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1980 720 failed emissions... Not sure what else I can do.


Dylan Berichon

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Hi all!

 

I've searched for similar topics but I haven't had any luck finding an answer that might help me.  Today I tried to get my '80 720 longbed through emissions in Portland and failed each time I went through.

 

The truck was way high on Hydrocarbons and Carbon Monoxide.  Years of experience with Volvos tells me that this means the engine is running too rich.  Each time I retried the test, I adjusted the carb mixture down.  Numbers got a little better but still not close to passing.

 

I screwed the mixture screw in all the way prior to the last test I tried today.  As before, numbers improved, but still not close to passing.  I was actually kind of surprised the truck would run at all (leading me to think that there might be a problem with the carb, but I'm not sure).

 

When I got this truck last year, it had been parked for about 4 years.  I rebuilt the carb shortly after getting it fired up after it's long nap, and in the past week I gave it a full tune (cap, rotor, plugs).  I also had the entire exhaust replaced about a week ago (which included a brand new catalytic converter.)

 

I've checked for vacuum leaks and replaced a number of hoses that looked tired.  I'm not really sure what my next step should be.

 

I never had any trouble with cars in the past, but they have all been fuel injected.  I've never had to take a carbureted vehicle though, and I have a feeling that the carburetor is where my problems lie.

 

Any thoughts on a weber conversion?  I can get a weber kit through my work pretty cheap, but I'm concerned that my emissions would be no better with the weber on the truck.  I don't have to pass a visual inspection, just the tailpipe sniffer.

 

I'm out of ideas.  Any advice would be appreciated.

 

And... if all else fails... does anyone know of a Portland, OR area mechanic that might be able to help me?

 

Thanks as always.

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I screwed the mixture screw in all the way prior to the last test I tried today. As before, numbers improved, but still not close to passing. I was actually kind of surprised the truck would run at all (leading me to think that there might be a problem with the carb, but I'm not sure).

 

Definitely something wrong. For sure. Fix the carb and it will pass emissions.

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The mix adjustments only really affect idle, so if it's failing the "cruise" section, it's not something you change with a screw.

 

There are a LOT of things that can cause that.  Too Rich at the carb could be fuel leaking in, such as a sunk float causing the fuel bowl to overflow, or even the choke not opening.  Or a plain worn out carb.

 

The fact that you got the mix screw turned all the way in and it's still running at idle indicates it's either dumping fuel, or not even operating in the idle circuit at all.

 

 

It can also happen with vacuum leaks, a worn out engine (lots of blowby), an EGR that's fouled or stuck partially open, misfire form all sorts of ignition issues,  timing not set right, etc etc.

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I think you need to start with testing the compression.   Adjust the valves.  Set timing, you know do a full tune up.  Get everything back to as close to factory specs.

 

Fuel injected, usually means there is O2 sensor, and a computer, that corrects many problems the engine has.  Carburetors do not do that. you have to get everything right.  Make sure the float level is good.  Get all the emissions equipment hooked up.  Many times, people who do not understand emission controls just rip it out, or disable it, and that makes the engine run worse, or not as efficiently.  you never know, until the dreaded smog check.

 

High CO, you are running rich.  High HC, that is unburnt gasoline.   Pay close attention to the valves, are they sealing good.

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One thing to add here that might keep you (or someone else) from chasing their tail too much. I high HC can also be caused by a very lean running engine as well as rich. I know that it sounds wrong, but too lean is an inefficient burn as well. In other words, not burning enough of the gas. 

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I had similar problems with my 85.  And I have to pass smog in California!

 

After a lot of fiddling I discovered that my engine WAS running too lean.  I riched out the mixture and all of a sudden the numbers sunk like a stone.

Long story short, it has to be rich enough to light off the catalytic converter.....

 

I now pass smog in Los Angeles, one of THE worst, with flying colors.

 

Counter intuitive, but it worked.

 

Check your plugs.  If they are greyish, it's too lean.  Mine were nearly white, although the truck ran great.

If your plugs are black, well, that's too rich and you do need to look at the carb again.

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Thanks for the suggestions guys!

 

Is there an easy "on the fly" way to check the mixture on these carburetors?  (SU carbs have and "Adjustment pin" that makes this process easy, but I'm not sure if there is anything similar on this thing.)

 

I have a few more things I'm going to try before I give up and try to find a mechanic.  Fingers crossed.

