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Thread: New Rider with a problem

  1. #11
    Junior Member mtn_'s Avatar
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    Nov 2016
    New Hampshire
    Quote Originally Posted by fender_fella View Post
    What a way to introduce myself! I'm a brand new rider and I have a 1980 Honda CM400A. I've had it about 2 weeks and it ran great at first but now it won't even idle, there's no acceleration and as I put along in 1st gear, all of a sudden it will backfire and rev up to super high RPM's , to where I have to just kill the engine or it may blow up.

    So Hello everybody, and what the heck is going on with my bike LOL!?
    welcome! i'm new as well but have some insight on the problem. the fact that it backfired and then revved could be two different problems. a misfire is a spark timing issue and the engine is getting fuel. an idle rev to max rpm without touching the throttle can be caused by many things but i've found its usually a stuck gas float pin in the carb, the one that is closed by the floats as they rise and the bowl fills with gas. when this sticks fuel can overflow the jets. common problem with rusty tank bikes with or without in line filters. if you pull the throttle it could be as simple as a sticking throttle needle slide in the top of the carb.

    either way, if it's a new bike to you and you don't know the service records the only way to truly track down ALL the problems is to start from ground zero. many helpful guides on the forum. i like to treat the electrical and fuel separately. unscrew both plugs and take them off the bike, but put them back in the plug lines connected to the coil. place the plugs on the motor just touching any metal to ground it. turn the bike on, kick it over while looking at the plugs to check for spark. remember both plugs need to be ground to spark as that is the ground of the coil. if it helps leave one screwed and plugged in and check one at a time. if you have a full bodied spark on both plugs, the coil, condenser, points, and advancer are at least working. however, this does not mean they are in time with the valves. adjusting the cam chain tensioner and timing the valves with the contact points is more than i want to write here, so check up on the guides the forum offers. cleaning the contact points always helps wit arc-age and out of time fires.

    if you have a full spark, its most likely a fuel issue. start at the tank. pull of the lines from the carbs and check that fuel properly flows through both lines. it is a great idea to clean or replace the metal screen in the fuel petcock if the tank is dirty at all. will help save your carbs and fuel flow. in line filters if you think you need them. if proper fuel is flowing into the carb, then its a carb issue. every single bike I've gone through has needed the carbs gone through at least a couple times. any issue with idling, revving, cutouts, high speed loss of power, etc is almost always a carb issue. learn how to clean your own carbs because it will save you $$$ down the road. they are truly easy to disassemble and with a set of extremely thin needle cleaners its a quick job to tackle the carbs. check the throttle slides for stickage while you're in there.

    all in all, first things first get or download the shop manual and parts catalog for your bike, very easy to find. then you can confidently reassemble. with two screwdrivers, a 10, 12, 14mm set of wrenches and a manual impact wrench you can disassemble practically the entire bike, spare the big lugs with an adjustable. if you grab a can of carb clean and really rebuild your carburetors there will be no more fuel issues. the cam chain tensioner, ignition timing and valve clearance are pretty mandatory tune ups and will maintain great life to your bike. most motors i open are neglected with the cam chain digging out the side of the case throwing aluminum everywhere it shouldn't be. the little hondas love the abuse though

    sorry for the length! have to at least be thorough if nothing else. we're here to help
    Last edited by mtn_; 12-23-2016 at 09:28 AM.
    ctrider likes this.

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  2. #12
    Member fender_fella's Avatar
    Join Date
    Dec 2016
    Thanks mtn for that. I will honestly have to look up most of what you said because I have no idea about cams and coils and points, but I know I'm gonna have to learn to keep a 36 year old bike running. As for carbs I think I will start there. Yesterday a neighbor loaned me an 18mm wrench and I went down and bought 2 new NGK D8EA plugs and installed them. The old plugs were black. I also added about 3 ounces of seafoam to a full new tank of gas. Youtube has been pretty good about showing me how to work on carbs. As it turns out I have two mikuni carbs, not sure which model though. I screwed up and tinkered with the air screw so now I guess they are out of sync. I think I can handle taking them apart and cleaning them. That would be a good start, or maybe just try that seafoam treatment in the bowls to see if it makes a difference. Also I think I have worn down the battery from trying to start it so maybe I'll trickle charge that too. In the span of my ownership of a couple of weeks I've managed to take a bike that I drove several miles home to having it not even start anymore. I am probably a mechanics dream customer LOL. Thanks again for the pointers.

  3. #13
    Senior Member Bill H's Avatar
    Join Date
    Aug 2015
    Waverly Ohio USA
    When you buy your tools be sure to pick up 6 point box end wrenches. I have a cb 400a and dont know how close the the two bikes are but on mine the left side carb inside bolt is hard to get to and an 8mm box end 6 point was all I could get in the area. Allso when removing the carbs I found that after everything is loose pull the carbs to the rear and tilt down then move to the left to remove. The cables will be off but on my bike I found that closing the choke will keep the lever from catching on the rubber boots . Be sure to mark the position of the cables and how they are threaded on the frame they are easy to hook up wrong.

    The clutch on the trail 90 was depressed by the shift lever when you changed gears the 400as have a tourque converter. Just a thought.

    Good luck Bill

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  5. #14
    Member fender_fella's Avatar
    Join Date
    Dec 2016
    This is my first bike and my first attempts at anything mechanical so I do appreciate all the advice and I'm sure I'll be posting a lot of "stupid" questions on here LOL, but I'm so grateful to have found this forum.

  6. #15
    Super Moderator longdistancerider's Avatar
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    Oct 2011
    Vancouver Washington (text 408-239-9580)
    There aren't any "stupid" questions, just a lack of knowledge that we'll help correct. Lot's of information in the SOHC section waiting for you
    dtsmjr8dan likes this.
    Jim O'Brien
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