cdg

OK first let me introduce myself. I'm 22 years old with a rather substantial collection of vehicles. I have been working on vehicles for a rather long time that being said. Alternatively you might find me framing as a carpenter, playing guitar, or finding cool landscapes to photograph (all chemical darkroom).

So my girlfriend needed a truck, and I needed a project to busy myself with. I bought a 1980 pickup 2wd 5 speed and set to work. A round of tires, new steering center link, starter motor, got the truck running, but with a backfiring issue (used to backfire between shifts).

The truck ran fine but once run down the highway or run at higher RPMs the truck would start backfiring louder than a gun through the tailpipe. After these loud backfires the truck would never idle right even though it would always accelerate just fine from about 1100 RPM on to near engine redline. Allowed to sit and cool down, the truck would always restart and run fine for a while, until it would get hot again. Trying to restart the truck while it is hot does not generally work without a lot of playing with the gas pedal.

So I tagged and plugged all of the vaccuum lines and built them back to the point where the truck would run well. Not even a hint of a backfire now.


However, the truck still insists on running badly after run hard or being run at freeway speeds.

Now here is what I have found to be the case so far:

1) The timing is set just right. The Vaccuum advance is still a little suspect, but during the times when the truck runs "badly", I have verified that the base timing is still set just where it should be, about 8 degrees BTDC. I have tried advancing and retarding the timing slightly, and while the truck feels livlier on the road with the timing more advanced, it does not seem to affect the truck's ability to restart.

2) The spark plugs, wires, distributor cap, and rotor are all new. The air filter is very recent.

3) The vacuum choke pull off does not seem to be the problem.

4) None of the vaccuum plugs that I installed are leaking.

5) When the truck is running badly it seems to be running rich. Smokes some black soot.

6) The EGR valve is physically blocked off and thus not the source of concern. All associated vaccuum lines, including the vaccuum modulator are plugged off.

7) I had suspected a sticking valve. The valve clearance is correctly set - all of the valves have been clearanced to .010 inches.

8) If the valve cover is removed when the engine is hot and running poorly, there is no evidence that any of the intake valves are sticking.

9) Even though I do not see a sticking valve, I will mention that the cylinder number 1 intake valve has quite a bit of clatter even though it has been double checked for the correct clearance.


If anybody here has a lot of familiarity with these trucks it would be greatly appreciated. I have been trying to attack this problem logically but it seems to be eluding me. It feels like a vacuum leak but I'm nearly certain it isn't.


That being said I've been thinking about a Mallory dual point ignition system for it along with a nice new Weber 32/34 carb conversion kit.

My next step to double check the sticking valve is to perform an engine compression check immediately after the truck starts running poorly. After that though it's got to be ignition (i.e. a faulty coil) or some random carburetor/intake/vaccuum issue.

Any thoughts are appreciated. Thanks!


Also, if I get this issue fixed up I need a front grill, and a radio (hoping for a working original FM radio. Any leads on these are also appreciated.

Scooby098

tryed setting the timeing to 8 deg?

marshm4ll0w

try spraying like carb cleaner around the vacuum lines and if it picks up you have a vaccum leak. done this in my 90 runner. found my vacuum leaks real fast

cdg

I might be chasing my tail on this one but perhaps it really is the intake valve?

Carb cleaner on the vaccuum lines is an old trick that I tried at first. Even when the truck is misbehaving though I cannot find any leaks. Remember most of the vacuum lines are plugged off good. I only hooked up what is absolutely necessary to get the truck to run good (most of the time).

The timing is set at 8 degrees BTDC. I did try diddling one way or the other too, and while advancing to about 15 degrees BTDC did seem to liven the truck's performance substantially, it did nothing to stop it from intermittently running like crap.

The vacuum advance unit on the distributor is probably bad but I don't think it's related to this particular problem. Applying vacuum to the retard end of the distributor does absolutely nothing, even with the advance plugged off. However, I have disconnected the vaccuum advance on the distributor and plugged it off securely and still the truck will intermitantly run badly. When the truck is running badly I have connected a timing light but it still reads right where I set the truck at 8 degree BTDC. Seems like you have to really diddle with the timing to get the truck to run badly anyway - I've had it advanced pretty far off the scale and retarded beyond 0 and still the truck runs (albeit not great at either etreme).

I had thought about a random misfire issue in the ignition, but when the truck is badly I have had an inline spark checker on each cylinder and it happily chirps away, even when the engine is near the "death rattle" low idle.


So there is no logical ignition problem present here from what I can tell. That pretty much leaves valves. Tomorrow I will rent a compression checker (I don't have one in this town) and go to work. Low compression will pretty much mean a faulty valve, meaning I will be hitting up either the local machine shop or Cylinder Heads International.



Luckily I already have the necessary gaskets, excepting the head gasket to do the job if it turns out to be necessary.



Another little clue here is that after changing the oil the problem seems to be a lot less prevalent. I changed it and put 5w30 in it - on the thin side for summer temps in Flagstaff but I am hoping to promote some oil burn and to only run this oil for a few hundred miles before changing it again (if it is in fact a sticky valve I'm hoping to work it free).

cdg

My dad always taught me to attack a problem by trying to diagnose all the symptoms collectively so if my posts are very long and rambling I apologize. I have been really scratching my butt over this one - since my butt and my head are pretty much interchangeable most of the time.

cdg

Here is the update:

After changing the oil, running every gimmicky engine flush and octane booster in the oil and gas, and finally adding about a half pint of MarvelMystery oil to the new 5w30 engine oil, the truck started to only intermittantly run badly.

2 days later now and I can run it up and down the highway and flog it as hard as I can (not that I do this regularly anyway but in the interest of diagnostics...).

The truck runs great, although still too rich up here in Flagstaff. I plan on doing the Weber conversion on it though and eventually still redoing the cylinder head for a little extra performance but at least now the truck has a new lease on life. Changed the transmission oil and the transmission is now noticeably smoother and doesn't growl when going down hills as much.