High Speed Miss

Discussion in '4-Stroke Engines' started by HoughMade, Sep 22, 2008.

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  1. HoughMade

    HoughMade Guest

    I have a high speed miss of some sort. At lower rpm, and with the throttle open less, runs great. When I open the throttle a certain amount- closer to wide open, but before wide, it starts to run rough. I have cleaned out the carb twice.

    The engine is a Honda GXH50, 4 stroke. Any ideas?

    Also, getting a new spark plug today- I really do not expect it to be the plug, but now that I've taken it out and broken the insulator....
    Last edited by a moderator: Sep 22, 2008

  2. Bigwheel

    Bigwheel Member

  3. Honda50

    Honda50 Member

    Is this something new and out of the blue or did it build up over time? If you are getting a good "snap" spark at the plug, we may have a fuel starvation problem at or near WOT (wide open throttle).

    Float level should be checked against specs and set correctly to eliminate that possibility. Here is a good test to determine if it's ignition or fuel related:

    Get yourself one of those plumbers sweating propane set-ups (like a Burnz-o-matic at home depot) and connect a vacuum hose to the valve assembly. So, now we have a propane bottle with a valve and hose attached. Turn on the valve and propane comes out the hose. Now, rig the set-up so you can introduce a little propane into the air cleaner assembly WHILE RIDING YOUR RIG AS THE PROBLEM OCCURS.

    After warming up the engine to operating temp, go on a test drive and get the engine to act up under load. At the point the engine falls on it's face, slowly open the propane valve. This will artificially enrich the air/fuel mixture. If upon enrichment, the engine 'picks up' and runs regularly, we have a fuel starvation problem. Either caused by a delivery (from tank to carb) or internal carb problem with the main metering circuit not feeding the hungry engine.

    The propane tool can also aid in diagnosing a vacuum leak at the carb gasket. As propane is introduced to the area around the carb with the engine running, a lean engine will pick up and smooth out. This would indicate a vacuum leak.

    To diagnose a No Start condition, check for spark in the normal fashion. If you get a HOT spark, place the propane hose in the carb throat or air cleaner housing and start the propane flowing lightly with the bottle in the upright position. If the engine now starts and runs on propane, we have a fuel problem. To confirm, while the engine is running, close off the propane. The engine should die if the carb is not functioning properly.

    To see if the idle mixture is set too lean on the carb, warm up engine, introduce propane slowly into the air cleaner with engine running. If the idle picks up and increases, we have a lean condition at idle and the idle screw can be adjusted accordingly to optimum idle quality.

    Hope this helps with your diagnosis.
  4. Honda50

    Honda50 Member

    Also, is your fuel tank mounted on top of the engine or remotely, like to the side, etc?
  5. Mountainman

    Mountainman Active Member

    HM -- have you played around with your carb adustments screws ?

    that propane idea from Honda50 -- I found to be --

    VERY INTERESTING !! Good food for thought there...

    Ride That Thing - Mountainman
  6. JemmaUK

    JemmaUK Guest

    Valve bounce, valve timing, valve gaps - condition of plug I would look at first

    unless you had done something before you got the problem it would have to be something that might drift in adjustment over time.

    It might even be something to do with the engine running in - stranger things have happened.

    I dont like the idea of pumping propane into a motor when the same test can be done in safer ways - like cleaning the plug right up - replacing it - and trying a run as much in the misfire condition as possible and then looking at the plug... simplest is always best.

    jemma xx
  7. HoughMade

    HoughMade Guest

    Good help found here- here is what I have so far:

    Last time I rode before the engine was pulled off for paint, it started bogging- I figured fuel starve and I think I was right. I completely cleaned the carb- and since I had never checked the valves, I checked and adjusted them too (we'll soon see if I screwed that up).

    When I got it reassembled, it ran much better than the last ride before it was disassembled. However, it had this higher rpm stumble- runs perfect up to a certain throttle opening, but then starts to miss when I open it a little more.

    I took the carb off and cleaned it again. The fuel filter is new. The spark plug is new.

    I will try richening to diagnose, good tip.

    On the stock carb, there is not much in the way of adjustment....none in fact (other than idle speed).

