EVO Missfire/Popping

Discussion in 'Engine, Fuel and Exhaust' started by Doug S, Sep 12, 2009.

  1. Doug S

    Doug S Member

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    Hi, first post here, very long so I can explain..

    I have a 94 FXSTC with 24K on the clock, picked it up earlier this year. The PO installed a crane CV3 cam and some sort of adjustable push rods, drag pipes with HD Chrome Werks baffles. Since day one it sort of popped a little and I assumed it was because of the drag pipes and backpressure issues. I added a lolly pop to the exhaust, and that quieted it down some. After awhile I noticed that the rear pipe was making all the noise and lately it has been fouling that plug with thick carbon, to the point it starts to miss. If you hold your hand on the rear exhaust you will feel-(cold idle, enricher on a little); fire,fire,pop/poof,fire, fire, pop/poof. The noise is not a true backfire, it kind of barks or pops and at the same time it sounds like a pooof. There is also a little smoke out of the rear pipe, you have to catch it in the right light it's brownish black and does not linger in the air like oil smoke would. It's very slight and goes away completely when the bike warms and the enricher is in. The noise seems to go away at higher revs and when warm it is not as noticable but still there, you can feel it with your hand over the exhaust. The front runs even/no smoke/plug looks OK(maybe a tad rich) and in general the bike starts easy and runs strong through the gears. But it fouls that plug quickly at slower speeds and I'm sure it's carbon.

    Here is what I have tried with no positive or negative results:

    Switched plugs front to rear and tried a known good set, genuine HD and then Champions..same=rear cylinder issue

    Tried a known good wire on the rear, switched the wires at the coil...same

    Found an intake leak on the rear, changed gaskets-OK now, took carb apart, cleaned bowl and jets-everything, reinstalled and leaned as much as I could, runs exactly the same. (I did get a couple of carb coughs after this but that seems to have stopped).

    Compression: (No oil consumption in the last 1K miles)
    cold: F-150 r-154
    warm:F-148 r-153

    Gas is good-screen clean, all hoses are new including vacume, stock ignition and coil, zero oil consumption and zero oil leaks. For a 94 this bike is in amazing condition, frame clean top to bottom, tins look new, chrome near perfect, just a well cared for bike.

    Even though switching the wires at the coil did not change anything, I took it off, the primary resistance looks high but I suspect my meter has gone (EDIT) because it is different each time I check it. Gonna borrow a buddys this morning. I took the plugs out and grounded them and cranked the engine. What should the spark look like? Mine looks weak, more red than blue, I've seen better (bluer) on lawn mowers.

    I'm also going to get a clear plug and check timing.

    Any ideas??

    Thanks, sorry it's so long but I wanted to get everything out there..
    Doug

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    Last edited by a moderator: Sep 12, 2009
  2. glider

    glider Veteran Member

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    It sounds like you have two issues. First is the weak spark and second would be either jetting is off or the rear cylinder is pushing some oil, more the jetting off here.

    Take a compression test and then check the jet sizes in the carb.
     
  3. Doug S

    Doug S Member

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    Glider...u have mail...

    The compression is about 150 for both, see above. It will miss/pop/poof from initial start up with clean plugs so I'm thinking oil would take awhile to foul them and if that much oil got in there would be more smoke and a different color. Also, the fouling is dark black, dry, and looks like carbon to me. But I'm a noob to EVOs/Harleys so I'm just speculating here. Would a leaky valve seal leave that kind of fouling?

    The carb is running a 45 low and 160 high and I do think the low is a little rich for it but maybe they put it in there to stop deceleration popping..Maybe sould try a 44??
    The carb is a CV and the guy also installed a Thunder slide kit(forgot about that), I don't know what it does but I don't like it being there...Drilling holes and such. How does it work? Can I just plug the hose and eliminate it as a possible issue?

    Thanks,
    Doug
     
  4. glider

    glider Veteran Member

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    Thunder slide and Hose??? They are generally a replacement slide for the CV carb with no hoses associated with it. This may be part of your problem.

    The CV carb does it's best work on a fairly stock bike with just a jetting change. The 45 should be OK but what I would look for is an intake leak on the FRONT cylinder that someone tried to richen the mixture to over come and thereby richening the rear cylinder too much and fouling the plug. It's a good possibility that this could be your problem with the good compression in both cylinders.

