78 Emissions and tuning Qs...
Thread Starter
Melting Slicks
Joined: Oct 2011
Posts: 2,748
Likes: 90
From: Sacramento California
C2 of Year Finalist (performance mods) 2019
2018 C2 of Year Finalist
78 Emissions and tuning Qs...
So first the symptoms:
78 L82.
Bad fuel mileage (150 miles on a tank) and a chugging in low revs up to about 2000 rpm.
I have not detected any vacuum leaks, and it idles at 650. Idle is not what i would call lumpy or rough, but it does shake the car. Maybe a little rough. It will rev to red line, but even with 3.55 gears doesnt feel very fast.
It also occasionally will start on high idle and not come back down.
I have replaced fuel filter, air filter, and have checked operation of EGR valve and heat riser. Timing is set at 12 degrees.
Plugs look good, wires look good.
Two things that I observed, 1) at idle only the front air cleaner flapper is open, the side flapper stays closed until vacuum is removed.
2) With vacuum on the heat riser it raises, which I believe is open.
All hoses are routed correctly.
Multiple issues? all related? Any insights?
78 L82.
Bad fuel mileage (150 miles on a tank) and a chugging in low revs up to about 2000 rpm.
I have not detected any vacuum leaks, and it idles at 650. Idle is not what i would call lumpy or rough, but it does shake the car. Maybe a little rough. It will rev to red line, but even with 3.55 gears doesnt feel very fast.
It also occasionally will start on high idle and not come back down.
I have replaced fuel filter, air filter, and have checked operation of EGR valve and heat riser. Timing is set at 12 degrees.
Plugs look good, wires look good.
Two things that I observed, 1) at idle only the front air cleaner flapper is open, the side flapper stays closed until vacuum is removed.
2) With vacuum on the heat riser it raises, which I believe is open.
All hoses are routed correctly.
Multiple issues? all related? Any insights?
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
Sounds like you're running rich on the primaries to me. How does it behave if you block vacuum to the flappers? The front one should provide more than enough air to keep it from running that horribly rich, but you may as well rectify it.
It sounds to me like the choke could be sticking shut. How long has the car been like this?
It sounds to me like the choke could be sticking shut. How long has the car been like this?
Thread Starter
Melting Slicks
Joined: Oct 2011
Posts: 2,748
Likes: 90
From: Sacramento California
C2 of Year Finalist (performance mods) 2019
2018 C2 of Year Finalist
I disconnected flappers with no improvement.
With the air cleaner off, the choke is open...even when on fast idle.
The choke closes when its first started, then opens as it should.
With the air cleaner off, the choke is open...even when on fast idle.
The choke closes when its first started, then opens as it should.
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
A lean miss is more of a "snap-vroom" whereas a rich bog is more of a "glugglugglugvvvvrrrrooooooom" sort of deal. When you say chugging I think rich.
As long as the choke flapper is completely open it sounds OK(as far as this condition is concerned).
Throttle shaft bores can cause issues but I'd expect that to be a lean issue (unmetered air) not a rich issue.
Can you look down the carburetor while it's idling and tell me what you see? You should see virtually nothing. I'm suspecting fuel dumping from around the walls of the primary bores - flooding. If that's the case, then I'd say probably a sunk float, if this is something that has recently come on.
I've had a stuck EGR valve feel like a spark / lean miss before, though, so I wouldn't rule that out even if you've tested operation of the valve at idle. I like to have an EGR blockoff plate on hand so that I can disable that system to rule it out.
As long as the choke flapper is completely open it sounds OK(as far as this condition is concerned).
Throttle shaft bores can cause issues but I'd expect that to be a lean issue (unmetered air) not a rich issue.
Can you look down the carburetor while it's idling and tell me what you see? You should see virtually nothing. I'm suspecting fuel dumping from around the walls of the primary bores - flooding. If that's the case, then I'd say probably a sunk float, if this is something that has recently come on.
I've had a stuck EGR valve feel like a spark / lean miss before, though, so I wouldn't rule that out even if you've tested operation of the valve at idle. I like to have an EGR blockoff plate on hand so that I can disable that system to rule it out.
Racer
Joined: Jul 2009
Posts: 337
Likes: 7
From: South Bend IN
NCM Sinkhole Donor
You should also get a timing light and check your timing. You want about 10-12 degrees initial timing with the vacuum advance disconnected. That is where I would start, then definitely take a good look at the carb. and make sure it is functioning properly.
