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Inline 4 carburetor tuning

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by mattizie, Jul 8, 2014.

  1. Hi guys,

    Does anyone have any resource for how to tune the pilots on an inline 4 engine? Neither Google nor the service manual yields any answers, and it is a dying art because almost all new bikes now are fuel injected..

    By pilots I mean the air-fuel ratio. At the moment it's running very rich and like dog balls. I'd rather do it myself than have to take it to a mechanic.
  2. Before changing the pilot jets you first need to eliminate all other possible causes for running rich. Perhaps by air-fuel ratio you meant adjusting the idle mixture screws? If so then it depends on what carburettors you have. If the screw is at the front of the carbs, then it is a fuel screw, and turning it clockwise will make it leaner at idle. If screw is towards rear of carb, then it is an air screw, and turning it clockwise will make it richer at idle.

    Need a bit more information. What bike is it? Is it standard or modified? Has it just all of a sudden started running poorly, or only since a modification?

    Pilot jets effect mixture from closed throttle to roughly 1/4 open. It overlaps with the idle mixture screw at closed throttle, and with the slide needle, which progressively takes over from 1/8 throttle to 1/2. Float level will effect mixture across the board from closed to fully open. Main jet has progressively more influence from 1/3 throttle onwards. This is a rough guide only.
    • Like Like x 1
  3. What he said +

    The pilot screw will be felt when you crack the throttle. So sitting on maybe 70 in top gear with the throttle almost closed and then open it. If it's initially sluggish, this is an indication of rich.

    If it pops on closing the throttle, this is an indication of lean.

    Most carbs are set 2 turns out from closed. So if you don't have the factory setting, then start there.

    Go for a ride.

    Wind all 4 half a turn and take for another test run. Once you are within half a turn, go to 1/4 a turn and then 1/8. make sure you wind all the way in every time and then wind out again. It's not hard to turn 1 1/2 turn the wrong way and then that stuffs up you whole procedure.

    I use the screwdriver bit off a battery drill to do it. These are small enough to allow you to get your hand in under the carbies whilst they are still on the bike.

    Also, you will need to adjust the throttle stop as you adjust the idle mixture. A more correct mixture will tend to raise the idle. So the closer you get to ideal, the more you will have to lower the idle speed.

    But as noted above, clean the carbies, get you float height right and sync them first, otherwise you are wasting your time.
  4. If you have a friendly vet you can grab four rather large syringes and some fish tank airline tube and make your own flow meter. Very helpful little tool for tuning carbs and I wouldn't start the job without one
  5. get 4 little needle valves whilst you are at the aquarium.
  6. Thanks for the replies:

    To clarify, the bike in question is a HONDA CBR 600 FX (1999 model).

    The bike was running perfect. This only happened when I took the carburettors off to clean them. First time doing the job, and didn't count how many turns out the original screws were before I took them out to soak them in carb cleaner with the rest of the jets. When I put them back I used the factor setting of 3 & 1/8 turns out for each carb.

    The problem is most apparent at low throttle; at higher throttle the problem vanishes entirely.

    Bretto and ibast, are you referring to carb synchronisation or something else, I'm not sure how I'd use a flow meter for this application. The carbs have already been synchronised.

    What I'm not sure about is how do you know which of the four screws needs to be screwed in or out, or do you just take them all out evenly and they'll all hit the sweet spot at exactly the same amount of turns?
  7. So were they synchronised after you did the carbie work of before?

    You adjust the screws all exactly the same.

    You haven't really told us what the problem is. You began with the assumption it was the idle jet screws.

    tell us a bit about what the bike is doing exactly.
  8. A bike running rich is more than the pilots. Taking carbs off, maybe having them upside down on the bench or even bumping them at an angle can cause problems. Cleaning them will.
    Kinda one of those ones that if it's not broke don't fix it.
    For mine I would set the set up and sync them again. That usually includes pilots.
    Basically you go back to factory specs and work from there.
    The syringes have mil increments so you can visual whats happening and adjust..... even at idle.
    There's plenty of utubes of it if you search, but most manuals run you through it step by step
  9. Bretto and ibast, thanks for your information.

    The carbs were synced after I'd cleaned and put them back together. I'm fairly confident that it's the pilots that are causing this. Like I said before, the bike was running perfectly, pretty much the only thing that I stuffed up was forgetting to count the pilot screws' turns before taking them out.

    In terms of it running, I misdiagnosed it, I think it's actually running lean. When idling or at low throttle it has very low power and "gurgling" kind of sound coming from the exhaust, I need to rev it higher to take off, and the engine cuts out if driving up a slope (my driveway) at idle, before it would climb on idle no problem. When accelerating the "gurgling" sound is present at up to about 1/4 throttle, and goes away with more throttle. If I accelerate and the chop the throttle to slow down, it backfires a fair bit. It won't do I try to replicate the backfire in neutral, but in gear, on the road, it will. Also, the problem is most prevalent when the bike is cold, but still persists after riding the bike for around 30 mins on the freeway.

    I did some more research into using a flow meter and I think I get it now. Would one of these http://hyperphysics.phy-astr.gsu.edu/hbase/fluids/imgflu/venturi.gif
    across each air inlet do the trick?

    So what I need to do is:
    Turn all screws out at the increments you've specified and keep turning them all out evenly until it sounds about right. Then use a flow meter across each inlet and fine tune the mixture until the air inlet flow is the same across all of them. Correct?

    Can't believe I almost forgot about Bernoulli's principle, we did it to death back in the earlier years of uni.
  10. Sorry read it like they'ed been done before..... Seems correct but it's Friday and a few have been had.
    My thunking is it's a pita to have to take all the tank, shrouds, airbox and crap off again.... So yeah your 95% all's good before closing the patient.

    Make sure the front manifold to the head seal is good, no great. It's always the last thing and quite often the problem
  11. Popping on closing the throttle is an indication of lean mixture screw.

    You don't go near an airflow meter after you've synced

    Just wind all the screws in until lightly finger tight, then wind them all out the same. Start with half a turn more than what they currently are.

    Take it for a ride and see how it feels. Then try 1/4 turn either side of where it is. Keep going until it feels right. I take it to 1/8 turns.

    Adjust the idle speed as needed.
  12. Most pilots I've set are between 1/1.25 out to 1.75
  13. are you talking about this bike? if so, they should all be the same. 2 turns out is common