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Thread: Percolation...at what temp?

  1. #1
    BlackoutSteve Guest

    Question Percolation...at what temp?

    Hello, I have a 454 with a 1000HP annular (80514-1) on a Victor Jnr intake with a 108-70 heat shield and the thick 3/16" gasket..
    The pump is a Holley HP150 with a Holley 12-704 reg mounted where the mech fuel pump would normally be. I have lowered fuel pressure from 7psi to 5psi in an effort to help the issue, along with lower the float levels 1/8" below the sight plugs, but no change.
    The engine has a 180°F thermostat and sits on 180°F all day whether in traffic or out on the highway.
    When the engine is fully warmed up and has sat in traffic for a minute or so, light throttle response is not very good and it stumbles. If let to sit and idle and then removing the air cleaner lid, quite a lot of vapor can be seen exiting both the primary and secondary vent tubes as if on the verge of boiling over. "Simmering" the fuel if you will.. When a finger is placed over the tubes momentarily, fuel spills out of the boosters as well as the air bleeds and naturally, that stalls the engine. When the engine is cooler and running smoothly, covering the vent tubes has no effect.

    I once had severe percolation where the fuel would boil over and stall the engine while idling in traffic, so I know what percolation is. However, given the components I currently have, I am surprised to see a mild form of it happening now.

    I guess my question is, at what temp is percolation expected to happen? Is what I have percolation in the first place?
    With my infra red temp gun, when percolating, the Holley's fuel bowls are ~135°F and the factory brass fuel distribution block, just before the carb is ~155°F. To me, that shouldn't be any real problem. I expected symptoms of percolation to be around 180°..
    It's just turned spring here (Australia) and I am wondering if it's the lower Reid Vapor Pressure of the fuel at this time of year.

    Has anyone got any advice or perhaps tell me what is going on. Hopefully I have over-looked something, or perhaps misdiagnosed the problem.
    Thanks a heap.


  2. #2
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    Quote Originally Posted by BlackoutSteve View Post
    When the engine is fully warmed up and has sat in traffic for a minute or so, light throttle response is not very good and it stumbles.
    You're right, this indicates the carburetor (or the fuel) is overheating due to a hot engine compartment.
    Just curious, what's the ignition timing advance at idle? Is it a vacuum advance distributor?
    If so, is the vacuum hose connected to the timed spark port or the full manifold vacuum port?
    (Exhaust headers can radiate a tremendous amount of engine compartment heat...if the idle timing isn't optimal.)

    With my infra red temp gun, when percolating, the Holley's fuel bowls are ~135°F and the factory brass fuel distribution block, just before the carb is ~155°F.
    To me, that shouldn't be any real problem. I expected symptoms of percolation to be around 180°.
    I agree, but I wonder what the actual fuel temperature was; especially with lousy fuel.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & MT 245/45R17 tires.

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