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Old 11-22-2020, 09:52 PM   #1
rpmerf
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Running rough

1970 Chevrolet C20 Custom Camper 350 Quadrajet, points ignition.

I'm half asking for help / suggestions, half pouring out my thoughts.

I have a wideband with the sensor after the driver's side header. I normally read about 14.6-14.8 at idle, lean out to as high as 15.5 at cruise, and hit somewhere around 12.5-13.0 at WOT.

Drove the truck to my parent's earlier today, and it was fine. When I left, it started out fine, then after about 2 minutes, it started running super lean. Anywhere from 16-19:1 at idle and cruise. As I opened the throttle, it would lean out more. I'm surprised it was still running at that ratio. The odd thing was, if I got the secondaries to open, it would go to about ~13.0:1 and drive fine. I got home mostly by cruising super lean, then going WOT any time I needed to accelerate.

I poked at it for a couple minutes, but didn't really find anything. Accelerator pump looks to be working fine. Checked for vacuum leaks at a couple places, but didn't find anything. Putting the choke on manually results in about 14.5:1 AFR. Choke full on normally gives closer to 12.5:1. I have a vacuum gauge on my wideband, teed into the vacuum advance line. It was reading about 15" at idle in park, which seems low. Pretty sure that is typically more like 18-21".

It seems almost like I am not getting fuel from the primaries. I would normally blame fuel delivery, but the secondaries seem to work fine. I don't expect an ignition issue, but it is always possible. I would think an ignition issue would have me running rich on the AF gauge. It's always possibble the AF gauge isn't reading correctly, but what the gauge reads, seems to me in line with the engine not running right.

I had the top off the carb a couple weeks ago to change out the primary spring, but i have driven it probably 50 miles since then, and it has been fine. This truck has been my daily for about 2-3 weeks, since I am doing suspension work on my Suburban.

Another thing - Last week when I got to my parents house, it was idling around 1300 RPM in park. I poked around a bit and it went back down to around 900 in park / 650 in drive. No idea what caused that. This has me thinking vacuum leak somewhere.

Plan for tomorrow:
Verify every vacuum line and vacuum device using a vacuum gun
Start it up and spray the carb to manifold and manifold to head areas with carb clean to attempt to find a vacuum leak.
Check timing
Fuel pump flow test
Change carb filter (last changed about a year ago. Maybe 500 miles since)
If I don't find anything else, pull the top off the carb to see if I see anything suspect.

Any other thoughts / ideas?

Thank you for reading
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Old 11-22-2020, 10:22 PM   #2
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Re: Running rough

What was the ambient temp when it acted up?
High or low humidity?
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Old 11-22-2020, 10:35 PM   #3
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Re: Running rough

50's maybe? Low humidity. Maybe a 15 degree difference from when I went to my Parent's house, and when I left. This seems outside the range of change due to temperature and humidity. I've gone years without even changing the idle screws for winter / summer
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Old 11-22-2020, 11:19 PM   #4
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Re: Running rough

Cool enough can cause fuel puddling in the intake. That’s why you showed lean. Less fuel vapor getting burned. Getting into secondaries increases air flow which pushes excess fuel thru. Runs better then starts to puddle again.
Around that temp of 45 to 50 it will do that.
Your strange high idle could also be from excess fuel suddenly getting burned. Intake might have finally warmed up at that point. Vac leak usually doesn’t add that much rpm.
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Old 11-23-2020, 09:45 AM   #5
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Re: Running rough

If that was the issue, I would think it should had started running better once the intake got up to temp. I drove home about 5 miles, with a bit of idling at lights, so the intake should had been warm by the time I got home. It is the original intake. I'll keep it in mind, but I think there is something else going on.
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Old 11-23-2020, 11:12 AM   #6
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Re: Running rough

Might need to check distributor internals too.
Sticky weights, broke spring pins maybe.
Stock motor should have near 20hg vacuum.
Is yours stock cammed?
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Old 11-23-2020, 05:28 PM   #7
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Re: Running rough

As is standard, it drove fine today. Started it up and it sounded good. Let it idle til the choke came off. AFR looked good. Drove it to the store and back, about 2 miles each way.

Idle in park - 23"
Idle in drive - 18"
Choke on AFR - 12.5-13.5
Choke off idle AFR ~14.85. Getting colder out, so this is expected.

The vacuum is measured off my wideband. I rigged together a 2 bar MAP sensor I had laying around and scaled the output voltage to inches of vacuum, so they might be off. Reads 2 with the engine off, so they are likely closer to 21 / 16. The MAP sensor is teed into the vacuum advance line.

