I am trying to diagnose a problem which I believe is some kind of misfiring.

The symptons happen when the truck is not yet fully warmed up and has only been in city traffic for about 10 minutes. When I enter the freeway and go into 3rd gear and above and if I depress the gas more than halfway I can feel occasional shaking and when that happens occasionally I can hear a popping sound coming out of the exhaust. If I let go a bit of the gas pedal it will go away. Once I am driving on the freeway for about 5 or 10 minutes more the problem disappears and everything works fine.

If I stop the truck soon after the problem occurs the exhaust smell is more "disagreeable". ;-) Probably due to a more incomplete burning of fuel.

I am thinking that one of the cilinders is misfiring, causing the shaking due to decreased power, the popping in the exhaust I believe is igniting of unburned fuel.

The problem did go away for a while after replacing the spark plugs (needed replacing anyways), but then returned. I have recently also replaced the spark plug wires and cap (it also was about time to do so). Neither solved the problem, though the new plugs, wires and cap do of course help in the usual way.

I am thinking it can be either the carburetor, the valves or a leak in the exhaust system. It seems unlikely a leak can cause it though, I have had that before, a bad gasket near the cat and it only caused a change in sound, no problems as described above. However I will be looking for leaks and if necessary fixing it.

The carburetor is a rebuilt unit I bought and installed about 3 years ago, I am planning to install the original one I rebuilt myself and is known to work. As a side note that one actually appears to deliver more power, but tends to choke the engine more when full throttle and not warmed up yet. I am suspecting it is because one tiny part in some fuel passage, a kind of filter which had disintegrated after removal and there was no replacement, is absent.

I did measure and adjust the valves a couple of years ago as well, this improved engine idling especially. I can imagine though that build up of gunk can cause this, and once everything is properly heated up it disappears.

Am I looking in the right direction? Does anyone know what exactly the problem could be?

Note that it goes away after about 15 minutes of driving, i.e. when the engine is fully warmed up.

 

Sounds like you have a lean miss at part throttle. Probably a clogged jet or the spring loaded air valve over the secondary barrel is sticking. I'd spray down the air valve with carb cleaner to make sure it's moving freely. For the possible clogged jet there is a good hillbilly trick for that. Remove the air cleaner assembly and with the truck running rev it up and choke it with your hand over the carb opening at different rpm ranges to clear all of the jets. Like I said, it's hillbilly but effective.

 

Thanks, sounds like possible culprit. I'll go and get a fresh can of carb cleaner. That other trick sounds like a fun job as well. ;-)

 

Sounds about right to me.Might also try a fuel additive to clean out everything.I've heard great things about Seafoam.Maybe give that a try.Here's a link to some reviews Opinions on SeaFoam cleaning additive? - Yahoo Answers

 

Thanks for the suggestions. I did carefully spray about 1/4 can of carb cleaner through the intake whilst it was idling and it worked like a charm. I didn't think it needed it because I installed this rebuilt carb only a couple of years ago. I was clearly overcomplicating this :-D