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  1. #1

    Default 351w Idle Issues

    I have a '71 351w with a lunati bootlegger cam, Holley 80458 4160 600cfm carb, Stealth intake, MSD Igniton, headers, the usual bolt on stuff. Motors stock otherwise. I got the engine pretty well tuned up where it will idle very well and runs decent, doesn't load up. I'm having issues with the exhaust stinking and burning my eyes at idle.
    I am tuning with vacuum gauge and I have an AFR gauge installed, just calibrated(in open air per innovate). I have my idle set to roughly 550-600rpm. Initial timing is 14*. The cam is pretty radical, making only 8-9psi of vacuum, so I changed from a 6.5 to a 4.5 power valve. I have glass sight glasses in my carburetor, so the fuel level is 1/4 of the way up as per Holley. Setting idle mixture to highest vacuum and then trying to tune with the AFR gauge keeping everything as even as possible. Engine will stall when idle screws are turned in any more than 1turn from bottom, they're about 2-2.5 turns out. It is reading between 12.5-14.1 at idle. I tried tuning it higher and lower. Still stinks to high heaven. I am aware that a lean condition can cause exhaust to smell as well as rich, but i am not sure which way to go. Reading the plugs, #1 is ok #5 looks a bit rich. I also tried adjusting the initial timing between 12-16* but didnt see much of a change. I definitely notice changes in engine performance while adjusting the idle mixture but I can not get rid of the gassy haze coming out of the tail pipes, Im not sure if my wideband gauge is lieing to me or I have something else going on. Any input would be appreciated, thanks.

  2. #2

    Default

    I've got a couple ideas, but they require opening the carburetor up for a look around at what parts are in it and some measuring of orifices. The problem with all newer (and even brand new) carburetors I have had opened up (and had to fix) is that they are a goofball calibration, (lack of) quality control, and sometimes awesomely (sarcasm) even include machining debris that blocks things... all for the good money you paid... a nightmare in general that needs to be fixed right off the bat to just FUNCTION somewhere near right. Are you up for removing it and opening it up? There really isn't an external-only fiddling with settings answer to the possible reasons for the problems you're having...
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  3. #3

    Default

    As well...

    Regarding the following: "The cam is pretty radical, making only 8-9psi of vacuum, so I changed from a 6.5 to a 4.5 power valve. I have glass sight glasses in my carburetor, so the fuel level is 1/4 of the way up as per Holley. Setting idle mixture to highest vacuum and then trying to tune with the AFR gauge keeping everything as even as possible. Engine will stall when idle screws are turned in any more than 1turn from bottom, they're about 2-2.5 turns out. It is reading between 12.5-14.1 at idle. I tried tuning it higher and lower. Still stinks to high heaven."

    IMHO, especially with a radical enough Windsor that only generates 8-9 inches Hg of idle vacuum, much more carburetor should have been installed. Regardless, with a 351 size engine and a borderline size carburetor, I wouldn't dare install a 4.5"Hg power valve, that may well begin closing at the top end (a very VERY bad potential WOT lean situation) due to the unnecessary vacuum generated by the engine size and the small carburetor. By the way, power valve activation point has absolutely nothing to do with what happens at idle or off-idle. You want further elaboration about that, ask away.

    The ability to stall the engine with idle mixture screw adjustment is a good sign that they are functioning, and still for the most part in control of the idling engine, but when you're at 2-2.5 turns out for an acceptable idle and AFR, something is very wrong... either the idle/transition circuit is way too lean (I seriously very highly doubt that... all the recent carburetor "designer" morons do is make all vehicles use far more gasoline than necessary, always), or there is restriction of idle air bleeds, and/or blockage or partial blockage of idle fuel inside the carburetor. (See debris above...)

    No matter what you do and it still producing eyeball-burning stink out the exhaust tells me that there's too much idle fuel coming from somewhere at the same time as it's obviously not providing enough idle fuel (2-2.5 turns out... 1-1/4 to 1-1/2 turns out with the idle mixture screws is ideal) for the radical and ratcheting action on the idle circuits by the camshaft/overlap. Idle feed restrictions located up high by all of the "new and improved" morons also serves to make this varying idle AFR and stink-rich idle and low-speed phenomena even worse, by allowing idle air bleed air to on-and-off get behind the idle feed restriction, playing hell, never allowing a stable and predictable and effectively adjustable idle mixture. I don't know if your carburetor has the idle feed restrictions up high or not, but idle feed restrictions BELONG submerged, in the back face of the metering block where they mostly always were, so as to not get affected by especially radical-cammed engines producing ratcheting action on the idle circuit, sometimes getting effed up with idle air bleed air, sometimes not.

    As for idle especially, don't put too much faith in whatever the wide band is telling you. There's far too much chaos going on with a radical camshaft. You need to tune the idle (and low speed and light part throttle acceleration and cruise fuel mileage later if you wish to optimize the transition circuit and primary jetting) by what the ENGINE (and how it runs) tells you. Ignore the vacuum gauge and WB, readjust the idle mixture screws to 1-1/2 turns out, start it up and warm it up and see how it idles. If it won't, back them out 1/4 turn and try again. When you find the point where it will idle on it's own when warmed up, turn each idle mixture screw slowly in until the engine slows/roughens (so prior to the stalling points you found earlier), and then back the screw back out 1/4 turn, and that's normally damn close to best. Set the other the same way...

    Anyway, there's my continued rant for saying what I said above. Seems like you've got most external remedies to try covered, as to float level and idle mixture etc... so, I believe you're going to have to remove it to see and report back how much primary transfer slot is showing with the choke and fast idle fully off and the carburetor set at the base idle with the idle screw, and maybe go inside too, and do things like verify passages behind sized air bleeds are actually bigger than the air bleed, as well as look for and remove potential debris blocking orifices/passages, etc...
    Last edited by Walking-Tall; 10-02-2018 at 04:03 PM.
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  4. #4

    Default

    Any progress with this?...
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

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