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Technical Backfires

Discussion in 'Traditional Hot Rods' started by dwleo, Mar 25, 2017.

  1. dwleo
    Joined: Aug 10, 2014
    Posts: 47

    dwleo

    Hi guys,
    Almost finished my T-bucket.
    One problem I have is the engine. Its a 350 sbc and when I start it, it fires immediately and runs strong for about 30 - 45 seconds then starts to misfire and backfire through the pipes and ends with (sometimes) a carb backfire then dies. It will startup right away and repeat itself.
    I am running a slightly mild cam and a fuel slayer 4 barrel. Changing the timing seems to make it worse so I am leaning towards incorrect valve lash.
    Is this a reasonable assumption?
    Thanks.

    - Daryl
     
  2. what carb you running (fuel slayer) cfm? not familiar with it, does it have a power valve
    new built motor or used motor
    new cam or one that was in motor
     
    Last edited: Mar 25, 2017
  3. When my avatar was a new build, the fuel filter got plugged from the junk in the tank and the HEI coil was weak. 2 problems at once! :confused: I liked to pull my hair out trying to figure it out!

    It would run just like you describe.
     
  4. Sounds like a coil going bad to me.
     

  5. How is your ground between the engine and chassis? I was putting a stock car together years back and got it running before I got a good ground in place, it ran just like what you describe.
     
  6. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 6,250

    Ebbsspeed
    ALLIANCE MEMBER

    What fuel pump are you using? If it's electric it sounds like it might not be keeping up.
     
  7. dwleo
    Joined: Aug 10, 2014
    Posts: 47

    dwleo

    All new parts. Fuel Slayer 650
     
  8. dwleo
    Joined: Aug 10, 2014
    Posts: 47

    dwleo

    Everything is new except the engine bare block. Yes, even the gas tank.
     
  9. dwleo
    Joined: Aug 10, 2014
    Posts: 47

    dwleo

    Grounds are good as near as I can tell. Never hurts to have more grounds, though
     
  10. dwleo
    Joined: Aug 10, 2014
    Posts: 47

    dwleo

    Manual
     
  11. My tank was new too...
     
  12. dan griffin
    Joined: Dec 25, 2009
    Posts: 505

    dan griffin
    Member

    Could be the coil is it one of those fine off shore systems?
     
  13. Just went thru something similar, engine would start and when rpm held high for cam break-in; would run til fuel bowls empty, then backfire thru carb and die. It would fire back up and same thing would repeat. We pulled the line and checked pump output and it was very low, my kid checked and found rubber line out of tank was partially pinched when I set the body on. Fixed line problem gone.

    Sent from my Nexus 5X using The H.A.M.B. mobile app
     
  14. dwleo
    Joined: Aug 10, 2014
    Posts: 47

    dwleo

    Now that's really interesting. So much to check tomorrow.
     
  15. Do you have a fuel pressure gauge at the fuel inlet? I swear by these things when sorting out a fresh engine.
     
    Montana1 likes this.
  16. boring-hop-yard
    Joined: Feb 24, 2008
    Posts: 69

    boring-hop-yard
    Member

    (New) does not mean good!
     
    bobss396 and Montana1 like this.
  17. dwleo
    Joined: Aug 10, 2014
    Posts: 47

    dwleo

    No fuel pressure gauge
     
  18. tinkirk
    Joined: Jan 16, 2017
    Posts: 111

    tinkirk

    Also what distributor are you running and, what are your plugs gaped at
    if you are running an HEI the plugs need to be at least .040 to .060 not sure if this would make it back fire and stall but sure will not run good with a smaller gap!
     
  19. Have Petronics on it?
     
  20. Check for vent in tank or gas cap vent if it starving for fuel. Start with the easy stuff first.
     
  21. Only thing I can think of is the timing is grossly out, distributor a tooth off maybe. How does it look with a timing light on it? Is the coil matched to the distributor?
     
  22. tinkirk
    Joined: Jan 16, 2017
    Posts: 111

    tinkirk

    what is your plug gap?
     
