4-stroke runs OK for a few miles, then starts to bog...grr.

Discussion in '4-Stroke Engines' started by salfter, Aug 11, 2007.

  1. salfter

    salfter New Member

    I've run into a bit of a performance problem with my project. It runs strong for 5-6 miles, but then it starts bogging down and surging. Twisting the throttle doesn't cause it to speed up like it should...sometimes, letting off on the throttle a bit causes it to surge and pick up a little bit of speed. It starts out having no trouble moving along at 20-25 mph, but then it has trouble just limping along at 10. After a check ride today, when I got home and lifted the rear wheel up for a no-load test, it exhibited the same behavior: instead of revving all the way up to speed, it'll go so far and no further. It even stalled out on me.

    Adjusting the screw on the side of the carburetor has made no difference. Remounting the engine so it's closer to level (can't get it completely level in this frame) has made no difference. Sealer on the gaskets between the carburetor and the intake port has made no difference. Removing the sheet metal around the cylinder so the engine might run cooler has made no difference.

    My daily commute is going to be about 10 miles each way, so getting only halfway there before running into these problems isn't going to cut it. There are a couple of things I haven't tried yet:

    1) I've never pulled the spark plug. Once the engine cools down, I'm going to pull it and see what it tells me about how the engine is running. In the meantime, is there something that could be happening with the spark plug (or the rest of the ignition system) that would cause this problem?

    2) The fuel line that came with the engine kit was 1/4" i/d vinyl tubing, not much different than I would use to move fluids around when making beer. The hose barb on the carb was 3/16", so I had to add some additional tubing and fittings in between to neck down from 1/4" to 3/16". There are sometimes air bubbles in the fuel line; could this be causing fuel starvation? Also, the clear tubing seems to change translucent after a short time, which would indicate that the gasoline is reacting with it. I'm guessing that a switch to rubber fuel line would be a Good Thing.

    Is there anything else that I'm missing? How do I get this thing running right so it's at least a mostly reliable commuter?

  2. OldPete

    OldPete Guest

    Wrong! Removing the sheet metal will make the engine run hotter. :)

    Your issue sounds fuel related.

    No kinks in the fuel line? If the answer is no...I bet the fuel tank cap is either unvented, the vent is clogged or the filter is clogged at the shut off valve. You might have to remove the fuel shut off valve for inspection.
    I have read where these tanks arrive new with much filth inside. Was the tank cleaned internally before it was installed?
  3. OldPete

    OldPete Guest

    You could have water in the fuel...try draining the flote bowl into a glass and look for water.
    Remove the flote bowl and look for dirt in the main jet.

    Are you using an inline fuel filter?
  4. salfter

    salfter New Member

    How does that work? I'd think that the sheet metal would've blocked air flow to the engine, and that removing it would've let air get to it as you're moving along.

    That's what I've been thinking...I've been throwing up this other stuff as possibilities.

    Nope...it's a straight shot from the tank to the carb.

    I didn't do anything special, but there wasn't anything rattling around inside it either. I could check the screen on the valve...it was clean when I installed it. I have fresh gas in the tank, so that shouldn't be the problem.

    Unvented fuel cap? That wouldn't make much sense, unless it's a manufacturing error of some sort. I suppose one way to test that would be (while the tank's empty) to try blowing in through the valve and listen for air coming out the cap.

    No...there wasn't one in the box. I thought maybe a fuel filter would be a good idea and checked AutoZone to see what they had that might work, but everything seemed a far bit larger than what would be appropriate for this small an engine. I also checked Lowe's (seeing as how they sell mowers and such) as well, but they didn't have any fuel filters of any sort.

    FWIW, I've attached a closeup of the spark plug. I've not gapped it yet; my gapping tool only goes down to .040", but my understanding is that these are supposed to be set to .030" or so. I'll need to get another gapping tool, along with some anti-seize, before I put the plug back in.

    Attached Files:

  5. Get a fuel filter on it ASAP.

    I think OL' pete hit it right on with a fuel cap venting problem.

    It sounds about how my motorcycle was acting up lately, it would run fine then start to bog and then stall out and after sitting a little while it would run fine for a little bit and do it all over again.

    I cleaned the carb and as I was messing with that I just let the fuel flow out the fuel line for a minute and it slowed down to almost to a stop.

    So I knew then that there was a restriction in the fuel supply so I cleaned the fuel valve and the screens and finally checked/freed up the fuel cap vents that were plugged and it has been fine.

    You have to be sure that you have a good consistant fuel flow from the fuel tank.If the vent is cloggged, the fuel will flow for a bit then slow down to a drip or may even stop completely it all depends on the air restriction in the cap.

    Dont do this above:
    I prefer to test things as they are supossed to work so just get a clean GLASS jar and put the carb end of the fuel tube in it and let the fuel run out into the jar and watch how the fuel flows making sure it has a good solid consistant fuel flow.
    You may have to run out a lot of fuel for the problem to show up.

    If the fuel slows down or stops then remove/loosen the cap and it should flow very well if the cap was clogged.
  6. srdavo

    srdavo Active Member

    the flywheel works very much like a vacuum cleaner. It draws air in, to cool the engine. (much more air than the wind can blow across it.) Without the sheet metal, It draws nothing.

    the pic of your electrode on the spark plug looks really light in color....almost white. this indicates a lean air/fuel condition. (bad) check for air leaks. running too lean makes your engine run hotter & can do permanent damage.
  7. thatsdax

    thatsdax Guest

    I had to.

