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Thread: stumble on hard accelleration

  1. #1
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    stumble on hard accelleration

    So I've been fighting this issue for a while and rather than flail around more I thought I would ask for advice.

    Since getting the car back on the road earlier this summer its run for ~400 miles fairly well other than one issue. A hard acceleration will cause it to stumble. This will happen either from a dead stop or when I am at speed and want to, say, pass another car. It will even due it under no load (just revving it in neutral). Its just fine if I am a bit leisurely with my foot. As long as I don't stomp on the throttle, it runs just fine at high speeds (70+ down the freeway).

    Things I have done:
    * check pretty thoroughly for induction leaks.
    * replaced spark plug wires (didn't expect much, but it was cheap and easy)
    * replaced my distributor with a 123Tune+. Made the car run much better in general, but didn't do much for this particular problem. I will probably start another thread about how much I love this thing.

    Things I'm planning/considering:
    * replace the coil. Doubt this will make a difference, but its easy.
    * install a fuel pressure gauge to check for low fuel pressure. I have only the engine driven fuel pump.
    * dismantle my Mikuni carbs and inspect the accelerator pump circuit


    What else should I be looking at?
    Frank Stutzman
    '56 190sl (chassis 01186)
    Done with paint, let the wild re-assembly begin!!!
    Boise, id

  2. #2
    Gerry Lazo 56 190SL owner
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    Re: stumble on hard accelleration

    Frank,

    Is the Mikuni like a Weber? If it is what pump jet are you us ing? I had the same issue on my webers and I had to increase my pump jets and replace my pump bypass valve in the bottom of the float bowl to closed.
    Gerry Lazo
    56 190SL (6500870)

  3. #3
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    Re: stumble on hard accelleration

    Gerry,

    I dunno. I've never been intimate enough with Webers to make a comparison.

    I bought these Mikunis from Will Samples (S&S imports) 20+ years ago. They were set up explicitly for the 190sl, so I assume the jetting is good. I don't remember having this problem 20 years ago, but, well it was 20 years ago. I'm thinking that if it accelerator pump circuit, the problem is a bit of dirt or varnish rather than the jetting. Would have thought a couple of hundred road miles would have caused this to work itself out though.
    Frank Stutzman
    '56 190sl (chassis 01186)
    Done with paint, let the wild re-assembly begin!!!
    Boise, id

  4. #4
    Gerry Lazo 56 190SL owner
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    Re: stumble on hard accelleration

    Mine were down for 8 years and they had varnish and I even drained mine first. I had to clean them at reassembly.
    Gerry Lazo
    56 190SL (6500870)

  5. #5
    23K Original Miles slover's Avatar
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    Re: stumble on hard accelleration

    I'd check the accelerator pump calibration
    John Lewenauer - Newsletter Editor - Regional Director
    1961 190 SL
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  6. #6
    Registered User Vernon Harrison's Avatar
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    Re: stumble on hard accelleration

    Check what position the rod is in in the accelerator pump.

    My 190 stumbled in the standard position when I rebuilt the carbs.

    Moved it to the hole with lower throw. Believe it was the lower hole.

  7. #7
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    Re: stumble on hard accelleration

    These symptoms are typical of a multiple carb setup where the carbs are out of sync. At full throttle both carbs should be against the their full throttle stops. Worn linkages and bearings can contribute to this as well as misalignment of the throttle rods. If they are out of sync far enough, one can expect backfiring through the carb that is opening late. On my setup I can expect your symptoms when a linkage rod on either the front or rear carb gets off by as much as 1/2 turn, which is what, about .002"? With tolerances like this it is easy to see that any wear in the ball/sockets and bearings in the rod that connects the two carbs will affect synchronization.

    Replacing the ball/sockets with aircraft cable ends gets rid of the slop that is inherent with the original parts. The latter use bearings with no measurable play.
    Robby Ackerman

    Leftover parts are proof you've made it even better

  8. #8
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    Re: stumble on hard accelleration

    Thanks for the suggestions everyone. I'm a little slow on following up on this as my heater in the garage can't cope with the temperatures we've been having. I'm lucky to survive 20 minutes out there before my fingers are too cold to hold tools.

