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How big of a cam is too big for a quadrajet?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Dunc03, Mar 10, 2003.

  1. Dunc03

    Dunc03 1/2 ton status

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    I have read online that quadrajets perform poorely at idle if you have too large of a cam duration. What I'm wondering is what is too large. I have a Gm Crate 350HO with 330 hp. The cam specs are

    Lift (I/E): 0.435"/0.460" Hydraulic
    Dur. @ 0.050" (I/E): 212/222 degrees.

    Is this too large of a cam to have a descent idle with a quadrajet. I know that this cam will not produce a smooth idle, but I think that mine is too rough, and it also seems like it misses at idle. I have heard that you can drill the fule/air mixture holes larger to help with this problem. Has anyone on here tried this technique, and does it work. I just bought a new Qjet and I hate to eat the cost on it, about $380 bucks. Any help would be greatly appreciated. Also I have a spare Qjet that i took off at the advice of a mechanic because he thought it was causing the problem. Anywasy the problem was, and still is, the timing advacne curve. This leaves me with a spare carb to test things out on, so if you have any ideas I'm willing to try them. Thanks in advance
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    With those kind of numbers, I can't see needing to physically modify the carb...I've got a 214/224 474/496 cam on a 403 (yes, bigger by a bit) that has an almost imperceptible lope to it, something that isn't worth messing with.

    You've made sure the idle mix screws are set correctly?
     
  3. Dunc03

    Dunc03 1/2 ton status

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    I have adjust the Idle mixture numerous time and it never seems to help smooth out the idle. The only time it did seem to help was when I was getting an open egr valve at idle. The tempature switch for the egr valve went bad and opened the egr valve at idle. At that point i didn't know it was happening so I was playing with the mixture. I backed out both screws 8 turns and it seemed to run alot better, but as you can imagine I was getting like 6 miles to the gallon. I think the mixture is currently set at about 4 turns out each. How far out have you got your set? I know different engines will have different needs, but your engine has a comprably sized cam so I'm looking for a ballpark figure to judge by. So in essence I'm not sure if the idle mixture is set right or not. Since fixing, eliminating, the egr valve problem playing with the mixture has not had much effect. It runs about the same, at idle, from 3-6 turns out. The exhaust actually smells richer the fewer number of turns out. I have to replace the distributor to fix my timing issue before I can determine what it is doing, fuel wise, above idle. The carb i have is off a 86 with a 350, and is the one with the dual capacity pump. maybe that will help in any diagnosis. I found the article I was refering to earlier here Qjet tips. I think you might have been the one to point this article out to me when I asked a question about the dual capacity pump. Anyways thanks for the help and any more you provide in the future.
     
  4. 84_Chevy_K10

    84_Chevy_K10 Banned

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    That cam should produce a relatively smooth idle.
     
  5. Doug M

    Doug M Registered Member

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    Yes, these mods work well. However instead of drilling out the idle channel restriction I chose to drill out the idle tube orifice size. http://www.500cid.com/mts/tech/Quad.htm Go here scroll down to where it begins "Whenever engine mods are added that adversely affect idle" I also drilled out the idle discharge ports but found that you will not be able to kill the engine with the screws turned all the way in because the threads on the af screws bottom out. I also got a thread die and added 3 or 4 more turns on the threads.

    All in all yes these mods increased not only idle quality but responsiveness. I only have an RV cam and used 48M rods, drilled out the idle tube orifice to 0.037, idle discharge to 0.089. Got the DR rods at a Chevy dealer and G hanger through Jegs. I have noticed considerable improvements, it worked well for me. The 0.037 drill I obtained at Bass Tool here in Houston. It is 2 inches long. I got 2 and would be glad to send you one, they're not easy to find. Send me a pm. I'm done with the thread die as well. It's a funny size, 4.5x5 IIRC.
    One more thing I found a local shop that rebushed the throttle shafts for $35. All of these are worn out by now and will provide air leaks and lean conditions.

