No power after engine rebuild - starts but won't rev or stay running

PlanetMinis Forums

No power after engine rebuild - starts but won't rev or stay running

This is a discussion on No power after engine rebuild - starts but won't rev or stay running within the Z50, CT70 and JDM Monkeybikes forums, part of the General Talk category; Greetings! It has been an ongoing struggle getting this bike running. I finally got the cash for the parts. It had low compression so i ...

Page 1 of 2 12 LastLast
Results 1 to 15 of 18
Like Tree1Likes

Thread: No power after engine rebuild - starts but won't rev or stay running

  1. #1
    Junior Member
    This user has no status.
     
    pcbisnice's Avatar

    Join Date
    May 2016
    Posts
    18

    No power after engine rebuild - starts but won't rev or stay running

    Greetings! It has been an ongoing struggle getting this bike running. I finally got the cash for the parts. It had low compression so i replaced the piston, rings, jug, and i got a brand new complete cylinder head so i could completely rebuild the top half of the engine. I got everything together today, and the bike basically behaves the same as before the rebuild. Hard to start... if it starts it revs very slow and not very high in rpms. Underload it almost won't move. I can keep it idling for 20 seconds and it will shut off. I check the spark plug and it is almost black from just the 20 second run, as i used a brand new plug. I clean up the plug, checked timing on the cam and rechecked the gap on the points and everything looks good. So i replaced the points and condenser, as well as the ignition coil just to make sure it wasn't those parts since I had them laying around. No change. The only thing i don't have is a compression tester to recheck compression. Just seems like there is no power STILL. Is there something I have missed maybe? Any ideas and suggestions would be greatly appreciated.

  2. #2
    Senior Member
    This user has no status.
     
    kirrbby's Avatar

    Join Date
    Aug 2014
    Location
    Davison Mi.
    Posts
    389
    What kinda bike do you have?? CB750?? Z50? What carb do you have on it?? Sound like it's running super rich. Raise the clip to lower the needle.

  3. #3
    Senior Member
    This user has no status.
     
    50race's Avatar

    Join Date
    Apr 2004
    Location
    North Vernon, IN
    Posts
    397
    cam timing, valve/tappet clearance, valve to seat seal, ignition timing, ignition bad, weak spark, bad spark plug the list goes on and on.

    Engine needs air, fuel, spark, compression, timing. Try to eliminate some of these as the issue and focus on the others.

    good luck

  4. Remove Advertisements
    PlanetMinis.com
    Advertisements
     

  5. #4
    Junior Member
    This user has no status.
     
    pcbisnice's Avatar

    Join Date
    May 2016
    Posts
    18
    Thanks for your responses. I have a 1984 z50r.

    Sorry for the confusion, I figured since this forums is for bikes that all have very similar setups/engines (different displacements) that it wouldn't be a problem. I don't think cb750's go in this forum

    I checked my spark today and for some reason it is more yellow in color. Rather then the bluish white it used to be. I am googling it with little results. Any ideas on this?

    Found out my compression tester is broke, so I may have been chasing problems that never existed.

    I will post later when i run more test.

  6. #5
    Junior Member
    This user has no status.
     
    pcbisnice's Avatar

    Join Date
    May 2016
    Posts
    18
    Ok so....

    I triple checked cam timing and it is perfect. I triple checked the points gap and it just starts to open at the F mark, and at the T mark its about .014 gap... which is within specs.

    I borrowed another compression tester from a friend and I ran 3 compression test and all 3 got me 70ish PSI. I sprayed in some oil through a straw and ran another compression test... got 89psi.

    This sucks because i just rebuilt this engine. The head is brand new, as is the piston, rings, and cylinder. I started this rebuild due to low compression... and i still have it. Maybe I need a new hobby?

  7. #6
    Super Moderator
    is grateful again...
     
    dirtbkr188's Avatar
    Supporting Member

    Join Date
    May 2004
    Location
    New Jersey
    Posts
    8,179
    Check the valve clearances, even though the head is new doesn't necessarily make it within spec.
    - dirtbkr188 -



  8. #7
    Junior Member
    This user has no status.
     
    pcbisnice's Avatar

    Join Date
    May 2016
    Posts
    18
    i doubled checked the valve clearance again and both intake and exhaust are at .002. anything else?

    Could i be 180 degrees off? Would it start like that?
    Last edited by pcbisnice; 03-11-2017 at 01:52 PM.

