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De-smogged 350 detonation

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by zombie, Oct 3, 2001.

  1. zombie

    zombie Newbie

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    I replaced the entire motor in my 85 Blazer about a year ago with a 350 from Jasper. Lately I started having carb troubles, so I decided to just yank all the smog do-dads (most of it was unplugged anyway) and get back to basics (inspections are not an issue). Anyway, I now have an Edelbrock carb and a set of headers and all of those ugly vacuum hoses and pumps are gone. EVERYTHING in the engine compartment except for the intake is now new stuff. I had to block off my EGR valve to get the carb on. Now I have a detonation problem I can't get rid of. I've installed colder plugs and jetted the carb richer, but I still can't make it go away even if I unplug the vacuum advance and set the timing to 2 degrees AFTER tdc.
    I've ordered Edelbrock's EGR adapter, which will hopefully allow me to reinstall the EGR valve. So then my question is what do I connect the EGR vacuum to? I'm thinking the timed port on the carb, but maybe that's not right. Can anyone give me a little insight into this problem? Please spare me the "You'll burn in smog-control hell" sermon.
     
  2. BurbinOR

    BurbinOR 3/4 ton status

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    Its the missing EGR causing this detonation in my opinion.

    When you get the EGR adapter it should tell you to where to take the vacuum source from.

    <font color=blue>'79 ONE TON TPI K5 - See it at---&gt;<a target="_blank" href=http://www.blazzinor.rockcrawler.com>www.blazzinor.rockcrawler.com</a>
     
  3. 84_Chevy_K10

    84_Chevy_K10 Banned

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    You have too much mechanical advance it seems, or they gave you the wrong harmonic balancer or timing cover and your timing is running way more advance than you think.

    My engine is totally de-smogged except the PCV valve and it does not detonate a bit.

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
     
  4. dragricinMT

    dragricinMT 1/2 ton status

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    What kind of gas are you running? Premium or the low grade? Might be a combination of things. Taking the egr and smog off won't cause detonation by itself. Try running premium gas and possibly an octane booster plus set your timing.

    78-K5 4"Lift 35"Tires 400SB TH350 NP205
    73 Vega 350SB TH350
     
  5. HarryH3

    HarryH3 1 ton status Author

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    Figure out how to use the EGR valve or back your timing off or get used to buying higher grade gasoline. [​IMG] EGR lowers combustion temps at part throttle, which greatly reduces the tendency for detonation. Without the EGR in place you have to come up with another way to reduce the detonation.

    <font color=black>HarryH3 - '75 K5</font color=black>
    <a target="_blank" href=http://www.angelfire.com/super/ThunderTruck>www.angelfire.com/super/ThunderTruck</a>
    It's a great day to be alive...
     
  6. zombie

    zombie Newbie

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    I didn't have a detonation problem before I changed the carb and removed the EGR, so I'm pretty sure having no EGR is the problem. As I said, even with timing at 2 degrees AFTER tdc I still have detonation. The only schematics I've found for vac hose routing to the EGR don't help because none of the other smog controls are there anymore.
    I get the Edelbrock adapter tonight, so I'm just gonna try connecting the EGR to the carb's timed vacuum port. It's my understanding though, that EGR does NOT need to be open when:

    a) The engine is cold
    b) The engine is warm, but at idle
    c) The engine is warm, but at full throttle

    Which is why I asked the question to begin with. I've still got a vacuum solenoid thingie sticking out of the coolant inlet neck, so I may see if I can route the EGR vacuum line through one of those if I can tell which combination will provide the functionality I described above. I'll let you guys know what I find.
     
  7. Burbinator

    Burbinator 1/2 ton status

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    Hose routing for the EGR should be : Valve to TVS (on coolant inlet), TVS to carb base. Make sure that all unused ports on the carb and the TVS are plugged. Sounds like an EGR problem to me. Good luck...keep us posted.

    - Alan

    '91 2500 Suburban Silverado 4X4
    '82 K10 Suburban Silverado (FOR SALE!)

    Always do sober what you said you'd do drunk...that will teach you to keep your mouth shut!
     
  8. KShortell

    KShortell Registered Member

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    Get a timing light that lets you measure advance- something like a ditigal or dial-iin analog unit. You need to check timing at around 3,000 rpm to see what kind of full advance you are getting. I had this problem in my boat (GM motor BTW). The advance mechanism's stop had worn allowing 42 deg of full advance when timing was set at 8 deg. BTDC. It'll also help you see if your vacuum advance unit is working. You should see a difference with the vacuum connected vs. disconnected.

    BTW, had to recurve my distributor to solve my problem- a roll pin fitted to the adv. stop pin with an interference fit...

    Hope this helps...

    1986 Chevrolet C10 Suburban Silverado
    Stock 350 w/ K&N Air Filter & Flowmaster Exhaust
    Edelbrock Performer IAS Shocks (These rule!)
     
  9. zombie

    zombie Newbie

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    Well the EGR isn't it. Using Edelbrock's adapter, I installed a new one tonight. That little learning experience alone cost me over $100. I'm running out of options here. I've tried almost every combination of timing and fuel that I can think of, but still no luck. I'm beginning to think I must have a vacuum leak on the intake that only opens up when the motor gets warm, cuz the bastid runs fine with no detonation when it's cold. As it warms up in the morning, it goes through a spell of coughing and sputtering as I drive to work which eventually gets better as it reaches full operating temp (that's what prompted the carb upgrade in the first place). I'm gonna get a stethoscope and listen around the area very closely to see if I can confirm this, because I really don't want to yank that thing off.
     
  10. Burbinator

    Burbinator 1/2 ton status

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    How's your cooling system and thermostat? Try pulling the thermostat out and see if it still detonates when warm.

    - Alan

    '91 V2500 Suburban Silverado
    '82 K10 Suburban Silverado (FOR SALE!)


    Always do sober what you said you'd do drunk...that will teach you to keep your mouth shut!
     
  11. 84_Chevy_K10

    84_Chevy_K10 Banned

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    I think when you changed the carb, you hooked your distributor to full vacuum where it used to be hooked to ported vacuum, and that's why you have detonation, not because you don't have EGR.

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
     
  12. dyeager535

    dyeager535 1 ton status Premium Member

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    So you *aren't* having problems once it warms up? Only the transition from cold to operating temp period? Or are you saying that the coughing/sputtering alone goes away when it warms up, and the detonation is totally seperate issue, that never goes away?

    Dorian
    My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</a>
    No anti-theft measures on your truck? No pity when its stolen
     

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