    One more thing, I don't profess to be a QJet tech. A year ago I would have shied away from even attempting a rebuild! Just sharing with you my experiences and IMHO success.
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    Almost sounds like you are running way rich for some reason...screwing the mix screws in all the way should kill the engine as I recall. Anymore, I start with mine only about 2-2.5 turns out after a rebuild, although originally I was turning it out 4 to start with.

    Something you may want to check more in-depth is that dual capacity pump. I did a short write-up on that from my GM service manual on this board, doing a search for dual capacity will probably bring up a couple of my posts, one of which describes how it works. If it's working incorrectly, I believe it could dump more fuel into the carb than you need.

    Still don't think physical modification of the idle circuit should be necessary in your case, something else seem to be going on here. As you lean or richen the idle mix, do you adjust the timing and idle speed to match?

    First thing I'd do though is understand what the dual capacity pump does, and rule it out. Worst comes to worst, you can always find an earlier, $5 Q-jet to rebuild to replace the dual capacity one.
     
  7. Dunc03

    Dunc03 1/2 ton status

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    I read your original post on how the dual capacity pump works. The dual capacity pump on mine is functional, but I have yet to check the ground circuit to see if it is working correctly. I tried to order a new one to take it out of the equation, but it is back ordered and ungodly exspensive. 60 buck for something that just grounds a circuit. Anyways I think mine is working properly, but I have yet to test it with an Ohm meter. I'm nt sure how exactly to test it so I have to wait for my grandfather to show me how. If I do find out it is bad I'll just ground the circuit so that it is always on and effectively bypass the dual capacity pump. It will hurt me on starts in the winter, but there's not alot of winter in Texas anyways. I think for now I will steer clear of the drilling until I eliminate the air/fuel mixture and timing issues. I have not been adjusting timing in conjunction with the air/fuel misture. I have left the base timing alone and only adjusted the air/fuel mixture and the idle speed. Would I need to readjust timing if I changed the air/fuel mixture? I have it set to the manufacturer's spec of 10 degrees BTDC at 700rpm's. It has not change since adjusting the air/fuel mixture. The mechanical advance doesn't kick in till above 1100 rpm's, so I can't see that adjusting the air/fuel mixture would effect it until it reved above 1100. In that case i could just readjust the idle speed screww to lower the idle back down. If I'm going about this all wrong please let me know. Thanks again for all the help guys.
     
  8. HarryH3

    HarryH3 1 ton status Author

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    I haven't messed with a carb in way too long... But does a Q-Jet have a power valve like the Holley carbs have? A stock power valve can be tricked into dumping extra fuel into the engine at idle when you have a cam that doesn't provide a good idle vacuum. On the Holley carbs you can get power valves that open at various vacuum levels, to prevent this from happening. Just a thought from a guy that has been running EFI since 1989 or so... /forums/images/graemlins/wink.gif
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    No power valve...at idle with most engines (GM DID run Q-jets with no primary rods at all lol) the engine is running off of the idle circuit, which is just vacuum through ports.

    The dual capacity pump was an attempt to get around some cold engine problems, but it was certainly nothing more than a crutch until they could (were forced?) to fuel inject the trucks.
     
  10. HarryH3

    HarryH3 1 ton status Author

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    The Holley carbs are supposed to only run on the idle circuit as well when the engine is idling. But then when the vacuum drops to a calibrated level, the power valve opens to provide additional fuel on accelleration. But a lopey cam can fool the valve into opening at idle and causing a very rich idle. Does the Q-Jet just deal with this by using the tapered metering rods to control how much fuel is introduced, based on throttle position?
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    Q-jet uses vacuum to keep the primary metering rods "in check".

    The problem with people "fine tuning" the Q-jet is that the power piston spring (which is what forces the primary rods to the rich position when vacuum drops) is different among different applications.

    I personally haven't messed with it either, but I'm sure some change can be had that way as well. Just another aspect usually not thought about when changing cams/vacuum.

    With low engine vacuum, you'd need to change to a *weaker* spring, so that vacuum hold's the primary metering rods down when they should be. But in this case, that cam should produce plenty of idle vacuum to keep the primary rods/power piston fully seated in the lean position.
     

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