PMOs - tuning a twin plug 3.4L

[Glenfield]

Team,

Running into some unusual tuning conditions on a new-to-me carbureted motor that I just installed in my SC.

See specs below.

Twin plug 3.4L
10.5:1 compression
PMO 46s
- Venturi: 42
- idle: 55
- idle air corrector: ~140
- main: 175
- air corrector: 175
- Fuel pressure: 4-4.5 PSI (fuel levels in float centered on sight glass)

Motor ran exceedingly rich during idle cruise - ~9.5:1 AFR (albeit with fuel levels in floats out of spec). Improved to ~11.5:1AFR with the larger idle air correctors and fuel levels corrected but still doesn't check-out.

What makes me think something other than jetting is at play is that the above is with mixture screws apparently turned almost fully lean.

In order, my mind goes to:
1. Position of throttle plates vis-a-vis progression holes at idle (progression hole(s) too exposed at idle)

2. Idle air bleed holes are gummed up / blocked (idle and idle air corrector jets are new so they should be clean and clear). Carbs purportedly thoroughly cleaned ~4 months ago but this is an unknown variable.

3. Spark plugs aren't hot enough ... need to confirm tomorrow but believe they are race plugs as this started life as a race motor. Could it be they're not getting hot enough at low RPMs to burn all fuel? Mechanic has complained about a particularly noxious fuel smell and tailpipes are pretty sooted up.

I'd hoped to get motor in fine fettle and then slowly fall in love with tuning carburetors. That's not my lot and will now spend the whole day tomorrow learning how to take apart, clean, re-install/reset and then trouble shoot a "can't get lean" condition.

Keen for any insights or thoughts as I look to make progress tomorrow.

Cheers,
Tom

[snbush67]

I think you should try 60 idle jets.

[stownsen914]

If it’s rich down low, not sure I’d want larger idle jets.

Fuel pressure is a little higher than recommended. Richard always said 3-3.5 psi, some say even lower.

I doubt this is your issue, but no solid motor mounts? Richard hated those too.

[Glenfield]

Yeah, I’d want to stay at 55 idle jets. Would be odd to have to go to 50s.

If all else checks out, I thought to open idle air correctors to 145. Mixture did respond to opening this from 130 to 140. Know this sorted out Gordo’s rich idle.

I actually do have wevo solid motor mounts. Can that really upset the idle circuit?

[Glenfield]

I should also say that I have a stock CIS pump with a return line to tank. I know that’s not ideal. It is causing some oscillation in pressure.

I expect I will go ahead and replace that with a low pressure pump.

[snbush67]

Like you said, something else is at play. Your description of the car running rich at idle with the mixture screws all the way in sounds like you’re getting fuel from somewhere else. Check the throttle plates, to make sure they are all equal and they are all closing fully when off throttle.

At idle, the idle jet is what is fueling the engine. Check the throttle screws and the position of the plates at idle.

[r lane]

Has been a while since I have dealt with carbs, but recall thinks 175 mains seem large and the venturi as well. Bob

[snbush67]

PMO 46 mm carbs on a twin-plug 3.4L, 10.5:1, and aggressive street use at Chicago elevation.

Assumptions

• 1 5/8” headers (SSI or similar) with a free-flow sport muffler (M&K, Dansk Sport, Bursch, etc.)
• Twin-plug ignition, properly curved
• Pump premium (93 octane)
• Target AFRs: cruise 13.8–14.0, WOT 12.8–13.0

Jetting

Circuit Jet/ Size/ Notes
Venturis (Chokes)/ 42 mm /Best compromise for torque and top-end
Main Jets/ 160 / 155 will be slightly lean above 6500; 160 is safe
Air Correction Jets /180 /Keeps top-end AFR stable
Emulsion Tubes / F11 / Smooth transition from idle to mains
Idle Jets / 60 / Crisp throttle response, no lean surge
Idle Air Bleed /Stock PMO / Don’t change unless AFR at idle is unstable

Expectations

• Peak Power: ~325 HP @ 7200 rpm
• Peak Torque: ~272 lb-ft @ 5000 rpm
• AFR Trace: Flat 12.8–13.0 WOT, 13.9–14.0 cruise
• Driveability: Instant throttle pickup, no hesitation, strong midrange

• If you run 1 3/4” headers instead of 1 5/8”, you may need mains at 165 / air corrector 175 to keep high-RPM safe.
• In cool fall weather in Chicago, you might go 155 mains if cruise is too rich.

