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wirihana 02-01-2006 02:42 AM

2.4L dyno and carb jetting question
Hi all, a question from a frequent lurker, occasional poster

The engine in question is a 2.4 with 2.2S pistons and E cams with apx 4k miles on it. It runs strong, pulls hard on the throttle with no flat spots but has been 'weak' at cruise. More throttle pushes you through this easily, but you can't accelerate all the time! An in-car narrow band O2 sensor showed it was running rich, so I went and organised a dyno to establish a baseline and get more accurate A/F readings.

results as follows:

Do the HP and Torque graphs indicate a healthy engine? - I think they are about right, and the A/F curve seems to be o.k. (just on the richer side of stoich.)

However, when we ran the car at very light load/cruise from apx 2500-4000 RPM, it read very rich - staying between 9-10 A/F ratio. We could look at the results on the computer screen as it ran, but could not record it to post here.

How can i tune these carbs to run leaner at cruise while retaining the WOT (or near to it) power?

The carbs do not spit, pop, and it idles very cleanly. It has a yellowish tinge around the exhaust on the bumber. It has PMO's with the following jets

idles 57
mains 120
airs 200
emulsion tubes f11

What do you guys think? Before the dyno I was thinking of larger air corrections (up to 210's) this the right track?

thanks in advance


jluetjen 02-01-2006 05:28 AM

Hi Nathan;

One piece of data is missing -- what sized venturi? I assume that the carbs are PMO-40's -- right?

How about the obvious -- did you try leaner idle jets while you were on the dyno? I've generally heard (and followed) the concept of being lean at idle.

wirihana 02-01-2006 01:11 PM

hi john
they are PMO 40's - venturi's are 32mm
how lean at idle should i be? it was around 14:1 at idle when we tested.
It was my understanding the idle circuits are no longer employed over apx 3000RPM.
Why am very rich at cruise at 4000+ rpm? I thought at these higher RPM it was running on the main circuit - by-passing the idles.

jluetjen 02-01-2006 01:33 PM

I've been collecting jetting information as it's been posted on this board and to be honest F11 emulsion tubes have never turned up until you posted your configuration. For a discussion about emulsion tubes and the affects -- have you seen this thread from a while ago? The data on that thread suggests that F11 are for "High RPM weakening with air corrector jets larger then 2.0 mm (which you have btw)". If your carb is set up to be OK at higher RPM's, then you might find that you're out of whack at lower RPM's. Note how at 2500 RPM you're mixture is rich at 2500 and lean at 3500 RPM. Also, have you confirmed that the accelerator pumps are set correctly? BTW, F3 or F26 seem to be more common in "happy" engines with a similar configuration.

Mr Beau 02-01-2006 02:01 PM

Hi Nathan,

Your power run actually look a little lean. Typical wisdom is to run 12.5-13.2 for best power. A Dynapak will let you hold basically any RPM you want so you want to use this to your advantage. Pick an RPM (say 3000), go wide open on the throttle, and then start the sweep up to max RPM (~7000 RPM). The goal of this is to minimize the influence of the accelerator shot and have a long enough sample time as the engine accelerates so you have a good idea of the jetting.

Idle may be best around 14:1, and part throttle would be find anywhere from 14 to say 15.5:1.

I'm not a carb expert, but my understanding is that the WOT mixture is mainly the main jet and emulsion tubes, and to a lesser extent the air corrector jets. Part throttle is largely the idle jetting.

And if you change the choke size, your jetting will also be affected as the signal strength changes.

wirihana 02-01-2006 02:05 PM

F11 are what PMO recommends for e-cams, I also have some F3's which we tried with the intial set up, but got bad hesitation at about 4k rpm.

accelarator pumps have been checked, along with float levels, and carbs synched and balanced

thanks for the links. the big spikes on the dyno around 2000-2500 were from him ramping up the load a bit quick i think - we did an earlier run we did not record at it was much flatter.

At high RPM cruise (above 35000rpm) which would be at less than 1/4 throttle - does the idle circuit control the mixture? or is it the main and therefore emulsion tubes?