    The tank is above the engine- see the pic in my sig.
  8. RdKryton

    RdKryton Active Member

    I guy I work with has a Honda lawn mower that had very similar symptoms. After multiple attempts at cleaning the carb I got out my torch cleaning file kit. I carefully selected the file that fit the high speed jet and gave it a few strokes. That worked for him the problem was gone. You must be very careful that you don't increase the jet size by filing some of the jet itself. If you try it make sure you remove the jet just to make sure you don't get anything in the passages. Hope this helps.

  9. HoughMade

    HoughMade Guest

    I think that will help- I ran propane in last night- smoothed it out considerably- so I think I am still working with fuel supply. I will do what you said. Down deep inside, I thought it was fuel supply.

    However, I will also check the valves when I have the carb off....what does one have to do with the other? In my case, my throttle linkage is retained by the valve cover bolts.
  10. HoughMade

    HoughMade Guest

    Checked the valves- I adjusted them, but I don't think that was the issue- they were prety close to spec.

    I took the carb off and completely went through it. Put it all back together- starts and runs perfectly at idle and a somewhat above, but it still will not rev.

    I'm stumped.
  11. HoughMade

    HoughMade Guest

    By the way, I called this a high speed miss- that's not really right. It's bogging.
  12. HoughMade

    HoughMade Guest


    I took one more stab at it last night. I took the air cleaner housing off I had built. I built the first housing a year ago and used it without trouble. I made a new one because I wanted a slightly different look, but I THOUGHT that I did not change the functionality at all. I had tested it without the air cleaner cover before, but not without the whole housing.

    Here's the thing. Take a look at picture 1, below (this actually a pic of a HS carb originally posted by Salfter, but it looks the samw other than the finishing on the Honda looks a bit finer).

    On either side of the intake runner, you can see a hole (not the mounting holes). The hole on the left melds into the runner. The hole on the left is separate. When I ran without the air cleaner housing, it ran perfect and smooth. When I looked closely, I saw that the gasket I had me (me screwing up), had the hole on the right in the wrong place so that hole was covered. As soon as I modified the gasket and opened that hole, made sure the housing was fully unrestricted in that area, and reinstalled everything, it ran perfectly. I took it for a 4 or 5 mile ride before work, ran very nice.


    Attached Files:

  13. RdKryton

    RdKryton Active Member

    Good catch. Glad you have it correct now.

  14. HoughMade

    HoughMade Guest

    Ran perfectly on the 16 mile ride to work this morning. Pulled well- so happy about this, I don't know what to do!
  15. Honda50

    Honda50 Member

    Great diagnosis!!
  16. Irish John

    Irish John Guest

    From what you say I think it's most likely the air\fuel mix. There are 2 screws on the carb - the black idle speed one and the air\fuel mix one below it to the left. It's easier to tune the air\fuel screw from new but if you turn it one way you'll starve the motor and the other way you'll flood it. By turning it until it begins to starve you then go half a turn to 3/4 turn back until it runs smooth again. If your valve gaps are correct which they are it should run sweet. Forgive me if you know all this but that's how I solved a similar problem on my HS. I messed about with valves and screws etc until a mechanic showed me how to get it sweet. I think fuel line probs are rare. Your tank would have been cleaned during your assembly and it would take a really serious blockage to starve the carb of the tiny amount of fuel flow it requires.
    Could also be that now that your motor is Imperial measure and the carb is still metric the motor gets confused doing the conversion tables!
    Sorry HoughMade, I see you solved it. Gasket problem - that's what I meant to suggest!!
    Last edited by a moderator: Oct 5, 2008
  17. Irish John

    Irish John Guest

    Why don't you get rid of your 56T sprocket to celebrate and try a 48T if you haven't already done so.
  18. HoughMade

    HoughMade Guest

    Did it a long time ago.
  19. Irish John

    Irish John Guest

    That accounts for the fact that our speeds are so similar. I should have counted the sprockets on your great pictures before I wrote that - sorry. I wish I could find a use for all the 56Ts I have lying around and I wish I didn't have to shell out for a 48T plus postage every time I bought a Grubee kit. There are very few people in MBc it seems still using that 56T sprocket yet Grubee still keep including them as standard in their kits.
  20. HoughMade

    HoughMade Guest

    I guess I lucked out. My kit came with both. At my size (6'3" 220#+) and the performance the 48 returns, seems like it would work for 98% of people.