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308 fuel pump trouble?

Discussion in 'Technical Q&A' started by gary308gtb, Jan 5, 2004.

  1. gary308gtb

    gary308gtb Rookie

    Oct 9, 2003
    41
    My 1978 308 GTB has been in the shop for more than a month. Here's my problem : "When I accelerate and hit rpms of 1000 to 4000 rpm, the acceleration is quite smooth. However, when I accelerate and hit rpms of 4100 to 4500, then I feel that the throttle is slightly suffocated, then some backfires can be felt and heard.

    Everything has been fixed and adjusted by professional mechanics : the cam belt timings, the distributor settings, the carburator settings. My fuel filter is new, my gas tank has also been cleaned and emptied.

    Can the fuel pump be causing this fuel starvation at 4100 to 4500 rpm?
    Do you think my MSD ignition has any effect on the fuel management since my tachometer doesn't work since the MSD was installed ? Is there any connection with the MSD and the fuel management ?
     
  2. pma1010

    pma1010 F1 Rookie
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    Jul 21, 2002
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    Chicago
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    Philip
    Gary
    Sounds more like a timing or jetting issue than a fuel pump issue from your description.

    Some general guidance:

    The 308s have a constant circulation system for fuel (I assume running at 5 - 7 psi). If you've got fuel above 4500 and below 4000, you've got fuel in between. In fact, if you are experiencing a backfire, this is fuel exploding in the muffler so it doesn't sound like a FP issue.

    If the car is falling on its face at 4000 (is it?), this is about the transition from idle to main jets at partial throttle. How is the car jetted? Specifically, are the main jets right sized? Too small and you'll get starvation at over 4K when running with constant (and larger) throttle openings. Too rich and you'll get a backfire through the muffler.

    Third, does the condition occur when accelerating or on constant throttle? If an acceleration issue, check the pumps (I am told when they go they'll tend to leak fuel all the time so you'll run rich).

    Last, (and I think most likely from what how you describe the symptoms) you mention you've had the distribs serviced (and I assume the advance curve checked). Have you put a timing light on the car and verified you are recording linear advance to 34 degrees at 5K?

    HTH
    Philip
     
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  3. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
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    Hans E. Hansen
    This is a common misconception. The accelerator pump *ONLY* functions when the gas pedal is moved. Once it's on the floor, the pump does nothing more. The pump is only to fill a temporary gap in fuel delivery just at the moment that you floor it. Steady WOT does not move the pump diaphragm.

    When they leak, gas seeps out of the external linkage, not internally. Definite fire hazard. I'm in the process of replacing mine this week.

    Hans.
     
  4. joeyy

    joeyy Karting

    Nov 11, 2003
    190
    long island
    Full Name:
    joe
    sounds like a timing glitch. check both dizzys on a machine. it is at the point the advance starts to come on. also MSD makes a booster for the tach fix. you can get it at Summit or Jegs or any good speed shop.
     
  5. pma1010

    pma1010 F1 Rookie
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    Jul 21, 2002
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    Hans, yes I phrased it poorly. Concur.
     
  6. gary308gtb

    gary308gtb Rookie

    Oct 9, 2003
    41
    Philip,

    Thanks for your helpful insites regarding my query. My mechanics checked the timing and it is okay. Regarding the jets, I have not checked their condition.

    The problem occurs on acceleration at the 4100 to 4500 rpm mark. At constant throttle from 1000 rpm to 4000 rpm no problem. The fuel starvation can be felt when I floor it at 4100 to 4500 especially at 3rd gear and 4th gear.

    I'm sorry I forgot to mention that at 1st and 2nd gear even at high rpm I don't experience the fuel starvation. It's more on the high rpms of 3rd and 4th gear.

    I checked my fuel pump and there are no fuel leaks.

    I am not sure about the advanced curve setting and if the linear advance is 34 degrees at 5000rpm. Will have this checked ASAP.

    Thanks again.

