Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: Seff on February 16, 2015, 12:21:10 PM
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Car: 72 Cutlass
Engine: Olds 380 stroker 9.5:1, roller cam, Ebrock performer RPM intake, stock 7a heads with 2" intake valves and a bowl blend port job. HEI off a 403 Olds connected to ported vacuum, set to 19 btdc initial timing.
Carb: 17057253 rebuilt for the old engine, a stock '71 Olds 350 8.5:1. I've just fitted it with an electric choke.
Symptoms:
- Engine won't idle, but will run at 2500 RPM on the top step of the fast idle cam.
- On the second step it runs, but stumbles.
- Off the fast idle cam it dies immediately.
- The GEN light remains on even at 2500 RPM, and the electric choke is slow to open, if at all. (suspect the field terminal is loose, will re-tighten).
- Backfires up through the carb when cranking.
I suspected that the idle mixture was too lean for this engine, so I turned the screws out - no effect. Haven't moved ATP, but that should't hamper idle quality.
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What are the specs on the cam? Was the cam degreed? Did you check for vac. leaks?
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(http://i.imgur.com/8Wa62s6.jpg)
The cam was degreed by the builder. Didn't check for vacuum leaks, but I'll hook up a vacuum gauge.
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Pretty hefty cam for the little Old's 380 at 9.5 to 1 compression. The carb will need idle system modifications to make it happy at low rpm's, but that is a nice carb for what you are doing. I use a near identical Pontiac unit on my 455 over 550hp.....Cliff
https://www.youtube.com/watch?v=6zVdoLR-VzM
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That's a cool video for sure!
I double checked, the final calculated CR was 9.3:1, not 9.5:1. Sorry for the confusion and misinformation. Originally and ultimately the engine is built with EFI in mind in conjunction with a TKO 500 and a 2.56 rear, which is what the cam choice is based upon.
I have the book, is here a recipe in it that describes the idle system modifications needed? It's back at my shop, so I can't check it at the moment.
Thanks.
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My engine builder says to specify that it's a 221/228@.050 on a 112lsa. That shouldn't be so extreme?
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Chased vacuum leaks today, every intake bolt got tightened about a quarter turn. Still rough idle.
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Like Cliff said, you are going to need idle circuit modifications to get a proper idle without nozzle drip.
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Well, I have two carbs, so I might as well. Can you tell me what kind of modifications, or what page of the book shows this kind of modification?
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I would read the chapter on how they work, then go to the high performance chapter for specific guidance.
Bigger than stock cams reduce signal (engine vacuum) at idle and low rpm's and require minor idle system modifications so the engine can get enough fuel from the idle system to make it happy.
I'd look at recipe #3 for idle fuel, for what you are doing.
If you haven't done so I would also install one of our kits, and I can supply tuning parts, jets, metering rods and PP spring, plus sized idle tubes for what you are doing.
I'd also make sure that you blocked off the vacuum supply to the choke housing when the E-choke was installed, or there is a vacuum leak at that location.
Also vacuum test the pull-off, and time it. It will need to fully release in less than 2 seconds for what you are doing. We have quick release pull-offs available as well...tks.....Cliff
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Gotcha, will start reading.
Have a HP kit in one of the carbs already, rods and jets as well. Sadly can't remember which.
Choke housing is NOT blocked off, just sucking air. Will fix that immediately. Or are you talking about the port at the rear of the air horn?
Choke pull-off will be checked.
Thanks!
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Choke housing itself. The hot air choke pulls air/vacuum from the carb body. When converting yo electric choke, that vacuum passage needs to be blocked.
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Welded shut an old tube for the choke vacuum, car runs much better at high rpm. Will try recipe #3 on the 77 carb now.
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For reference, it's 74 jets and D4 primary rods.
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What's the cranking compression? Let us know how it turns out with the other tweaks.
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It runs pretty well with the mods to the '77 carb. Allows it to idle decently if not super smooth.
Now I have a strange sound, though. Comes from the front of the engine, it disappears (but may still appear) at moderate throttle but is there at idle. I disconnected PSP, ALT and water pump, and it's none of those three, as the engine makes the same sound without those connected.
My old engine had the same noise after a timing chain change, and I assumed it was due to the cheap timing set. But nothing other than the carb carries over to this engine that hasn't been ruled out already in the above paragraph. So I'm perplexed, and of course worried that it's a rod bearing or something.
Video of the noise: http://youtu.be/2usEjphiR-w
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Turned out to be the hose running from the valve cover to the carb baseplate. Replacement eliminated the sound.
The engine runs at 180-190 F when cruising, starts decently, but idles a little rough, at least until it's warm (crossovers blocked, so that's to me expected). When cruising it gets very "loud" and sounds like it's sucking a lot of air at anything below 1100-1200 RPM. A lot happier at 1500 RPM. Downshifting in all gears results in choppy motions at around 1100 RPM, like the engine's stalling (which I guess it technically is, when you downshift hard enough?).
Thinking about getting a wideband O2 sensor to aid me in tuning.
The only modifications I made to the 77 carb was drilling out the upper idle air bleeds as prescribed by recipe #3 - everything else was already up to size or bigger than suggested.
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I'd look at recipe #3 for idle fuel, for what you are doing.
I take it this means I shouldn't worry too much about the rest of recipe #3? I did the idle system mods, and the only part I identified that wasn't on spec for a #3 was the main body main air bleeds, which are close to 0.030" than the prescribed 0.070". Should I go ahead and open them up?
As for roughness and vacuum leaks, I went over the choke assembly again and noticed that the housing was out of round, meaning the E-choke didn't hold air. So, I glued the little plastic tube shut instead, fixed the problem that way.
I drive the car mainly between 1000 and 1500 RPM, should I still be adjusting my APT at 2200 RPM?
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I'd leave the MAB's alone.
When converting to E-choke, the vacuum to the choke housing must be blocked on the suction side, as the e-choke grounds directly on the housing without a gasket......Cliff
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I'd leave the MAB's alone.
When converting to E-choke, the vacuum to the choke housing must be blocked on the suction side, as the e-choke grounds directly on the housing without a gasket......Cliff
So you'd ONLY modify the idle system? Well, then I'd better bring the rest of the carb back to specs. I modified everything to third recipe specs.
Right now the fuel economy is pretty lousy - could that be because I'm working at too low RPM? What kind of vacuum should I be expecting at idle with that cam?
Thanks.
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That cam should idle with a min. of 14 inches maybe even 15.
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Gotcha, thanks. I'll let you know what it idles at.
When should I be considering idle bypass air? The book says "larger camshafts", but doesn't specify how large.
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The engine idled at 18-19" when I put on the gauge, and now idles at 19" after adjustment. Are you guys SURE this engine needs that many carb mods? I had a vacuum leak back when I initially tried to start it up, mind you.
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What's your idle rpm? Seems like a lot of vac for a 221 cam.
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It was 900 at that time. Any lower and the engine shakes and gets rough. Or, it did. I've since added idle bypass air (0.080" both sides), and have yet to calibrate the idle mixture and idle speed since.
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Is your timing stable or does it drop off below 900 rpm?
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Ugh, forgot to check timing. Anyway, dropped it to 800 RPM and vacuum decreased to 17-18.