Motorcycle acceleration help!!?

So i have a 1985 honda rebel 250 (cmx250) with 26k miles on it, the last owner didnt treat it right so im trying to fix it all up ride it for a bit then sell it. I am new to motorcycle mechanics and have some experience with car mechanics but this is bugging the crap out of me. So, when im riding and there are times where the acceleration force of my bike drops by almost half, it happens out of nowhere completely random. It doesnt matter if my bike is hot or cold, whether ive been riding for a short time or long time, or how much gas is in the tank. I have put the carb through 3, 48 hour cleaning sessions of only Sea Foam in the bike and no gas, i have changed the battery, the spark plugs, the distributor wires and the air filter. I have checked all fuses and wires and yet i cannot figure out why the power drops on me. I could be riding for over an hour and have minimal acceleration and then the next day full acceleration out of nowhere. I take the same route every time and i ride 3 times a week. Please tell me what is wrong with my bike??

9 Answers

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  • Anonymous
    4 weeks ago
    Favorite Answer

    One: Check if fuel Flows, not dribbles.

     Remove cap-- is it Breathing?

    Remove tap, clean the inlet sock. I had to remove mine on my Honda, and put a filter inline.

    .Two:  possibility carbs out. Get the Haynes bonk, set the floats.

      LOOK at needles. Cracked, swollen/stickling?

     Remove and  LOOK at all jets. Especially the pilots,. they control off- idle  acceleration. . Blow them out with spray cleaner, See Daylight thru them and emulsion holes?

     Clean Idle needles.  Springs/o rings  still There? A creeping g screw from vibration can solve your mystery. My new Harley had loose Throttle plate screw that advanced idle on me

    . Borrow an air compressor, blow all ports out. 

    When together, use a ,015" piano wire under each throttle plate,adjust  idle speed screws' until  wires Just Slip.   .Set idle mix screws to 1 1/2  turns out or what book suggests.This rough syncs the carbs

    Use an air tip on that hole on vey top of venturi, before the slides. The slides should move up freely. and d own, quickly with air removed

    . Push slides up with finger, move Smoothly? Should shut  in about one second. Too fast means holed diaphragm(s). or other vacc leak, maybe the 4 screws on top the vac chamber  "mushroom" are loose?

    3: Maybe Igniter?. My '83  CB650  had it t under seat, it got Wet once, and bike acted like you describe.

    4  Any  vac lines loose? My CB650 had a troublesome vacc-actuated fuel tap that I tore off, I  replaced it  with a  std on-off type from an old CB 750.. CA models are aMess  with vacc  lines to a carbon canister.

    My '83 would die on me, sometimes. . There were 2 buss rails on fuse box just crimped together at factory. I had to solder them. Kill switches get dirty, too, Use  yours and also  flick it back and forth with engine off, occasionally.

    EDIT: An '85 has NO  auto choke.NO "brass" in an '85  Keihin carb   the bodies are pot metal and floats are plastic ,All carbs I've  seen,  including aircraft Schebler-Marvels, are pot metal. Old  Amal   monoblocs had brass floats, I still have a scale to tell if they leaked and were too heavy.

    @Geo, Plug  vacc  line to the new petcock, as its now not going anywhere. Or, just  put a cap on the manifold source. 

    REF Hassled with 4 carbs on an '83 CB650 from 2010 until I sold her 3 weeks ago.

  • 3 weeks ago

    Why did you check so many electric components? Riders should periodical check the thick negative cable under the bolt that attaches it to the frame, close by the engine or at both. Ignition cables are usually twisted onto screws that stick out of the spark plug cap part. Your problem seems to be the result of one cylinder not igniting. Coils can do that when their winding shorts out and they will when warm because the resistance is higher but also when sparks start heating the coils. Usually this leaves traces of heat burn on the outside of them. Look for YouTube video's on how to do the resistance test. Forgotten might be the carb boots leaking and sucking in air. This will be constant in effect but the seams do not always leak equally much. It is tested by a wet spray of burnable aerosol, lighter gas or brake cleaner (take care not to let it explode and catch fire). The refs will go up when a boot is leaking. Then a gas slide can stick in open position or on some bikes an air intake enhancer stays open because the carter vent makes sticky under the air filter but when you have not opened the carbs the movement of the floater or the needle above it could be restricted. The hinges may cause more problems then Seafoam can fix and the floater can develop cracks from being dry so that it sinks down but on some bikes two days of not use may be enough to dry out the carbs and the process of sinking repeats. There is something that you can inspect from the outside: carbs have drain screws, put a glass pot under the rubber hose that hangs down, pull out a stop if there is any on the hose and drain the carbs to see if there is any water in that gas.

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  • fuzzy
    Lv 6
    4 weeks ago

    when the power drops are both cylinders still firing or has one cut out? Should be able to hear this. If this is happening probably spark but could be carby.

    Does the bike still idle under these conditions? If still idles Ok then almost certainly dodgy carby/carbys. Get Haymes manual & strip clean & inspect them properly the nset up a recommended in manual (forget Seafoam)

  • adam
    Lv 5
    4 weeks ago

    seafoam. Thats not how you clean the carbs. You need to remove the carbs and actually clean and blow them out with air. When you install them back on the bike they need to be synched. In addition the next time it has no power open the gas cap. Its possible you have a venting problem. Gas tank usually has a filter with its petcock. Clean or replace it. 

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  • Geo
    Lv 6
    4 weeks ago

    Here's what I would do: Replace the black fuel with clear or tinted fuel line to confirm that you're getting fuel at all times thru the petcock. Speaking of which, if it's a Vacuum Petcock replace it and the Vacuum line to it. On a bike that old replace ALL of the Vacuum Lines. And put a Vacuum Gauge on it and see what it does, they're cheap and very useful tools.

  • 4 weeks ago

    Rebuild the carb as in completely disassemble, clean in a dip-type carb cleaner, replace all of the rubber parts, replace the needle / slide and jet(s) if needed, replace the float and adjust everything per the manual. Also make sure the cables and throttle are cleaned, lubed, and work freely. Coils can become intermittent, also. 

  • Anonymous
    4 weeks ago

    Cleaning a 35 year old carb will only help if the tolerances are correct. Brass is a soft metal, it does wear. Strip it, clean it properly, measure the components. It should not cause intermittent problems though, have you checked the integrity of the rubber manifolds? Have you checked the coils? With a Honda of that age it is worth checking the reg/rec too.

  • CB
    Lv 7
    4 weeks ago

    There may be a filter in the fuel tank just above the petcock - have you looked in the tank to see if there is rust or scaling (I would be suspect of a 35yr old fuel tank). Might be blocking the filter (the one in my VT1100 was pretty fine). If not filter at least give the tank a good look and remove the petcock. You can always but a hand full or two of pea gravel in and shake it with a little water of kerosene (shake for a long time) drain add some gas repeat until clear. (I would get thumbs down if I told you to use water then follow with liquid Heat or seafoam - but that is how I did it).

    Of course do the fuel filler cap vent test first per 'they keep killing suzy' (keep it simple and cheap). 

  • It might be something as simple as a blocked filler cap bleeder. Next time you lose acceleration stop and open the cap to let the air in. This this cures the problem either clean your fuel cap or replace. 

    PS...also make sure the choke is off. If it's automatic that could be an intermittent fault. One day the choke is off, another day it doesn't go off and you lose acceleration when the engine gets warm because the air/ fuel mixture isn't right.

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