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Old 10-05-2009, 08:52 PM
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Jet Performance Q-jet tuning

I have a 72 Chevy 350 that I put on an alum EGR intake and a Jet Stage 2 Q-jet. When I first installed it it idled fine but fell flat on its face and died when I floored it taking off so I made few changes. I changed the secondary rods to DAs and the hanger to a G. I read a G and DRs are a very good combination but didn't have the DR but DA is leaner than the ones that were in it. It still fell flat on its face but I don't think the choke was all the way open locking out the secondaries. After driving a bit it was performing better but not great so I changed the vacuum can to one that is a lot quicker. That didn't work and created a bog at the transition. I also tried disconnecting the squirt pump and there was not really a lot of difference. Could have a bad pump but it seems to pump fine and I can see a mist of gas coming out of the horn. I also set the AV spring to 7/8 turn. I don't know what the primary rod and jets are. Somebody from Jet could probably tell me. I know longer have the little afterfire when I let off the gas I had with the other Q-jet I had on it. I don't know what to do next. I suppose a new squirter. I have a old stye performer coming that I will install incase there is a problem with the stock manifold. I didn't find any oil in the intake so I might not have a problem there.

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Old 10-05-2009, 09:52 PM
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Matching a carb to a performance motor can be tricky. Figure out how many cfm you are pumping with your motor, and then get the specs for the carb. If the carb is too big for the motor combination, then you won't have any luck with any combination of parts. If you have a stock configuration, then get the parts list for your motor (by serial #) and start with that. You did mention "mist" coming from the acceleraetor pump, well there should be a solid stream of fuel coming out into the throat.
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Old 10-05-2009, 10:02 PM
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The mist was coming out of the top when I hit the pump when it is not running. So it seems to me that I am getting a good squirt from the pump. BTW my vacuum reading was 19 at idle and when I hit the gas it went below 10 and when I put the pedal to the medal I went to near 0. I was getting much more vacuum with the Edelbrock and very rarely got to 0 at full throttle unless I was under a load pulling a trailer.

Anyone want to trade a stage 2 for a stage 1?
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Old 10-05-2009, 10:13 PM
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Quote:
Originally Posted by Dogbreath077
Matching a carb to a performance motor can be tricky. Figure out how many cfm you are pumping with your motor, and then get the specs for the carb. If the carb is too big for the motor combination, then you won't have any luck with any combination of parts. If you have a stock configuration, then get the parts list for your motor (by serial #) and start with that. You did mention "mist" coming from the acceleraetor pump, well there should be a solid stream of fuel coming out into the throat.
Q-Jet is a variable cfm carb up to 750-800 cfm.

I had mine rebuilt by a professional shop in CA to my engine specs. It runs like I have FI.

Dogbreath077, you can not have a properly set up Q-Jet that is too large for the engine!
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Old 10-06-2009, 10:53 AM
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I wouldn't be messing with the secondary rods or hangers if I was tuning off-the-line performance. You won't be in the secondaries at that point.

I would start with the basics. In no particular order:
Can you look down into the primaries (with the choke blades open) and see the accelerator pump squirting fuel when you move the throttle? You should get a good stream.
Have you correctly adjusted your idle mixture screws?
Have you checked for vacuum leaks?
How much fuel pressure do you have?
What is your timing?
<<EDIT: While a bit more invasive, check your float level.>>

FWIW, the stage II qjets (per JET) are good for cams of up to 220 duration at 0.050". I'm not sure what they lower limits are, but my stage II was not that far from stock when I dissected it.
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Old 10-06-2009, 11:05 AM
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I tried a Jet q-jet and it was terrible. sent it back 3 or 4 times and never got it to run. stage 1 or stage 2 didn't make a difference. Don't know what the hell jet does to their carbs but it doesn't work. They should at least try one of their own carbs first before selling that junk. That's just my opinion with jet q-jet carbs.

get a new 3310 holley if you want a carb that works right out of the box.
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Old 10-06-2009, 11:18 AM
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Your the second person in a week to get a jet carb that had problems from the git go. The other guy's secondary throttle shaft holes in the base plate were worn out creating a noticeable vacuum leak. Kind of say's something for the quality of workmanship, huh? He had to pay shipping to send it back for repair. If your engine is stock, an edelbrock performer rpm and a 4160 holley with a 1" 4 hole spacer would have been perfect for your engine.
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Old 10-06-2009, 12:02 PM
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I put a jet stage 2 on my son's camaro 355cid z28 and had great response. you probably need to lean the apt valve on the carb- the instructions were in the box with the carb manual! there is a Allen plug on the top of the carb just in front of the choke that you remove, it sounds like it needs leaned out. I have had no problems whatsoever with mine after adjusting the idle mixture screws and rpm, that camaro will run like a raped ape in comparison to the 1406 edelbrock that we had on it before!
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Old 10-06-2009, 12:10 PM
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The 1st thing I would do is contact jet performance! I am a firm believer in tech dept contact with the manufacturer before taking everyone's "expert advice" on someone else's product. you paid a good $ for the carb, CALL THEM!!!
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Old 10-06-2009, 01:37 PM
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Quote:
Originally Posted by streetrodderbn
The 1st thing I would do is contact jet performance! I am a firm believer in tech dept contact with the manufacturer before taking everyone's "expert advice" on someone else's product. you paid a good $ for the carb, CALL THEM!!!