    I haven't unmounted my carbs yet to check the acceleration pump, but per Vernon's suggestion I took a mirror and checked the accelerator pump actuator rod. Its a little hard to see, but I think it is already on the lowest of the three holes.

    Robby's suggestion is really good. I am reusing the original solex linkages that came with the car. There is no slop in it at idle, but I'm not so sure at other positions.

    I do have a carb airflow meter similar to http://amzn.com/B00CMC57R0 that I used to sync the carbs at idle, but it is a little useless on something like this that is a little transient. There is no way of switching it between air horns fast enough to catch this stumble.
    Frank Stutzman
    '56 190sl (chassis 01186)
    Done with paint, let the wild re-assembly begin!!!
    Boise, id

  9. #9
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    Re: stumble on hard accelleration

    So a little update on this issue...

    I picked up a cheap Mr. Gasket fuel pressure gauge and plumbed in in between the fuel pump and the carbs. The jiggling indicator needle showed something like 1.5 psi regardless of RPM. That didn't make sense as the Mikuni manual I that I have says that the fuel pressure has to be between 2.84 and 4.98 PSI. Had to be instrument error. Rented a more sophisticated gauge from a parts store to double check. Unfortunately that gauge had a 1 to 100 range, but the needle did move off the peg so I'm thinking that the 1.5 PSI is vaguely correct. Still scratching my head on why my carbs are working at all.

    I moved on the the throttle linkages per Robby's suggestion. I had picked up three more airflow meters, so now I could stuff one into each throat. While the carbs were fine at idle, they were way off in the rest of the throttle range. I tweaked them around and got them much better, but I think I have some wear in the linkages that is causing a bit of slop. I could tell the carbs were working better, but the silly carbs still stumbled on a hard acceleration. None the less, a tip o' the hat to the master.

    Today I decided to un-mount the carbs. Their bowls were full of fuel, so I figured if I gave them a quick jab on the throttle lever I should get a squirt of fuel into the carb throats. And I did - on exactly one of the throats. Looks like I tearing into the accelerator pump circuits.
    Frank Stutzman
    '56 190sl (chassis 01186)
    Done with paint, let the wild re-assembly begin!!!
    Boise, id

  10. #10
    56 & 60 sl & 67 Sedan 230 Andre Hudon's Avatar
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    Re: stumble on hard accelleration

    Remove filter and check your fuel pressure.
    ________________________________
    André Hudon

  11. #11
    Administrator wpuryear's Avatar
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    Re: stumble on hard accelleration

    If you have enough height in your garage, you can check supply pressure by teeing in some clear tubing hung from the rafters. Gasoline will rise about 3.2 feet in the tube for each psi.

    Walt

  12. #12
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    Re: stumble on hard accelleration

    With engine off, and the accelerator pedal to the floor (I press a 22" stick between the pedal and seat), check the linkage. One will be against its full throttle stop, and the other will not be. Adjust the shorter linkage so that it's also against the full throttle stop of that carb. Forget about how tight they are at idle -- we don't drive the cars at idle. See if this is an improvement or not. Everything else can be perfect, filters, plug wires ..., and as long as the carbs are not sinc'd you'll have these flat spots. Note the adjustment must be made using the accel pedal and not grabbing & moving the linkage by hand in the engine compartment.

    BTW, Just finished installing new front CV axle half-shafts, differential seals, headlight assemblies, PCV valve ... on my daughter's Outback, and have a number of left over parts. There is still quite a ways to go though, to get the Outback's weight down to the 725 Kgs of a Lotus Elise, ha.
    Last edited by robby ackerman; 02-14-2018 at 07:40 AM.
    Robby Ackerman

    Leftover parts are proof you've made it even better

  13. #13
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    Re: stumble on hard accelleration

    Walt

    I'll remember the 3.2'/psi.

    My garage floor is 32' above sea level, the fuel pump is 21" above the floor and the rafters are 10' 9" off the floor. Yes, this test method will work for me, but I'll need to drill a hole in the metal roof.

    Robby

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