  23. dfriend
    Joined: Mar 7, 2012
    Posts: 20

    dfriend
    Member
    from B.C.canada

    Hi
    It sounds like the timing is to far retarded ( adjustment or dist. weights sticky or seized or in wrong) as it is back firing out the exhaust. Also if the power valve is blown (back fire--if it has a vacumn p/v) it could load up the engine and cause it to stall. Just some thoughts.
     
  24. dwleo
    Joined: Aug 10, 2014
    Posts: 47

    dwleo

    I think it's best to give the engine details first then some history

    Intake: ProComp 22000
    Heads: Aluminium
    Cam: Elgin e921p
    Lift: intake .450 Exhaust .460
    Duration @.050 224/224 lobe center 114
    Lifters: Hydraulic
    Carb: Quickfuel Slayer 600
    Primary Main Jet: 68
    Secondary Jet: 74
    Idle Air Bleed: 70 / 39
    High Speed Bleed: 31 / 31
    Pri Nozzle: 31
    Sec Nozzle: X
    Needle & Seat: 100
    Power Valve: 6.5

    I assembled this engine and had it running smoothly at 2000 rpm for 20 mins or so to break in the cam as required about 6 months ago. I then changed the oil and then worked on other things. I had reason to pull the dizzy and relash the valves as a looseness occurred. Nothing has been right since. I would crank the engine until the marks would show on the damper and both valves would be closed and the piston is at the top of the cylinder. I set the distributer to hit plug #1 at that point. When I finally got the setup almost right, the engine started right up albeit now with the aforementioned backfiring that would happen after about 20 - 45 seconds of running. The engine would either die or I would turn it off. Hit the gas once and crank it and it would start. But again for the limited time then backfire would start. Over the past two days I have checked the valve lash and readjusted the mixture screws to 1 full turn out. I regapped the plugs (disgustingly fuel dirty with soot) to .040 and rechecked the vacuum lines. I hooked up a vacuum gauge a short while ago to the full vacuum line on the carb bottom and started it up. I kept it running (through the backfires) by upping the rpm's a bit but the vacuum showed nothing. I popped off the cap for the PVC line (don't have one) and the gauge read 6 immediately and drop to zero when I put my finger on it.
    So, because it was running ok during break-in period and prior to when I pulled it, the distributer is naturally suspect however that would not explain the backfiring. There is a fuel smell to the exhaust, plugs are sooty and it almost seems (to me) that the carb is getting overcome with fuel and is flooding into the cylinders leading to misfiring and backfires. If I play with the timing setting, it doesn't get better but does get worse.
    I'm flummoxed.
     
  25. Make sure you didn't cross the 5 and 7 plug wires. That is exactly what it does when you have them crossed, and it is common especially if you run the wires under the headers.
     
  26. I agree,a fuel pressure regulator would eliminate several issues. Maybe even solve your issue.Would be money well spent.
     
  27. The first thing I'd do would be to plumb a fuel pressure gauge into the carb feed line. I usually place the gauge under the windshield wiper so I can actually drive the car and watch the gauge. This will tell you if you're fighting a fuel or spark problem. My hand held vacuum gauge has a fuel pressure gauge in it.
    If it has good fuel pressure I'd swap the coil for a known good one to test. I've had several coils that tested out OK with an ohmmeter and when they warmed up, they were shorted internally...total junk.
     
  28. dwleo
    Joined: Aug 10, 2014
    Posts: 47

    dwleo

    No.. They are correct and separated.
     
  29. Trouble shooting 101 , when a new problem shows up, go back to the last thing you "fixed"

    Now, what the hell exactly happened to creat a calve train looseness and what is a looseness exactly

    Valve trains don't just happen to get loose. There's a problem there for sure.
     
  30. dwleo
    Joined: Aug 10, 2014
    Posts: 47

    dwleo

    They do tend to get loose. Depends on whether you are dumb enough to adjust with lifters loaded or not.
     

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