Lean out at WOT. Fine at 3/4 throttle

Propane, Butane, LPG, GPL, C3H8, C4H10
BigBlockMopar
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Lean out at WOT. Fine at 3/4 throttle

Post by BigBlockMopar »

Started noticing for a while now my engine leans out at somewhere above 3/4 throttle. Pretty much irregardless of engine rpm or acceleration force.
The car keeps accelerating during the lean-out situation but the AFR-gauge is pegged lean. Letting off the throttle it feels as if the engine gets/produces more power for an instance.
Getting on the throttle right after that makes the mixture lean out again.

Things I've changed/checked;
- Vapourhose checked for delaminating on the inside. Checked out fine.
- Vaporizer lever-height checked and even set it a minor tad higher than spec. The coverplate is just barely against the lever now when installed. No effect.
- Lock-off and vaporizer-inlet fitting changed for a larger diameter (from 2.5mm to 6mm). No change.
- Installed a large IMPCO filter-unit/'storage pot'. No change.
- Enriched the power setting on the 425cfm. Had a minor positive effect on acceleration, but AFR still leaned out the moment the throttle is at WOT.

The car has 2 propane tanks in the trunk, both fuel-locks are 'open' during operation. They are connected together with a T-connector with anti-reversion checkvalves, into a 6mm outside diameter fuelline to the front of the car.
There it enters the IMPCO fuel filter-pot. After that is a fuel lockoff and an 8mm outside diameter coppertube to the vaporizer.

I've thought about changing the 6mm to an 8mm fuelline, but I doubt very much this is the issue at hand here.

Impco 425 carb, 300E vaporizer.
Don't really think I'm already at the limits of the carb or vaporizer yet.
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

storm
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Location: NSW, Australia

Re: Lean out at WOT. Fine at 3/4 throttle

Post by storm »

Sorry to say this but if you go by the HP & TQ outputs shown in this post in your other thread you probably are at the limits of both units. If you take a 1/3 driveline loss your engine is making close to 350 HP at 4500 RPM and that is way past the advertised capability of a 425 which is rated at approximately 300 HP. Try fitting 2 regulators, 1 off each tank outlet, and see if that helps. I'd also be going 8mm fuel line from the tank to regulators but we have had this discussion before.
Fuel flow requirements calculations
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456

BigBlockMopar
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Re: Lean out at WOT. Fine at 3/4 throttle

Post by BigBlockMopar »

The AFRs on the dyno-run started to go lean at around 3750rpm.
Currently I see AFRs going lean (way lean) a far below those rpms now.
Since this is just a 360ci engine I still think something is still blocking/limiting the fuel flow somewhere.

I might have to dig into the vaporizer and have a look at the foam-filter on the primary side of it.
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

C3H8
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Re: Lean out at WOT. Fine at 3/4 throttle

Post by C3H8 »

You don't have to take it all apart. The foam filter is unlikely to cause this. It's main function is to trap liquid LPG on start up which is then released as the vapourizer warms up. To test the vapourizer to see if there is an issue with it or somewhere before it you need to put a pressure gauge on it. There is a primary pressure port beside the vapour outlet. It is marked on the bottom of the vapourizer. If you were looking directly at the model E vapour outlet with the unit in a horizontal position you will see a 1/8" pipe plug, 7/16" wrench head at about the 4:00 position. Put a pressure gauge that can read 0 to 10 PSI on there. In theory the primary pressure should read 1.5 PSI. In reality it will probably only read 3/4 to 1 PSI. That's fine as long as the pressure is steady and not bouncing all over. Once you begin your power test watch the primary pressure. If it remains steady the problem is after the vapourizer. This would indicate an issue downstream pf the vap. If the pressure drops all of a sudden then the problem is in the vapourizer (likely the primary seat) or in the fuel supply leading up to it.

C3H8
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Re: Lean out at WOT. Fine at 3/4 throttle

Post by C3H8 »

Further to my previous response.
The fact your mixtures are leaning even sooner now points to one of several problems.
1. The primary seat has separated from the backing and is plugging the vap inlet.
2. Problem with the lockoff or filter (you say you checked those already.
3. Collapsed inner liner in the fuel delivery hose.
4. Faulty excess flow valve in the tank service valve.
5. Cracked delivery pipe inside the tank causing the system to draw vapour instead of liquid.

Fastest way to see if problem is 2 through 5. Attach a forklift tank straight to the vap with a different hose and run test again.