    On my 4 stroke I had to pull the carb and clean it out to get mine to run decent. They use a lot of cosmo when they assemble these carbs and motors. Enjoy the ride.. Expect a DAX 4 stroke by spring. Thanks...Enjoy the ride..
  8. OldPete

    OldPete Guest

    Remember, this is a 4 stroke that is having fuel delivery issues. So yes it does look a little too clean but once the carb is working right that plug color will darken a tad but it will never darken like a 2 stroke's. :)
  9. salfter

    salfter New Member


    A trip to Checker secured some anti-seize, a spark-plug gapper (one that goes down to the .030" gap needed here; the one I already had only went down to .040"), and a can of carb cleaner. They had no fuel filters that looked suitable for a small engine, but the Lowe's on the opposite corner had a filter that looked like it'd work.

    I started by checking the spark-plug gap, since it was still out of the engine...turns out that was OK. I smeared some anti-seize on the threads and reinstalled it. A couple of pulls got the engine running, but now it was bogging right away...even under "no load" (lift the rear wheel and twist the throttle).

    I then checked the flow from the fuel tank. It was slow, but steady...near as I can tell, the shutoff valve imposes a significant restriction. Loosening/removing the fuel cap made no difference, so that appears to be working properly.

    Knowing that the whole engine was covered in some sort of slimy stuff (cosmoline? oil?) when it arrived, I went with the theory that the carburetor was gunked up during assembly and needed to be cleaned. I disassembled it, soaked it in the can of carb cleaner (it's a gallon paint can with 3 quarts of cleaner and a basket), and put it back together. When I reconnected the fuel line, I put a filter inline (the package says Briggs & Stratton part number 5018, but it looks more like this than this...maybe they redesigned it and still have an old photo up).

    While I was at it, I went ahead and reinstalled the sheet-metal baffle around the cylinder. There's another piece that looks like it maybe goes on the front of the cylinder, but it looks like some of the bends in it go the wrong way. Since I'd have to pull the header and muffler off to put it on and since I already had some gasket sealer in that connection, I left it alone.

    After tightening up the motor mount, replacing the old, worn-out kickstand, and reinstalling the chain tensioner I thought I didn't need, I set out for the office. Where the engine previously started acting up after about 10 km, this time it went the whole 19 km to the office without a hiccup. I cruised at about 30 km/h most of the way, crossing a couple of freeway overpasses with ease. After pedaling a bit to get going, twisting the throttle causes it to quickly get up to cruising speed. Twisting the grip to WOT never bogged, but instead you could feel it start to pick up more speed right away.

    After pausing for a couple minutes or so at the office, I started taking a slightly longer way home. I had shut off the engine and fuel valve at the office, so I only got about 1-1.5 km away before the engine bogged and died. I had forgotten to open the fuel valve again...guess it doesn't run too well on air alone. :p

    All in all, I put a bit over 40 km on it this evening, and the only problem I had was not related to the engine. A few klicks from home, the left crank fell off the bottom bracket. :eek: Vibration from the engine must've caused it to work loose; since the nut that holds the crank to the bottom bracket is right-hand-thread even on the left side, it seems that I'll need to hit it with some Loctite to keep it from coming undone again. When I got home, I also found that the bottom bracket assembly had also started coming loose, despite having been tightened down quite a bit. Some more Loctite (of the removable variety, not the permanent stuff...same for the crank nuts) should take care of that. (I never would've had that problem with the one-piece crank that was on there before, but those seem to have gone the way of the dodo.)

    I think it ran well enough (with the minor problems noted above) that I might try using it for the commute tomorrow. It'd be a good idea to bring some wrenches and screwdrivers; I have an under-the-seat bag that can hold those. It looks like my project has turned the corner and is getting to where it might be usable on a more-or-less daily basis.

    For future reference, under installation instructions and tips, it might not be a bad idea to suggest that you should rebuild the carburetor before installation. I suspect it'd save someone else a fair bit of grief.
    Last edited: Aug 13, 2007
  10. Irish John

    Irish John Guest

    electric probs from kill switch.

    One other thing Salfter that I found happened similar to the bogging probs you describe was in fact caused mainly by the really shonky kit kill switch - at the throttle control button end. It is a total piece of crud and the wiring is so bad that it interferes with the electrics and causes intermittent cutting out for a split second at a time. I disconnected it and it improved now I'll clean the carby as per your excellent advice above.
    Russian2Ride likes this.
  11. loquin

    loquin Active Member

    Also, check that there aren't any air leaks at the intake manifold. It's possible that as the engine heated up, the thermal expansion shifted the gasket enough to cause the leak to develop.

    And, as srdavo mentioned, the flywheel is also a blower - it pulls air in and forces it over the engine. The sheet metal directs the air flow, keeping the engine temperature stable. If the shroud is removed, this will interfere with your engine's temperature regulation.
  12. Alaskavan

    Alaskavan Guest

    I'm experiencing a problem, and this thread seems like as good a place as any for it.
    1. The Lifan motor on my trike doesn't idle consistently. It starts up and idles a little fast, then as it warms up, the idle falls off.
    2. When the engine is cold, as I accelerate, the engine will bog occasionally until I'm doing about 20 - 25 mph.
    3. Once I get going, engine cold or warm, the engine surges at speeds between 20 - 35 (perhaps it's bogging a little?). The engine runs smooth at low and high speeds.

    I changed the plug, and tried running without the air filter. No effect. I'll change the fuel filter tomorrow. Anybody have any other suggestions?
  13. bughuggger

    bughuggger New Member

    I had a pinhole sized hole in my fuel line but it was on the place where it covered the metal nipple. So I cut the bad part off and everything was fine again. Maybe a tiny leak. Go for a thicker hose if you're goint to commute.