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308 "stumbling" after slow right hand turns

Discussion in 'Technical Q&A' started by Birdman, Jul 12, 2004.

  1. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
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    Hi Everyone,
    Here's the scenario. My '77 GTB with a recent carb synch and running perfectly is exhibiting an occasional strange behavior. When I take a SLOW speed right hand turn (say, a 90 degree turn in second gear at less than 20 MPH, low RPM, the kind you make in your neighborhood, not on the on-ramp) the car will "stumble" or hesitate. I can hear and feel that one or two cylinders are not pulling their weight. At the same time I get backfiring. It takes between a few seconds or up to a minute to go away, long after the turn is completed. This is not repeatable however. It only does it once in a while. It does it infrequently enough that I can't figure out what it is. It actually took me a while to notice that it happens following right hand turns because it doesn't happen that much. At first I thought it was ignition related, so I pulled the plugs to see if one was getting fouled, and they are all fine. Even after replacing them, it still did it. Then I noticed the relationship to the turning. I'm thinking it might be float related except that I DON'T get the usual carb 308 fuel starvation issue at high speeds! I can go around a right hand offramp very fast, pressed against the door, 3000-5000 RPM and NO hesitation or fuel starvation. This is why I'm thinking maybe it's not floats. I know I need to pull the tops of the carbs and check the float levels, but I want to wait until the driving season is over and do it with the cam belt change. I hate to have the car out of commission in the summer. That blows.

    Anyone got any ideas where else to look?

    Thanks

    Birdman
     
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  3. Mike328

    Mike328 F1 Rookie
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    Jonathan,

    I've concluded that this is a characteristic of carbed 308s.

    Here's some data:
    http://ferrarichat.com/forum/showthread.php?t=7218&highlight=hesitation


    Mine has always done this, except one time when I had my mechanic do some jet work, it went away for a while. They increased the accelerator pumpt jet size and also the idle jets, but things got too rich.

    Not everyone seems to have it. A number of people, including me, have the exact same situation you had.

    I had my carbs rebuilt, manually reset float heights PRECISELY to the best spec I could find, tuned, tweaked, and still had the problem.

    None of the reviews I've read about the car mention this. There may be a common solution out there but I've NEVER found any solid leads on it.

    This is one of the reasons why I decided that, as awesome as they were, I couldn't deal with carbs on a day to day basis; I didn't like having the car basically die on me in slow-speed, part-throttle corners.

    Would be very interested in hearing the solution. The problem seems to be losing one or more (maybe up to four or six?) cylinders, could be ignition related or fuel supply related.

    Do some searches--there was some good info in the FerrariChat archives, as well.

    --Mike
     
  4. DN35

    DN35 Formula Junior
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    Nov 22, 2003
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    I have a '78 308 and mine does the same thing. It did it both before and after its recent major service. I just keep the revs over about 3k on a long right hander and it goes away.
     
  5. Jdubbya

    Jdubbya Two Time F1 World Champ
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    Exactly! These cars are known for this problem. The secret is keeping the revs high enough to avoid the problem.
     
  6. Spasso

    Spasso F1 World Champ
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    From what I have read it is a common characteristic of the carbureted cars. Mine does it too. Has to do with the way the floats swing in the carbs. my Chevy truck does it even worse with the Holley carb on it.

    The trick is to pump/feather the gas lightly to compensate with the accelerator pumps and all is well, either that or carry more speed through the corner.
     
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  8. dm_n_stuff

    dm_n_stuff Global Moderator
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    Dec 10, 2003
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    Dinos do it too.

    Mechanic's advice was, "just stay in the throttle."

    Helps, but doesn't make it go completely away. And I've seen it referred to in several road tests for dinos.
     
  9. Spasso

    Spasso F1 World Champ
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    Fuel injection makes it go completely away.:p
     
  10. BigTex

    BigTex Seven Time F1 World Champ
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    ...along with several horsepower!!!! lol!
     
  11. tuttebenne

    tuttebenne F1 Rookie

    Mar 26, 2003
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    I think these folks didn't read your post. Its NOT characteristic of carbed cars to have backfiring and up to a minute before getting back to normal. If you had the carbs apart and it started doing this after the rebuild, I'd be suspicious. The float level might be too low or you might have a float that is binding. I don't have a trick way to isolate which carb it might be so the only way to tell would be to pull off the air cleaner and inspect each carb. This doesn't take long. You might take two hours to complete the job (including adjusting the float(s) if they need it.

    Best wishes,

    Andy
     
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  13. Jdubbya

    Jdubbya Two Time F1 World Champ
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    First of all, yes I did read the post and second I will stick with what I said. The only part that sounds strange is the part about it taking so long to go away. He didn't say he had the carbs apart he said after a recent synching of the carbs, and that he needs to take them apart to check the floats.

    He also doesn't mention whether or not this is with the car cold or warmed up? That in itself could make a difference. I have experienced the problem at higher and slower speeds, but the key is the RPM's. If they are kept high enough I don't have the problem at any speed!!

    Still could be some other problem but I'm willing to bet it's just one of those little bonuses that we with the carb cars get to enjoy that all the others don't!
     
  14. Nuvolari

    Nuvolari F1 Veteran
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    The carb cars will hestitate like this sometimes in slow speed corners. To the best of my knowledge the solution lies in the ignition. At slow speed in the corners the stock ignition system has its spark plugs 'soaked' by the carbs and the recovery time are the plugs cleaning themselves out. I had the exact same problem with my car and it totally disappeared when I went to an electronic ignition setup. It seems as though the electronic ignition just gives the plugs enough more juice at low revs to keep them from getting soaked. Not highly scientific but this is what happened to me.
     