It has a 184/194 cam, dished piston, 80's heads, about 9.5:1 CR.

I set the timing a couple weeks ago, but that doesn't mean it's fine now.

I'll poke around a bit, but it's hard to troubleshoot when it is running good.

Thank you for your help!
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Old 11-23-2020, 05:38 PM   #8
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Re: Running rough

I started thinking - A vacuum leak should not have it running this bad. Last year, I changed the base gasket, and realized I had been running with the wrong base gasket for ~8 year. The base gasket that was in there left a decent size vacuum leak. Even with that leak, it still ran good.

I looked through my post history to figure out when that was. June last year. Truck was running bad which had me looking for vacuum leaks in the first place. I wonder if it was the same issue I had last night.
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Old 11-24-2020, 03:51 PM   #9
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Re: Running rough

Its possible the primary metering rods were stuck down. You can check them by putting a small screw driver down the vent hole. When it contacts the holder it should bounce up & down with light pressure. Coarse if it is running right now it must have came unstuck. If it happens again that would be the first thing I would check.
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Old 11-30-2020, 02:36 PM   #10
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Re: Running rough

Shouldn't the power piston and primary rods be all the way down at idle and light throttle up until enrichment starts?

Driven the truck a bunch of times since I started this thread. Still running perfect. I hate intermittent issues like this. At least I got this thread to reference if it comes up again.
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Old 11-30-2020, 05:50 PM   #11
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Re: Running rough

[QUOTE=rpmerf;8842814]Shouldn't the power piston and primary rods be all the way down at idle and light throttle up until enrichment starts

Exactly, I was thinking running this test with the engine shut off.
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Old 12-03-2020, 11:29 AM   #12
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Re: Running rough

Happened again today. Needed to run into the office to grab a new badge. First time I have been to the office since mid-March. ~30* when I left this morning. Did great for about 5 miles. I did notice the AFR starting to get into the mid 16's at cruise, but I figured that was due to it being cold out. Overall, the truck was running great. I had a large downhill I went down in 2nd, and a large uphill that I hit the secondaries on. When I got to the top of the hill, it started running bad again.

Limped it to work. I was in there about 20-30 minutes. Come out and it's still running bad. Turned the idle screws out half a turn each, and that didn't seem to richen it up. Didn't have time to mess with this more, otherwise I would had kept turning to see what happened. I used a wire to hold the choke closed, and that got me to a good AFR. Mid 14's at idle. It did enrich to mid 13's at part throttle, so that verifies the primary piston is moving. Even though the AFR was good, it still ran horrible. I had trouble getting over 40mph. I only got about 2 miles before I went WOT again, which pulled off my choke wire. Drove the rest of the way home bouncing between running horrible and WOT. WOT felt good, but I don't know that it was as good as it normally is, or just good relative to how it was running.

Back at work now, so I can't troubleshoot until later today. I'm hoping it is still having issues when I get off work.

This is feeling more like ignition. I managed to get the AFR about correct with the choke, but it still ran bad. The idle vacuum seemed low, so I likely didn't have as much vacuum advance as I normally would, but I still have around 16* base timing.

I'm looking through a Quadrajet Issues document by Lars Grimsrud. Lots of good info, but it's hard to determine if any of these are my problem with an intermittent issue.
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Old 12-03-2020, 11:50 AM   #13
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Re: Running rough

Still seems to be weather related to me!
Burn us off a pic of your engine bay so we can see your set up.
No rush.
When you got time.
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Old 12-03-2020, 12:15 PM   #14
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Re: Running rough

Yes, does seem to be weather related. It seems like it is fine above 50*, but starts having issues below that. The oddest part for me is how it was fine for 5 miles, then started run bad like I flipped a switch. When I started it up again, it started fine. Not like it was starving for gas. I didn't need to pump the gas.

I'll grab a pic at lunch.

I do have an open air cleaner. I was thinking I might switch back to the factory air cleaner. It has headers, so I would need to rig something up there for the hot air intake.
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Old 12-03-2020, 12:20 PM   #15
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Re: Running rough

Off the cuff guess is carburetor icing. Often happens with temps in the 30~60*F range and higher humidity. The OEM answer was the THERMAC system. Since you mentioned headers, likely this isn't installed/working anymore. THERMAC is the system that pulls hot air from the heat stove on one of the exhaust manifolds. If it is icing, possible the ice buildup is restricting the air flow in those small center venturies and causing your lean condition.
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Old 12-03-2020, 12:36 PM   #16
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Re: Running rough

X2 on the icing.
Takes a bit of driving sometimes to have it start icing up.
High humidity doesn’t always cause icing. When I lived in a colder and lower humidity part of the country it was a common occurrence for me.
Usually just shutting it off allows enough heat to rise thru the motor to clean it out.
One of the easier tricks to help was a heated plate under the carb. Kinda like what some fords used. Just need to re-route the heater hoses.
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Old 12-03-2020, 12:37 PM   #17
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Re: Running rough

I was wondering if something like that was happening. I was thinking if that was the issue, after sitting for 20-30 minutes with a hot engine, it should clear out, if only for a little bit.

