News

Breaking up at high end.

Discussion in 'Technical Q&A' started by docweed, Mar 31, 2005.

  1. docweed

    docweed Formula Junior

    Dec 8, 2004
    452
    Morgantown,WV
    Full Name:
    Chuck Stewart
    I recently purchased a 79 308 with 49000 mi. From what I understand it was pretty much idle for the past several years. It just had a major service done. After some carb tinkering and some fuel system issues it now runs strong up to 6000 rpm. After that it breaks up badly trying to reach 7000 rpms. Would some gas additive ie. Techtron be of any help. What other areas should I check to try to iron out this problem? Thanks.
    Charlie
     
  2. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
    Full Name:
    Russ Turner
    Techron is great.
    Ensure that both distributors are advancing to full advance per spec; the flyweights may have gotten gummed up if sitting around.
    Replace the fuel filters.
    Sometimes, sitting mufflers will corrode and collapse inside causing a very high restriction evident at high rpm function

    'It just had a major service' - did that include timing belts? cam timing may be off.

    Good luck - that's a great car!
    have fun
    rt
     
  3. geekstreet

    geekstreet Karting

    Feb 7, 2005
    220
    Sydney
    Full Name:
    Cam
    I'd put my $2 on an ignition problem (leads, cap, rotor, coils, etc).

    Partial fuel starvation shouldn't really stop a quick rev to 7000 although it would hurt in-gears acceleration to the same rpm. Try it in neutral (ie under no load). This uses only a fraction of the fuel of full-throttle, and also makes the spark at the plug stronger. If it won't get to peak rpm in neutral then it's something independent of engine load & airflow. Possibly distributor (points bounce), cam timing, electronics?

    Is it a sudden breakdown at much the same rpm (more like ignition) or does it get to an rpm but then won't hold it (more like fuel supply)?
     
  4. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    18,395
    Texas
    Full Name:
    Steve Magnusson
    If, when checking the advance vs RPM per Russ' suggestion, the firing in the coil wires or spark plug wires goes away at high RPM (and it's not a point problem), one other thing to check is the distributor rotor position relative to the cap/housing -- the stock 2-valve carb rotor tip is quite narrow (therefore sensitive to positioning as the advance changes with RPM). The later 308QV dist rotor 124288 is wider at the tip and fits the 2-valve carb distributor.
    Good luck with the hunt...
     
  5. Ken

    Ken F1 World Champ

    Oct 19, 2001
    16,078
    Arlington Heights IL
    Full Name:
    Kenneth
    It COULD be a fuel problem. Will it rev over 6k at idle? If so, that's it. If it still breaks up, I'd say it most likely the distributor (s). Points bounce.

    To be really pessimistic, could it be a broken valve spring? Or weak valve springs? I'll let the real experts address that.

    Ken
     
  6. pete04222

    pete04222 Formula Junior

    Nov 1, 2003
    613
    Maine, USA
    Full Name:
    Peter Cyr
    #6 pete04222, Apr 1, 2005
    Last edited by a moderator: Sep 7, 2017
    My guess would be ignition system also. The stock ignition is fairly weak when new.

    I've got a '79 also that I've had for about 3 years. Shortly after I got it the low speed points set failed, I simply disconnected them. A year later the high speed points started wearing out also and the car was hard to start. I decided to get rid of the points altogether and installed the Crane Cams XR3000 Fireball ignition and the PS-60 coils (this set up allowed me to do away with the ballast resistors also). I put in Magnacor wires and new NGK plugs. I was able to do the whole job myself and it cost me less than $500.
    Image Unavailable, Please Login
    Image Unavailable, Please Login
     
  7. docweed

    docweed Formula Junior

    Dec 8, 2004
    452
    Morgantown,WV
    Full Name:
    Chuck Stewart
    Thanks to everyone for your suggestions. If I understand correctly I should try to rev the engine to 6000rpm and beyond to 7000 while the car is in neutral and under no load. If it does not break up above 6000 then it is probably fuel supply issue. If it still breaks up above 6000 then it may be an electical problem. Please correct me if I my interpretation is incorrect. Will I risk any harm to the motor at that high rpm's? I'm chicken.

