1971 CB350 - Engine gets "bogged down" at higher throttle
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Thread: 1971 CB350 - Engine gets "bogged down" at higher throttle

  1. #1
    Junior Member dslenz's Avatar
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    1971 CB350 - Engine gets "bogged down" at higher throttle

    It was a beautiful spring evening in Wisconsin, so I took my CB350 out for a ride through America's dairyland. Once I got out onto county roads, I noticed some weird activity at higher speeds/RPMs. In 4th and 5th gear, I could not get above 5,000 RPMS, nor could I get much above 42 mph. If I continued to throttle up, the engine sounded bogged down - like it wasn't getting enough air. I would also start to lose speed/RPMs. I haven't tested exactly, but I'd say it happened at the final 1/3 of throttle travel.

    By way of background: I've owned this CB350 for about a year. Last year, I definitely did not have this issue. After a long winter, I drained and refilled the tank, changed the oil, checked tensions, etc. but otherwise the bike is stock. The only other issue it has is electrical (which I'll post in the proper forum).

    Any ideas?
    '71 CB350 (Stock)

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    Junior Member dslenz's Avatar
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    Two other notes:

    First, I note that this is in the incorrect forum. If a moderator could move it, I'd appreciate it.

    Second, I've been riding around town without significant issue. I can rev to higher RPMs in lower gears without issue and felt like I had normal power getting up to 40 MPH.

    Thanks!
    '71 CB350 (Stock)

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    Sensei 66Sprint's Avatar
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    Ok....At 40 MPH, you should just be getting ready to shift from second to third gear (possibly just completed that shift)...Third is good to about 55 MPH, and forth to about 65.....
    IF you are in fourth or fifth gear at 40MPH, OF COURSE you are "lugging" the engine...... Throttle response will be dismal, and revs will build so slowly you will think you hit a wall.....
    You HAVE to use and run-out the gearing if you want good acceleration.....Your bike is likely simply responding to being in way too high a gear.........

    I use the approximate shift points I just mentioned on my 175 (which is basically a "high-powered-Hoover" compared to a 350) and neither redline or experience your "bogging" symptoms.....
    "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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    Junior Member dslenz's Avatar
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    Quote Originally Posted by 66Sprint View Post
    Ok....At 40 MPH, you should just be getting ready to shift from second to third gear (possibly just completed that shift)...Third is good to about 55 MPH, and forth to about 65.....
    IF you are in fourth or fifth gear at 40MPH, OF COURSE you are "lugging" the engine...... Throttle response will be dismal, and revs will build so slowly you will think you hit a wall.....
    You HAVE to use and run-out the gearing if you want good acceleration.....Your bike is likely simply responding to being in way too high a gear.........

    I use the approximate shift points I just mentioned on my 175 (which is basically a "high-powered-Hoover" compared to a 350) and neither redline or experience your "bogging" symptoms.....
    Thanks 66Sprint. Even if I'm slightly embarrassed, I'd be mostly relieved to find out this is operator error. I'll give this a shot, hopefully tonight.
    '71 CB350 (Stock)

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    Sensei 66Sprint's Avatar
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    Don't be embarrassed, we know that you were just trying NOT to abuse an old engine......

    I Shift out of first @ 20 to 25 MPH....Second @ 35 to 40....Third @ 50 to 55, and fourth @ 60 to 65....and that's probably STILL short-shifting slightly on a bigger bike....

    These engines LOVE to rev......
    Last edited by 66Sprint; 05-16-2019 at 01:38 PM.
    "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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    Junior Member dslenz's Avatar
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    Update: I tried this out this evening, again on back roads. Wound out the gears as described. I still didn't have enough power. I got back up to 42 mph in third gear at around 7200 RPM and couldn't push it to go any faster (except downhill), even with the throttle totally open. I tried upshifting, but it went back to sounding like it was underwater. Some other factors: One, I noticed that when I would back off the throttle (i.e. to take a turn), the bike sounded like it was missing. Two, I think my battery is going - despite an overnight battery tender, I had to kickstart.

    Any other thoughts? Could the whole thing be associated with a dying battery (it was left on the bike all winter in Wisconsin).

    Thanks again for your sage guidance. I will say that after about an hour riding in lower gear the engine was... hot. Like I could barely grab the key to pull it out of the switch hot.
    '71 CB350 (Stock)

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    Supporting Member Yendor's Avatar
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    A fully charged Battery is a MUST for these bikes.
    Anything less and the performance will suffer.

    That said I would check your timing with a Strobe light and make sure that both sides are even with the proper marks at idle and full advance 4000 rpm or better.
    Also pull your plugs and show us pic's of them - both sides, so we can see how it's burning - Rich/Lean or just right.
    1970 CB 350 CAFE - Current Project on the bench,
    1972 CB 350 K4 Red - Now a Happy Rider ! !
    1972 CB 350 K4 Green (My Sons) DONE - YES !,
    1st Bike 1970 SL 350 (Brought home in Parts - Trailer/Trunk/Back Seat - I miss that bike),
    2nd - Bike Kawasaki 750
    3rd - '73 XLCH 1000

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    Sensei 66Sprint's Avatar
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    Quote Originally Posted by dslenz View Post
    Update: I tried this out this evening, again on back roads. Wound out the gears as described. I still didn't have enough power. I got back up to 42 mph in third gear at around 7200 RPM and couldn't push it to go any faster (except downhill), even with the throttle totally open. I tried upshifting, but it went back to sounding like it was underwater. Some other factors: One, I noticed that when I would back off the throttle (i.e. to take a turn), the bike sounded like it was missing. Two, I think my battery is going - despite an overnight battery tender, I had to kickstart.

    Any other thoughts? Could the whole thing be associated with a dying battery (it was left on the bike all winter in Wisconsin).

    Thanks again for your sage guidance. I will say that after about an hour riding in lower gear the engine was... hot. Like I could barely grab the key to pull it out of the switch hot.

    ​IIRC, 45MPH in third on my 175 is only about 6K/6.5K RPM (hard to be more exact as it's immediately after shifting into third and still accelerating)...You MUST be running on JUST one cylinder at least part of if not most of the time....OR your compression is very low.....

    Yes, let's see the spark-plugs.......

    The "Burbling" (underwater sound) is/could be the non running cylinder pumping air.... The excess heat is due to the extra work the one running cylinder has to do to both move the bike and pump the non firing cylinder's piston up and down.....Therein lies the loss of power......This theory explains a lot...
    Last edited by 66Sprint; 05-16-2019 at 09:49 PM.
    "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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    Supporting Member crazypj's Avatar
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    After I 'played' with it my 1968 CD175 would lift front suspension at 60mph in third going to 4th.
    OK, Back to normal program, What oil are you using and have you fitted heavy duty clutch springs? Do you have cheap pods or stock air-box
    There are three types of people in the world, those who can count and those who can't
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    Junior Member dslenz's Avatar
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    Thanks again team. I'll pull the plugs this weekend and send pics as a start. If that doesn't answer the question, I can get it in to a shop with a strobe (sounds like I might be headed to one anyway). After yesterday, I'm more confident that the bike is actually underpowered.

    To answer the other questions posed: The battery was fully "charged" when I started out on both trips - tender indicated full charge, had been on overnight. I haven't tested the battery to see if it's no longer holding charge. I changed the oil with motorcycle oil - I don't remember the label, but I used the viscosity recommended in the original manual, and an oil specified for motorcycles. Oil change within the last 100 miles. I have not fitted heavy duty clutch springs, and I'm 95% confident the previous owner did not either. I have the stock air box. The bike is completely stock as far as I know.
    '71 CB350 (Stock)

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