Which vacuum lines are a must on my q-jet?

Discussion in 'The Venerable Q-Jet' started by sean Buick 76, Apr 4, 2005.

  1. sean Buick 76

    sean Buick 76 Buick Nut

    This is a unknown year cheby j-jet on a stock 81 Buick 350. It has been fully rebuilt a few thousand miles ago. The idle air screws are set at 4.5 turns out and the vacuum fo the brakes, trans, and vacuum break are hooked up It runs fine when driving but idle speed needs to be set high (800+ rpm) to idle smooth in gear. The timing is set at 12 deg with vac. advance plugged and total timing is 32 deg at 2200 rpm with crane adj. advance and yellow mechanical springs. No smoke out exhaust and exhuast doesn't smell rich.

    I need to know which ports I need to hook up and which ones are just for smog stuff. I will get it all sorted out later on but for now I need to make it run ok. Suggestions?
     
  2. jamyers

    jamyers 2 gallons of fun

    (i really need coffee...) Without knowing which Q-jet you've got, it's hard to say exactly, but as a minimum, you'd need the dashpots, PCV, vacuum advance, and should be able to block everything else off. Sorry I don't have a picture handy, but you could search the board and prolly find one.

    Anyway, looking at it from the front, there should be a LARGE port at the lower left, that's for the pvc hookup. Above that, often at the top of the main body, is a port that is full vacuum all the time, lotsa folks run that to the distributor vacuum can. On the right side of the front, there are 2 (?) ports, one is "ported" vacuum, which means no vacuum at idle, vacuum when the throttle blades are open. Most late-model engines were intended to use this for the distributor (lack of advance at idle = hotter engine = reduced emissions), but lotsa folks switch to the full vacuum. The other one on that side I forget (???).

    On the back, there are 1 or two that may go to the tranny and vacuum-operated stuff like a/c doors, or not, depending on the model.

    On the passenger side, the 1 or 2 vacuum dashpots need to be hooked up to the nearest vacuum ports, they should be pointing right at the ones.
     
  3. sean Buick 76

    sean Buick 76 Buick Nut

    I was planning to use the Thermal Vacuum Valve/Switch off the manifold for the dist. vac. advance. I've tried both the ported and non ported off the carb but I have heard I would need to recurve my dist. to make it happy with anything but the stock (Thermal) conection. I'm sorry I don't have the numbers off the carb, I am out of town right now. I just can't stop thinking about getting it to idle and run right. Please forgive my ignorance about this subject, I am going to get the carb book suggested on other threads.

    More info: Removed and cleaned tank recently, replaced fuel sock, blew air through fuel lines, new filter, new air filter, oil change, no vacuum leaks.

    Thanks James, Sean.
     
  4. Nicholas Sloop

    Nicholas Sloop '08 GS Nats BSA runner up

    You SHOULD re-curve your distributor. :Smarty: Not hard to do, and critical to getting maximum performance out of your motor. Lots of info on doing that as a sticky in the Buick FAQ forum.

    http://www.v8buick.com/showthread.php?t=63475
     
  5. jamyers

    jamyers 2 gallons of fun

    Although the idle screws don't have a "right" number of turns out, 4.5 seems to be a bit much, most I've dealt with like about 1.5 to 3 turns out, depending...

    If I remember, that thermal switch blocks all vacuum until the engine warms up, since retarded timing = more heat, then it would open and allow for the ported vacuum (only advance above idle). Not sure why that would have an impact once the engine is warm...

    Might look down the carb primaries while it's idling, see if you see fuel dripping out of the venturis. If so, that's your problem. Too much fuel from somewhere, and you've got to open up the throttles to let more air in, which screws up the idle mixture, which means you're in a problem spiral. The solution is to coax the idle speed adjustment down, while re-setting the idle mix screws.

    Hmm...Get the Doug Roe book as you plan, then get a carb kit ($12-15), and then get a clean spot and pull and rebuild it, paying close attention to the float height and other settings.

    Got me wondering, too! :bglasses:
     
  6. sean Buick 76

    sean Buick 76 Buick Nut

    Thanks again for all your help James. I'll look into your suggestions next time I have a chance. The carb was just rebuilt so I shouldn't need to do it again, but I see your point about the float level. I'm really busy with my schooling at BCIT up here in Canada but I am trying to get my car ready for summer work further up North. Nick, I will get the dist. recurved but not till motor rebuild time. Thanks, Sean.
     
  7. Nicholas Sloop

    Nicholas Sloop '08 GS Nats BSA runner up

    I re-read the initial post, and have another thought. Since it is a late motor it has an EGR valve. Is it connected at all? Your problem may be that the EGR valve is slightly open at idle. If it is connected, try disconnecting it.
     
  8. sean Buick 76

    sean Buick 76 Buick Nut

    The EGR is not connected. There is a bit of smoke out the EGR so I left it unhooked. I thought it would be bad for the motor. I tired shaking the EGR to see if it was working and I can't hear anything. Should I ? Anyone have a part # for the EGR valve on a 350? I also need the thermal vacume switch that threads into the intake any #'s?
     
  9. Nicholas Sloop

    Nicholas Sloop '08 GS Nats BSA runner up

    That doesn't sound right...
    Out of where? The vacuum nipple? The vent holes in the bottom of the canister?
    Either way, it does sound like it is stuck open, and maybe has a hole in the diaphram too. Get a new one. Should be maybe $40 at the local auto parts store. Hook it to a ported vacuum source, if you don't have access to a diagram of the original hose routing.
     
