Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: 87mcss on March 21, 2017, 01:19:43 PM
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I have a 400 sbc engine. I put a 17059508 Q-Jet on the motor as I upgraded the cam heads and intake. I went through the carb and made some adjustments per Cliff's book. The engine runs ok and the cam break-in was fairly uneventful. The engine revs up and the high idle seems fine, but the idle is nonexistent. I can't seem to dial in the idle mixture screws whatsoever. It doesn't seem to matter if they are 3 turns out or 12 turns out. It BARELY want to idle, somewhere between 400-500 rpm. Cranking idle speed screw does very little as well until you finally get it so far open that the main metering kicks in and it's humming along at 1300 rpm.
Here is what I have done to the idle circuits:
Idle tubes: .039
Idle down channel: .059
Idle bypass air: I forgot to write down but I'm thinking it's between .106 and .110
Mixture screws holes: same here but I'm thinking about .090
Upper and lower idle air bleeds: .070
I've tried adjusting it every which way I can think. I've tried to add timing and that made no difference, overall the engine seemed best with about 15° initial advance. Since it is just stumbling along I'm concerned that the power piston is up and making this even more complicated. Now, the cam in this motor is a little wild... okay it's probably too wild, admittedly. It's 235/243 at .050" w/ 109LSA. I know Cliff will hate this cam. So a rough, smelly idle is expected but I'm just trying to get it to idle at all. As I said as of right now no matter of adjusting the idle screws seems to make a lick of difference once they're open enough to keep the motor running. Where should I be looking?
Thanks!
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I'm not going to be much help as I'm still learning myself, buy some of the guys will want to know what compression you are running and maybe what intake. Do you know what the camshaft part/grind number is?
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About 9.4:1 compression. Edelbrock Performer RPM intake. Cam is Howards 118041-09.
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Have you tried more initial timing? Probably doesn't have much vac. Just keep upping the initial & see if it helps.
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Yeah, adding timing seems to have little to no effect either. I got as far as making it hard to start because it had so much timing but it still wouldn't idle with a lick.
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What heads/piston used on the 400 SBC and what is the compression ratio?....
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Stock pistons, 64cc chamber heads. approx. 9.4:1 compression.
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I'm not sure if it's my whole problem but I definitely think the power piston spring is too strong. It is fully lifted while the enginet is trying to idle. Is modifying the spring advisable or do I need to purchase a selection?
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Just for giggles you could pull the power piston spring out and put the piston and metering rods back in. Then try running it. That would tell you if the spring is the problem, but I'm thinking you need less camshaft, wider LSA or more compression.
I looked the cam up and its 297 duration seat to seat, I think it would be happier with closer to 10.5:1
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Shouldn't affect the idle at all unless fuel is pulling out of the boosters. How much vacuum are you getting?
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Not much. About 5" but again, it's struggling to idle at 400 RPM. At an 800 RPM idle it would obviously be better. At high idle, while warming up, it's humming along at 15"-16" or so.
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The cam you have will need about 8-900 to idle decent.
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Which is what I was shooting for.
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I decided the engine was starving for more air and fuel. I opened up the idle down restrictions from .059 to .064, the idle mixture holes from .099 to .104, and the idle bypass from .101 to .109. It is MUCH closer now. I can get it to idle out of gear around 750 but that is still with too much throttle angle. At this point it really seems like it wants more bypass air. That's gonna be my next step.
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Still think it needs timing. How high have you had it?
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Yeah, I've been playing with the timing. Now that the motor is getting fed, it's responding better. I've got it at 22° initial, 35° total right now, and she's running rather nicely, but I'm thinking that's a bit much initial. I'm gonna play with that some more.
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Something you should check, get a positive piston stop to check real TDC. I found that my balancer has slipped, and didn't know it till after I changed intakes and had to retime the engine and couldn't get it right. My timing tape was off by 10 degrees. Found the new tdc to get it going and just have to get a new balancer now
https://m.youtube.com/watch?v=PD0ZRc7Dy28 (https://m.youtube.com/watch?v=PD0ZRc7Dy28)
Link should take you to the video.
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I checked that while I was assembling the engine. If it's off, it ain't by much.
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Do you have vac. advance hooked up? Manifold or ported?
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It's hooked up to manifold. It's running much better now. I've now opened the idle bypass up to .112. It idles ok, but it is still taking a lot of throttle angle to get it there. Out of gear it's in the 900s and in gear is around 650. Sounds MEAN! Throttle response is very nice. Pulls great, lights up the tires. But as I said it needs a bit much throttle angle to get the idle where I need it and the mixture screws have little effect once they're open. 1 turn or 10 turns doesn't sound any different. It sometimes diesels when I shut it off, so I feel like I need to find a way to get a better idle with less throttle opening. More bypass air? The timing is still set at 22° and I think that's about as much as it will want.
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How much advance do you have at idle with vac. adv. hooked up?
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I don't remember exactly. I want to say it was around 30-32°.
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Yeah, the 22° initial is too much. It kicks back against the starter sometimes. When I start it cold it is WAY too rich. It needs to be cleared out before or will idle well.