    Also readjust the idle mixture to it's highest and steadiest point, then just close it a hair to the lean side.

    I assume that you did the compression test the proper way with the throttle full open and the slide held up.
     
  5. Doug S

    Doug S Member

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    Glider,

    I didn't hold the slide up but the throttle was locked wide open, I cranked about 5 strokes but it was at max in two or so..hard to tell. Is that gonna make a big difference in the compression results?

    OK, looks like I have something besides the thunder slide on this carb. I'll try to explain it and break out the camera later if I have to..

    There is a fitting under the fuel inlet on the bottom of the bowl, drilled and tapped in, 90deg fitting. Clear fuel line goes from that to a "device" that is mounted on the right side of the carb as you look at it. This thing is about an inch long and 1/2 wide/round and is drilled/tapped into the carb in front of the slide. A thin hollow brass rod sticks into the opening of the venturi to about the center of the slide as you are looking at it. The slide has a small drill hole just started, more of a little dent. I think this thing is supposed to bring extra fuel up under certain conditions. The PO said he had installed a Thunder slide and I just though this was part of of it...guess not...LOL
    Anyone know what this is and what it is supposed to do?

    I replaced both intake gaskets and will recheck for leaks, the rear one was leaking initially. I'll also reset the mixture. I'm gonna get a new gasket for where the carb mounts and a clear timing plug to check that.

    Thanks
    Doug
     
  6. glider

    glider Veteran Member

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    That sounds like a thunder jet kit that takes fuel from the bowl and adds it to the carb up above. Totally un needed on a near stock motor and could be your problem with too much fuel flow.. The normal jetting in a CV carb is more than enough without the thunderjet. IMO unless you have some performance mods in the engine, I would remove and plug the holes in the carb.

    It also will make a difference holding up the slide when doing a compression test.

    [​IMG]
     
    Last edited: Sep 12, 2009
  7. SpringerSteve98

    SpringerSteve98 Active Member

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    I'm gonna have to ride with glider on this one. He helped me through my CV carb problems. I had a dynojet kit in mine when i got it. I replaced the slide( with one not drilled) , emulsion tube, needle, and went back to a stock emulsion tube and needle with a few washers and it never ran better. I have a late 90's EVO as well with a cv carb. I ended up using a 42 pilot jet and a 190 main... Hope this helps.
     
  8. Doug S

    Doug S Member

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    Glider,

    That's what I got, a Thunder "Jet", from looking around on the net I have the Thunderslide also. Too bad, I'd rather it be stock and I'd take it from there. While I probably have carb work to do, I found that my spark is indeed weak and that may not solve my problem but it's a good place to start.

    I did a resistance check on the coil and it was right in the middle of the specs on both sides (2.8 & 11K). I have 12.3 volts comming to the primary with the ignition on so it's getting enough juice. I took a known good ignition wire,hooked up a plug and Grounded to a head bolt. Checked the 2 sets of plugs I have along with a spark tester. Lights off, garage doors closed and I had a thin little redish/white spark. I then went over to my buddys stock (99 FXSTC) and ran them on his bike the same way. They were on fire compaired to my bike and the spark tester also showed a better/thicker spark.

    Can anything else but the coil cause this difference?
    Maybe my starter draws more than his and my spark is ok once running?

    This is the original coil so I'm gonna change it, should I get another stock one or one of those hi-performance ones-(do I need new wires or have to do other work).

    Thanks
    Doug
     
  9. glider

    glider Veteran Member

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    Being a '94 I would say the wires should be replaced too maybe first before the coil. The coil could be weak or the cranking volts may be low which would also cause a weak spark. Check the spark when running.
     
  10. Doug S

    Doug S Member

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    Yeah, probably the wires due to their age but I was using a different wire for the tests. What type of coil?

    To check spark while running I guess I run it on one cylinder and pull the plug on the other and watch it??

    BTW: I just did a leak down test..The rear cylinder showed less than 5%, unreadable on the guage, I did hear a tiny hiss in the intake. I redid the test and could barely hold the wheel with the brake and no hiss until the wheel moved a little. LOL,reason I used the brake is when I did the front first, I had the bike balanced on the stand and had lowered the tire to the ground after finding TDC..so it would not spin..Well that sucker spun that wheel, dragging the metal wheels of the stand, bike went about 10" with me sitting there having a heart attack...Front cylinder was perfect near as I could tell.

    Doug