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Thread: Demon 625 Issues

  1. #1

    Default Demon 625 Issues

    Hi Everyone. First time posting on the Holley forum. I wanted to see your guys' thoughts on what could be happening with my Nova.
    First the setup:
    1970 Nova 4-speed Muncie
    1977 Chevy 350, stock bore and heads
    Demon 625 Carb
    Comp Cam
    Weiand X-Pert Single-Plane
    Headers
    Points Ignition (Upgrading today to HEI.)
    Factory MECH fuel pump

    SO I just got the car back on the road after sitting for 10 years. The engine and transmission I purchased late 2018 and spent 2019 refreshing the engine with new gaskets (except heads) and paint. I also rebuilt the Muncie. I put a brand new 625 Demon on it bought through Advance Auto. The car has a great idle so I figured it's ready for a test drive. Well that did not go well. About every 10 mins or so of steady driving, it would all of a sudden loose power then backfire and sputter to shutdown. It did that about 4 times going the distance of about 15 miles. I looked at the plugs when I got home and they were pretty black. I cleaned them up and then let the car idle for about 20 minutes and no problems there. No black smoke or anything like that. I did change the fuel filter that's after the pump and it was pretty dirty. It was brand new when I first dropped the engine in last year. I believe it was picking up some rust and dirt from the bottom of the tank. I have not ran the car since changing it because I'm dropping in the HEI distributor today. I adjusted the carb to the factory setting for the idle mixture screws and before I installed it. I adjusted the Goggle valve to just barely showing the small rectangle opening.

    All that being said, any ideas of what's happening? It's very strange that it seems to run OK for a bit then just die like that. One more quick note, it takes it about 15-20 mins to start again after that happens. Thanks guys.

  2. #2
    Join Date
    Jan 2010
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    Brisbane, Australia
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    Default

    Sounds like your fuel tank needs a clean out. Keep replacing the fuel filter for now. You could also have a fuel tank breathing issue.
    The "Goggle valve" is the secondary throttle valve(s), not the primary side.
    What camshaft do you have in your engine?
    Can you test your spark output from a plug lead? Gary
    Regards, Gary

  3. #3

    Default

    Thanks Gary. Yeah I was afraid the ethanol ad started to mess with the tank once I changed out the fuel filter. I've changed the filter, so hopefully it will clear up over time. It is interesting that you say I could have a fuel tank breathing issue. I have the EEC style tank, but the EEC system is long gone so as I was changing to a new sending unit, I plugged off the two passenger side tube ends and have the divers side vented. I'm going to get a new vented cap as well. Could this be why it's backfiring?
    I turned the engine over to TDC in prep for the HEI distributor. I removed all the plugs to make it easier to turn over and the plugs are all pretty black. I wonder if the ignition system just doesn't have a good enough spark for complete combustion? The cam is a Crane Cam that came installed when I bought the engine. Here are the specs:
    PowerMax Hydraulic Flat Tappet Camshaft
    Lift: .459''/.486''
    Duration: 268°/280°
    RPM Range: 2000-5000
    What would be the best way to test the spark plug output? I have a inline tester that shows if it's getting juice but that's it.

  4. #4
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
    Posts
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    Spark should be able to jump a gap of 1/2 inch.
    I have HEI that can jump over 1 inch, but you need to be well insulated when testing that type of output.
    Your backfiring is bad timing (out the carb), or intermittent misfiring (out the exhaust). Gary
    Last edited by Gaz64; 01-14-2020 at 05:09 AM.
    Regards, Gary

  5. #5
    Join Date
    Dec 2009
    Location
    Connecticut
    Posts
    24,187

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    And even greater distance with a distributorless ignition system:
    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, 232°-244° duration/.623" lift/114° LSA camshaft, 11.8:1 comp, TFS R-Series FTI ported intake, BBK 80mm TB, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 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, B&M Hammer shifter, 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 & 245/45R17 MT tires.

  6. #6
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
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    Quote Originally Posted by Danny Cabral View Post
    And even greater distance with a distributorless ignition system.
    Yes, DIS, I see that a lot.
    I stopped an engine in a workshop emergency by pulling the coil lead.
    External HEI coil fired to heater hoses as the engine wound down, (the key was still on).
    Wish I had a photo of that one (about 6 sparks across a 2 inch gap in parallel to rubber as ground, through the rubber and then coolant?). Gary
    Regards, Gary

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