Theory

• You can run slightly leaner at low rpm cruise with twin-plug than a single-plug, but still need enough main fuel for high compression at WOT.
• Chicago elevation (~600 ft) means you don’t have to lean out like at altitude.
• Aggressive street driving benefits from slightly richer idle and pump jetting to avoid hesitation in transient throttle.
• 42mm chokes keep airflow high enough to feed the 3.4 at upper RPM but not so big that you lose midrange torque.

Tuning

1. Idle mix — Warm engine, adjust mixture screws for peak idle vacuum, then back out ~¼ turn.
2. Cruise AFR — Aim for 13.5–14.0 at light load (can go leaner with twin-plug if no surge).
3. WOT AFR — Target ~12.8–13.0 at peak torque, ~12.5 near redline.
4. Air corrector — Fine-tune top-end AFR; larger = leaner high-RPM.
5. Main jets — Fine-tune midrange & upper RPM fueling.

[Tom_in_NH]

All good points. However, under your Theory heading, you said that twin plug allows more ignition advance. In fact, it is exactly the opposite. Because it ignites from two opposing areas of the chamber, you run less advanced than is typical with single plug. If you were to run more advanced with twin plug, peak pressures would have occurred way too early, and would get very expensive very quickly.

[snbush67]

Originally Posted by Tom_in_NH

All good points. However, under your Theory heading, you said that twin plug allows more ignition advance. In fact, it is exactly the opposite. Because it ignites from two opposing areas of the chamber, you run less advanced than is typical with single plug. If you were to run more advanced with twin plug, peak pressures would have occurred way too early, and would get very expensive very quickly.

Maybe my comas are in the wrong spots, I was thinking more advance at cruise, low load, detonation is less likely. Does that sound right or do I have things backwards?

[stownsen914]

Re: fuel pressure, it would be good to have that stable in the 3-3.5 pain range. It’s one of the first things Richard used to ask when troubleshooting with him.

Re: solid engine mounts, that was the second or third thing he’d ask. I believe he said they caused unpredictable behavior due to transmitting vibration to the carbs. Maybe it’s the needle valves that are impacted?

[Glenfield]

Pelican email

Thanks all. Great feedback and help.

Another clarifying detail - 1.75 headers and a GT3 exhaust (only bc it was gifted to me).

Spent day with motor yesterday.

1. Started with measuring idle jet size to make sure it was a true 55 … I’m 90% sure
2. Inspected mixture needles to rule out any damage there … all god
3. Moved to throttle plate location … backing out idle control screws brings motor to a bare idle and pronounced stumble. Not perfect, but suggests throttle plates are properly seating at 80:20 confidence
4. Checked airflow … achieved lean best at ~5kg / hr and 950RPM with mixture screws 1.5 turns out from full seated (and only fractional cracking of throttle via idle screw)

I am giving most weight to #4 above. Airflow a little low for a bigger motor but within range with throttle plates seemingly properly positioned. 1.5 turns out isn’t abnormal.

On mixture screw … motor stumbles about a half turn in from seated. So really, should only back it out a half turn to 1 full turn out. It runs measurably smoothly with 1.5 turns and so that’s why I landed there.

I’m now reading ~11.5 at idle and then progressing a little lean during transition (~14-15).

I’m still exclusively focused on idle circuit and so won’t worry too much about lean transition yet (albeit I expect to increase mains to 180, leaving air correctors at 175).

The big Q I’m grabbling with … do I go down to 50 idle jets? Text book says no, but motor is definitely calling for less fuel in idle circuit.

Other option is reaming out idle air correctors from 138 to 145.

[Tom_in_NH]

Originally Posted by snbush67

Maybe my comas are in the wrong spots, I was thinking more advance at cruise, low load, detonation is less likely. Does that sound right or do I have things backwards?

No, even at low load, cruise, you'll be running significantly less advanced with twin plug.

[snbush67]

Originally Posted by Tom_in_NH

No, even at low load, cruise, you'll be running significantly less advanced with twin plug.

Tom, I’m sorry I edited my initial post and now I don’t even remember how I wrote it. But, thank you for the correction, Im on board now.

[Glenfield]

I’m about 26 / 27 degrees all in on timing. So less advance than single plug. Surprisingly, distributor is curved to have about 15 degrees advance at idle (JB racing twin plug). I have twin CDI+ boxes that enable some customization of curve. Will play with that last once I’ve dialed the tune.

