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Monkeys mess with my engine

Discussion in 'Mechanic and Repair' started by CowDoc1, Sep 8, 2010.

  1. CowDoc1

    CowDoc1 LawnSite Member
    Messages: 8

    Hustler Super Z with 25hp V-twin Kawasaki FH721V-AS12.

    Had one vented fuel cap and one unvented. Both tanks 3/4 full. Mistakenly tightened down unvented cap. While cutting level ground, vented tank self filled, unvented tank almost emptied. Yes, that seems backwards. Smelled gas "squirting" out of vented cap. Loosened unvented cap, tank levels equalized. Problem solved... I thought. Few minutes later lost power, as if on one cylinder. Some white smoke.

    Pulled plug wires, found cylinder near fuel pump was culprit. Both plugs carbon (not oil) fouled. Replaced plugs and that coil, adjusted all valves. No change. Swapped coil positions. No change. Still same cylinder. Coils properly gapped with no wobble on flywheel. Woodruff key undamaged and in place. Compression 120 & 170 (problem cylinder). Minimum spec 57 psi. Ran Sea Foam to remove carbon. Started backfiring through carb.

    Disassembled heads. Mild carbon build up. Sea Foam hadn't done anything to remove it. Don't waste your money. Cleaned heads and piston faces, lapped valves, reassembled engine. Valves, guides and seats look OK. All valves set 0.004" (0.003-0.005 spec). No change in performance.

    Disassembled and cleaned carb. No debris noted. Jets look fine. Did not pull caps. Blew compressed air through all passages. Changed fuel filter. Checked fuel pump. Fuel output OK, but hose to cylinder head is hard and should be replaced. Can't say that's the problem. Reset the governor arm. Verified no air leaks around carb or intake. Tried cutting grass. Weak at first and gradually lost power over 30 minutes. Almost didn't make it home. Again the same cylinder.

    The engine runs fairly well cold but has miss on bad cylinder. Can consistently locate bad cylinder by pulling plug wire. Spark plug tester light "looks" like plug is firing normally even when engine misfiring. It's hard to tell. The governor searches a little. Probably due to misfiring.

    Air filter is for externally vented carb and is correct. Both plugs carbon foul within minutes. Tried plugs with 2 step higher heat range. No change. Was going to replace carb jets if available, but read in forums of people with similar problems. Replacing whole carb didn’t help them.

    So... Why is this thing fouling plugs so quickly? And why is it losing power on one cylinder consistently when hot? It’s not the coil, plug, flywheel or top engine bearing, timing, fuel pump, fuel filter, valves, compression and “probably” not the carb. As a side mystery, why did the unvented fuel tank empty and the vented one overfill? That's just wierd.
  2. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    From the pics below which looks like the condition of your plugs ?


    Your fuel tank mystery is easy, The un-vented sealed tank built up pressure inside (sun/heat/fuel sloshing around) and pushed the fuel over to the other side. Both tanks must be vented.....
  3. CowDoc1

    CowDoc1 LawnSite Member
    Messages: 8

    The plugs are "carbon fouled". Restricted air cleaner? No, it's a new filter and it happens even with the air filter off. Weak ignition? No, its a strong consistent spark. Poor compression? No, it has 3 times the minimum spec. Over rich fuel mixture? Ah Ha... maybe.... but the jets have no adjustment.

    I thoroughly cleaned and blew out the carb. It is possible debris is under one of the caps. I'm reluctant to remove those. I'll tear into the carb again tomorrow. Don't know what else could be causing this. Do fixed jets wear out? Dirt is more likely.

    Sorry Restrorob, but I disagree with your fuel tank assesment. I agree the fuel would expand slightly, but we are talking about pushing 1/4 tank of fuel to the vented side and at the same time burning 1/2 tank only out of the unvented side. Logically, the unvented tank would form a vacuum and no fuel could come out of that tank... except a little due to expansion. It should have sucked the vented tank dry and nothing out of the unvented side. It has to be monkeys.
  4. 44DCNF

    44DCNF LawnSite Bronze Member
    Messages: 1,564

    If there is a hose between the two tanks for equalization, make sure it is not obstructed. I had a similar experience in a Mack due to that crossover line degrading internally, and a plugged vent on one tank (unvented cap in your case).

