Carb Cough

Discussion in 'Engine, Fuel and Exhaust' started by FLDL02, Jun 25, 2013.

  1. FLDL02

    FLDL02 Member

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    Gents,

    I haven't posted for a while but something new happened.

    I have a carb cough when rolling throttle off then back on slowly the first 1/4 turn. It happens in all gears, at all speeds, under all loads, warm or cold engine (worse when cold engine).
    I ride at sea level in Vancouver, BC (i.e. Pacific northwest climate).

    I just rode the coughing 2002 FXDL to Vegas and around the southwest desert for a week. The first day the normal regular cough happened with the normal frequency, 5 to 10 times every tank of gas. Same the second day until we got to Ely, NV. From then on not a single cough for 5.5 days of riding from Vegas to Roswell to Tombstone, Phoenix and back to Vegas. High elevations, dry air, day temps in the 80's etc. We left Vegas and by Idaho the cough was back with the same old frequency.

    One solution to the cough is to ride with enricher out 1/3 all the time.
    The other solution to the cough is to move to Vegas.

    Since these are not viable options, what jetting/mixture/washer settings do you think would improve the coughing for riding in the Pacific Northwest?

    The desert had high elevation, hotter and probably dryer air.
    In British Columbia the coastal air is cool and probably more saturated. The BC interior climate is dryer and hotter with higher elevations. The cough is rarer in those climates.

    Is there a guide for tuning to different climates? I want to eliminate the cough in the coastal areas even if it means slightly lower mpg in the interior. I get 45 mpg with the current configuration. Stage 1, CV carb from prior owner with I believe 46 pilot jet. I tried a stock 45 last summer with no improvement. Idle mix screw set where it runs best. I can't remember exactly but between 1 and 3/4 to 3 turns out.

    Keep in mind that no coughing occurred in the desert conditions with this configuration.

    Thanks
     
  2. Slo-Ryd

    Slo-Ryd Junior Member Contributor

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    Have you removed the air cleaner and looked into the throat to verify the accelerator pump is giving a squirt of fuel when you twist the throttle? Also, an intake leak or a small hole in the top diaphram that lifts the slide can cause some of the issues you describe. I don't believe the problem is in your jetting and chasing around changinging jets may only mask one of the suggestions i mentioned. A thorough rebuild of the carb may be in order. You shouldn't have to run it with the enrichner pulled out slightly. Coughs are usually a lean condition and as the air temp gets warmer, the motor requires a little bit less fuel to run efficiently. My money is on one of the three areas i've mentioned.
    [​IMG]
     
  3. Bodeen

    Bodeen Well-Known Member Staff Member Moderator Contributor

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    I agree with BigAl. Put a new fuel pump diaphragm in it. Also look down the throat of the carb and make sure the slide moves freely when the throttle is engaged. Also make sure that the fuel stream that enters from the fuel pump is shooting straight down the throat. If not then it can be twisted so that it does so. You may even try a Yost Power Tube if nothing else works. Good luck and let us know what you find.
     
  4. FLDL02

    FLDL02 Member

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    Thanks but,

    The only variables were the climate and the enricher knob. Shouldn't that rule out intake leaks and diaphragm holes? Twist squirts fuel apparently in the right direction and slide seems fine. So if my climate is causing the cough what "tuning adjustment/jet/washer/mixture screw" combination is required to richen the mixture just enough to eliminate the cough?

    Adjusting the mixture screw alone does not eliminate the cough. I haven't tried washers yet and haven't taken the carb completely apart. I guess that's the next step.
     
  5. Slo-Ryd

    Slo-Ryd Junior Member Contributor

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    No, not really. A cough will more times than not lean towards the bike being too lean. Your having to use the enrichner pulled out slightly re-affirms that it wants more fuel. A 45 or 46 slow jet should be more than enough for it unless you were running a high compreassion radical build. Try spraying the intake seals with WD-40 or carb cleaner to confirm it doesn't have an intake leak. I've battled intake leaks on my Softail since the day I got it and it exhibits the same symptoms when one develops.
     
  6. Rockdrummer

    Rockdrummer Member

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    check the intake seals. with the bike running spray some carb cleaner around them. If the idle increases you most likley have leaks at the intake seals. This is a common problem. I would do this with the engine cold. The carb cleaner is flamable. Use with caution.
     
  7. FLDL02

    FLDL02 Member

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    I have sprayed around the intake seals with no changes observed. Must I use a fluid sprayed through a tube touching the entire circumference of the seal to be sure or is a flammable vapor sprayed in the intake seal vicinity enough to reveal a leak?

    What does it mean that the cough did not happen even once in the southwest desert? A leak goes away there... or the lean mixture is fine there?
     
  8. Bodeen

    Bodeen Well-Known Member Staff Member Moderator Contributor

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    I'm betting on the air density being different and therefore changing the mixture.

    The tube would be better specially if the leak is small.
     
  9. Jack Klarich

    Jack Klarich Expert Member

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    The costal cooler air will be a major issue in coughing and possible carb icing also
     
  10. tourbox

    tourbox Senior Member

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    You said you tried a 45 jet last summer but no change. You thought the PO had a 46 jet. By going to the the LOWER 45 you leaned it out even more. Is the 46 jet in it now? Also do you know what the Main jet is? You can go larger on either jet to richen it up. I would still look for intake leaks also.
    tourbox