weird engine problems

[truckinprogress]

so i have a 350 with impco propane system and its been acting up these days. it runs perfect except when i go up hills(on highway). if i start goin up a at say 2200 rpm, and i keep it there, there are no problems. but if i start at 2200 rpm and i advance the throttle, the engine just goes to ****. missfires and feels like its gonna blow up. all i have to do is take my foot off the gas and go again and it will be fine unless i accelerate too fast. would this be a vacum leak on my timing advance or something??

 

[73k5blazer]

You might perform a compression test. I had a chipped valve once, you'd never have known unless you severly loaded the thing. Unless it was floored or towing a heavy load (4 place enclosed snomo trailer), the thing ran fine, even got ok gas mileage. Just missed under heavy load.
Just a suggestion......

 

[78Suburban]

You might perform a compression test. I had a chipped valve once, you'd never have known unless you severly loaded the thing. Unless it was floored or towing a heavy load (4 place enclosed snomo trailer), the thing ran fine, even got ok gas mileage. Just missed under heavy load.
Just a suggestion......

How much of a different compessin did you have in that cylinder?

 

[73k5blazer]

7 cylinders were 160ish, the bad one was around 140 IIRC. Noticably off looking at the numbers. A leak down test furthur confirmed it was in the head (air coming back thru carb), pulled the head and found the chipped (very small chip) in the valve.

 

[78Suburban]

7 cylinders were 160ish, the bad one was around 140 IIRC. Noticably off looking at the numbers. A leak down test furthur confirmed it was in the head (air coming back thru carb), pulled the head and found the chipped (very small chip) in the valve.

Any tech articles on there about how to do a leakdown test? Can you just put an air compessor fitting in a plug hole and listen for air coming out of the carb?

 

[73k5blazer]

A leakdown test tells you exactly how much air is escaping and where it is escaping from. You could just hook an air supply to the cylinder and see if it's esacping somewhere. If you want to do a true leakdown, I built my own using instructions from here:
http://www.xs11.com/tips/misc/misc3.shtml

Worked great. Remember cylinder to be tested needs to be exactly at top dead center (don't leave your wrench on the damper and your face in the way when hooking up the air supply, if the cylinder isn't exactly to TDC, the engine turns very quickly.)
I'd do a compression test first, if a cylinder is off, then do a leakdown on that cylinder.