    GARY
     
  7. ferrarifixer

    ferrarifixer F1 Veteran
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    Jul 22, 2003
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    Phil Hughes
    Most likely to be an ignition fault.

    I'd suspect a breakdown of a high tension component in one or two cylinders....plug, lead, coil, cap etc etc.

    Carb butterfly spindle seal air leaks can cause popping and missing, as can air leaks in exhaust manifold and many others.

    fuel pump highly unlikely to give starvation until extreme use.
     
  8. gary308gtb

    gary308gtb Rookie

    Oct 9, 2003
    41
    thank you ferrarifixer will follow your advice. Gary
     
  9. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
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    Hans E. Hansen
    Something that I'm not clear on from your description: What happens above 4500rpm? Does all work OK - both steady cruise and acceleration?
     
  10. gary308gtb

    gary308gtb Rookie

    Oct 9, 2003
    41
    When I start to drive and accelerate at 1st gear even from 4500 rpm up everything is ok, then at 2nd accelerate to 4500 rpm and up everything is fine, at 3rd gear cruising from 1000 to 4000 its normal, then when i accelerate and reach 4100 to 4500 rpm the car feels as if it is being suffocted like a fuel starvation, small backfires start to come out and u can feel a percolating when i press the throttle and the car doesn't want to accelerate any further.

    Actually when I reach this point i am forced to shift to 4th, then the same thing happens again, then i am force to shift to 5th. So basically, I haven't forced going to a higher rpm than 4500 at 3rd, 4th, & 5th gear.
    At cruise even at 3rd 4th & 5th gears, the car is ok at 4000 rpm & below.

    (Mind you, i only drove the car in this condition on test drives after the mechanics in the shop have tried to improve the settings and adjustments).

    As of now, the technicians at my Ferrari service are about to test another fuel pump for my car, and they want to also change the high tension wires. They checked the distributor settings and timings and everything is at the proper settings. No air leaks in the exhaust and carburators.

    It's really frustrating because it's a trial and error thing. We can't really pinpoint yet what the exact defect or malfunction is.
     
  11. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    18,159
    Texas
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    Steve Magnusson
    Your description does seem like the fuel supply side just isn't keeping up and the bowls eventually get drained by the time you hit the higher gears. If you've still got a generally stock US version '78 308 fuel system there's a pressure regulator (59) on the output of the fuel pump so you might check that as well as the fuel pump. For a test, I believe if you manually block line 60, the full fuel pump pressure will be available to the carbs (but the pressure regulator could still have a blockage problem in the normal line 61 to line 65 path, or for that matter you could have a kinked line somewhere else -- my recollection is that it's a very tight package and the line lengths/fitting directions must be tuned to avoid a kink). Anyway, just wanted to point out that there might be more than just the fuel pump to consider on the supply side of the equation if it's a US version.
     
  12. Ken

    Ken F1 World Champ

    Oct 19, 2001
    16,078
    Arlington Heights IL
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    Kenneth
    One of our Weber Europa guys had a similar problem. He pulled spark plug wires and determined which carburetor was at fault for spitting and backfiring at a specific rev range. Webers have a lot of tiny places that must be immaculate. Are your carbs clean? Can your problem be replicated in the garage revving the engine? If so, then you should be able to see if you can trace it to a particular carb, if indeed that is where the fault is. At worst, you can determine that the carbs are not to blame.

    Ken
     
  13. gary308gtb

    gary308gtb Rookie

    Oct 9, 2003
    41
    Thanks Steve for your very detailed response to my query. Yes my car still has the stock US version '78 308 fuel system. Will check the pressure regulator, I never thought of that. Will also check the fuel lines for any kinks.

    Gary

    Thanks Ken, will also check the carbs and spark plug wires to find out which carb is backfiring and spitting. My carbs are quite clean already. However, I can't replicate the problem, in the shop by just revving the engine. When revving the engine at neutral even at maximum rpms, there is no backfiring or spitting of the carbs.

    Gary
     

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