250 w/Aluminum - Troubleshooting help - running roughly,rich

[62Ranchero200]

Background: This is a solidly built 250 with a Clay Smith 274/274/108 cam, aluminum head, 500 CFM 2 BBL Holley, Ci headers, DUI.

The Ranchero is a weekend driver only, so it's common for me to go a week or two between driving it. It ran great until ...

Two weeks ago Saturday, I started it to go to my Falcon club meeting, then a cruise-in. It ran extremely roughly, wouldn't idle, and was extremely rich. The wideband was pegged out at 10.0, and the exhaust was eye-watering. I let it run for about 15 minutes, hoping it would warm up and run better and leaner, but it never did and I finally gave up and took my daily driver to the car club meeting. At the time, I thought that either the float or needle valve was stuck, flooding it with gas, or that the power valve had blown out and was leaking.

Tonight, I pulled the breather off and cranked it a good long time with a remote starter switch ... no gas was overflowing from the bowl vent and no drips from the venturi boosters - so apparently it's not a float or needle valve issue. I pulled the bowl off and replaced the power valve, but the issue remains. With unlimited time, after warming up it will idle ... but it still runs extremely roughly and is still extremely rich. I can see the squirt from the accelerator pump discharge nozzle when I work the throttle, so it definitely does have gas in the carburetor bowl. I don't hear any whistling sound, so I don't think there's any significant vacuum leak; and if there was it would probably be lean, not rich.

I put a timing light on it and the timing is spot on ... 20 degrees BTDC (yes I know that's quite a bit, but it's always been set there and the DUI is custom curved by WSA111). Oddly though, the timing light is only firing intermittently. Could this be a sign of an issue with the DUI? If the module or coil was going out, could it still run, but only fire intermittently? That could cause it to run rough and might cause the rich exhaust, since a non-firing plug would leave a charge of unburned gas in the cylinder. If I remember correctly, the timing light was steadily on the last time that I used it.

I thought about a bent pushrod or other valvetrain issue, but it would still fire correctly in that case, as evidenced by the timing light, right?

Any other ideas?

* Update: I checked the voltage at the BAT lead of the coil: 12.5 V. So apparently there's not an issue with my ignition switch or wiring.

Thanks,
Bob

 

[rbohm]

it sounds like an ignition problem to me bob. start with all the usual suspects, the plug wires, the plugs, etc. also look for carbon tracking in the dist cap.

 

[cobraguy]

Ignition miss will cause a rich exhaust. Fire it up and feel header tubes the cooler ones will indicate the missing cylinder(s)

 

[62Ranchero200]

it sounds like an ignition problem to me bob. start with all the usual suspects, the plug wires, the plugs, etc. also look for carbon tracking in the dist cap.

The Autolite (#3922 if I remember correctly) plugs and Live Wires were new as of a year ago, with maybe 1,500 miles since then. Since the Ranchero was running fine up until two weeks ago, it's hard for me to imagine how the plugs and/or wires would fail so suddenly. The wires are attached at both ends and show no signs of any damage. If the wires are in question, I could pull them and measure the resistance of each wire, right? They should be consistent. Could also pull the plugs, but if they plugs are fouled now, that could be the result of the ignition miss, not the cause - hard to tell. Checking for carbon tracking in the cap is a good idea and I will do that.

Could any components of the DUI, such as the module and the coil, fail partially such that the engine would run, but would fail to fire intermittently?

Thanks,
Bob

 

[rbohm]

the control module could be failing.

 

[vssman]

I had, what I belived to be, either the coil breakdown or a module start of fail in my DUI which would cause it to run really rough and then die - I was loosing a good spark. I replaced the cap, coil, module and condenser which fixed the problem on mine.

Edit: FWIW, this happened on a distributor that was over a year old with about 100 miles and an additional, maybe 4 hours, on it.

 

[JackFish]

Have you tried backing off the timing?
And what have you done with that 500cfm Holley?