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What RPM is this idling at? The reason I ask is because if you have it set too high, you could be pulling fuel somewhere other than the idle circuit. This would make it not shut off by turning the idle mixture screw in and also would make it fat at idle, if that is where you are failing at.

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Checked the choke, it's fully open once the truck is at temperature.

 

Hooked up a tach, the idle is about 800rpm at idle.

 

Idle is where the truck is failing, that's the only part it has to pass in Oregon.

 

I did more tweeks today: Adjusted timing, played around with the carb some more, I even changed the coolant and thermostat in hopes that might make a difference.  Same result. HC and CO are still too high, and it didn't pass.

 

Checking Valves is the next step, I suppose.

 

Is there a recommended Portland area Datsun mechanic I can take this thing to?  I'm starting to run low on spare time to try and get this thing to pass emissions.  I'm about at a point where it would be worth it to let someone else figure it out.

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The thing is, it is pulling fuel from somewhere it is not supposed to be. This is your issue. Sort of like if you totally shut the valve on your propane BBQ and it still stayed lit, you would know that the valve is faulty and leaking. Similar deal, by turning it in all the way to seat, you shut off the fuel, or at least it should have been and it should have stopped running. Since it did not, you have an un-metered leak. This is causing you to run rich. So something is haywire in the carb.

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I have a question here, what is the maximum highest idle speed your allowed to have in Oregon?

I would turn it up just under the highest idle allowed, put the highest grade gas in it you can find, and have it fully warmed up when going for the test, and do not sit in line idling, go back when there is no line.

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It's sounding like a carburetor replacement might be in order, if I can't get this one to work correctly.  Are original Hitachi carbs still available for this thing?  I've done some digging, and so far I've only found a "Canadian" version available (and it's very expensive).  How would a Weber compare?  Would a weber carb allow the truck to pass emissions?

 

Thanks for the input so far guys.  I hope I can get to the bottom of this.

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Since it's not a visual inspection, I don't see a reason why a weber would not pass.

 

My only concern was being able to get a weber jetted correctly to run lean enough at idle.  I've helped convert a few Volvos from SUs to Webers, and even after jetting them, I found the webers seem to run a little richer at idle for whatever reason.  (I actually like the SU carbs better than the weber, for the record.)

 

From what I've read, a weber is a much better choice than the original Hitachi carb for performance reasons, but I don't think I've ever had the Hitachi carb on this truck set up correctly during the time I've owned it, probably because it is just worn out.  A golden rule I've learned is that you can't tune a warn out carb, no matter how hard you try.

 

It's sounding more and more like a weber would be the way to go...  I'd just hate to drop the cash for one and have it not pass emissions.

 

 

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Easy. Idle emissions are controlled by the idle speed screw and idle mixture screw. If yours was working correctly, adjusting per best lean idle will let you pass.

 

Its no different with a Weber. Unless the carb is broken, it is adjustable in exactly the same process as the Datsun manual outlines (best lean idle adjustment).

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I've ordered a replacement Hitachi carb that is Federal spec, and got it a pretty good deal on it through my work.  (A weber kit was a little more expensive, as it turns out.)

 

I'm confident after reading through the form that the truck will pass once I get the new carb dialed in.  Here's hoping, anyway.

 

Thanks for the input all, especially smoke and ggzilla.

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Please do let us know the outcome on this.  And more importantly, if the carb is GOOD, where you got it from!

 

I ordered a rebuilt California spec carb for mine a few years ago and it failed so badly I almost made the gross polluter list.

Long story short, I ended up rebuilding my original carb and have had no more issues.

 

I have dealt with rebuilders in the past, and it's a total crap shoot getting something that's actually done RIGHT.

 

Here's crossing my fingers for you.

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What was your timing at idle? I had this issue last year, except my CO was fine and HC was high. CO is A/F mixture, and you can set this to lean best idle with a vacuum gauge, and more than likely pass. HC, as stated, is usually ignition, as it was in my case. The centrifugal advance in my distributor was hung up in an advanced position because the plastic pieces were broken. My initial timing was 25 degrees BTDC. Ran great, except it pinged on hills and smelled like a BBQ grill. I reset it to 12 with a reman'd distributor and had no problem passing. Get the carb to where it idles smooth enough, THEN recheck the timing and set it, THEN go back to the carb. You can really lower HC by lowering the initial timing to less than the spec to where it barely runs, but honestly, it really should pass at the factory number.

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