Gordo's PMO Carb Tuning

[KTL]

Some comments related to your comments

-Yes the idle jets have a major influence on the AFR/mixture below 3000. Why they've always been called them idle jets is strange. Its a misnomer to call them that because they aren't just for idle! 2000-3000 is typically referred to as the transition range of rpm. A lot of people have trouble getting that transition dialed in and that results in some hesitation or flat spot while the carbs are going from the idle circuit to the main circuit.

-So when you changed your idle jets, yes w/out question you should have to readjust your mixture screws for each and every carb. I once had a problem with my former Weber 46 setup where one mix screw was out of adjustment (I think somebody purposely messed with it to see if I could diagnose it- real nice, eh?) and the engine ran like it had a rev limiter kicking in. Once I was shown that the mix screw was out of whack, the engine came to life. So those mix screws are super important in that if they're off, you can't get going because they set the tone for the main jets to carry forward in the higher rpms.

-I would go back to the 55 idles, go back to the 180 air correctors and go with the 170 main fuels. That should get you into a more comfortable AFR for hard acceleration.

-With the new hatchets, don't forget that you can adjust your hatchet actuator rod to work out any initial leanness you might experience on acceleration.

-Yes you shouldn't be looking at the mains (air corrector and/or main fuels) with regard to your cruise richness. To get that cruise mix adjusted you have to focus on the idles and idle airs. The idle circuit has tiny idle airs- one of PMOs numerous enhancements to the original Weber IDA-3C system

[moneymanager]

Kevin, I went from 140 to 145 mains recently. Seems a bit rich. Per your statement a few emails back, could I go back to the 140s then decrease the air corrector (mine are 190's) by 5 to effectively give me about a 141-2 main jet size? Any downside to that? Don't mean to derail this discussion which is of great interest to me at the moment as well.

[KTL]

Yep if you're too rich with a one increment increase on the main fuel jet, i'd definitely recommend trying an air corrector decrease with the original main fuels.

I don't see any downside to adjusting the air correctors. Decreasing some air bleed into mix is similar to increasing the main fuel jet. However the air correctors have a weaker influence on the mix and therefore you typically have to take a "bigger swing" at the adjustment to get the desired effect if you're quite a ways off. In this case where you're pretty close, I think the air correction by one increment down may get you where you want to be

The main fuels and air correctors can be juggled in whatever fashion works for you. They work together to create a mix of air and fuel. It's just that the main fuel jet has a greater influence on the mix than does the air corrector.

[Gordo2]

Gents,

I know you know this, but for clarity purposes of others who may read this thread in the future - a smaller numbered air corrector corrector size makes the mixture richer and vice versa.

As such, moneymanager wants to lean out his mixture, therefore he would change his air corrector jets from 190's to 195's or greater...

Thanks again for the feedback Kevin,

Gordo

[KTL]

Gordo,

Thanks for the clarification. Keep in mind moneymanager's issue is he wanted to get a richer mix and his choice of one increment larger on the main jets was too much. So an alternative choice to get richer is to go back to the previous main jet size and also install a smaller air corrector. This approach should get him richer but not as much as what he got with the small main jet increase

[Gordo2]

Well,

Today I attempted to tune the carbs a bit better with the smaller 50 idle jets installed. Not happening - no matter what I do, they still end up too lean at lower RPM's.

As such, I swapped the 55 idle jets back in and decided to mess around with the air bleed screws to see if they helped lean out the cruise. Took this approach based on the following comment in the PMO instructions:

"NOTE Some engines will run rich at idle and or cruise. In these cases, it’s worthwhile to set the Air Screws one or two turns open for initial setting."

I went to 2 turns out on the air screws and adjusted my idle mixture screws. Again, no joy; still hovering in the 11-12 AFR range:

Below graph: Cruise in 5th gear ~ 75 MPH on a relatively flat/straight road:



Meanwhile the combination of the larger main jets (from 160's to 170's), smaller air correction jets (190's to 180's) and the hatchets has resolved my acceleration lean issues:

Below graph: hard acceleration starting from idle, 1st to 2nd gear then off throttle:



I also upped the rev limiter for the run; a bit higher than intended ... Under hard acceleration, it's very strong now - I could feel the tires spin a bit when I stomped on 2nd gear taking the above recording.

It runs great, but I'm clearly not very efficient at cruise. Will leave this configuration as is for now, and see if the 1 5/8" headers I will be installing early Nov have any impact.

Thanks again folks,
Gordo

[JJ 911SC]

Originally Posted by Gordo2

... It runs great, but I'm clearly not very efficient at cruise. Will leave this configuration as is for now, and see if the 1 5/8" headers I will be installing early Nov have any impact...

Enjoyed the next few weeks without being "lean worry"

[KTL]

Keep in mind that you can refine it even further with a smaller jet change. That is, you could try a 165 main jet and a 185 air corrector. Reason I say that is because you're dipping below 12 under hard acceleration, which is probably a bit rich. 12-13 is typically the target, leaning more towards 12 than 13.

[Gordo2]

I think my wideband sensor is filthy or something odd is going on.

My car sat since I last adjusted the carbs - which had it running very nice, but still on the rich side for cruise at ~ 11 AFR, and good #'s for acceleration.

Meanwhile, last weekend I fired up the car and went for a short drive - my AFR at cruise was dropping into the 8's (vehicle was fully warmed up, LM-2 fired up with a reading of 20.9). I assumed it must have been a fluke and didn't touch a thing.

Tonight I went to fill the car up - again I'm seeing the cruise drop from an idle of ~ 12.5 AFR to the 8 range.