 

[bubba22349]

Though since everything is near new I don't think you will find a problem with wires or plugs but to be sure it's fast and easy to test them. No need to pull the wires completely you can test them hooked up to the cap put one contact of your volt / ohm meter on plug end and other on the distributor contact. While you have the cap & wires off look at the caps center contact button &'the rotor, also for the signs of carbon tracking as "rohm" mentioned. Over the years of working on the GM's with stock HEI systems found that often the mod's would fail from heat, especially the cheaper ones it's a good idea to carry a spare with you. Good luck

 

[62Ranchero200]

Have you tried backing off the timing?
And what have you done with that 500cfm Holley?

I haven't tried adjusting the timing yet. The engine ran great for about a year with that timing, including several dyno pulls. If the engine "doesn't like" the timing now, something else must have changed. The DUI was custom curved to work with the 20 degree advance.

The Holley is extensively modified, with a Quick Fuel metering block drilled for custom, replaceable idle bleeds and PVCR jets. All of this has been in place for about a year and ran great until two weeks ago. I did crank the engine to check for overflowing fuel due to a stuck float or needle valve, and I replaced the power valve in case it was blown, but that made no difference. I see no indication of leaking fuel, inside the throttle bore or outside the carb, and no indication of a vacuum leak.

Thanks,
Bob

 

[wsa111]

Bob, did the power valve have raw fuel on the vacuum side of the diaphram??
Did the rubber o'ring seal on the power valve to metering block look intact. Upon re installation of the valve did you put lube on the o'ring to prevent tearing??
Hook your timing light pickup on one plug wire at a time. If intermittent flashing of the light is present on each plug wire then need to ohm the pickup coil.
You can try another module if you have one. If you need to purchase one at a parts store get one for a 78 corvette.
Remove the cap & inspect the rotor for proper fitting & as mentioned before for carbon arcing.
If you can't google the ohm reading for the pickup coil let me know & i will check my old HEI troubleshooting manual.

 

[pikesan]

Sounds like a really cool engine! Good luck getting it fixed!
I'm don't think I read anything about pulling the plugs? If you have an electrode broken off, or it's one cylinder that's not firing, you'll see it in the plug. If they're all drenched with fuel (black) then probably back to square one. (fuel or module problem)

 

[62Ranchero200]

Bob, did the power valve have raw fuel on the vacuum side of the diaphram??
Did the rubber o'ring seal on the power valve to metering block look intact. Upon re installation of the valve did you put lube on the o'ring to prevent tearing??
Hook your timing light pickup on one plug wire at a time. If intermittent flashing of the light is present on each plug wire then need to ohm the pickup coil.
You can try another module if you have one. If you need to purchase one at a parts store get one for a 78 corvette.
Remove the cap & inspect the rotor for proper fitting & as mentioned before for carbon arcing.
If you can't google the ohm reading for the pickup coil let me know & i will check my old HEI troubleshooting manual.

No fuel on the vacuum side of the diaphram, no sign of any issues with the rubber o-ring.

I found the following on the Performance Distributor web site: http://performancedistributors.com/professional-tips/

I put a multimeter on the ignition components and found the following readings. They all seem within spec to me, although the magnetic pickup's resistance is very slightly high compared to the spec and the coil's primary resistance is very slightly low:

#1 spark plug wire (about 10" long) - 340 ohms (should be 300-350 ohms)
Primary side of coil - 0.5 ohm (should be 0.6 – 1.5 ohms)
Secondary side of coil - 9.09 K ohms (should be 6.0k – 10k ohms)
Magnetic pickup - 992 ohms (should be 800 – 910 ohms)

So I'm hoping it's the module.

@wsa111,

If looking for DUI replacement parts at a local auto supply, do they need to be for a six cylinder GM car? Obviously the distributor cap would. What about the module, pickup (if purchased separately from module), coil, and rotor? Performance Distributors doesn't even answer their phone on weekends.

Thanks,
Bob

 

[vssman]

DUI told me to ask for parts from a 1980 Chevy van 250 6cyl. For a coil, I went with an Accel as that doesn't matter on # of cyls.