BigBlockMopar
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Re: Lean out at WOT. Fine at 3/4 throttle

Post by BigBlockMopar »

Thanks for the tips.
I recall I already have replaced the foamfilter earlier when I had the vaporiser open at one time. The old one was caked and brittle and broke when touched.
I'll see if I can install a pressure gauge and route it near the windscreen to view it.
Maybe if I can find a suitable pressure sender I could feed the readings into the MegaSquirt ECU and have it log along with engine RPM and manifold air pressure/vacuum.

Having 2 fuel tanks in the trunk, I could disable one or the other, to rule out any tank-related issues.

I also have a OHG X-450 to play with, but don't think I have the right spring to put into the vaporiser.
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

C3H8
Posts: 1062
Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada

Re: Lean out at WOT. Fine at 3/4 throttle

Post by C3H8 »

Regarding
Having 2 fuel tanks in the trunk, I could disable one or the other, to rule out any tank-related issues.

Unless you have sperate hoses from each tank going all the way to the engine compartment switching tanks will not reveal if there is a collapsed inner liner in the hose. That's why I recommend a temporary tank and replacement hose for testing. This also eliminates the filter and fuel locks by attaching directly to the model E. The pressure sensor is a good idea if available.

BigBlockMopar
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Re: Lean out at WOT. Fine at 3/4 throttle

Post by BigBlockMopar »

The high pressure lines are all copper-tubing.
Both tanks are connected together in the trunk.
I've fueled up the car yesterday and I'm noticing (as always), the car has some more power. But still everytime I depress the throttle-pedal fully, the AFR goes very lean after about a second orso. Everything stays fine with 3/4 throttle.

Didn't have time today to get anything done, so hope to be able to do some wrenching tomorrow.
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

PropaneIH
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Joined: Wed Nov 13, 2019 12:40 pm

Re: Lean out at WOT. Fine at 3/4 throttle

Post by PropaneIH »

How lean is very lean? Are you running gas scale AFR, propane scale AFR or Lamda?

Tom68
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Location: Australia

Re: Lean out at WOT. Fine at 3/4 throttle

Post by Tom68 »

BigBlockMopar wrote:
Sat Mar 28, 2020 2:15 pm
The high pressure lines are all copper-tubing.
Both tanks are connected together in the trunk.
I've fueled up the car yesterday and I'm noticing (as always), the car has some more power. But still everytime I depress the throttle-pedal fully, the AFR goes very lean after about a second orso. Everything stays fine with 3/4 throttle.

Didn't have time today to get anything done, so hope to be able to do some wrenching tomorrow.
Sure sounds like a restriction before the vaporiser.

I know the answer to this will be yes, but the tanks are picking up liquid and not vapor aren't they.

BigBlockMopar
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Re: Lean out at WOT. Fine at 3/4 throttle

Post by BigBlockMopar »

PropaneIH wrote:
Sat Mar 28, 2020 5:26 pm
How lean is very lean? Are you running gas scale AFR, propane scale AFR or Lamda?
The gauge goes off the scale to lean. At least for sure leaner than 18-19s.
If I 'feather' the throttle across 3/4 to full, I can see AFR's going from 11-12s to quickly fluctuating leaner and go full lean right after.
Backing off the throttle slightly brings the AFR back to around 12s again.

The AFR-gauge is still on the gasoline-range I recall, but the one-point of AFR-difference is overshadowed by the large drop in this case.
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

PropaneIH
Posts: 10
Joined: Wed Nov 13, 2019 12:40 pm

Re: Lean out at WOT. Fine at 3/4 throttle

Post by PropaneIH »

Are you running electric lockoff or vacuum lockoff? Vacuum lockoff flow more than electric ones.

PropaneIH
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Joined: Wed Nov 13, 2019 12:40 pm

Re: Lean out at WOT. Fine at 3/4 throttle

Post by PropaneIH »

Ive reread your post several times. This happens regardless of RPM? I wonder if the fuel valve and diaphragm in the 425 mixer is lifting all the way or there is a burr. Horizontal installations cause accelerated wear on 1 side of the fuel valve.

BigBlockMopar
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Re: Lean out at WOT. Fine at 3/4 throttle

Post by BigBlockMopar »

Electric. Vacuum lockoffs aren't at all common over here.
The internal diameter of the current lockoff is 5-6mm.
The old one I removed from the car had barely a 2.5mm orifice.

EDIT:
Yes, regardless of RPM. The 425 carb is mounted vertically.
I've manually checked the movement of the large valve but didn't notice any rough spots.
Will open it up tomorrow and have a closer look.
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

PropaneIH
Posts: 10
Joined: Wed Nov 13, 2019 12:40 pm

Re: Lean out at WOT. Fine at 3/4 throttle

Post by PropaneIH »

Any update on your issue?

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