Fine Around Town, Dies at Higher Speeds
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Thread: Fine Around Town, Dies at Higher Speeds

  1. #1
    Member JimmyRiot's Avatar
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    Fine Around Town, Dies at Higher Speeds

    Hi everyone, I’m reaching out here because I don’t know what to do next, & my search skills don’t seem to be good enough to find the answer on here, although I’m sure it is somewhere. Please feel free to link me to the elusive thread(s).

    I found these close-ish threads, but I'm still stumped. I guess I'll check my gas cap for ventilation, but I'd be surprised if that was the problem & the fix. 78 CB400T sputter and backfire only after up to temp
    Engine Bogs and dies under load
    1970 CB350 K2 power cutting out intermittently
    Runs, stalls and repeat

    My ’71 350 was running pretty well the first 50 miles or so, -at least I could get it up to 70mph on a 12 mile trip & it wouldn’t die. Lately it’s been quitting once I get to high speeds, and this has become a repeating pattern 3 times in a row now. Here’s what happens:

    Runs fine around town, but when I get the revs up & go 45-60mph & the engine really gets warmed up, first it sputters a little when I crank the throttle above 45mph, and somewhere along this timeline my idle jumps from 1.3k to 4 or 5k. Then it loses 50% power, as if I lost a cylinder, until there's NO power but engine’s still “running” & then it’s just a short amount of time before it quits on me. I pull over & wait a few minutes and it’ll start back up & run just good enough for me to barely hobble home at low speed if I’m close. Today I had to push it the last few blocks.

    At first I thought it was a fuel delivery problem, so I took off the petcock & the screen was semi clogged so I cleaned it all & though hey, this’ll solve it. My plugs didn’t show bad colors or signs. I was ready to buy carb rebuild kits, but the previous owner used an ultrasonic cleaner on them, and I trust him and am still in touch. He has a lot of pristine restored Hondas and is a mechanical engineer, but I bug him enough as it is– I don’t want to be a nuisance. So I’ll be one here haha. Thanks everybody.
    Last edited by JimmyRiot; 04-14-2017 at 02:50 AM.
    CB350K3

  2. #2
    Super Moderator outobie's Avatar
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    There are several things that could be your issue but Before we go through them systematically try this...
    Do you have inline fuel filters?
    if so try removing them and running fresh new oem Honda fuel line 5.5 mm from your newly cleaned petcock to the carbs with no filters
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  3. #3
    Senior Member 76TWIN's Avatar
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    Hmm sounds like a classic clogged fuel cap vent to me. Seriously when it does this next time, open the gas cap and see if it will start. If it does then that's your problem.
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  5. #4
    Member JimmyRiot's Avatar
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    Thanks Outobie & 76. I don't run inlines and my lines are see-thru clear. All was clear to the carbs & I even pipe-cleaner'd the carb nipples the fuel hose goes on. Maybe it is the gas tank cap- I wish I opened it yesterday to hear a vacuum noise. How do I improve the venting on my gas cap? I had it apart this winter & cleaned it. The rubber seal was cracked so I flipped it over. I have a new seal but I almost don't want to put it in for a tighter seal, y'know? There's the spring & screw, I guess loosening the screw improves venting even though this allows the spring to push the disc closer to the tank. I'll take a look & test it tomorrow.
    CB350K3

  6. #5
    Senior Member Carnivorous Chicken's Avatar
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    Cap's got a vent in it, and it can get plugged, especially if the tank's been recoated and the cap as well.
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  7. #6
    Member JimmyRiot's Avatar
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    Yes guys, it was the clogged vent in the gas cap, & it doesn't quit on me anymore, thanks.
    Fine Around Town, Dies at Higher Speeds-fullsizeoutput_d79.jpegFine Around Town, Dies at Higher Speeds-img_0950.jpgFine Around Town, Dies at Higher Speeds-fullsizeoutput_d78.jpegFine Around Town, Dies at Higher Speeds-img_0952.jpg

    However, it's not running great. The new spark plugs turned black so I guess I have an oil problem which lines up w/ the hesitation & sputtering (misfire) symptoms. Plenty of info about that on here, so on to my next case!
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    CB350K3

  8. #7
    Super Moderator outobie's Avatar
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    Wet black plugs are oil fouled dry powdery fouled plugs are carbon fouled

    different causes
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  9. #8
    Member JimmyRiot's Avatar
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    Cap vent fixed :) now high idle when warm, then dies

    I've been reading & researching similar problems like in When hot gets stuck at high idle.

