'65 305 dream with strange surge....
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Thread: '65 305 dream with strange surge....

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    Senior Member OldTwinsRock's Avatar
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    '65 305 dream with strange surge....

    I mentioned this before in the thread in "pictures and vids" but since I've been unable to nail it down, thought I'd start a dedicated thread. Not sure whether this should be in engine, or fuel/ carb, so I put it here. She'll start and rev just fine in neutral, but once I'm riding, she surges real bad. I've set the float height, changed the float valve and seat back to the one that was in it. Once I take her out of gear, she'll surge one more time, then settle down. Sitting still, she'll rev up then settle down like she's supposed to. Very strange behavior. TIA !

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    Sensei 66Sprint's Avatar
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    Please phone me to discuss......
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
    Contact info: E-mail; [email protected] Phone; 540-525-5199

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    Senior Member OldTwinsRock's Avatar
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    Great, I see you're online...is this a good time ? Don't want to call too early.

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    Senior Member OldTwinsRock's Avatar
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    This thread is as much for myself, my memory being what it is.....
    I talked with Sensei ( Mucho gracias). I described the issue this way:
    Riding along at say three quarter throttle, it's as if I quickly twist the throttle to full, and back to 3/4. The bike lurches forward and the back, rinse and repeat - as long as it's in gear. Pull the clutch in, she surges once or maybe twice more, and settles down.
    Sitting in neutral, throttle response is normal.
    Sensei sez, carb float height is mission critical. Also, float valve not leaking is very important.
    First, I pulled the bowl, and discovered much less gas in it than there should be from running two days prior (~1/2-3/4").
    Next, I put a cup under the carb and turned on the gas. Decent flow. Slowly raised the float until the spring loaded tit started to move in the valve. Gas stopped,and did not leak. On, off, on off. Seems fine. So I must have float height wrong, right ?
    Take carb off (placing rag under carb to catch fumbled nuts/washers !) Carb is PW22. Float height 26.5 mm from bottom of bare gasket groove to far end of float (bottom if carb is right side up) when float tang just touches valve tit using card stock cut to dimension to measure with. So the question in my mind is, why would I have so little gas in the bowl if I have it set right, and I'm getting sufficient gas delivered. Must be doing something WRONG.

  6. #5
    Sensei 66Sprint's Avatar
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    This is one theory...... The 26.5 mm float height is incorrect or incorrectly set.... It allows too little volume in the bowl and at continual higher throttle openings, the usage further depletes the fuel height and allows air to enter the jetting, causing the acceleration surge.... Because the float pivots at the rear of this carb, and the acceleration would "throw" the fuel in the bowl rearwards (allowing the front of the float to drop further and to a wider opening of the shutoff needle) it rapidly dumps in enough "extra" fuel to refill the bowl to above the jetting, ending the acceleration......This suggests reducing the float height would be helpful (25 or 25.5 mm?)

    This is the OPPOSITE theory ..... While float height is correctly set when bowl is off, a SLIGHTLY heavy float (RELATIVE TO FUEL DENSITY) raises the fuel level slightly above normal within the jet tubes.....When the carb is operating at slower rev ranges (on the idle/slow jet circuits) the effect is somewhat minimized and could have been corrected for with mixture and throttle stop screw settings.... However, once on the main jet, and into the more radical needle taper, the higher actual fuel level allows enough extra fuel into the venturi that you would experience a surge that would accelerate the engine but would be somewhat "momentary" because the jetting orifice is now controlling how fast the jet tube refills and you get the return to slower revs.....As it gets fully refilled the effect repeats.......
    In this case, I'd try a 27 mm height first......OR, as a test, raise the clip, dropping the needle slightly (one notch at a time) until the effect stops or changes the rev range/throttle position at which this occurs .....

    This is not the first example of how modern fuels demand slight modification of the previously standard float height settings.......
    The problem with either theory is that neither one can fully account for why the surge only happens when the bike is in gear, but not in neutral......