Mr Beau 02-01-2006 02:26 PM

I believe the idle circuit is more throttle dependant than RPM so it could still be the idle circuit if the load is low. Playing with the main jets would likely have little effect, but I can't really say about the emulsion tubes. It's been a while since I played with Webers...

davidppp 02-01-2006 03:12 PM

Hello there.

IMHO< the mains are small for 32's on the 2.4.

I agree that you are running mainly on the idles when bigger on the mains and smaller ideles would be the direction I'd try..

But Richard at PMO can likely give you something off his head which will work well..

I'd call/fax him..

Kind regards

TonyG 02-01-2006 11:44 PM

I would try two notches bigger on the main jet, one notch smaller on the air correctors.

How does the throttle transition from the idle circuit to the main circuit feel?

I wonder how a 30mm venturi would work on that engine. Seems like you're in between a 30mm and 32mm. It might be worth a try (especially on a primarily street driven car).

Can you get a 30mm for a 40mm PMO?

(Not to hijack.. but I need a 42. Anybody know if you can bore out a 41mm properly?...)


wirihana 02-02-2006 01:38 PM

Many thanks for all you replies and information

the transition from idles to mains is fine.
At cruise however it actually feels like it is almost bogging - sort of struggling. You can push through it easily enough - but running at apx 9-10 a/f ratio at cruise cannot be good for the engine.

I will start by going down on the idles to try and lean out the idle circuit, maybe also take a look at the idle air screws as well.

next would be to try and richen up the mains for more power - either by larger mains or smaller air corrections or a combo of both. no doubt this will have some effect on the cruise again....I will focus on the idle jet aspect first.

i am very close to PMO spec jetting as reccomended by Mr Parr - so perhaps another fax to him is in order.



Mr Beau 02-02-2006 02:24 PM

Hi Nathan,

I don't think the mains/airs will have much effect on the cruise so I wouldn't worry much. My experience with air correctors is that they have a pretty small effect compared to mains, and their effect increases with RPM.

In any case, it sounds like you have a handle on the situation and you're moving in the right direction.

kucharskimb 02-03-2006 10:21 AM

okay... here's one more opinion. i'm also battling my webers at this time, and am going back tomorrow for another group dyno armed with some more main jets.

my setup is a 2.2E stroked to a 2.4. in essence, your motor with about 9.6:1 compression (yours is probably 10:1).

for the carbs, i have:

idle: 60
main: trying 135 as i was lean at 130, have 140s on hand
34 venturies

from your graph, i'm guessing that your A/F looks like it does because you were at cruise and then jumped on the gas. it momentarily went lean until you got the fuel shot from your accelerator pump, which then sent you super rich. not what you were talking about with the narrow band sensor, but a good lesson to remember to be smooth on the gas.

when setting up the mixture at idle, it's all on the mixture screws. you can do all sorts of compensating for the wrong size idle jets with the mixture screws. after closing them, turn them out 1/2 turn at a time until you get max rpm, then another 1/4-1/2 turn out.

the mixture screws will continue to influence your A/F until about 1500rpm, then you are all into your idle jets.

if you want to see if the idle jets are too big, you can test them by opening all of your air correctors at 1/2 turn increments. this is the same as leaning out the idle jets. PMO suggests this. Bieker Engineering does not.

around 3500, you are transitioning to the mains at partial throttle.

at WOT, you are ONLY using your MAIN jets. so, yes, you have the ability to leave WOT alone, and only correct your cruise (part throttle) A/F ratio.

couple troubleshooting things that i have had to deal with:
1. are your idle AIRWAYS clean and unblocked?
2. do the gaskets that go between the carb body and the float bowl covers have the holes pre-punched to allow air into your idle circuits?
3. are any of your float needle valves stuck open.
4. are any of your accelerator squirters stuck open (there are several small check balls in that system)

IMO, your setup is too lean, but the dyno shows otherwise. also, not sure why you are not making more power. DIN v. SAE is less than 1% difference.

i'll post my results from the dyno tomorrow with the final jetting setup. maybe it will give you something to go on.

wirihana 02-06-2006 10:47 PM

How did the dyno tuning go Matt?

please post your results on jetting and A/F graph if you get a chance. If you can share what your A/F ratios were (or targeted) at idle & cruise etc that would be grand.

