Honda CB350 losing power at higher RPMs

Discussion in 'Old's Cool' started by secondbreakfast, Apr 30, 2011.

  1. secondbreakfast

    secondbreakfast n00b

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    Hey yall. So i've been wading thru the internet the last few weeks trying to solve the riddle of the month. I've been seeing a lot of information and am hoping someone can help pin down what might be going on with my bike.

    Basically i can get it to idle fine but when it hits beyond 4k it loses power and starts to make strange noises as if maybe its out of sync of something. I'm new to bikes in general but have learned a lot about mine in the last month as I've thus far:

    changed the oil and cleaned the centrifuge
    changed the spark plugs
    set the points and timing
    cleaned and oiled the spark advancer
    cleaned the carbs and set to manual settings

    the carbs open at the same time and the mix and idle screws are set equal so im not sure how else to sync them. i do notice that the left exhaust has more pressure coming out of it. oh and its a 1973 Honda CB350G if that helps.

    anything helps! thanks!
    #1
  2. lrutt

    lrutt SILENCE.....i kill you

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    Make sure you gas cap is venting properly. Next time you have it oiut and are running it up, have the gas cap loose to allow more air in. You could have a partial obstruction.

    Other thing could be bad diaphrams in the CV carbs. Pull the slides, clean them good, then use WD40 and spray liberally on the top side so it puddles. Check for leaks on the bottom side.
    #2
  3. PTC

    PTC Turd polisher

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    search plug reads. make sure you do this on a safe road, engine warm, kill the motor when the problem happens and safely coast to a stop. read your plugs.
    recheck your timing with an automotive type timing light. set your valves. its likely to ba a carb issue. i think your plug read may reveal wet plugs.but study plug reads and you will learn alot. good knowledge for anything with spark plugs. is there a mouse nest in the muffler or air filters...
    http://www.advrider.com/forums/showthread.php?t=189436 - check in there too
    #3
  4. tbirdsp

    tbirdsp REMF

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    I think the books actually say to synch them by judging how much is blowing out of each exhaust:huh I just did as you did.

    I second the notion of checking the carb diaphrams - I've seen a lot of them bad on 350 twins.

    What exhaust is on the bike? I had a '69 CB that I put different mufflers on. It would cut out at higher rpm, I think I ended up going richer on the main jets.
    #4
  5. Harry Backer

    Harry Backer Ran when parked!

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    Try pedaling faster thats what I do :D
    #5
  6. Wandering Dane

    Wandering Dane We are the luckiest guys

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    I had a couple of CB350s when they were new, a '68 and a '70, as best as I remember.

    Here are a few items that I experienced:

    * One problem in my experience was similar to yours, no power at mid-to-higher RPMs. The plugs were dark and for a while I was convinced that somehow the mixture had gotten richer all on its own. Of course it hadn't. The problem was very high resistance in the spark plug caps. New caps solved the problem after about a month of head scratching.

    * Another problem was starting difficulties. Turned out that I had set valve clearances too tight and burned several valves.

    * One of the bikes developed a vibration and noise. Oddly, the alternator rotor had started to break up in small pieces and was out of balance.

    * The points can be tricky to set and don't seem to maintain their position for any length of time. Often, misfiring at higher RPMs can be traced to bad points or point-springs that have lost their tension. New points are cheap insurance and then check and re-check timing.

    Good luck.
    #6
  7. crspep

    crspep Circling the drain

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    Maybe check the gas flow through the petcock [maybe the tank screens are dirty/full of rust] Are the petcocks vacuum operated [maybe closing down if you are under load and engine vacuum drops] Air cleaner clean ? Points adjusted ? Advance working ? [timing light]
    #7
  8. Berck

    Berck Been here awhile

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    So, at the risk of being captain obvious... Are you sure the choke is off? That means the choke lever is down? I only ask because, well... I got a bit confused when I first got my CB350... thought it was running like crap, turns out it ran a *lot* better with the choke off. And I'm not the only one this has happened to, so I mention it just so you make sure, not to insult you:)
    #8
  9. wavygravy87

    wavygravy87 Been here awhile

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    ^^ What he said. I screwed with mine for 2 days before finding out that the bike was pulling the choke "ON" when i rode it. It would run perfect until it hit 4,000 rpm then it would start poppin and fartin. Ziptie the choke open and go for a ride. I promised I wouldn't tell anybody what was wrong with mine to keep from lookin like a goober. :D Hope yours is this simple.
    #9
  10. Al Tuna

