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Unfortunately, I failed smog, although I have new oil, high grade Chevron fuel with additive, new fuel cap and (i thought) all vacuum lines connected. It ran fine before smog test, but after, it wouldn't stay running, I had to keep revving to keep it going. Tester said I PCV valve is defect, and measured 2848 NO emission @ 15mph which makes me a gross polluter. I'm not happy to pollute, so I'm looking for a good vacuum hose diagram for the 1978 20R truck (CA smog). The diagram on the hood is partly faded. Online search didn't give the right one.

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can't help it having a carb. What bothers me for example, is that my positive crankcase ventilation (PCV) valve tested fine, while in the smog report it's 'not functioning'. I have no idea what to do. Thinking about selling it. This way, I do not have any pleasure from the vehicle, while it already costs me tax, insurance, not to speak about the investment. When confronted with the California smog rules (that are massively bypassed once vehicle passes; just lookup 'desmog' on youtube) I'm very very sad, to the point of simply wanting to leave this state. The government should at least offer the right vacuum hose diagrams. Also, the 'visual inspection' has nothing to do with the reason smog once was started, which was more functional than car fashion. I include the vacuum diagram that's on the hood. Which seems different from the ones I see here. It's also different from the hose connections in my reality. 

1978vacuumhoses20R.jpg

Edited by bas_vossen
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Yes I hear you but that is why they have so much stuff to try and clean them up. Over the years I'm sure some experimenters have tinkered with stuff expecting to "make it better" or "who needs all this junk"? The visual inspection is to basically see how badly hacked it has been because there would be no chance of making it right without a lot of parts and expense. Back when your truck was made I was still very active in the trade and it was hard for us to some times to make these thing run well and still have low emissions. Just a thought but will CA allow you to title it as an antique and by pass the emission testing? It's old enough.

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After a complete cleaning of my carburetor, an original AISAN 38151, and routing all vacuum lines according to the diagram above, I started the engine, and at first, it ran extremely high idle so I ran out of the car to adjust the 3 screws until it was low again. I stopped the engine by testing the EGR valve, and that must work since the engine immediately stalled. Unfortunately the next time I started it, it wouldn't hold any idle so I screwed in the screws. Now even with those 3 screws (low idle, high idle and the mechanical throttle-stop on the front side of the carb) I can't get it to keep running on idle. When I unscrew the air cleaner cap, I see both valves are open all the way, and I suspect my secondary throttle diaphragm to be leaking. Could this be the cause of these probs?

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  • 3 weeks later...

After finding a completely rebuild Aisan carb in Craigslist, I mailed the advertiser who was in Napa, he called back on Monday, and I picked it up the same day. Installed it, and my problems were gone. New plugs, cables, distributor cap and rotor, timing checked, and no more gross polluting. Unfortunately, I forgot about the leaking fuel filler hose, so still have to repair that before I pass smog. I think my old carb was rebuilt by an amateur, who forgot a couple of ball valves. The rebuild kit has all of those, but I'm not going to put time in it anymore. If anyone wants to buy my old carb and rebuild it, it's for sale now, incl. the kit for $150. Pics on reply. 

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The factory Aisan carbs are extremely difficult to rebuild. My toy had an aisan 38290 2bbl (1978). I purchased the rebuild kit intending to to do the work myself. I quit halfway though the build. Too technical for my mechanical abilities. There must have been a zillion tiny pieces that had to be installed. This is why the rebuild kit only costs about $30 and a rebuilt carb costs $275.

I ended up throwing the carb away and purchased a Weber 32/36 for a replacement. The Weber will pass California smog requirements. Nobody wants the old Aisan carbs anymore. 

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I read Webers wouldn't pass smog, and consume more while indeed adding some extra hp. The Aisans aren't that hard to rebuild, the kit comes with complete exploded view. I'll continue my rebuild and make someone happy. It's CA smog specific. I also found the exact way of connecting all vacuum lines.

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My apologies if someone already covered this. I am going to point my finger at your EGR valve. If it sticks open you will not be able to idle because it will create a vacuum leak. They can seal or not seal randomly. The EGR valves are notorious for getting a build up of carbon and gunk inside. They open up at higher rpms via a vacuum line. I think it is mounted on the back of the cylinder head next to the firewall. If you were in a different state you could remove it and install a delete plate. 1016038-3.jpg?1515491448

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Ok hear is what happens and egr dumps exhaust gas back into the intake system it is inert what it does is it takes up space in the compression cycle. If it stays open it allows exhaust gas into the intake when it shouldn't it's not really an air leak but there is nothing in it that would promote combustion. Carburetors like things like down hill no throttle because it dump in fuel with a deep vacuum this is where the egr comes in to clean things up.

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