I do have the 1/4" carb gasket, which insulates the carb from the intake more than the thin paper gasket.

I'll try to get the original air filter assembly connected, and see if that helps
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Old 12-03-2020, 01:11 PM   #18
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Re: Running rough

I’ve made header heat stoves from sheet metal duct parts found in the plumbing/heating/air cond section in hardware stores. Some come with a round stub that’s just the right size to fit a stove tubing piece also.
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Old 12-03-2020, 02:10 PM   #19
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Re: Running rough

Took pictures and started it at lunch. Sounded rough, but I didn't drive it. It would barely idle. At that point it had been sitting about 3 hours, and it was 60* outside.

Ignore the mess of wires in the background. That's mostly for the wideband. I'm still using the factory ignition wire.

My vacuum plugs are vacuum lines with .177 or 1/4" BBs in them.

Let me know if there are any other angles you'd like, or something for my to specifically focus on.
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Old 12-03-2020, 02:24 PM   #20
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Re: Running rough

How’s it run with the air cleaner off?
I’ve had the air cleaner short out the coil before.
Have you checked the distributor internals?
I’ve had the point plate come loose and move up/down changing the points gap.
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Old 12-03-2020, 02:48 PM   #21
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Re: Running rough

Runs the same with the air filter off. I haven't gone around the block with it off, but idling in the driveway, it sounds the same and the wideband reports the same.

I poked around in the distributor at some point. I think the day after it was running bad before. Everything looked to be in order. I set the dwell about a month ago. If it's still running bad when I get off work, I'll hook up the dwell meter and the timing light just to be sure everything is in order. I'll poke around in there again. Use a vacuum pump on the vacuum advance to be sure it all looks like it is moving correctly.
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Old 12-03-2020, 04:14 PM   #22
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Re: Running rough

When I found the point plate loose I could grab the points and pull them straight up.
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Old 12-03-2020, 05:22 PM   #23
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Re: Running rough

Done some poking. Still running bad.

Pulled the distributor cap. Points seem solid. Vacuum advance moves back and forth smoothly using a vacuum pump.

Started it up. Dwell seemed pretty solid around 30-31. Worked the vacuum advance with the vacuum pump and the dwell didn't move. Timing base is around 16-17*. I was at about 1K RPM, so base might be a little lower. Vacuum advance adds about 12*. Currently at idle, I was around 24*. Hooked the timing light up to each wire to confirm that it is firing. At least there is power going through the wire, it could be shorting somewhere.

Playing with the choke, even with the AFR showing correct, it still felt rough.

While I was working, I kept hearing a rattle that was driving me crazy. It was the AC bracket to the intake (factory AC). I checked the bolts, and a good bit of them were loose. I tightened them down, but it didn't make a difference. Now I wonder if I should be looking at new intake manifold gaskets. I might even have a set in my gasket pile.

I'm wondering if I should try swapping out the coil.

Thinking I'll pull the plugs and see if anything stands out. At this point, I'm looking for any hint or clue that can point me toward something odd.
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Old 12-03-2020, 06:07 PM   #24
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Re: Running rough

Pulled the coil. Spec is like 1.24-1.46 on the primary and 7.2k-9.5k on the secondary. My coil was more like 0.7-0.8 on the primary and 5.5k on the secondary. I grabbed the coil out of my Daytona, and that was in the spec range.

It runs better, but still not perfect. I went around the block. It is close to stoich at idle, around 15.0. It was leaning out a bit during cruise, around 16.8. I don't know if I ever got into primary enrichment.

Overall, a lot better than it was doing before.
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Old 12-04-2020, 12:32 AM   #25
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Re: Running rough

That bit about the intake bolts kind of raises a red flag. Mine sucked oil when they went bad and my plugs got fouled. I could see oil wicking up my intake and tightening the bolts didn't help me. In fact mine even puffed blue smoke when I left a stop light. Don't know if that helps, but that's the symptoms I had with my intake gasket problems.
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