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  1. #1
    Master Lancista Professorgearhead's Avatar
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    Lurching/surging carb (?) problem.

    My '72 1,600 runs really well except between about 2,200 and 2,800 RPM. There it lurches and surges. In other words, it’s not smooth, quite unpleasant, and downright embarrassing . It’s not throttle linkage which I’ve fixed with bearings. It’s not fuel pump. The fuel pump is new and I just pressure tested it. It shows 6 lbs. and hold the pressure nicely. Float levels are correct. Ignition (Pertronix coil, module, and wires), timing, plugs, valves, all carefully set and checked. New fuel filter, and this winter I redid the gas tank and blew out the lines. I’m still convinced it’s a fuel delivery issue.

    Some ham-fisted previous owner tightened the carb emulsion tubes to the point where the holes of two tubes were partially crushed. I “uncrushed” them as best I could and redrilled the holes to the same diameter as the “good” ones seemed to be. I’ve recently been told that returning damaged emulsion tubes to their OEM configuration is essentially impossible, and that this may be causing the surging/lurching because they must be perfect—incidentally, the are all marked “Sole 775”, which I believe is correct. I’d appreciate any thoughts/advice before ordering new ones.
    Thor Frohn-Nielsen
    '72 Lancia Fulvia 1600

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    Legendary Lancista 1,6 HF's Avatar
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    Re: Lurching/surging carb (?) problem.

    6 psi is too much fuel pressure; you really don't want more than about 4.5 psi. I can't say that this is your problem, but it may be related.
    Ed Levin
    Fulvia 1,6 HF

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    Legendary Lancista lancialulu's Avatar
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    Re: Lurching/surging carb (?) problem.

    Sole 775 is Solex 175 which is correct. Do you have the brass washers underneath? These set the height of the emulsion hole .......

    I suspect the emulsion tube can be improved with more holes below which in theory richens the mix during early transition from the idle circuit to main jet. The 35mm carbs fitted to the 1300 has these extras 2 rows (but the are just drill through and not cruciform like the ddhf, and they are the smaller diameter type).

    The 42ddhf is improved by increasing the idle jet to somewhere near 60. This helps the above situation of transition too.

    Fuel is another issue. Shell Vpower seems to suit my 1600’s. Supermarket pump fuel is of dubious quality wrt ethanol which makes the fuel more volatile.

    Finally, have you carefully checked the actual cam timing opening and closing wrt to specification, a 20degree out on inlet timing can cause real performance issues in the rev band.

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    Master Lancista Professorgearhead's Avatar
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    Re: Lurching/surging carb (?) problem.

    Yes, I have the washers under the emulsion tubes. I have not checked the cam timing but will put that on the list. Is there, by chance, a link to some good instructions for doing so?

    From a a bunch of reading last evening, there seems to be pretty compelling evidence that too high fuel pump pressure can lead to the surging I’m experiencing. The argument is that the carbs are simply overwhelmed—particularly at lower speeds/throttle openings. Thus I’m going to start there. The adjustable pressure regulator is on its way.

    of late I have been using fuel with ethanol, and did wonder whether that could be a contributor. Non-ethanol-laced fuel is harder to find in Canada, but Shell's high octane stuff is guaranteed not to contain any. I’ll try it.

    i am puzzled that the fuel pump would be pumping more than spec. (The pressure meter is accurate). It’s new last year, and bought from a well respected Lancia specialist.
    Thor Frohn-Nielsen
    '72 Lancia Fulvia 1600

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    Legendary Lancista Taddraughn's Avatar
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    Re: Lurching/surging carb (?) problem.

    Quote Originally Posted by Professorgearhead View Post
    I have not checked the cam timing but will put that on the list. Is there, by chance, a link to some good instructions for doing so?
    If you search around on the forum there are a few threads around that talk about it in detail including both the Lancia method of using feeler gauges and the other method of using an indicator and degree wheel. I found the degree wheel method to be easier for me, but this probably won't be the case if your engine is in the car (not much room down by the crank pulley). The shop manual provides instructions for the feeler gauge method. It also provides cam opening and closing specs when .4 mm of tappet clearance is set.

    I'm at work at the moment and don't have time to search through and find the right threads to link in this reply but I'll edit this post later when I have time to find the threads I was reading prior to putting my cylinder head back on.

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    Legendary Lancista lancialulu's Avatar
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    Re: Lurching/surging carb (?) problem.

    Aim for 3psi fuel pressure....

    Timing cam is with workshop manual method using a 2.2 mm slipguage.

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    Master Lancista Professorgearhead's Avatar
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    Re: Lurching/surging carb (?) problem.

    Thanks for all the suggestions. I’ll report back later next week, after I’ve installed the Holley adjustable fuel pressure regulator. Please stay tuned.
    Thor Frohn-Nielsen
    '72 Lancia Fulvia 1600

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    Re: Lurching/surging carb (?) problem.

    Thought I should report back, though later than I intended. I bought a Holley adjustable fuel pressure regulator, and it immediately became clear that too much fuel pressure was indeed the problem. There’s no gauge on the regulator, so it’s trial and error, but the lurching is gone and the car drives smoothly throughout the RPM range. I’m happy! What does puzzle me, however, is that the new fuel pump which I bought from a reputable dealer, and which is supposedly to original specs, puts out 6 lbs. pressure when 3 is appropriate. Thanks again for the advice.
    Thor Frohn-Nielsen
    '72 Lancia Fulvia 1600

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    Legendary Lancista Marnix's Avatar
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    Re: Lurching/surging carb (?) problem.

    I am assuming the 6lbs means 6psi?

    OK, this is actually a bit over the limit for the carbs, normally max would be 4.5psi (the float valves are specified to be able to hold 5psi).

    I am not sure what version of the car we are talking. S1 cars (from a certain serial No on, when the carbs started to have the spring load ball point float valves) would have a pressure regulator. S2/3 cars didn't have one, but the fuel return line acted as a crude fuel pressure regulator. I a few cases (the S1 Fanalone), both were there.

    Normally either of them would be able to reduce the fuel pressure of the fuel pump to "normal" values. The original fuel pump contained a (crude) pressure regulation internally. Aftermarket pumps, well...

    I normally provide a fuel pressure regulator in any case. Too much trouble to try and find what is wrong otherwise if the engine behaves strangely.
    Lancia Fulvia 2C 1964
    Lancia Fulvia Coupe Rallye 1,3 1968
    Porsche 924 1979
    Alfa Romeo 33 1.4IE 1993

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    Master Lancista Professorgearhead's Avatar
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    Re: Lurching/surging carb (?) problem.

    Hi Marnix: yes, I did mean 6 lbs pressure. The car, a '72 series 2 1,600, has the original 42DDHF Solex carbs—which I have gone through. I’m dialing the fuel pressure back and have immediately noticed a big difference on the lurching. Where it used to be quite considerable between about 2,200–2,800 rpm, it is now far less, and really only present at lower rpms now: 1,800–2,300. The Holley pressure regulator has 15 complete turns from open to closed, so I’m dialing it back bit by bit, looking for the sweet spot.
    Thor Frohn-Nielsen
    '72 Lancia Fulvia 1600

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