Author Topic: 305 Runs Rough on Propane  (Read 2091 times)

Offline CJGuy

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305 Runs Rough on Propane
« on: August 22, 2017, 11:25:49 AM »
Hey all,
Not sure if this is the correct place for this, but I just grabbed me an '80 screw cab



My second one in as many years. It's a Cummins swap candidate in a few years, but for now I need it running and moveable as I work on my main truck ('93 Dodge W350). I haven't personally started the truck, but the PO said the truck will run if you "choke it out" by depressing the button on what I believe is the regulator



Does anyone here know what could be the issue here?  I'm a diesel mechanic, I've zero experience with propane. The truck is not set up for dual fuel, strictly on propane. I also noticed that headers dump right by the transmission, there's no mufflers or even an exhaust system past the headers. Could this be a contributing factor?

Any and all insight is welcomed


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Offline impco lpg

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Re: 305 Runs Rough on Propane
« Reply #1 on: August 22, 2017, 12:44:36 PM »
looks like a impco modle E vaporiser/reducer, the brass button in the middle is a choke. when pressed it will give a little bit more lpg when cranking ONLY. and the mixer on the carb doesn't look complete  300a/50 series and filter.
do you have a model number on the mixer ie; (CT425M 2)
headers and exhaust have know effect
is there a lpg filter before the vaporiser ?
is the vaporiser leaking around the seals ?
rebuild kits can be purchased 


hope this helps neil
« Last Edit: August 22, 2017, 12:57:09 PM by impco lpg »
1983 cucv m1009  blazer 6.2 4x4
1991 1500 suburban 5.7 4x4 lpg
2003 dodge ram 5.7 4x4 lpg
1985 chevy c10 stepside  2wd 5.7ltr 350 lpg

Offline Jason S

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Re: 305 Runs Rough on Propane
« Reply #2 on: August 22, 2017, 06:07:55 PM »
Check the items that impco lpg suggested...

How long has the truck been sitting?  The regulator and the mixer each have a diaphragm that can go bad.  If it's sat for a while, it's likely they're both in need of a cleaning and a fresh kit.

You might check with some of the local propane supply companies to see if they stock kits.  Kits aren't real expensive and rebuild is pretty straight forward.

Agree on the Regulator/Vaporizer as an Impco Model E.

The mixer looks like an Impco 425. Used standalone, they'll bolt directly to a Holley throttle plate.

There are air filters offered for the 425 mixer, Wix filters has one in their catalog (at least a few years ago they did).  I put together a filter setup for my 425 using a cheap aftermarket air cleaner and K&N filter (see link below).

Here is a thread with some additional photos of an LPG setup with both the 300 and the 425 mixers. 
« Last Edit: August 22, 2017, 06:10:12 PM by Jason S »
1973 GMC K2500, Super Custom, Camper Special, 350, TH350, NP203, 4.10's
1974 Chevrolet K10, Custom Deluxe, 350, SM465, NP203, 3.73's

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