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FrankGRUN 07-22-2014 08:11 PM

Carb Jetting with Dual Fuel Setup
I'm finally about to finish restoring my 1982 E350-based Dual Fuel Motorhome. I had to replace the stock motor with a remanufactured 460 long block. The replacement was built with a Melling MTF-3 RV cam, and the cam timing has been set to 2 degrees advanced from the 8 degrees retard factory setting. I'm using an Edelbrock 1411 750 cfm carb with an Edelbrock Performer 2166 manifold. The smog hardware is disabled (EGR and air pump) and the exhaust system consists of Headman Elite Long Pipe ceramic coated headers feeding a 3" single exhaust through a very large high flow muffler. The right and left side exhaust pipes from the headers meet about halfway from the right side exhaust manifold to the inlet to the muffler. Duals were not possible because of interference with the motorhome holding tanks. The ignition system uses the Ford Duraspark II distributor, recurved for RV applications, and a Crane Hi-6s ignition multispark system. The plugs are Autolite XP-25. The transmission is a C6 automatic with RV torque converter and a Gear Vendors Overdrive. In overdrive I'm turning 2300 rpm at 55 mph. The motorhome is 24 ft. long, weighs 11,000 pounds and resembles a large barn door moving down the highway.

When I added the headers, the airflow in the stock engine increased to the point that the 1411 was running rich and I had to rejet to 1 stage leaner main jet and metering rod.

Now as I convert to dual fuel, I will be bolting an IMPCO 425 to a right angle adapter to the top of the 1411. I will use an IMPCO Model E converter and a Dual Curve Timing adapter as well as the IMPCO Commander Controller for the 425. Another right angle adapter will be used to connect to the high flow in-line air cleaner.

My question is this: With the right angle connection to the IMPCO 425 and the air flow restrictions intrinsic to the 425 mixer, will this impede the air flow seen by the Edelbrock 1411 to such an extent that the 1411 has to be rejected?

Any help would be greatly appreciated.


FrankGRUN 07-24-2014 10:33 AM

Error and typo! My question should be : given the airflow restrictions of the IMPCO 425 mixer when the propane system is fully retracted (that is operating on gasoline) will the Edelbrock 1411 have to be rejetted or recalibrated compared to its operating condition before I install the 425?

Sorry for the confusion.

gasman6674 07-25-2014 12:27 PM

It might. The 425 will only pass 425 cfm air if that's not enough you will need to adjust gas carb or re-jet. I would try it first then change as necessary. Where are you mounting the vaporizer?

FrankGRUN 07-25-2014 03:46 PM

I'm mounting the 425 towards the rear of the engine right along the engine transmission axis, then a right angle air cleaner adapter pointing to the passenger side. I made up a cold air intake using components from the later EFI E350 models and bring a 4 inch brake duct tube (compatible with high temperatures) from an inline Spectre air filter to the right angle adapter. The vaporizer is mounted in the front of the engine compartment towards the passenger side to give access to the card adjustments and distributor. It all just fits under the doghouse, but mixer adjustments require removing the doghouse.The vaporizer is plumbed into the heater circuit with an inline thermostat.

gasman6674 07-27-2014 08:01 PM

Sorry been camping, You should be good, You should only need 140 to 160 deg water at convertor any higher and you will have issues with heavy end build up.
Good Luck

FrankGRUN 07-29-2014 09:17 AM

I've now picked up an Innovate Wide Air Fuel ratio meter, so I will first calibrate the 1411 with the new engine, then redo the calibration with the mixer and air cleaner path in place. Thanks for the advice.

FrankGRUN 08-07-2014 11:16 PM

After some research, I've found that the IMPCO 425 mixer will cause a pressure drop of between 0.6 and 1.4 inches of water over the maximum rpm range I anticipate using. The right angle adapters are much less of a problem. Calculations at 500 cfm and atmospheric pressure for a 4" diameter round duct into a 5.25 inch plenum show less than a 0.2 inch of water impact. I have a low pressure manometer and I intend to measure the full pressure differential from my cold air intake/ Spectre in line air cleaner approach to the plenum above the IMPCO mixer when I finish this thing. In the final configuration, I'll be using the IMPCO Commander feedback control to adjust the operating air/fuel ration with the LPG operation.

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