Mechanic installing motor wants to swap NGK8 plugs for NGK6s. He thinks 8s are too cold for a street motor and then I’m not getting full burn. I’d be more inclined to go down to 7 and then try the 8s once I have my idle cruise leaned out.

Was very eager to keep learning and tuning but won’t have a chance for a few days … as an aside, I have quite enjoyed how logical, systematic and easy it is to materially alter the tune on carburetors (for better or worse mind you). It’s very helpful that it’s all mechanical and right there staring at you.

[Glenfield]

Picked this back up today and could do with some help. Quick upstate first on what’s changed and where it stands.

Mechanic who installed motor swapped in 40 chokes (down from 42) to get a stronger signal (will come back to this as i wouldn’t have gone there for the conditions car is experiencing).

Other changes I made today:
- 50 idle jets (text book says no, car says maybe)
- 130 idle air correctors
- 160 main air correctors (text book says no, car says maybe)
- 185 main jets

AFRs at Idle / light cruise / higher rpm cruise = 11-12 / 12-13 / 13-14 … I need to work on transition and higher rpm which are 14-15. I’ll come back to these but good enough to move onto the larger problem at hand.

The problem … Any sharper throttle induces a very sharp stumble and backfiring. The spike in AFRs to 17-20 confirm a dangerous lean condition. That has to be accelerator pump …

I richened it out fully and using a vile, measured volume at full throttle actuation. I can only produce about 0.5 of a cc. I presume I’m too light? I’ve seen 0.5-0.6 and also 0.8 given as spec.

I have the hatchet cam and am all the way rich in the screw. Is there anyway I can get more volume?

[snbush67]

The 40’s will provide a stronger signal and will tune easier. Use the 50 idle jets and tune for economy then try the 42’s.

Idle circuit first.

You went down to 50 idle jets which would have leaned out the idle circuit but then you also went down on the idle air corrector, so now your back to about the same mix from the idle circuit just through a smaller hole. With the stronger signal from the 40 Venturi’s you’re about the same place (AFR)you were when you started.

Keep everything as is, except change to the 140 air correctors to pull more air into the idle mix, just switch those and then tune your idle to best lean try to get it at least 12.8 but don’t go higher than 14 for now. You don’t want to lean out the idle too much, the reason your getting the lean stumble at progression is that the idle circuit has to have enough of the right mix to get through progression and onto the mains, maybe you have enough signal to pull enough through with the 40 Venturis but I think that 60 idle jets with 165 idle air correctors is the engine really needs to run 42 vents.

Now the main circuit.

What emulsion tubes are you using? You’re using 185 fuels and 160 airs and still going lean? Something isn’t right maybe a typo?

Accelerator pump jets are only going to squirt with rapid throttle. You want to test your tune while driving with light and slow throttle movement so you don’t activate the pump jets. You want to find the places that stumble, cough and bump so you can tune those out.

Shane

[Glenfield]

Thanks Shane.

Going back to 130 idle air correctors - thought here was to feather the idle circuit better into the main circuit. My understanding is air correctors have more impact towards end of their respective circuits. I’ll heed your advice and go back to 140 idle air correctors snd see if I can make them work.

Mains … still a work in progress. I was running 185 mains and 170 main air correctors (excuse typo) and yielding the below AFRs. It’s close and I intend to come back to this.

All of the above is being tested at idle and in light throttle 3rd gear accelerations to 4500.

The real acute issue is the accelerator pump … any throttle angle and it stumbles and backfires very badly and goes dangerously lean. I’m thinking along two lines:

#1. volume of shot only 0.5cc with adjustment all the way rich. Tested on 1 and 4 (need to test other cylinders)

#2. timing of shot … perhaps it’s not actuating with throttle plate in right sequence

[Glenfield]

The time series at bottom is 120s under a mix of idle and gradual acceleration. 12-14 is green. The red spike is the throttle condition.

This was with 50 idle / 130 idle air / 185 mains / 170 main air correctors.

I have an early SCRS airbox to put on which I expect will richer things up so focused now exclusively on that red spike and accelerator pump.

[snbush67]

Originally Posted by Glenfield

I have the hatchet cam and am all the way rich in the screw. Is there anyway I can get more volume?

To get full volume the nut on the adjustment rod should be almost all the way out, about two or three threads showing, so that the pump lever can rest in the neck of the hatchet at rest. That allows the to get the full ramp of the hatchet for a full squirt.