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High idle, burning fuel


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Bought an 83 720 last year that had all the smog stuff pulled off; aftermarket headers; Webber carburetor. Man she ran great for a little four banger but being in California and needing To get it smogged, I went to pick and pull on half off day abs pulled all the smog equipment, and original everything I could find to get it to pass smog.


After a couple of days of running vacuum line and spending some bucks to get cat put in the pipe I took the old girl to a smog shop to get a pass or fail (if you fail in California, they give you a 60 day permit to drive it while you fix it) and I figured the test would let me know what I needed to address.


Smog guy said I passed everything except I was running to rich. Which seemed like small issue. I was idling at 1500 so I knew it needed some tweaking.


But it seems the idle screw is factory sealed, so that's a no go. The throttle cable is all the way in, so that's a no go as well (unless I bend the baracket a bit?) So I figured, maybe it's the timing. But wouldn't you know I lent my timing light to a neighbor kid who got arrested. That's Ok, I've set timing with a vacuum gage on lots of old cars, I'll try it with this one. I had to bring the vacuum pressure all the way down to about 15 psi (not sure where that puts the timing though) to get the idle around 1000 rpm. Still to high. And no power at all. (No way I'd take her one the freeway like this) I'll pick up a new timing light tomorrow.


Anyway, another guess would be the choke? But I've only worked on trucks with manual chokes and I think the carb I picked up was from an 84 or 85 and has an automatic choke. So if it's stuck closed then it would run rich but would that effect the idle?


I'm outta idea's and wanted to see what you guys thought


Thanks for reading this novel.



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The idle MIXTURE screw is sealed but the idle SPEED is not. Try turning the idle speed screw down. Be sure there is enough slack in the cable.


When warmed up take the top off th4 air filter... is the choke plate fully open?? The choke also forces a fast idle cam setting or the fast idle cam linkage is stuck and it isn't comming off when warmed up. I'll post a few pictures when I get off work to show you where to look. In the mean time WD-40 the linkages around and just above the throttle cable where it meets the carb. This may loosen it up (if it's stuck)


Another thing is the secondary throttle plate is not fully closed and air leaking past it. I've had this happen.

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I appreciate the responses. Got called into work today so I'll pick up a timing light at lunch today and pay with the carb tomorrow.


I did have a question about the timing that I neglected to ask yesterday. The timing indicator mark is a four toothed saw. This confuses me (doesn't take much nowadays) when using a timing light before, I've always seen a stationary point (or arrow) on the block that lines up with timing marks on the harmonic balancer or pully. I'm not sure which one of the teeth on this timing indicator to use to set timing. Is this engine backwards to my thinking and maybe there is a single mark on the pully and each tooth on the stationary timing indicator is 5 or 10 degrees?


Thanks again

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  • 2 weeks later...

The idle screw may be sealed but u can use a ounch or screwdriver or whatever to break threw the seal and adjust it. I bought 85 720 and it idled at 2000rpm. I fixed a few vacuum lines still had same problem.. replaced o2 sensor still no good adjusted the idle screw... the previous owner had already broke through the seal to access the screw. Got it turned down to a smooth 800rpm.. on the bottom side of your hood ahoukd be a sticker twlling what rpm

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If the mixture is still plugged then leave it as it is set at the factory and does not change nor will it lower the idle or fix bad fuel consumption. When smogged if it's removed you may fail just for that.

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