Carburettor leaks | FerrariChat

Carburettor leaks

Discussion in '308/328' started by pedders, Aug 21, 2022.

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  1. pedders

    pedders Karting

    Apr 25, 2010
    69
    Hi all

    The idle is too fast on my 308 and it gets worse as it gets warm.

    A squirt of brake cleaner on the gaskets where the carb meets the manifold (as advised in a different thread - thank you) causes the idle speed to drop significantly, so I am pretty sure this means they are leaking.

    So, I replaced the gaskets, including the spacer plate. I note that originally this was a one piece item, whereas the replacement (from Superformance) specifies that a paper type gasket is required to be used in conjunction with the spacer. I have put those gaskets either side of the new sandwich plate.

    I still have the same issue - high idle, significantly reduced with the brake cleaner.

    The throttle cable is totally backed off, as are the throttle stop screws on the carbs, so in theory the butterflies are fully closed - but it still idles, presumably due to the leaks.

    I am wondering whether I need to tighten the 4 mounting nuts on each carb, or whether I have perhaps overtightened them. I can’t find a torque setting.

    Also, in the jnterest of full disclosure, I did not find it a fun job - getting to some of the studs to tighten the nuts was a total pain, and a few spanners were chopped up to try and get to some of them, so if I had to guess, they need further tightening.

    Any suggestions, though, before I break something, would be much appreciated. Thank you.
     
  2. bitsobrits

    bitsobrits Formula Junior
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    Nov 12, 2011
    746
    Omaha, NE area, US
    Full Name:
    Steve
    #2 bitsobrits, Aug 21, 2022
    Last edited: Aug 21, 2022
    With new gaskets and spacer plates, IF you have unwanted air leaks at the carb/manifold interface THEN you likely have warped carb bodies or simply under tightened carb mounting nuts. Also, you may instead have an issue (leak) with the small vacuum pipes or hoses that attach to the manifolds.

    To make removing/installing/tightening the carbs much easier, replace the factory hex nuts (13mm wrench) with reduced hex flange nuts (10mm wrench-search Amazon for 8mm flange nut or use your goto fastener supplier). I would suggest non stainless versions so you can retrieve them with a magnet. And buy a combo (box and open end) stubby 10mm wrench and Bob's your uncle.

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  3. Rifledriver

    Rifledriver Three Time F1 World Champ

    Apr 29, 2004
    37,106
    Cowboy Capitol of the World
    Full Name:
    Brian Crall
    Webers are famous for base gasket leaks. The 1st idea is always to tighten them more. The carb is made from a pretty malleable grade of aluminum and that remedy has probably already been done multiple times over the cars life and warped the bases. Any time one is off carefully examining the degree to which they are warped is in order. They can only be filed so much to straighten them. Someone, I think it may have been Pierce manifold was able to actually straighten them to some degree. Good idea, it keeps the throttle shafts from locking up from being so far out of alignment.
     
  4. mike996

    mike996 F1 Veteran

    Jun 14, 2008
    6,872
    Full Name:
    Mike 996
    I have NO experience with Weber carbs but I do have considerable experience with a lot of others and it is not unusual for over tightening to warp a carb surface so that it will not seal properly. Resurfacing on a glass plate with some wet/dry sandpaper is a common "repair." Further tightening an already warped carb base will just make it worse.

    Again, I can't say this is a problem with Webers but it's a simple thing to check with the carbs on the bench...
     
  5. pedders

    pedders Karting

    Apr 25, 2010
    69
    Thanks for the replies all - that’s a great idea on replacing with smaller nuts but with a flange (and non stainless as I have repeatedly fished out the 13mm hex with a magnet during the course of refitting).

    I’m going to try tightening them up a bit, but I am already resigned to getting them off the car and checking the base…

    Thanks again.
     
