, usually.

If your light/progressive throttle is still too rich, try lengthening the throw of the acc' pump lever.

I'll give it whirl tomorrow, but to confirm, do you mean so the accel pump comes in a little later or lengthen the rod so it becomes fully compressed earlier?

[TNT ANDY]

I love this - there are only a few who still really know the art - good on ya Mart