    Other opinions of these possible scenarios would be appreciated ........ One is wrong, but I can't yet find the flaw in logic that makes it wrong.......
    Last edited by 66Sprint; 09-19-2019 at 06:07 PM.
    OldTwinsRock likes this.
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
    Contact info: E-mail; [email protected] Phone; 540-525-5199

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    Senior Member OldTwinsRock's Avatar
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    After reading and rereading your two theories, and thinking about it, I'm sure you are on to the problem. Something is causing air getting into the jetting process. When I adjusted the tab on the float, bringing the gas level in the bowl up, I was sure that would fix it. It seemed to do a little better, but at higher revs fell into the same annoying behavior. Took the carb off again ( getting good at this) and brought it to town to make a cork bowl gasket as the rubber one that came w/ the kit has square corners that don't fit the round corners of the carb and is a total PITA. Decided to take out the jets and screws and triple check every tube and orifice. Main jet is a 125 and low speed jet is a 35. While examining the main jet tube, the one with tiny holes in it, I noticed there were four holes right by the threads which were stopped up. Details, details.... Another thing, there was a tiny O-ring came with the carb kit. I had read somewhere that this was supposed to be on the air screw. Well when I dug it out of the hole, it was totally buggered. Not sure if that is critical to good adj. or not.
    In the coming week, I'll get the carb back on and see what happens. I'm also wondering If what "looks" like good flow from the petcock might not be good enough at higher RPM's to keep the gas level correct. Something else to double check.
    Thanks again for the help.
    Last edited by OldTwinsRock; 09-23-2019 at 06:23 AM.

  8. #7
    Sensei 66Sprint's Avatar
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    Your carb DOES NOT get an o-ring on the mixture screw........just a spring.......
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
    Contact info: E-mail; [email protected] Phone; 540-525-5199

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    Senior Member OldTwinsRock's Avatar
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    Put the carb back on. Still had the surge. Took the "can-o-worms" petcock off. The rubber seats appeared swollen, the holes small. So I used a sharp utility knife to open them up a bit, put it back together, waited over night for the seepage to fix itself (love it when things fix themselves), and started her up. Ran like crap. Determined no sparky from one side of new (cheap) coil. Since I hadn't taken the old OEM coil off, I hooked those wires up, and she ran like a champ w/no surge. Rode over to the neighbor (1/4 mi) shut her off, bs'd, she would not start. Thinking might have to re check float height. Bit of drip from overflow tube. It's been a long slow haul with this bike, but for a bike 54 years old, not too shabby. Thanks for the help !

  10. #9
    Senior Member OldTwinsRock's Avatar
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    Have never claimed to be mechanically inclined. Sometimes I feel mechanically challenged.
    Here's my report:
    The surge settled down after "feddling" with the "petcock". Make of that what you will.
    I'm not exactly sure what the float height is now, but I think it's close. To being close to being close.
    I put the new float valve and seat back in in hopes I wouldn't get the slight overflow out the carb bowl, since the petcock now leaks a bit.
    I lowered the slide needle two marks after having raised it one mark for no good reason, or more to the point, faulty reasoning.
    I mounted the new, newer coil on the old bracket. The new coil only seemed to be sparking on one side. The new, newer coil seems to work well. It's fun to feel her run well after so much trouble shooting / miss-interpretation of the symptoms. Like I said, not bad for a fifty four year old bike. Thanks for the help.
    Today she ran like a champ in first gear for at least three miles.

  11. #10
    Junior Member Tom Deluxe's Avatar
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    Tom Deluxe Here! We would set the f/level by sighting across the bottom of both floats and getting 7or/8 m m of main jet and tube showing. This was the only way known to us the first 25 years or so. In the interest of good running,you could have a strobe light look at what the ign.timing is actually doing when running.
    When new, it is to be at the F mark at idle and at the two forward marks at max. However older worn jobs can vary some,(since theres no external adjustment pos.
    it may be nec. to fudge some on both ends.--- In doing several engines of both types,the usual procedure during rebuilds was to correct this while apart. But it's not something most folks want to attempt..Cheers "T"

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