Cliff's Quadrajet Parts and Rebuild Kits
General Category => Quadrajet Carb Talk and Tips => Topic started by: Cfrench192 on July 14, 2016, 01:41:34 AM
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So I have a 6000 lbs truck. When I put in better gas, the problems go away. My camshaft has 250 degrees duration at 6 degrees . I fail smog for hi nox. (Double the limit). New cat. 3 inch single exhaust. Lots of air flow. How can I modify my carb. Holley rebuilt m4me. I believe 71 jets with 51 m metering rods. It has a cruise/ off idle stumble when I'm cruising around 15-25 mph. On the freeway, if I floor it. All I hear is a bag of marbles. Please help. It has an electric edelbrock 120 gph fuel pump. The air fuel gauge almost always reads 17.2. Around town and on the free way. The timing is happy usually around 2-4 btdc initial and 91 octane. This is all with the a.i.r. Pump functioning . Is there anything I can modify or should I try to modify. (Idle channel restriction???)
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EGR valve working? Ports clear?
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Yes. It's a universal egr with no restrictions ( no orfices). The motor has swirl port heads and runs great. It seems like the camshaft requires more fuel. At cruising / part throttle.
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What are the other emissions readings; HC and CO.
Also, how have you verified the EGR is working; ie...applied vacuum to valve to stall or stumble engine?
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Vacuum pump on egr stalls engine. Hydro carbons 31 ppm at 15 mph and 21 ppm at 25 mph. Co % is 0.00 for both speeds. Nox is at 3292 at 15 mph and 1996 at 25 mph. Co2 % 11.1 and 10.6. %o2 is 4.9 and 5.5
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Ok, the bag of marbles situation sounds like your pinging under load due to ignition timing or a lean condition which explains the HI Nox numbers.
What is the factory timing spec? 0 degrees?
Do you have oversize wheels and tires on that rig? I've seen them cause Nox failures before...too much load on the engine.
I would put a vacuum gauge in line with the EGR vacuum line and stick it on your windshield and see if you are actually getting enough to pull the valve open.
Cliff should be able to tell you if your carburetor is too lean, that also would explain the HI Nox.
What year is the truck/engine?
Those emissions readings are actually quite good, with the Nox exception.
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Ok so the truck is a little bit lifted with larger than stock tired. It has a th350 transmission with 4.10 gears in the axle. The truck is a 1986 c20 3/4 ton. The motor was out of a 93 suburban. It is a 5.7. I had to put in an electric fuel pump because there was no mechanical provision in the block. The factory spec for this year of truck is 6 degrees. I'm at like 2-4 to prevent misfiring. Total I only get like 20- 22 w/ vacuum advance. From what I have read. These carbs were set up lean for emissions purposes. This is what I believe my problem is.
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Definitely sounds like the motor is looking for more fuel.
Hopefully Cliff will chime in with a calibration recommendation for the carb. Also, post the carb number so he knows what you're running.
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17085000 is the carb number. You may be correct about insufficient vacume to egr. At these rpms the egr only gets a couple of inches of vacuum. I have a theory that if I open up some restrictions for the idle circuit, more hydrocarbons in the combustion chamber will help cool the motor down a little bit. As far as coolant temperature my Guage only reaches about 120. I have a really big radiator I'm this thing.
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I'd wait to see what Cliff says about that carb, and then I'd focus on the EGR vacuum source issue. Does it come from a ported vacuum switch or an EGR solenoid vacuum switch.
Either way, I don't think that valve is lifting enough to allow enough exhaust gas into the combustion chamber.
What T-stat temp are you running?
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the first thing you need to do is get the temp up . make sure the gauge is right and get it to 195. it doesn't matter if you have a ocean for a radiator , if the thermostat works it will hold it at 195 and that should help with your problem . fwiw.
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It does have a 195 degree thermostat
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does the gauge work . do you have a temp gun to check it . if you are only running 120 degrees you are not burning the fuel and causing terrible wear to the engine.is it a new thermostat.
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Other thermostat is new. I have an electric fan on my radiator. The truck gets up to 195 around town. On the freeway, it stays cold because of the 3 core radiator. On the sniffer machine the truck gets up to 195 because it doesn't have any wind blowing through the hood. I would also like to add that when the truck is on the sniffer, I can here it off idle stumbling, it sounds like a misfire. What I am trying to do here is get the internal combustion temperature lower than 2,500 degrees by adding more fuel to the off idle cercuit. I think I'm going to try cliffs 1st recipe in his book. I'm pretty sure the hot combustion gas is worse for the motor than the low coolant temperature ( on the freeway), where it idles around 3,200 rooms. If I were to start at 0 mph and accelerate to freeway speeds gently, my air fuel gauge reads 17.2. All the way up to freeway speeds. I also have to take into consideration that the air pump blows air into the exhaust system. All I am trying to do add more fuel for my camshaft. My idle screws are like seven turns out. Almost all the way out. When idling in park. The air fuel also reads 17.2. The jets and rods are perfect for the carb. Somewhere in the idle channels and passageways , there is too much air. This carb was set up for a stock 350. My heads have higher compression because they are swirl port, the intake is an aluminum edelbrock performance series, and my camshaft is a medium sized cam . The motor is starving for fuel. Fuel filter is new. No clogs in the fuel system.
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I think you may have a couple of issues, one being a fuel starvation problem, and two, not enough manifold vacuum for the EGR system due to the camshaft your running.
You can try the fuel solution first, but those NOx numbers were HI.
Let us know the outcome.
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I can get 20 inches of vacuum with this cam at idle. It's around 15 inches to prevent missfiring. The cam was chosen by a machine shop. The machinist knew that this truck needed to pass smog. The cam he chose for me was a cam that was able to pull the necessary vacuum in the manifold. That's why I'm believing that this is a fuel issue. The carb I bought from summit may have been calibrated for a stock 350 cam. However I really have no idea. The truck hates hot days. It loves cold days.
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What are the cam specs, advertised, .050", lobe separation, etc?
Difficult to provide specific advice on commercially "remanufactured" carburetor, because they "butcher" them up and change most of the items involved with the calibration.
One BAD habit I see with them is opening up the lower idle airbleeds to HUGE dimensions then putting in huge idle tubes to compensate.
71 jets would be pretty small for on of those carburetors, but I don't know the MAB size it may have been changed from stock?.....Cliff