Our engines are very similar, mine has a calculated comp. ratio of 9.5:1. Your webbers will be jetted up larger with your 34 venturies compared to my 32's so the comparisons will be interesting.

Unfortunately on my dyno runs the ratio is wrong at 5.00, i think it should be 3.50 - so thats why my HP seems meagre i think and comparisons won't mean much.

The fourth gear in the 901/03 gear box is not quite 1:1 and not sure if the dyno operator got the final drive ratio right either - does anyone know what this "ratio" box should be on a dynojet?

thanks for the tips - the original air filter bases were modified to fit the PMO's by drilling the appropriate holes for the idle circuits among other things - much quieter than the K&N's on the long trips.

I swapped out the 57 idle jets for 55's on friday and have noticed an improvment at cruise, although the narrow band meter has not changed its reading. Idle & transition are unaffected, but it seems to run slightly hotter.

I have not touched any of the mixture screws or idle airs yet. Next step is to try 52 idle jets after some more driving, and i will need to look at them at that stage I suspect



Mr Beau 02-07-2006 05:30 AM

The ratio you enter will likely change the torque but not the HP figures. Not sure what you need to enter, but it shouldn't really change things that much as you can cahnge the RPM rise rate with the dyno (thus negating any intertia effects).

Ignore the narrow band meter to tell you how rich or lean you are. It's only good to let you know if which side of 14.7 you are. Cruise leaner than 14.7 is OK (it won't hurt anything) but it may affect the drivability if you go too lean.

Have fun tinkering!

RoninLB 02-07-2006 05:50 AM

Just a guess.

Mains seem to be a tiny bit lean according to the dyno. A dyno is not real world conditions.

With 55idles how far out are the idle mix screws? If you have them at 2.5 turns then at 2.25 turns it'll run like crap. Adjust all the same. Adjust all the same. Adjust all the same.

Adjust accelerator volume leaner till it bogs then richen slightly till it doesn't. Too rich cools combustion then gives a false sense of acceleration as it lags and eventually zooms.

Air correctors are nice to adjust the mains when in 5th & 4k foreward. At 4k there shouldn't be any idle jet interference. If it feels like it's leaning at 4.5-5k then you reduce air corrector size.

Where the mains comes in varies. Mine come on at around 3.2k and the idles stay with the rpms till around 3.8-4k.

I like to road test in 5th. It should be fixed pedal ok at 3.2k and 4k. When that's nice 4th gear pulls should be perfect.

32 venturis should be fine.

What distributor is installed? What is your initial advance? What is full advance?

RoninLB 02-07-2006 05:56 AM


Originally posted by TonyG

I wonder how a 30mm venturi would work on that engine. Seems like you're in between a 30mm and 32mm. It might be worth a try (especially on a primarily street driven car).

nice thinking but 32 should be doable for nice street and over 5k rpms.

kucharskimb 02-07-2006 09:06 AM

attached is the run from this weekend.

the hand written numbers in the legend are the main jet sizes. all other info is the same as above.

for some reason, the 140's were actually leaner at 3500. i'm going to pull them and see if they are drilled correctly.

i could probably try to drop the AF down further, but i figure the air will be warming up soon enough...

i'd like to email you directly for some questions. thanks

kucharskimb 02-07-2006 09:08 AM

yeah, yeah... premature...

jluetjen 02-07-2006 10:17 AM

Could you mark up the lines on the chart? I don't know about anyone else, but I can't make out the colors. I don't see where any of the A/F lines are crossing either.

Mr Beau 02-07-2006 12:33 PM

Agreed, tough to make out the data on the graph.

One influence could be the accelerator pump. Ideally you could go WOT and use the brakes at the same time to 'steady' the engine at low speed, and then release the brakes to allow the RPM to climb. Though maybe this isn't too practical on a Dynojet.

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