    Have you tried to run it at a little less than full throttle to see if the miss goes away? Have you looked in the intake to verify the choke is not closing any while at full throttle? I don't know if there a need on that engine to synchronize the throttle and choke adjustments, as there is on the 12.5 Kaw, but this sounds like what happens (except the fuel transfer) when the choke and throttle are out of adjustment with each other? On the 12.5 it's a matter of aligning the choke and throttle linkage via a drift pin or drill bit at a certain throttle setting, and adjusting the throttle cable. You might check that. If out of synch, it could allow the choke to come on slightly, a little too early (while in full speed detent rather than just past it) and that could be the cause of carbon fouled plugs, smoke, and miss or stumble.
    Last edited: Sep 9, 2010
  5. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    Well Doc, SHOOT those monkeys !!!

    Your thinking is scewed, If you have two tanks tied together with a 'T' or 'Y' going to one fuel pump the fuel pump will pull from the least restricted tank. Therefore the pump will not pull a vacuum on the sealed tank. As I said, Pressure built up in the sealed tank and the pressure pushed the fuel into the vented tank. I have seen this happen myself while test running a unit not knowing the customer had closed one vent.

    Now for your run/miss issue,

    Take that "light" spark tester and throw it in the trash, All their good for is sending people on witch hunts as it did you. Pick up a REAL spark tester and check for intermittent spark again.

    If you don't want to do that test the coils as below with a multimeter;


    My quarter is on a bad coil, Everyone knows Kawi's eat coils like cows eat grass......

    On to your carbon fouled spark plugs, Pull the plugs on the low speed adjusters then remove and clean the low speed circuit properly.

    Good Luck
  6. CowDoc1

    CowDoc1 LawnSite Member
    Messages: 8

    I pulled the caps and all jets and cleaned the carb again. In the barrel, just behind the pilot jet, are three holes that are the outlet for the main jet. The holes in the right barrel are all the same size. Two of the holes in the left barrel are much smaller. To equalize the barrels, the factory set the left pilot at 2 1/4 turns. The right pilot was only backed out 1/4 turn. That would correct for the different hole sizes. I'm not sure if the different size holes are a manufacturing flaw or if it was designed that way. If someone has a comparable carb, please take a look. The left side does have a smaller main jet. I don't understand the reasoning behind that. Both cylinders are the same and the same distance away. Why?

    The left main jet looks like a #138 (8 hard to read). The right is a #140. Neither of those sizes match the parts drawing. What size main jets are needed for an externally vented carb with a standard (not canister) air filter? Maybe too large of jets is why it's running rich and fouling plugs.

    There was no change in performance. I tried tweaking the pilot jets at idle, but no joy. The left cylinder won't smooth out.

    I checked compression while it was warm... 97 (on bad cyl) and 160. It was 170 (bad cyl) and 120 before I cleaned heads. Those measurements were cold. I'm not sure why the cylinders are different but they are above the spec of 57. I'll run it until it loses power tomorrow and check compression in the "failed" state.

    How does automatic compression release work on this engine? Could it be releasing compression on the left cylinder when it shouldn't? Doesn't seem likely, but I'll have to rule it out.

    Restrorob, you lost a quarter. I already checked all three coils with a multimeter... twice. I replaced the one on the bad side as a precaution and I tried swapping the right and left coils. The left cylinder is always bad. The right is smooth and steady. Sorry, but it's not the coil.

    That is a neat spark tester and I may have to buy one, but the Matco tester is fine for this use. It was more of a verification since changing coils and plugs had no effect. Since it lights at the right times, even when stumbling, the spark is most likely OK. You are right. A spark problem isn't completely ruled out.

    44dcnf, the choke and throttle cables are on seperate levers on this mower. I did verify the choke isn't closing at full throttle. Also, the fouling even occurs at idle. Nice try though. I do believe fouling is a carb problem.

    The two fuel lines T before the fuel pump. If one line were clogged, the pump would drain the other tank (least resistance as Restrorob said). But that tank shouldn't over fill. Hmmm... unless there was a one way valve. Maybe a leaf over the tank outlet acting like a flapper valve. It would let fuel in but not out. I suppose debris in the line could do the same thing. A possibility. But something would still have to force the fuel into the tank. Little or no fuel should come out of an unvented tank or clogged line. All of it did.