  9. #8
    Super Moderator
    is grateful again...
     
    dirtbkr188's Avatar
    Supporting Member

    Join Date
    May 2004
    Location
    New Jersey
    Posts
    8,179
    All of the ring ends staggered at either 120° or 180° apart from each other? New head gasket? All four of the engine stud nuts torqued evenly to 9 ft.-lbs?
    - dirtbkr188 -



  10. #9
    Junior Member
    This user has no status.
     
    pcbisnice's Avatar

    Join Date
    May 2016
    Posts
    18
    I tried extremely hard to keep the rings 180 degrees off when installing. I did use a new head gasket, and i torqued the headbolts down with a torque wrench to around 8lbs each.

    Maybe i need to pull the cylinder and check out the piston and rings?

  11. #10
    Super Moderator
    is grateful again...
     
    dirtbkr188's Avatar
    Supporting Member

    Join Date
    May 2004
    Location
    New Jersey
    Posts
    8,179
    It sounds like all of the mechanicals are in order, is it possibly an electrical issue. You did mention a change in the spark color, maybe it is just barely firing off and not generating much power in the cylinder. Just a thought...
    - dirtbkr188 -



  12. #11
    Junior Member
    This user has no status.
     
    pcbisnice's Avatar

    Join Date
    May 2016
    Posts
    18
    Thanks for all your help. But, spark shouldn't have any effect on compression, right?

  13. #12
    Super Moderator
    is grateful again...
     
    dirtbkr188's Avatar
    Supporting Member

    Join Date
    May 2004
    Location
    New Jersey
    Posts
    8,179
    Quote Originally Posted by pcbisnice View Post
    But, spark shouldn't have any effect on compression, right?
    That's right, and since everything that does have an effect on compression is within spec, you should have tons of compression. What procedure did you use to take a compression test, if I may ask? My reason for asking is that these little engines are usually in the 160-180PSI range, and will run (minimally) with 100PSI of compression. If you're only getting a 70PSI reading, compression is seeping away from somewhere.
    - dirtbkr188 -



  14. #13
    Junior Member
    This user has no status.
     
    pcbisnice's Avatar

    Join Date
    May 2016
    Posts
    18
    Quote Originally Posted by dirtbkr188 View Post
    That's right, and since everything that does have an effect on compression is within spec, you should have tons of compression. What procedure did you use to take a compression test, if I may ask? My reason for asking is that these little engines are usually in the 160-180PSI range, and will run (minimally) with 100PSI of compression. If you're only getting a 70PSI reading, compression is seeping away from somewhere.
    i have a snap-on compression tester i borrowed. I inserted it with the correct size adapter snuggly where the spark plug goes. Gave it full throttle and kicked it over till the compression gauge stopped moving.

  15. #14
    Super Moderator
    is grateful again...
     
    dirtbkr188's Avatar
    Supporting Member

    Join Date
    May 2004
    Location
    New Jersey
    Posts
    8,179
    Well, I'm just about out of ideas to help you get to the bottom of this. At this point, I really don't have an answer for you...sorry about that.
    - dirtbkr188 -



  16. #15
    Senior Member
    This user has no status.
     
    50race's Avatar

    Join Date
    Apr 2004
    Location
    North Vernon, IN
    Posts
    397
    valve to seat seal
    z501wheel likes this.

Page 1 of 2 12 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. Help! 1974 CT70 starts but won't stay running
    By JB MT in forum Z50, CT70 and JDM Monkeybikes
    Replies: 2
    Last Post: 09-27-2013, 12:12 AM
  2. Help me get this XR70 to stay running...
    By justy4897 in forum Honda CRF70
    Replies: 10
    Last Post: 02-07-2011, 01:15 AM
  3. Engine Difficult to Start and Stay Running
    By JaxFLCRF70 in forum Honda CRF70
    Replies: 4
    Last Post: 06-19-2009, 08:00 PM
  4. Engine won't stay running?
    By vegatron75 in forum Chinese/Import Minis - General Discussion
    Replies: 9
    Last Post: 04-21-2007, 03:19 PM
  5. Wiring issues? Bike starts but won't run?
    By Kyle_245 in forum CRF50
    Replies: 5
    Last Post: 03-08-2007, 12:11 PM

Search tags for this page

xr50 no power after rebuilt

Click on a term to search for related topics.

» Log in

User Name:

Password:

Not a member yet?
Register Now!

» Sponsored Links

Powered by vBadvanced CMPS v4.3.0