 

[bubba22349]

Bob, did the power valve have raw fuel on the vacuum side of the diaphram??
Did the rubber o'ring seal on the power valve to metering block look intact. Upon re installation of the valve did you put lube on the o'ring to prevent tearing??
Hook your timing light pickup on one plug wire at a time. If intermittent flashing of the light is present on each plug wire then need to ohm the pickup coil.
You can try another module if you have one. If you need to purchase one at a parts store get one for a 78 corvette.
Remove the cap & inspect the rotor for proper fitting & as mentioned before for carbon arcing.
If you can't google the ohm reading for the pickup coil let me know & i will check my old HEI troubleshooting manual.

No fuel on the vacuum side of the diaphram, no sign of any issues with the rubber o-ring.

I found the following on the Performance Distributor web site: http://performancedistributors.com/professional-tips/

I put a multimeter on the ignition components and found the following readings. They all seem within spec to me, although the magnetic pickup's resistance is very slightly high compared to the spec and the coil's primary resistance is very slightly low:

#1 spark plug wire (about 10" long) - 340 ohms (should be 300-350 ohms)
Primary side of coil - 0.5 ohm (should be 0.6 – 1.5 ohms)
Secondary side of coil - 9.09 K ohms (should be 6.0k – 10k ohms)
Magnetic pickup - 992 ohms (should be 800 – 910 ohms)

So I'm hoping it's the module.

@wsa111,

If looking for DUI replacement parts at a local auto supply, do they need to be for a six cylinder GM car? Obviously the distributor cap would. What about the module, pickup (if purchased separately from module), coil, and rotor? Performance Distributors doesn't even answer their phone on weekends.

Thanks,
Bob

Looks like everything checks out except for the magnetic pickup if your volt / ohm meter is a dial type make sure the scale has been zeroed so it reads correctly. With it on the ohm setting touch the two leads together and check that it reads zero, if it doesn't set the dial screw a little until it dose. If yours is a digital that doesn't have an adjustment you might need to calculate how much it is off.

X2 The Modules can interchange check in "wsa111 's" above post he recommended to use a Module for a 78 Corvette, the coils will interchange too. The magnetic pickup needs to be a six cylinder version see "wsa111 's" answer below. Good luck

 

[wsa111]

Bob, did the power valve have raw fuel on the vacuum side of the diaphram??
Did the rubber o'ring seal on the power valve to metering block look intact. Upon re installation of the valve did you put lube on the o'ring to prevent tearing??
Hook your timing light pickup on one plug wire at a time. If intermittent flashing of the light is present on each plug wire then need to ohm the pickup coil.
You can try another module if you have one. If you need to purchase one at a parts store get one for a 78 corvette.
Remove the cap & inspect the rotor for proper fitting & as mentioned before for carbon arcing.
If you can't google the ohm reading for the pickup coil let me know & i will check my old HEI troubleshooting manual.

No fuel on the vacuum side of the diaphram, no sign of any issues with the rubber o-ring.

I found the following on the Performance Distributor web site: http://performancedistributors.com/professional-tips/

I put a multimeter on the ignition components and found the following readings. They all seem within spec to me, although the magnetic pickup's resistance is very slightly high compared to the spec and the coil's primary resistance is very slightly low:

#1 spark plug wire (about 10" long) - 340 ohms (should be 300-350 ohms)
Primary side of coil - 0.5 ohm (should be 0.6 – 1.5 ohms)
Secondary side of coil - 9.09 K ohms (should be 6.0k – 10k ohms)
Magnetic pickup - 992 ohms (should be 800 – 910 ohms)

So I'm hoping it's the module.

@wsa111,

If looking for DUI replacement parts at a local auto supply, do they need to be for a six cylinder GM car? Obviously the distributor cap would. What about the module, pickup (if purchased separately from module), coil, and rotor? Performance Distributors doesn't even answer their phone on weekends.

Thanks,
Bob

Looks like everything checks out except for the magnetic pickup if your volt / ohm meter is a dial type make sure the scale has been zeroed so it reads correctly. With it on the ohm setting touch the two leads together and check that it reads zero, if it doesn't set the dial screw a little until it dose. If yours is a digital that doesn't have an adjustment you might need to calculate how much it is off.

X2 The Modules can interchange check in "wsa111 's" above post he recommended to use a Module for a 78 Corvette, the coils will interchange too, and so should the magnetic pickup. Good luck

Bubba on the DUI-HEI it will need a pickup coil for a six cylinder engine cause the pickup only has six pickup spikes.
Its different than the DS11 where you only have one pickup.

