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Still having engine issues. (Long)

Discussion in 'Shop Talk' started by TexasDoc, Mar 28, 2012.

  1. TexasDoc

    TexasDoc Member

    I'm still having trouble getting my rebuilt 351W running smoothly. First I wiped a 3 cam lobes on a flat tappet cam. Rebuilt with a conversion kit (see previous posts). Now I have it running and am going to take it to San Marcos this weekend. However, I am still having trouble getting it running smoothly.

    Problem: The car starts instantly. Runs a little rough until it warms up. Then it runs well for about 15 minutes or so, then starts to get rough. For those 15 minutes, accelleration is good and smooth (haven't gotten on it much since I only have 50 miles on it total). After the 15 minutes, the acceleration is not as smooth. No backfires, just some hesitations through the rpm range. Also, the lifters *seem* louder and more clackety. If I let the car sit for an hour and cool off a bit, it seems to run well again for a few minutes, then starts to run rough again. Temperature guage reads a steady 190 degrees throughout all of this once warmed up.

    What I have:
    1982 351W (non roller) block 40 over.
    Comp cams XE retro cam with hydraulic roller lifters.
    Roller rockers set with 1/2 turn after zero lash on cam base curve - warm engine.
    AFR 185 heads
    Edelbrock AirGap Performer intake
    Holley 670 carb with vacuum secondaries
    100amp alternator
    Proform distributor (HEI with built-in coil)
    Mallory 6AL ignition box
    Normal volume oil pump - warm idle pressure = 20, cruising pressure = 50
    Castrol dino oil 10W-30.

    The initial timing is set at 12 degrees with the vacuum advance disconnected and plugged. Intake manifold vacuum is around 14 at idle. Relatively stable with a mild fluctuation. It won't idle at less than 1k. HAve srayed everywhere with carb cleaner looking for air/vacuum leaks - none found. Starts instantly. I'm not sure what total timing is - not sure if I measure this with the vacuum advance attached or not. I'm also not sure if the 1000 rpm idle is causing some of the mechanical advance to come in at idle - not sure how to test this since I cannot get the idle below 1000 rpm. I have adjusted the air/fuel to be 1/8 turn out from maximum rpm - same on both sides. This is about 7/8 turn out from seated. Secondaries are closed. I tried to go with the square slot, but it would then idle at 1200 with the idle screw backed all the way out - and the air-fuel screws were inoperative. At this setting, I would place my hand over the secondaries and the car would bog down.

    So, I'm not sure what is going on. I'm not sure if this is a carb tuning issue, distributor problem, electrical connection fault, or mechanical engine problem. Why would it run well when it is warm, but rough when it is hot? Any thoughts? I am almost ready to change to an MSD distributor with external coil, buy a different carb (or fuel injection system). However, I would like to know what's wrong instead of just throwing parts at it.

    Any help will be appreciated greatly!

  2. IndyCobra

    IndyCobra Member

    Can't help much as I don't know a lot about engines but I can provide you a couple of data points. My 351W w/Holley was built by the Engine Factory. Idle mixture screws are out about 1 3/4 turns, and there's no way this engine will idle below 1000. I have it set right at 1000. That's where it likes to sit. Any lower, and it gets very rough.

    Also, have you adjusted the fuel level in the float bowl yet? Mine was too low and when the engine got hot, it would boil the fuel in the float bowl out the vent tubes. I still may need to install a phenolic block between the manifold and the carb but raising the fuel level seemed to help.
  3. TexasDoc

    TexasDoc Member

    Yep, fuel is right at the bottom of the sight windows.
  4. Lakeway Cobra

    Lakeway Cobra Member

    You may be having fuel boil over. Winter fuels have a much lower boiling point and our 80 degree temps don't play well with winter fuel in a carbed motor. Especially if it is a mechanical fuel pump with the line running toward the front of the motor. Electric fuel pumps put some pressure on the fuel from the tank so inherently raises the boiling point some.

    You could try to lower the float level about 1/16 below the bottom of the plugs to see if it helps. Or find some aviation gas to mix in.

    Note this was a huge problem last year at the San Marcos meet. I just go ahead and lower my float levels every spring to counteract it some. I also have a phenolic spacer which helps some.
  5. Copperhead

    Copperhead Member


    You made no mention of fuel pressure. Carb engine should be 5-7#'s.
    My Demon 750 mechanical is set right at 6#'s with a mechanical pump. 418W stroker built by The Engine Factory, 185 AFR heads, Victor Jr manifold, Comp roller cam, Pertronix distributor with no MSD box. Idles at 700 no problem - a little lumpy, but sounds great.
    Vacuum seems a little low. Might need to adjust on carb mixture screws some.
    Square slot is key at idle on primary plates - secondaries should be flush. Use the screw on throttle arm to lower the idle once all other settings are made.
    Could need an adjustment to the jet sizes in the carb too.
    Just some thoughts.
  6. fasteddie

    fasteddie Member

    Hows the oil pressure? You mention the lifters start making noise after about 15 minutes. I once had a problem with the lifters making noise after the engine warms up, oil gets thin with heat. In my case the gasket between the oil pump and pickup tube was left off, this caused the pressure to drop once the engine heated up.
  7. TexasDoc

    TexasDoc Member

    Figured out it was a bad plug. Running great now. Still gotta work on the carb tuning. Secondaries are closed at idle. No fuel running thru the transfer circuit so I have a hesitation when the secondaries open.
  8. IndyCobra

    IndyCobra Member

    Not sure how lowering the float level makes this problem better? I know that's what Holley recommends but it seems the more fuel there is, the more heat it would take to vaporize it?

    So this problem I am having where the engine stops for 5-7 seconds after I take off in a hot car is a problem with the fuel boiling? Will it go away with the summer blends? I'm about to rip off this Holley and replace it.
  9. Lakeway Cobra

    Lakeway Cobra Member

    I'd have to go back to some old posts to explain it exactly, but it has something to do with the fuel being bubbly/foamy and it does not raise the float enough... which causes more gas to flood into the motor through the vent tubes if I recall correctly. I'm sure there are some experts that can chime in.

    Do you have a phenolic spacer? That should help.

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