  10. RAbarrett

    RAbarrett Well-Known Member

    One other thing I recommend is to bypass the thermal vacuum switch. That will allow full vacuum advance when cold, which will provide better driveability, possibly less stalling, and possibly less fuel from the pull-off and choke, saving fuel. The smoke from the EGR concerns me. It is possible that the gasket is leaking, vacuum leaks, or is is simply oil in the exhaust being pulled into the intake from valve seals, etc. It is also possible that the diaphragm on the EGR valve is leaking, causing a vacuum leak. The EGR is connected to ported vacuum isn't it? It should be. Exhaust leaks and the resulting dilution will require additional fuel at idle, as will incorrecty adjustments. Be sure that when you adjust the mixture, the timing and idle speeds are correct, then adjust for maximum idle speed, and no richer. Let us know... Ray
     
  11. sean Buick 76

    sean Buick 76 Buick Nut

    Oh sorry guys its the pcv valve that has some smoke coming out. I've been told it's because I didn't use a belly pan when I changed the intake it didn't smoke with the other intake. I'm going to fix that asap. The EGR is also disconected so I need to find a ported vacume source, on carb? What problems would this cause? If I don't use the Thermal switch were should I get the vacume advance from? car aswell. Sorry for the mix up, and thanks for all the help. So I think I need a diagram of the whole vacuum system. Any ideas on were to find one?
     
  12. RAbarrett

    RAbarrett Well-Known Member

    Many aftermarket manuals have diagrams for the vacuum routing. I suggest not connecting the EGR unless it is necessary for emissions testing, as it typically causes part throttle surging, and tip-in sags. Normally the porterd vacuum ports are for the cannister purging, timing controls, and EGR. I suggest connecting the cannister purge through a temp controlled valve, but the timing controls should be direct as driveability will improve. PCV should be connected to the large port, typically in the front. If using the heated air cleaner, the straight vacuum source will be connected to the valve on the case, and the other port should go to the valve on the snorkel. Typically, the only other indicated vacuum controlled system would be the exhaust crossover, for heating the intake manifold, which can be ignored, as the heated air cleaner will suffice for this. Cannister purging and the timing controls should be the only ported systems. All of the other controls require manifold vacuum sourcing. Please ask questions if they will clarify efforts. Ray
     
  13. sean Buick 76

    sean Buick 76 Buick Nut

    That helped a ton, I got a Haynes manual that shows were they all go so that also helped. Everyones input has been greatly apreciated! So were exactly should I hook up the vacuum advance? is there a non ported connection on the carb? How do I check which are ported?
     
  14. Nicholas Sloop

    Nicholas Sloop '08 GS Nats BSA runner up

    Easiest was is to simply un plug it and put your finger over the port at idle. No vacuum at idle but lots of vacuum when the motor is reved means ported. You could also do the same with a vacuum gauge, if you have one.
     
  15. sean Buick 76

    sean Buick 76 Buick Nut

    Thanks. So should I advance or retard the initial timing from 12 deg. or should I add a stop to the vacuum advance to limit that. Will I get more or less advance from the ported?
     
  16. jamyers

    jamyers 2 gallons of fun

    The terms can be misleading, but the only difference between "full" and "ported" vacuum is only in when it's available. "Full" vacuum is available "full-time", and "ported" vacuum is only when the throttle blades are opened slightly. typically, full vacuum is taken from below the throttle blades, and ported comes from a slot right above the throttle blades when they're closed. Neither will give you more or less vacuum once it's available.

    I'd set it at 12 initial timing, then drive it and see if it pings, then go from there. Read up on recurving ignitions, that's a whole nother topic...

    Hope this helps!
     
  17. sean Buick 76

    sean Buick 76 Buick Nut

    I will get the dist. recurved when I get my 76 350 rebuilt. Untill then I'll try ported and non ported at compare the total timing with initial mechanical yellow springs, and the crane vacuum advance adjusted differently with both ported and non ported. Which is best for a stock low compression 4bbl. 350? Should I limit the vacuum advance with a pin as Larry has suggested?
     
  18. sean Buick 76

    sean Buick 76 Buick Nut

    I know this may sound like a weird question but... When my 350 4bbQ-jet is idling and I am checking my initial timing I disconect the vacume advance from the carb. The idle speed slows down a bit but idles fine. But idle gets rough when I plug the vacuum output from the carb with my finger. it seems to run better with the vacuum advanac conected somewere else and the vacumm output left open and sucking air. Whats going on? I'm lost.
     
  19. jamyers

    jamyers 2 gallons of fun

    Sounds like your vacuum advance can (or maybe the hose) is leaking air...

    Unhook the vacuum advance, your idle should drop something like maybe 100rpm, but still be steady. Bet yours isn't going that far down because the vacuum can isn't putting all the advance in it should.

    the idle mix is set to compensate for the leak, when you plug it, the mixture goes too rich, and the idle gets rough.

    Hook a hose up to just the distributor vacuum can, and suck on the end. It shouldn't leak any, if it does, you need a new can.
     
  20. sean Buick 76

    sean Buick 76 Buick Nut

    I tried what you suggested and it does hold. Its a 6 month old crane adj. canister and I put in the yellow (early mech) springs in the dist. So I'll go back to the idle mixture screws, they are at 4.5 out right now. I've read on here how to set that. I'm still confused about the vacuum thing, I'll read your post over.
     

Share This Page