SBC 4mpg

[4Ruster]

Hello all,
I am still very new to propane, but I am trying to learn as much as I can.

I have a 1981 GMC C1500 with a 305/TH350C. I only bought it a few weeks ago. The engine has an RV cam, a holley street dominator single plane intake, and an impco 425 mixer on a holley baseplate. It is fed through an impco model L vaporizer and a vacuum lockoff.

The tank stops at 90L, and I can eat through that in 150km just driving around gently.

It would start perfectly in the mornings cold, but you can shut it off and wait 30 seconds and try to start it and it would just crank and crank for 15 seconds until you gave it full throttle, then it would catch on and start. I replaced the vaporizer and now it starts perfectly no matter what. I thought my mileage would increase but it did not. There is still a strong propane smell in the engine bay too. I have the idle mixture screw 6.5 turns out I set that for the highest vacuum with a gauge.

Power is great though, it just cruises and accelerates with the slightest crack of throttle.

[4Ruster]

Quick update I just replaced the mixer with a known good unit. This one whistles when cruising and the throttle response is not as good, I need a bit more throttle to accelerate. Maybe my mileage will be better, I will have to see. I will have to adjust the idle screw still, I just turned it out until the idle picked up, I'd say it's about 9 turns out.

[4Ruster]

It looks like the hose between my vaporizer and the mixer has a crack in it, I will replace that hose tomorrow. Could that cause such a poor mileage number? I was expecting around 12mpg.

[Tom68]

It looks like the hose between my vaporizer and the mixer has a crack in it, I will replace that hose tomorrow. Could that cause such a poor mileage number? I was expecting around 12mpg.

Cracked hose would make it lean.

[4Ruster]

Seems like my mileage hasn't increased much. I filled up 90L on sunday and I'm at half a tank now

[C3H8]

You need to supply some additional information. Have you done any checks on the basic components.
Compression: If you have never verified the condition of the engine do a compression test.
Spark plugs: New and gap at .035.
Cap and Rotor: Condition and replace if not sure how old.
Plug wires: Same as cap and roto.
Most important: Timing. You need to check the complete curve. Mileage problems are frequently related to either stuck weights or improper advance.

[harmkekornelis]

I would say it is running rich, full throttle on start gives more air to the mixture to start is, in your first thread you said start runs and shutt of it takes a while to start, therefore i would say to rich, of a vacuum leak so it would not suck. anyway if i am not wrong : 4 mpg is 1 Gallon is 5ltr an a mile is 1.6kilometer so: 5 ltr to drive 6.4 km, that sounds rich to me,

[C3H8]

We would have liked to help but your not vey good at supplying any of the info asked for in a previous post. Since we haven't heard from you I have to expect you went back to the basics and solved your problem.