    Al Tuna doldrums

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    Battery! The CB has to have a strong battery to run properly.
    If the battery is weak, it will lose spark to one cylinder and bog at high RPM.
    #10
  11. secondbreakfast

    secondbreakfast n00b

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    but nothing worked!

    with everyones suggestions i have (on top of what i had already done)

    blew out the stock, reusable air filter with compressed air
    cleaned the fuel petock filter and installed inline fuel filters
    made sure the choke was closed
    pulled the diaphragms and inspected (they were in good cond)
    set the static timing and point gaps
    checked the timing with a timing gun (fine at idle, though not hitting the advance area at all when i rev it gets just before it)
    put in new plugs
    and adjusted the carbs again (the book says to get the right idle then toy with one of the mix screws to get that carbs max RPM then go back and forth until the mix is optimum and the idle is 1200 with equal exhaust pressure. messing with the mix screws didnt seem to do much so i left them at factory setting (3/4 out) and just messed with the idle screws until the exhaust pressure was equal at 1200)
    the exhaust smoke is not colored

    but it still does it! if i go much beyond 4k it actually dies when i release. the battery should be pretty well charged but i guess ill charge it again. the only other thing is that the advance must not be working right and not allowing the spark to hit when it should. the plugs are coming out dark and powdery-ish which i know means they are rich or not sparking on time. the thing is i checked the advance. i cleaned it and gave it a couple drops of tri-flow lubricant. the springs seemed strong. maybe i should stretch out the springs a bit? thats all i can think of. any more ideas? thanks!
    #11
  12. stru0121

    stru0121 n00b

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    Hey did you ever get your Scrambler running? I just picked up a pretty sweet '73 350 Scrambler with low miles and it's got a similar problem--idles all day long, but once I drive it and get up to 4,000 RPM or so, it loses power and almost sounds like it's missing.

    I haven't yet replaced the contact points or taken apart the carbs to clean them out (just new plugs, new oil, carb adjustment, new battery) but I plan to do carbs next. Found this thread and I thought I'd see if you got your working cleanly.
    #12
  13. motomike14

    motomike14 Thumper Crusader

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    For both of you, I'd check your carb boots. Old rubber tends to crack. Pull your carbs off and get a good look inside. Any sort of air leak or tear in those boots will wreak havoc.

    Second thing to check would be your air filter setup. Things like to live in there. A friend of mine had an XT600 that would do the same issues; we pulled the airbox, and sure enough, found a host of things in there, including a rats nest.

    Check for leaks in your air boots coming off the carbs as well.
    #13
  14. Thunderchild

    Thunderchild Rumble, rumble

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    Have you done a compression test yet? I would start there.
    #14
  15. LashLarue

    LashLarue Been here awhile

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    Check the advancer behind the points plate. I have seen them stuck. Note how they are assembled before you take them apart cause you can get them 180 off.
    #15
  16. JeffS77

    JeffS77 cheap bastard

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    sounds/looks like the timingisnot advancing..thats be your problem I bet
    #16
  17. gravityisnotmyfriend

    gravityisnotmyfriend Long timer

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    Yeah, check compression. These thing need rebuilding if the get below 160PSI The came from the factory at 185psi.

    If thats good, spray some ether on the carb boots while it's running. If the engine speeds up - or changes at all, you've got cracked boots.

    I've got a '71 cb350 that wouldn't rev past 5,000 RPM. Turns out it was just a clogged airfilter. I tried blowing mine out with compressed air, but it wasn't good enough. Replaced teh paper element with foam and she's running like a top. Pull your air cleaners off all together and take it for a spin to see if it'll rev up. Just stay off the dirt/gravel/mud til you get them back on.
    #17
  18. hook0924

    hook0924 Been here awhile

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    Hondas are nortorious for having weak valve springs on pre 83 models. This causes valve float and an eventual piston colision.
    #18
  19. kbasa

    kbasa Roubaix! Super Moderator

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    This is what happened to my CB750F when the alternator failed: It first started as missing at high revs.
    #19