Idle issues
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C2 of Year Finalist (track prepared) 2019
I checked the vent tube and it is not plugged. I know it's weird, but true. The moment I disconnected the vent tube it ran great. Is there a vacuum, albeit slight, that the air cleaner pulls on the tube and could that have an effect? Is there a reason it is connected to the air cleaner?
What happens when I disconnect the Balance Tube at the Fuel Meter is that turbulence from the engine fan swirls around the opening in the side of the Fuel Meter and plays havoc with a stable idle air/fuel mixture.
The reason the Balance Tube connects to a port in the air cleaner is that is where still-ish air at atmospheric pressure can easily be had.
Jim
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From: Greenville, Indiana
What happens when I disconnect the Balance Tube at the Fuel Meter is that turbulence from the engine fan swirls around the opening in the side of the Fuel Meter and plays havoc with a stable idle air/fuel mixture.
The reason the Balance Tube connects to a port in the air cleaner is that is where still-ish air at atmospheric pressure can easily be had.
Jim
Suppose his air cleaner is obstructed?
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I have good reason to believe the FI unit was not properly calibrated after it was rebuilt.
If I'm right, the experiment I suggested which involves disconnecting the vacuum line to the Enrichment Diaphragm ought to result in a stable, albeit somewhat rich, engine idle..... with the balance tube installed, of course.
Jim
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From: Greenville, Indiana
Sounds like it could be.
Last edited by MikeM; Nov 9, 2015 at 10:52 AM.
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From: northern california
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Yes, but not just at idle, at all RPMs. His unit was NOT calibrated by the rebuilder so the Economy Stop could be rich or it could be lean, but it is almost certainly wrong (and I'm betting on lean).
The fact that the engine speed picks back up once idle vacuum drops low enough for the Ratio Lever to swing to the Power Stop is a big clue that the unit is lean. Another is that Keith says the Idle Fuel screw has little effect, except that he can stall the engine when it's screwed all the way in.
Being a 4800 unit, there could be other conditions causing a lean condition besides just lack of calibration.
In a 4800 unit, the idle boost circuit is configured differently from ALL later units. There could be a piece of trash blocking an orifice. A restriction in the main diaphragm "T" fitting could be an inappropriate size. Who knows, one of the valves in the Cold Enrichment housing could be leaky, causing a loss of Main Diaphragm signal.
None of these potential problems are suitable for a Do-It-Ur-Selfer to diagnose, so I'm just focusing on calibration for now.
The fact that the engine speed picks back up once idle vacuum drops low enough for the Ratio Lever to swing to the Power Stop is a big clue that the unit is lean. Another is that Keith says the Idle Fuel screw has little effect, except that he can stall the engine when it's screwed all the way in.
Being a 4800 unit, there could be other conditions causing a lean condition besides just lack of calibration.
In a 4800 unit, the idle boost circuit is configured differently from ALL later units. There could be a piece of trash blocking an orifice. A restriction in the main diaphragm "T" fitting could be an inappropriate size. Who knows, one of the valves in the Cold Enrichment housing could be leaky, causing a loss of Main Diaphragm signal.
None of these potential problems are suitable for a Do-It-Ur-Selfer to diagnose, so I'm just focusing on calibration for now.
Thread Starter
Pro
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From: San Clemente CA
Ok, Jim, I ran the experiment by disconnecting the Vacuum tube from the Enrichment Diaphragm while leaving the balance tube connected. As expected (I think) the idle ran smoothly. Ratio level rested against the Power Stop. I also disconnected the Balance Tube while the Vacuum Tube was disconnected and it still idled smoothly but at a slower rpm.
In summary, with either the Balance Tube or the Enrichment Vacuum Tube disconnected, engine idles smoothly. When both are connected, idle is poor.
I have attached a couple of videos. I don't know if these will help, but they illustrate the two above points.
Keith
In summary, with either the Balance Tube or the Enrichment Vacuum Tube disconnected, engine idles smoothly. When both are connected, idle is poor.
I have attached a couple of videos. I don't know if these will help, but they illustrate the two above points.
Keith
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Keith,
The two videos got the point across very nicely. I thought the engine sounded pretty good when it got the richer mixture.
If you have a shop near you with a chassis dyno, I'd suggest you take the car there and I'll walk you through the calibration process..... which I think that FI unit sorely needs.
Alternatively, if you have a friend with a wide band air/fuel gauge you could borrow, same offer: I'll give you instructions on what to do.
Lastly, if you don't have access to either of those resources, we can attempt approximate calibration by using the tried-and-proven, if somewhat inaccurate, Seat-Of-The-Pants-O-Meter.
Jim
The two videos got the point across very nicely. I thought the engine sounded pretty good when it got the richer mixture.
If you have a shop near you with a chassis dyno, I'd suggest you take the car there and I'll walk you through the calibration process..... which I think that FI unit sorely needs.
Alternatively, if you have a friend with a wide band air/fuel gauge you could borrow, same offer: I'll give you instructions on what to do.
Lastly, if you don't have access to either of those resources, we can attempt approximate calibration by using the tried-and-proven, if somewhat inaccurate, Seat-Of-The-Pants-O-Meter.
Jim
Thread Starter
Pro
Joined: May 2014
Posts: 530
Likes: 360
From: San Clemente CA
Keith,
The two videos got the point across very nicely. I thought the engine sounded pretty good when it got the richer mixture.
If you have a shop near you with a chassis dyno, I'd suggest you take the car there and I'll walk you through the calibration process..... which I think that FI unit sorely needs.
Alternatively, if you have a friend with a wide band air/fuel gauge you could borrow, same offer: I'll give you instructions on what to do.
Lastly, if you don't have access to either of those resources, we can attempt approximate calibration by using the tried-and-proven, if somewhat inaccurate, Seat-Of-The-Pants-O-Meter.
Jim
The two videos got the point across very nicely. I thought the engine sounded pretty good when it got the richer mixture.
If you have a shop near you with a chassis dyno, I'd suggest you take the car there and I'll walk you through the calibration process..... which I think that FI unit sorely needs.
Alternatively, if you have a friend with a wide band air/fuel gauge you could borrow, same offer: I'll give you instructions on what to do.
Lastly, if you don't have access to either of those resources, we can attempt approximate calibration by using the tried-and-proven, if somewhat inaccurate, Seat-Of-The-Pants-O-Meter.
Jim
Keith
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Brett Foote
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Jim