Vacuum to fuel vapor can

Engines, Exhaust and Fuel Systems

  1. mgbeda

    mgbeda Member

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    Hi folks,

    The biggest problem with Marty ('87 5th) is it idles really rough. Right now I'm checking for vacuum leaks. I've been disconnecting and plugging one line at a time hoping I'd find one that made the idle smooth out. The weird thing is that there are three hoses leading to the fuel vapor can, two big hoses and one small hose. There is no noticeable vacuum in either of the big hoses. There is vacuum in the small hose. When I say no vacuum I mean I take the hose off the carb, plug the hole at the carb with my finger and feel nothing. That seems wrong. Shouldn't at least one of those two big hoses really, um, suck?

    Marty has a Holley 6280 2 bbl with all the electric controls. I can't remove or simplify anything because this is California and cars only 33 years old aren't smog exempt.

    Thanks,

    -mB
     
  2. Camtron

    Camtron Well-Known Member

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    I could be wrong, but I believe there should be two hoses off of the carb that go to the canister, one of them should have vacuum the other is a bowl vent tube. Theres a third hose that goes to a vacuum temperature switch that’s on the intake manifold. There should also be a fourth hose on the canister that would be a vent hose back to the gas tank.
    So again, I THINK there should be a total of 4 hoses at the canister, 2 of which should have vacuum; I think lol one of the other guys will likely know for sure. Just going off what was on my 89.
     
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  3. mgbeda

    mgbeda Member

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    Yes, there are 4 to the canister. I was only counting the three to the carb. The small hose to the switch has vacuum. It's obvious which one is the bowl vent. So the big hose from the bottom of the carb to the canister should have vacuum, huh? Well it doesn't. What could that mean?
     
  4. Camtron

    Camtron Well-Known Member

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    Did you only check that carb port for vacuum at idle? I think that it may only have vacuum off idle like the vacuum advance for timing. Trying giving ‘Marty’ some gas while you check that carb port, if you haven’t already. I bet you’ll feel some vacuum on your finger with the RPM.
     
  5. Aspen500

    Aspen500 Well-Known Member

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    The purge hose (small hose) between the canister and carb is ported vacuum and shouldn't have vacuum at idle, only part throttle like the vacuum advance, as Camtron thought. The big hoses are fuel vapor vents to the canister and will never have vacuum.
     
  6. Camtron

    Camtron Well-Known Member

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    Lol I’ve never even worked on a complete lean burn system lol
    Most things I’d say to check/adjust for a rough idle won’t make a difference with the lean burn trying to correct/compensate. So, I bow out with: is your EGR valve the factory stock unit? If so, it may not be open/closing all the way and is messing with the lean burn brain box. I also just assume the car has a good set of plugs, wires, cap, rotor ect...
     
  7. Aspen500

    Aspen500 Well-Known Member

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    Good call Cam. Make sure your EGR valve is closed at idle. Try unhooking the hose to be sure there's no vacuum at idle. If the vacuum amplifier is bad, it could be sending vacuum to the valve when it's not supposed. You may need to remove the valve and visually check that it's not mechanically stuck open, or has a chunk of carbon holding it open. I remember seeing that many times back when cars all had EGR valves.

    It all depends on what you mean by rough idle. Is it rough like a miss or rough as in erratic? A miss at idle but not at other times is most likely not an ignition problem. I mean, it could be but, it'd be down the list a ways. A burned valve can cause an idle miss but smooth out with higher rpm, as can a broken valve spring.

    Erratic idle comes back to an air (vacuum) leak, stuck open egr, problem with the carb idle circuit (misadjusted, plugged port, etc). A vacuum leak will only affect idle, not pt or wot, unless it's huge and then the car wouldn't idle at all. Try blocking the brake booster hose to eliminate a possible booster diaphragm leak. If you have speed control, check the vacuum hose from the booster check valve to the servo. Leaking intake gaskets are another possible.

    Those are all just ideas on things to look at and is by no means a complete list of possible causes. Hope it helps at least a little bit.:)
     
  8. mgbeda

    mgbeda Member

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    Thanks for the good info. I did only check at idle, so I will try revving it while I check the bottom big hose.

    The EGR does open and close. There is no vacuum to it at idle and if I apply vacuum the plunger moves and the engine dies. But it might not be 100% closed. I will take it off and check it.

    I did the usual tune-up first (cap, rotor, plugs), Need to double check wires. I did check the vacuum to the brake booster; no difference if I plug that hose.

    I would call this erratic; not like a constant miss on one cylinder. The RPMs surge up and down by ~200.

    Would a too-heavy float cause this?

    Thanks again,

    -mB