  15. Birdman

    Birdman F1 Veteran

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    Thank you all for your input! In fact, it usually only lasts the duration of the corner, but today the backfiring and general hesitating took about a minute to go away. That was unusual and got me worried because the car has been running so well since I syched it. It did not start with the carb synch. It did it before I synched the carbs, but worse. It has improved markedly. Today, the car was not warmed up when it did it. I had only driven about a mile. Even though all my logic says it has to be carb related because it seems to be triggered by a turn, I still can't help thinking that a beefier spark wouldn't help. I can't decide if it's going way lean in the corner, or if in fact the thing is getting a big dose of extra fuel dumped into one of the intakes and it's actually wetting the plug, causing misfires.

    Has anyone got any pics of the procedure to check the float heights? I was under the impression that it involved completely removing the carbs, now I have been told that it's just the "tops" of the carbs. I need more info!! It doesn't look like the "tops" come off without completely disassembling the whole linkage and everything. Am I wrong?? (I hope so...)

    Birdman
     
  16. Mike328

    Mike328 F1 Rookie
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    Carbs don't need to come out to get at the floats.

    1. Remove the airbox cover.

    2. Remove the air filter.

    3. Remove the air horns 4 sets of 4 8mm nuts, don't drop the nuts down the throat. Nut driver or 1/4" socket w/ extension or GearWrenches will make quick work of this.

    4. Remove air box cover (additional bolts). Note there are 4 carb gaskets which might have little round metal bushings in them (4 each), don't drop these either.

    You're now looking at the tops of the carbs. Each carb has five bolts, flat-head.

    5. Remove the tops of the carbs via the five bolts. Keep track of which top is for which carb.

    6. Carefully lift up the top of each carb once the bolts are removed. The float will come with it.

    Now you can set the float levels. If the needles and seats are not new you'll want to replace them.

    Setting the float levels is something I've done once, very patiently and it tooks some time. You'll need a nice set of pliers (I use sears craftsman professional "small" mini pliers, needle nose if I recall). You'll also need a good little tiny metal measuring blade/ruler, or even better a nice vernier caliper.

    This is a start...

    Archive search should reveal more about how to actually SET the floats.


    --Mike
     
  17. tuttebenne

    tuttebenne F1 Rookie

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    If you are running stock coils and want to get a hotter spark, try MSD "blaster" coils. They fit in the stock mounts, are relatively cheap and available, and put out a ton more power than stock. You could go to electronic ignition too but its a matter of how far you want to go. I've been running stock points and caps, with 8mm silicone wires and blaster coils for 18 years. Lots of track time, no high end misfires - but still has a bit of the low end stumble.

    Best wishes,

    Andy
     
  18. Birdman

    Birdman F1 Veteran

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    Andy,
    I thought these high output coils were bad for the points? Do you go through points faster than with stock coils?

    Mike,
    Thanks, obviously setting the floats is not as hard as I thought. I'll add it to my winter fix it list!

    Birdman
     
  19. Verell

    Verell F1 Veteran
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    I've got a pair of vernier calipers you can borrow.
     
  20. Birdman

    Birdman F1 Veteran

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    Hey Verell, welcome home!
     
  21. tuttebenne

    tuttebenne F1 Rookie

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    Birdman,

    I have eighteen years and lots of miles on the same set of points I installed with the blasters when I first bought the car. From my experience the coils have produced no negative impact (other than being red in an otherwise dark engine bay)

    Andy
     
  22. Birdman

    Birdman F1 Veteran

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    OK, here's my theory. See if this sounds plausible.

    I take a right hand turn. It's a slow turn, but because of that, it's pretty abrupt. So the gas all sloshes to the left in the carbs. That tends to jam the needle valves abruptly closed. (Would this happen? Not sure. This is where my theory could be wrong). One sticks. Coming out of the corner, I hit the gas, but one of the valves is stuck closed. It can't open until a little fuel pressure from the pump pops it back open, even though the float has gone down again. Since the gas is on and the fuel is flowing to accelerate, there is not enough back pressure to open the valve until all the other float bowls fill up and their needle valves close down enough to give me some fuel pressure on the sticky one. The thing pops and runs lean on two cylinders because one carb is starved for gas. Finally, a little while later, the sticky valve unsticks and everything works again.

    My test to confirm/deny this will be to try an experiment the next time it does it. I'll pull over, shut the car off, and let the fuel pump run for 10 seconds to pressurize the system, then restart and see if it goes away instantly.

    I'm thinking that replacing the needle valves and seats (as well as setting the floats of course) might do the trick.

    Birdman
     
  23. tuttebenne

    tuttebenne F1 Rookie

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    In theory, the fuel pump is running all the time and applying pressure to the valves simultaneously. Consider that if filling three carbs in a slow speed application is too much for the pump to catch up with, what happens when you are really accelerating and all four are drawing fuel at a more significant rate.

    I would put my chips on one or more fuel bowls being low on fuel. I recall adjusting mine many years ago to cure a hard cornering problem at the track. I set the heights 1 mm higher than what the book calls for. No problems and the hard corner starvation got better.

    You just might want to set aside a few hours one night to open up the carbs as per Mike's procedure and check the float heights.

    Andy
     

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