Triple

My truck has never idled correctly. It accelerates strongly and cruises smoothly, but it lopes at idle and backfires under deceleration. Last year it backfired so violently that the muffler completely exploded.

I'm in the process of partially de-smogging the truck. The EGR has been removed and capped, the previous owner already capped the air injection ports, and I'm plating the O2 sensor port in the exhaust manifold. Almost all vacuum lines will be removed and/or capped per the tutorials on this site. For the time being, I want to continue using the factory Aisan carburetor. When I replace the exhaust, I will delete the catalytic converter and run a straight pipe to the new muffler.

I've heard of other issues causing similar symptoms: faulty thermo switches and thermo switch connectors, for example. I want to eliminate all possible sources of the poor idle and backfiring short of rebuilding the entire carburetor. Suggestions?

This old truck shows just 122K miles. It should run a lot better than it does.

Creepycrawler87

Have you check the timing? if timing is to far advance or the vacuum advance is stuck it will do what your talking about

Triple

How do I determine whether or not the vacuum advance is stuck?

Creepycrawler87

get ya a timing light and check your base timing, then rev the engine and see if the timing advances with engine rpm

Triple

UPDATE:

I installed block-off plates for both the EGR and the oxygen sensor port on the exhaust manifold. Some previous owner had already blocked off the air injection ports. I removed the catalytic converter, found it to be completely destroyed inside anyway, and installed a section of straight pipe in its place. Behind the straight pipe, I installed a new, pre-welded Bozeman muffler and tail-pipe assembly.



I did my best to mimic the vacuum routing of this diagram:



Unfortunately, there were differences between my truck and the diagram that necessitated a little guess-work. For instance, my truck has two charcoal canisters; I plumbed the rear just like the one in the diagram, but the front originally had one line connected to vacuum and another to the smog pump... which I removed. I don't know if/how I can retain the front canister; I connected the vacuum line to the port the diagram uses to feed the TVSV and CB, and the smog line to a port on the carb of corresponding size, but I don't think it's working. I blasted both canisters with compressed air, but I'm still not venting the tank even with at least one canister plumbed correctly. I will try removing just the front first and if that doesn't work, I'll remove them both and vent the tank somewhere else with a breather.

I capped the thermo valve in my air cleaner assembly. The port the diagram uses to feed its thermo valve is connected to the BVSV and CB on my truck. The diagram shows a "TVSV" that has three ports like my BVSV, but it is of a different shape. I connected the lines in the same top to bottom order as the diagram and ran them to their respective components. The diagram doesn't even utilize a second BVSV beside the thermostat, so I capped mine. For the CB, I re-used a restrictor/jet from somewhere else in the old manifold routing but I don't know how to test its function (or if it is directional).

The vacuum advance lines are connected exactly as shown in the diagram.





Upon start-up, I increased the idle speed a bit via the adjuster screw and checked the timing, which was dead-on perfect. The BVSV appeared to control the choke opener and/or AAP, but once the engine was warm, idle was no better than before doing any of this work. The backfiring disappeared completely, however, though the he truck felt down on power under acceleration.

From here I decided to advance the timing from 0-degrees to 5 as recommended after de-smogging. I also adjusted the valves; none were loose, a couple were just ever-so-slightly tight, but nothing severe enough cause the type of driveability problems I was experiencing. This was the first time I had removed my valve cover, so at least I could confirm the condition of my timing chain guides. They look like OEM plastic, unfortunately, but they are intact and feel solid to the touch:



The truck runs/drives much worse with the advanced timing, so I will reset it to factory spec the next time I work on it. At 5-degrees I am now getting a fair amount of "dieseling" after cutting the ignition and fear I could start backfiring again as well. At either 0 or 5-degrees, however, the engine still isn't running much better than it did before the de-smog. It's still spitting too much smoke and still coating the inside of the exhaust with oily black grime.

I also noticed that the vacuum line connecting the intake to the BVSV gets so ice cold that water condenses on the outside:



Clearly, I've routed something incorrectly. I will start by removing the charcoal canister(s), but after that I'm not sure what to do. I could cap everything except the vacuum advance to see how it runs at operating temperature, but I'd like to retain some start-up and cold-weather assistance if possible.

This truck will be listed for sale as soon as I have it running smoothly. I don't want to invest in a Weber unless absolutely necessary.

Triple

Man, just crickets around this place these days. Facebook has officially killed the message board... sad...

Anyway, I removed both charcoal canisters, capped all their associated ports in the engine compartment, and drilled a hole in the gas cap. The dieseling effect has disappeared but now the truck barely runs at idle once at operating temperature. Upon cold start-up, however, it revs like a crazy bitch and even tapping the gas pedal won't bring the rpm down.

I haven't re-set the timing back to 0-degrees because I can't get the thing to idle while I do it.

Getting a little fed up with this thing. It has to make a 700-mile trip in a couple weeks and I'd rather not get 4mpg the whole way... and lawd help me if I get stuck in traffic...