Meanwhile it seems to run good, but does seem to be slower on throttle response and churning over at idle a bit rougher than it seemed to in the past - but not sure, it could be me overthinking the reading.

Since I didn't touch anything on the carbs since my last adjustment a few weeks ago, I'm assuming my LM-2 must be reporting some whacked #'s.

I'm going to pull the wideband sensor tomorrow to clean it and give it a fresh air calibration (it calibrates installed to 20.9).

I'm also going to pull the plugs to see if they are getting fouled.

Any other ideas?

Thanks, Gordo

[MetalDoc]

Loose connection.... the element is not holding temperature.

[Gordo2]

Fresh Air Calibration

The odd AFR reading problem is resolved. I removed the wideband sensor from the header, cleaned it with some carb cleaner, blew it out with compressed air and ran a "fresh air calibration". The sensor was covered with black soot, but not too excessive. I think the fresh air calibration did the trick (I tried recalibrating while it was installed - showed 20.9 but I was still getting whacky readings).

Road Trip

I drove the car from Virginia to Ohio this weekend and had a blast. Back-roaded the entire drive out (winding from VA - MD - WV - into Southern OH up to NW Ohio); ~ 570 miles / 13 hours (with a few sidetracks along the way). Mostly highway on the return trip. Pretty good break in run. Used just under a quart of oil.

I averaged ~ 20mpg; not too bad considering I was running 70-80 mph for a good bit the drive, and stomping on it every time I launched

On my way out, the carbs popped and coughed a bit when rolling through the mountains - LM2 showed I was a bit lean under load (up steep inclines). Roadside adjustment - a little turn of the mixture screws cleared it up. Other than that, the carbs performed great. Although the LM2 shows I'm still rich at cruise, I checked my plugs (NGK BPR 7's) and they look good.

I hid the LM-2 display after I ran for a bit following my roadside adjustment - too distracting / too easy to obsess / too nice of roads and fall colors to ignore...

I'm going to try to get it to the dyno in the next couple weeks - will post an update if I make it in.

Once again, thanks for all the help folks - nearing the end of the season and I'm feeling pretty good where things stand.

Gordo

[JJ 911SC]

Gordo,

Great, I guess you got a few more weeks to enjoy before it get cold.

Yesterday morning we were at 32F when I went out to et the paper :roll eyes:

Enjoy the rest of the season.

JJ

[obscene]

Thanks for the updates. Good motivation to rebuild and tune my Webers over the winter!

[Gordo2]

Well,

I've been enjoying the PMO's on the rebuilt 3.2L for nearly a year now.

The combo runs pretty solid - but you just can't help thinking you can make it a little better if you can just find / hit the sweet spot with the right combination of jets and settings.

I never sorted out the rich cruise issue (LM2 Wideband showing ~ 11's) and slight hesitation to mains. This weekend I gave another go at removing the 55 idle jets and replacing with 50's.

It tuned in pretty good, but my transitions leaned out too much causing my transition hesitation to worsen (and lean to 15's - I'd rather be on the rich side of things).

So 55's idles are rich, 50's are lean...

Meanwhile, I found a place that sells idles in a 52 and 53's - considering the 53's.

Also considering my emulsion tubes:

- I currently have F16's, considering trying some F11's.

Any thoughts?

Thanks, Gordo

[moneymanager]

Try bigger-number idle airs. This will give slightly leaner 4k cruise which, at partial throttle, is on the idle circuit. I started with 130s, went to 140s, putting part throttle cruise in 12's which Richard at PMO thought not bad. I may try 150's one of these days.

[KTL]

Yeah i'd look at the idle airs instead of the pure idle jets. The PMO design incorporated the idle airs for just this reason- finer tuning.

I'd also consider the F11 emulsion tubes. PMO recommends them for street usage.

[Gordo2]

Originally Posted by moneymanager

Try bigger-number idle airs...

Thanks gents - on the idle air bleed jets...

Are those the stainless jets on the top plate of the PMO's and if so, I assume they are swappable with the standard Weber IDF idle air jets?

Also - I understood from reading a bunch of stuff from Pelican VFR 750's experimentation (Zenith TIN Idle Air Jet Modification), and Paul Abbott's "Performance Oriented" site and posts, that the idle air jets "tend to delay or advance delivery response" as opposed to influencing the overall idle/progression AFR.

I've decided to do some more driving and and logging before I swap anymore parts - running Paul Abbott's recommended driving profile (3rd gear, slow throttle advance ~2k RPM up, on an inclined road).

Will update the thread with this data log and go from there.

Thanks again,

Gordo

[Harry998R]

Hi Gordo,

As it's been 12 months since you completed the carb project surely it's time for another project, how about giving EFI a go?

Cheers,

Harry.

[KTL]

Yep the idle airs are the removable guys on the top of the carbs. I don't know if there's an actual equivalent removable Weber idle air jet available. These might be PMO-specific.

[Gordo2]

Originally Posted by Harry998R

Hi Gordo,

As it's been 12 months since you completed the carb project surely it's time for another project, how about giving EFI a go?
Cheers, Harry.

Maybe a future engine - for now I'm still having fun learning the art and science behind the carbs... I gave serious consideration to going with a full up TEC-GT / TBI system, but opted for the carbs out of a preference to tune with a screwdriver vice computer (yea, I'm cheating with the LM2...).

------------------------------------
KTL,

I spoke to Richard Parr quite a while back - he mentioned the stainless idle air corrector jets that he had made for the carbs and that he had 2 sizes made. Meanwhile since they are not stamped with sizes - he couldn't tell me which size mine are.



I never thought to ask if they are the same as the idle air correctors on some Weber's - will ask if I give him a call in the future.

Gordo