Stumbling idle at running temperature
Page 1 of 2 1 2 LastLast
Results 1 to 10 of 11
  1. #1
    Senior Member alehman's Avatar
    Join Date
    Aug 2017
    Posts
    212

    Stumbling idle at running temperature

    Happy riding season everyone. I just pulled my 350 out of storage and had a great ride back to house. I fixed my top speed issue (wasn't going above 65mph) and my new exhaust sounds great. But as usual there is a new problem to deal with. Once at running temp, the engine stumbles below 2k rpm and cannot idle. It runs perfectly when I've just started it up. Always runs great above 3k rpm no matter the temp.

    I'll try to give a picture of the state of the bike. The engine is pretty solid and was rebuilt 600 miles ago. Valves set, head bolts retorqued this winter, compression 180/180. I can't imagine running temperature is affecting the carbs significantly. Which leaves the electrical. I put a pamco on this winter, which I know is a major achilles heel, but I want to rule out everything else before I put points on again. My right plug is brown and left plug is a little black but nothing serious. I had to rejet to 110/70 for my slightly more open than stock exhaust.

    What could cause an idling issue only at running temperature? Thanks in advance.

  2. #2
    Super Moderator Mydlyfkryzis's Avatar
    Join Date
    Jul 2012
    Location
    Northern NJ USA
    Posts
    5,916
    My bike had bad rubber carb mounts. Did not leak cold, leaked hot. Vacuum leak caused high idle and stumbling. Got bad enough that it would not run at all after 20 minutes.

    Sent from my SM-T830 using Tapatalk
    Richard

    Mydlyfkryzis = Mid Life Crisis
    My Build:http://www.hondatwins.net/forums/1-p...-ignition.html

    ______________________
    1991 Honda Nighthawk 750 "Big Red"
    1976 CB360t "Sophie"

  3. #3
    Senior Member alehman's Avatar
    Join Date
    Aug 2017
    Posts
    212
    Thanks, I'll look at my mounts. They're new though and I dont have a high idle issue... it just wont idle and I have to keep it revved.

  4. Remove Advertisements
    HondaTwins.net
    Advertisements
     

  5. #4
    Senior Member alehman's Avatar
    Join Date
    Aug 2017
    Posts
    212
    The problem seems to be electrical. When i hooked a timing light up to my right cylinder, the light does not flash at the same time there is a stumble in the idle. The left cylinder is fine. Then when I took the bike for a test ride and it died at idle as it is now doing regularly, it would not start back up. Its almost as though the battery was straining to trip the solenoid--- it wasn't even turning over. All this reminds me of what happens simply when your battery isn't charged. But when I plugged my battery into a tender the tender seemed to believe that the battery was at greater than 80% charge. I'm starting to suspect the battery is bad and I just have weak spark at idle, even though it's a 1.5 year old AGM that hasnt given me any grief before. However, it would explain why I don't have issues above ~2500 rpm--- above that the charging system would pick up the slack of the battery.

    I'm not sure why I would be having this issue only at running temp and not when cold. Is it easier for a weak battery to pass a spark across the plug gap when the engine is cold?

  6. #5
    Senior Member 76TWIN's Avatar
    Join Date
    Aug 2013
    Location
    Puget Sound/Seattle area
    Posts
    3,371
    Classic symptom of a bad coil which would once warmed up start to produce inconsistent sparks. Often times it will correct itself once it cools down.
    '76 CB500T Frankenbike
    '73 CL450 (basket case for now)


    Carpe narem.

  7. #6
    Senior Member 76TWIN's Avatar
    Join Date
    Aug 2013
    Location
    Puget Sound/Seattle area
    Posts
    3,371
    Also test your battery. Anything less than 12.5v at rest means it's on its way out.
    '76 CB500T Frankenbike
    '73 CL450 (basket case for now)


    Carpe narem.

  8. #7
    Senior Member alehman's Avatar
    Join Date
    Aug 2017
    Posts
    212
    Thanks 76twin. I tested my battery and with the key off its 12.9v. Key on and it drops to 2.3v. Thats 2.3, not 12.3.

    What on earth could be causing that big a power drain? It wont crank, none of my LED gauge lights come on. I think you might be onto something with the coil... when i was setting up my pamco my coils tested at 5.5 ohms. Im going to take my tank off and test them again.

  9. #8
    Supporting Member Yendor's Avatar
    Join Date
    Aug 2011
    Location
    Camden County, New Jersey - USA
    Posts
    2,790
    From your Voltage Tests you have a bad battery.

    No Battery that is good and fully charged or even 1/2 Charged will drop from 12.9 to 2.3 volts by just powering the coils.

    What you are seeing is it cannot handle a LOAD.

    Take the battery out and to a Auto Shop they can do an "OFFICAL" Load Test on it to determine it's health.
    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

  10. #9
    Senior Member alehman's Avatar
    Join Date
    Aug 2017
    Posts
    212
    Got it, thx. I will look into load testing. Both my coils are also testing 3 ohms cold, which is below spec for the pamco. I dont know why its 2.5 ohms lower than when i tested them in the winter. Ill address that after the battery.

    The bike wont even run when kickstarted... a few revs and it dies.

    So i can avoid it in the future, what would cause a new-ish battery to fail like this?

  11. #10
    Senior Member alehman's Avatar
    Join Date
    Aug 2017
    Posts
    212
    Yeah, im pretty sure you are right. Even with the coils off it cant even make the solenoid click.

Page 1 of 2 1 2 LastLast

Sponsored Links

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Similar Threads

  1. Stumbling engine
    By DrHonda in forum Engine Discussion
    Replies: 2
    Last Post: 08-11-2013, 11:03 AM
  2. Stumbling at High RPMs
    By SMKennedy94 in forum Engine Discussion
    Replies: 6
    Last Post: 03-22-2013, 02:55 PM
  3. stumbling/sputtering 3500rpm
    By skooter in forum Engine Discussion
    Replies: 15
    Last Post: 06-13-2012, 10:55 AM
  4. studdering/stumbling at 1/16-1/8 throttle opening HELP!!!!!!
    By B-40 in forum Fuel Supply and Carburation
    Replies: 22
    Last Post: 05-14-2011, 05:26 PM
  5. '73 CB350G Stumbling at 1/2 Throttle and Up - Please Help!
    By RatRob in forum Fuel Supply and Carburation
    Replies: 3
    Last Post: 04-12-2011, 01:33 PM