 

[bubba22349]

:duh: I guess had a case of brain fad or my second cup of coffee :rolflmao: thanks for keeping me honest Bill. I will also edit my above answer.

 

[62Ranchero200]

Bob, did the power valve have raw fuel on the vacuum side of the diaphram??
Did the rubber o'ring seal on the power valve to metering block look intact. Upon re installation of the valve did you put lube on the o'ring to prevent tearing??
Hook your timing light pickup on one plug wire at a time. If intermittent flashing of the light is present on each plug wire then need to ohm the pickup coil.
You can try another module if you have one. If you need to purchase one at a parts store get one for a 78 corvette.
Remove the cap & inspect the rotor for proper fitting & as mentioned before for carbon arcing.
If you can't google the ohm reading for the pickup coil let me know & i will check my old HEI troubleshooting manual.

No fuel on the vacuum side of the diaphram, no sign of any issues with the rubber o-ring.

I found the following on the Performance Distributor web site: http://performancedistributors.com/professional-tips/

I put a multimeter on the ignition components and found the following readings. They all seem within spec to me, although the magnetic pickup's resistance is very slightly high compared to the spec and the coil's primary resistance is very slightly low:

#1 spark plug wire (about 10" long) - 340 ohms (should be 300-350 ohms)
Primary side of coil - 0.5 ohm (should be 0.6 – 1.5 ohms)
Secondary side of coil - 9.09 K ohms (should be 6.0k – 10k ohms)
Magnetic pickup - 992 ohms (should be 800 – 910 ohms)

So I'm hoping it's the module.

@wsa111,

If looking for DUI replacement parts at a local auto supply, do they need to be for a six cylinder GM car? Obviously the distributor cap would. What about the module, pickup (if purchased separately from module), coil, and rotor? Performance Distributors doesn't even answer their phone on weekends.

Thanks,
Bob

Looks like everything checks out except for the magnetic pickup if your volt / ohm meter is a dial type make sure the scale has been zeroed so it reads correctly. With it on the ohm setting touch the two leads together and check that it reads zero, if it doesn't set the dial screw a little until it dose. If yours is a digital that doesn't have an adjustment you might need to calculate how much it is off.

X2 The Modules can interchange check in "wsa111 's" above post he recommended to use a Module for a 78 Corvette, the coils will interchange too. The magnetic pickup needs to be a six cylinder version see "wsa111 's" answer below. Good luck

I found an Accel four prong module at a local O'Reilly. After the swap out the car started and ran more smoothly. It still may not run as smoothly as it should, but it's been raining a lot here in Houston, so I didn't have a chance to take the Ranchero out today. I adjusted the idle mixture screws when it was running badly and may need to make more adjustments.

I have to say I'm disappointed in the DUI module quality - failure after only a year and about 1,000 miles.
Thanks
Bob

 

[bubba22349]

I used to like to use the NAPA Echelon Modules they were top quality and back then Accel was also made and owned by them. Don't know who makes the DUI parts. Glad you got it running for sure you will need to readjust the idle mixture screws but that should get it back running right. Good luck

 

[rbohm]

I found an Accel four prong module at a local O'Reilly. After the swap out the car started and ran more smoothly. It still may not run as smoothly as it should, but it's been raining a lot here in Houston, so I didn't have a chance to take the Ranchero out today. I adjusted the idle mixture screws when it was running badly and may need to make more adjustments.

I have to say I'm disappointed in the DUI module quality - failure after only a year and about 1,000 miles.
Thanks
Bob

thats the problem with the GM ignition module. they are all hit or miss as to longevity. the best you can do is install it properly, using the required dielectric grease, and hope for the best. the problem with that module is that it doesnt have enough surface area to dissipate heat properly. as such they tend to run a bit hot all the time. most modules can handle the extra heat, some cant. and it doesnt matter who makes the module either, they are all that way.

 

[vssman]

Did the original module have grease and were the screws tight? The screws were loose on mine and there wasn't a lick of grease under the module. That made me question DUI's quality control.