    I understand what Restrorob is saying, but I disagree. If you fill a coke bottle with water and turn it upside down quickly, the water stays inside. The vacuum won't let it out. That would be the unvented tank. If you heat the bottle, the water would expand and create pressure. A few spoonfuls would push out. The same with sloshing. The small change in pressure would not force all of the water out. My unvented tank was emptied. Makes no sense to me. The gas should have been used from the tank with least resistance. The vented one. Instead it was used from the one with most resistance. I may have to recreate the situation. Anyone got some monkeys? :)

    Still no handle on the main problem of power loss on the left cylinder. Best thought now is an intermittent compression problem that gets worse when hot.
  7. Ruben Rocha

    Ruben Rocha LawnSite Senior Member
    Messages: 577

    You may think it is a few drops of gas.
    But I have the same problem with vented tanks.
    No isolation valve between the tanks.
    Sitting in the sun the radiant heat builds pressure and starts pushing gas out of one tank to the other . Or in your case non vented would seem worse. as the gas vapors heat up they expand increasing pressure in the tank then the fuel has no where to go but release through the fuel line so where does it go but in the carb or to the other tank.
    Your example of a coke bottle is a static example. With only what 16 oz. And water is not gas . Now add heat to the bottle if it is 5 gallons and see what happens. As more fuel is pushed out of the tank it allows room for more fuel vapors to expand. So the problem exponentially gets worse.
    If you want a good example get a 5 gallon plastic gas can.
    Fill it up all the way while cold.
    Set it in the sun all day not much will happen.
    Then take a gallon out.
    The can will swell up in the sun. Then at night it will collapse some due to the vacuum.

    Rob is right on track with this issue.
    But it is your choice to ignore this suggestion .
  8. fatboynormmie

    fatboynormmie LawnSite Bronze Member
    Messages: 1,293

    Tried using this procedure on my kaw fh451v last week and was never able to get these readings on either the old coils or the new coils. I was testing with a simpson 260 and a fluke 73III so good luck.
  9. fixer67

    fixer67 LawnSite Silver Member
    Messages: 2,098

    Here is what I think happened to the gas tank mystery. The UNVENTED tank built up pressure from being shanked by you mowing. The pressure pushed the gas like a spary can into the VENTED tank. If the mower had been left out in the hot sun the heat would have done then same thing after a while even with the engine off and mower sitting still. Tank mystery=SOLVED.


    My bad. I see now that Ruben Rocha bet me to it
  10. CowDoc1

    CowDoc1 LawnSite Member
    Messages: 8

    There is a diode, a transistor and a coil in the circuit. It is not just a resistor. Depending on how much current your particular meter puts out, the measurements may be different than the chart. If the meter puts out too much current it can damage the circuit. The main thing is that the measurements make sense. For instance, diode D1 should block all current in the reverse direction... unless the diode fails. If you read an open or short somewhere there shouldn't be, you have a problem. Exact measurements are less important as long as they make sense. You have to look at the circuit and think about what you're measuring... unless you buy a Kawasaki meter.

    Service manual states, "Use only Tester 57001–1394 with new battery at room temperature for this test. A tester other than the Kawasaki Hand Tester should show different readings. If a megger or a meter with a large-capacity battery is used, the ignition coil will be damaged.

    I do thank you guys for your suggestions. I'm not brushing anyone's ideas or experiences off. That's why I'm here. As an engineer with some physics background, I do disagree with your gas tank assesments. I'll try to come up with a semi brief explanation why. I need to crunch some numbers on vapour pressure first. Might even set up an experiment. I may just find you guys were right... but I don't think so. It's just a curiousity. The problem was solved 5 minutes after it happened. I modified the cap... and fed the monkeys.

    My real problem is the loss of engine power in the left cylinder when hot and under load. I'll look at compression and valves again this weekend. Hard to do a good dry/wet compression comparison on a horizontal piston. The right to left compression difference has me worried.

    Also, I'm still looking for an answer about the proper main jet sizes. I bought it used and who knows what was changed. I haven't been able to find a reference. Local support is hit and miss. As a reminder, it's a 25 hp Kawasaki FH721V-AS12 with an externally vented carb and the standard (not canister) filter. It's carbon fouling plugs with a #138 and #140 jet.

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