    High on the list is the air-leak test on the intake manifolds (rubber short tubes between the carb & engine for onlooking new guys like me). After engine is warm but idling at 1200rpm (before it shoots up to 4 or 5k) I did the test w/ flammable spray (carb cleaner in my case) an no rise in idle. I sprayed from all angles, no rise, so I assume my seal there is tight. Maybe I should repeat this test at super-hot high-idle.

    Richard Midlife says: Mydlyfkryzis*
    Might be a warm engine vacuum leak.....If the intake manifold boots are old or cracked, they might leak more warm then cold. My manifolds were like that. Hot they leaked (after about 20 minutes of running. Cold, no problem detectable.


    We've determined that from the look of my R spark plug that I'm probably running rich, so I played w/ the adjustment screw, but apparently that's just for idle & you have to go inside the carb to adjust the rich/lean for running? A rookie question I know but right now I value "ride over pride". Yes I've RTGDM but either I missed it or looked in the wrong places & used the wrong internet searchwords as usual

    Another fix suggested on the thread up top was valve seating / clearance. Does this move as the engine warms? I could check again, but if it's unlikely I'd like to move on.

    Scratcher09 had this to say about maybe it's some felt washers: I have a possible fix for the original question. I have experienced this. Both times it turned out to be that the felt washers on the butterfly shafts were dried out. I used to dip the carbs without removing the shafts because I did not know about the felt washers. They need to be oiled to seal. And I know of no way to properly oil them without removing the shafts from the bodies. Just be sure to stamp the tiny screws that hold the butterflies on the shaft. This keeps the screws from finding their way into the combustion chambers. You can test to see if your felt washers are the problem by spraying solvent (wd-40, starting fluid, etc.) directly at the ends of the shafts. If the rpm's increase, then your washers need to be replaced/oiled.

    Yendor says: Re: When hot gets stuck at high idle.
    This could also be the Timing Advance assembly getting stuck (gummed up, weak springs or slightly rusty).
    If this is the case I might just go w/ a PAMCO because:
    wolfegm says: I had this same problem several months ago, read this thread, and tried the solutions that were suggested. I think the high revs was being caused by the spark advance because I did not detect a leak, and when I put in a pamco, the problem went away.

    I just don't know why for the first 50miles it was fine & ever since it's been bad.. Poor me I know but after fork oil, cable lubing, aluminum polishing, brake inspecting, shock replacing, electric wiring & spiffing it up all winter it's frustrating. I just want to experience a nice long ride. Thanks in advance for any help, and feel free to explain things to me like you're talking to a 6-yr old.
    CB350K3

  10. #9
    Senior Member doode's Avatar
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    Can you give an accurate play by play real life example travel log of how the engine behaves? The first post, as I understand it, your going 45, then suddenly idling? For how long? Then how far do you go? Then it dies.
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  11. #10
    Member JimmyRiot's Avatar
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    Quote Originally Posted by doode View Post
    Can you give an accurate play by play real life example travel log of how the engine behaves? The first post, as I understand it, your going 45, then suddenly idling? For how long? Then how far do you go? Then it dies.
    Hi Doode, yes, and thanks. Now it's where I don't even ride it. I have 91 no-oxy gas, clean lines & petcock screen, and it idles fine for like 5/6minutes at 1.2kRPM. Then without touching it the idle clinbs to 5k, where it stays, regardless of throttle reving, until it climbs down down, then it dies. I don't even ride it anymore. I was about to today, but then the idle climb happened, and then the die happened. Tks in advance for any insight
    CB350K3

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