  6. jmaienza

    jmaienza Formula Junior

    Jan 8, 2009
    628
    Massachusetts
    Full Name:
    Joe
    I have had luck with this procedure. I have a piece of granite countertop (go to a granite counter shop and ask for the sink cutouts). Before I begin, I check to see how flat the base is by seeing if it rocks side to side, end to end and corner to corner. In not, then I carefully check each corner and try to slip a piece of paper under each corner. If it is not flat. I oil the countertop to make the sandpaper stick, put down a piece of wet-dry paper and put some oil on the sandpaper and grind away. I usually go in a circular or figure 8 motion. Every now and then, wipe off the bottom and check for flatness.

    The smaller hex nut is also a good idea.
     
  7. dino1971

    dino1971 Karting
    Silver Subscribed

    Sep 24, 2013
    156
    Bordeaux, France
    Full Name:
    Dario
    Hi, I'm also struggling with this after an engine rebuild on a 308 GT4 and also found that the idle changes when you spray brake cleaner on the base. Removed carbs, sanded spacers (they get imprinted with the carb base) and re-installed to no effect. However, as note elsewhere, the vapors have a tendency to get sucked in by the engine which spoils the result. Sure enough, when I douse the bases with a water mister (which would also temporarily block the leakage), there is no effect.
    Are your air bypass screws completely closed? If they are not, then that is where the air is coming from.
    A crow's foot wrench was good for doing/undoing the nuts.

    Good luck, and keep us posted!
     
  8. cbmcdona

    cbmcdona Karting
    Silver Subscribed

    Jul 19, 2022
    150
    North Carolina
    Full Name:
    Caleb McDonald
    Make sure when you are spraying that it is the carb base that is leaking and that it’s not the vacuum ports in the runners right underneath the carb base that pull from the EVAP system. This would definitely be an easier leak to deal with if it was.


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  9. classicalfas

    classicalfas Formula Junior
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    Dec 5, 2012
    318
    Gardiner, NY
    Full Name:
    Peter D
    Use the smaller 10mm hex flange style nuts. If you don't have, get a stubby combination wrench, this worked great when I needed to remove and reinstall my carbs. I also use a hemostadt to place washers and the nuts holding them by the flanges to place on top of the studs. It's an invaluable tool, I use it all the time.
     
    kiwiokie and cbmcdona like this.
  10. pedders

    pedders Karting

    Apr 25, 2010
    69
    Thanks for the advice everyone.
     
  11. bl10

    bl10 Formula Junior

    Jun 8, 2011
    442
    Chatsworth, CA
    Full Name:
    Barry Leavengood
    1978 GTS Us with Pertronix, Acel plug wires, Pertronix coils, low resistance plug extenders with re-jetted carbs.
    All intake system gaskets are original including base (thick) gaskets.

    A few questions.
    Are you using the flammable brake cleaner? I always heard that if you sprayed brake cleaner at a vacuum leak the engine would speed up.
    How fast is it idling when up to temp?
    Are you sure the cold idle cam screw is not holding the throttle open?
    Are you using the idle retard micro switch?

    After I put the pertronix in I had a lot of trouble getting it to idle down. Keep in mind that without the idle retard switch you end up with the timing advanced about I2 -15 degrees at idle over stock which really speeds up the idle. I suspected a vacuum leak somewhere but couldn't find it so I built a smoke machine per the internet and couldn't see any leaks.
    So I did the following.

    1. restricted the vacuum line going into the carbon canister.
    2. Closed all the air by pass jets. Only one was 1/2 turn open.
    3. Messed with the flow synchronizer to insure all four carbs were closed as much as possible.
    After all this it still idled a bit to fast. About 1200 with AC off

    but I could put pressure on the rear carbs and get it to slow down (especially the drivers rear) so I put an extra spring on it and it works great. I now have to open the idle speed screws about 1/4 turn and get 975 idle with AC on and 1025 with it off.
    I think I should probably reset the butterflies in the carbs but that seems to be a miserable job as the butterfly screws are swaged and really tight. I'll save that one for another day.

    A couple of other things.
    I have cleaned the carbs multiple times over the years so there is probably no grease sealing the throttle shaft bearings, which I wasn't even aware of until I started reading this forum, however I didn't see any smoke coming from the throttle shafts.
    Maybe I need to grease the throttle shaft bearings as I never have.
    According to what I've read the throttle shaft springs a a little wimpy and maybe stronger ones would help.


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