yes, I did that. They kept sending me very weird running carbs. I got tired of sending carbs back and forth and gave up. Ended up using my old q-jet that I rebuilt myself for about a year, then switched back to my holley.
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Old 10-06-2009, 02:02 PM
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Timing hasn't change from when I had the Edelbrock on it. 14 inital, 37 with mechanical and 55 total at 3000.

I have looked to see the squirt but can hear a good squirt and see vaporized gas coming out of the horn when the engine is not running.

Idle mixture screws are set for highest vacuum and are 3 turns out. I am at 19" at idle. I read some where I might want to lean them out for better throttle response. Take them down to where it just stumbles.

To lean out the APT do you turn it out or in? Someone suggested I turn it out 1 turn.

I took my pressure regulator off but the guage did not work any way. Stock mechanical fuel pump with no signs that it has too much pressure.

I couldn't find any vacuum leaks. I suspected the intake manifold at the interior but don't see any oil in the bottom of the intake. I am getting a new manifold.

I assumed that the float was correctly set by Jet. In fact I thought everything was set by Jet except the mixture screws. That doesn't appear to be the case.

I called the Jet tech and they wondered why I was putting a stage 2 on a basically stock engine. I bought it with very little use and at a good price and heard that the Jet carbs were top of the line.
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Old 10-06-2009, 05:30 PM
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The vacuum at the base should be 21-22in on a stock 350. Is 19 the best you can do? You've got it too rich or too lean either way you need to find the happy spot by watching the vacuum gauge and not counting "turns".
turn the screw in till the rpm drops, and out to the best reading. Sometimes you have to do it 2 or 3 times to get the highest . Might want to check the PVC valve also. It works on the basis of a controlled vacuum leak and can effect the idle air /fuel ratio. I haven't heard anybody say anything about that yet. But here's probably something I've forgot too.
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Old 10-06-2009, 11:29 PM
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19 is the best I have ever achieved with both the Edelbrock and the Q-jet. I think I could get more at sea level but I consider this pretty good for 2500 ft. The PCV valve appears to be good. It is one of those billet units I believe Edelbrock makes I got at a swap meet. I was working fine with the Edelbrock carb. I am still trying to figure if I am lean or rich. There are times when it does smell rich. I am going to try to lean with the APT. I am also going to try to lean out my mixture screws by turning in until it just starts to run rough. 3 turns out was my base but I also got highest vacuum. The plugs don't indicate rich or lean.
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Old 10-07-2009, 08:28 PM
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I talked to the Jet tech guy who was very helpful. He told me the jets in a stage 2 are 74 and the rods are 41. He also said I could turn out the APT up to 3 turns. So now I believe I have been on the rich side. I did several tests this evening. I turned out the APT a total of 7 turns (The tech guy said base was 5 so I turned it all the way in then out 7) I also adjusted the idle mixture screws down to where it would just start to stumble and then opened them up a bit. It rev'd great in the driveway and idled fine at 900. I was off the idle speed screw before making the adjustments and now I had to turn it in to stay at 900. Vacuum was a great 20. I took it for a test drive and the off the line is much better. It no longer dies when I floor it. It takes off fine and accelerates fine until I get up to 45 which is when the secondaries start to kick in. There is a bog at that point but when I get above 50 to 55 it takes off again. I can hear the sound of the secondaries working. I also tried it with the pcv valve disconnected the port plugged and it might have been a little better but not very noticeable.

I have the secondary air valve spring at 7/8 turn with G hanger and DA rod. I might try putting the AU rod back in it or go to a even leaner rod. I also turned out the APT one more turn but didn't have time to test tonight.

I am making progress. I think I need to get a better transition into the secondaries and I might have it. A little better of the line wouldn't hurt either.
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Old 10-12-2009, 01:42 PM
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I have turned out the APT 2.5 turns and it made a big difference. It no longer falls on it face but still has a bog between 35 and 45 mph. I adjusted the secondary air valve to 1/2 turn and it made a small difference. I reinstalled the AU secondary rods. I come off the line great and above 50 it accelerates great. It is just very sluggish at 35 to 45. I also reset the secondary spring to 1/2 turn from 7/8.

I also did a test with the accelerator pump disconnected and it ran about the same as with it. I also noticed some fuel at the top of the pump rod. I am going to change the pump. It must still be running rich if it doesn't need the pump to get off the line. The plugs are dark brown.

Lars Grimsrud has told me that with a modified carb there is no baseline to start from and I need an O2 sensor to get it right. I might have to get a bung put in so I can install a sensor.
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