    Also, at first it was a fuel starvation problem as I would go to 6000 then the car would slow down and I could not accelerate again for sometime until the fuel cought up. We checked the fuel pressure at the inlet to the carbs and the pressure was only 1 lb. The techs then looked at the fuel pump area and deceided that the one way check valve was defective. They removed it and eliminated the three way hose situp and made it into two hoses. This helped greatly as the pressure went to 4 lbs. My question now is 4 lbs enough? My 911 has 5-6 lbs. Thanks
    Charlie
     
  8. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
    Full Name:
    Russ Turner
    Yes. Webers are quite happy at 3 - 4 pounds of pressure. As you've done so well, must also ensure to provide volume at this pressure as well.
    Great work! Looking forward to hearing of your progress!
    rt
     
  9. GTO84

    GTO84 Formula Junior

    Dec 13, 2003
    562
    Don't rev your engine to 7,000 rpm in neutral unless you want to buy bearings prematurely. You'll only be using the idle and accellerator pumps anyway. You need a load on the engine to get the float bowls to drain, so it will starve. Check your points. Check dwell and timing, and make sure the springs on the points are there and not broken. Points will float at high RPM, so I'd check them first. Correct fuel pressure for Weber DCNFs is 3 m.h20 which converts to 4.5 PSI. Put the check valve back in re install the 1/4" fuel return line, run 5/16" for the supply, because that's not fixing it. Don't jerry rig the fuel supply system. Did you check your fuel filter? When in doubt replace it. It probably hasn't been changed in 20 years. That will give you a gradual pressure decrease over a few months, and it seems to be the most overlooked part on carb 308s.
     
  10. GTO84

    GTO84 Formula Junior

    Dec 13, 2003
    562
    Why use the wrong part to correct the misaligned distributor?
     
  11. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    18,395
    Texas
    Full Name:
    Steve Magnusson
    It was more a suggestion to use a better part if you are replacing the dist rotor anyway. Just gave the part number for reference -- not as a specific fix for poor alignment.
     
  12. ferrarifixer

    ferrarifixer F1 Veteran
    BANNED

    Jul 22, 2003
    8,520
    Melbourne
    Full Name:
    Phil Hughes
    I'd go with points here....

    check the gap/dwell, but also condition generally. Make sure they align flat surface to flat surface, and open cleanly without "skidding" due to worn pivot etc.

    Gap 012-015', dwell about 38-40deg. Forget the idle set, remove them to save confusion.

    The actual contacts can also come loose within the points themselves, although this is rare.
     
  13. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,354
    NY
    I had a point contact come loose right after I got my car. I thought the engine blew up. It was backfiring like crazy with the whole bank quitting and then refiring. If he has this condition he will know it. The following week I threw out the points and put in a Pertronix's set up. Runs smooth all the way up to 7500 rpm. So usually a points problem comes into play at high rpm where they have a tendency to float . Also check the advance per the OM and verify if both dizzys are advancing as you rev the motor. Get rid of the points.
     
  14. ferrarifixer

    ferrarifixer F1 Veteran
    BANNED

    Jul 22, 2003
    8,520
    Melbourne
    Full Name:
    Phil Hughes
    Points will work to 8500 rpm in std form without float, but if condensors are poor or dwell wrong they struggle.
     
  15. geekstreet

    geekstreet Karting

    Feb 7, 2005
    220
    Sydney
    Full Name:
    Cam
    Which bearings? (engine? gearbox?)
    Are you suggesting that briefly running an engine up to peak rpm SHOULDN'T be done in the workshop?

    I don't believe that it is any worse than when running to the redline on the road. Granted on the road at full-throttle you have significant compression in the cylinders which will act to counter the piston decellerative inertia but this only happens every 2nd cycle (ie on compression stroke and not on exhaust stroke).

    Any other thought on the pros & cons of revving under "no-load"?
     
  16. GTO84

    GTO84 Formula Junior

    Dec 13, 2003
    562
    Good. Understood.
     

Share This Page