hello i have a verry frustrating problum. i have a 1980 cb650 custom. I started with two cycles took one from frame up tyook all the good parts. Any whoto the prob. I ha it running but it was only running on cylinders 3 and 4 . so i replaced cdi units with some aftermarket ones that have the same timing. no i have spark have gas and compresion but wont runn. went threw and did all the tests in both books i have ohm testing contanuity rtesting all all wires off of cdis and to them tested and resplaced the elec points. cleaned carbs im open toi many sugestions. ..............................Last nite we checked the timing turned the motor by hand and spark is where it should be cam is where it should be bc of the valves are set and at tdc the valves are where they should be. replaced the coiles tested them have full spark on all plugs. plugs are getting wet so its getting fuel. when i fully choke he carbs seems liek it firse out the back of the carb vrs out the exhaust. but when i take the choke off it fires out the exhaust and back fires realy loud. ............my email is rockford245@yahoo.com...........
Thnx alot
ShorTy
650custom will not run
Hmm... might be overly simple, but are you sure you didn't cross the plug wires? The left coil controls cylinders 1 and 4, and the right controls 2 and 3.
Did you rebuild the top end when you put the bike together? If so, perhaps you got the cam chain off by one link. It's very easy to do, and will completely throw off your timing (and perhaps leave an intake valve open, causing the backblow).
The backfiring is probably because your plugs are only firing intermittently. Raw gas vapor is exhuasted into the header and ignites, causing the backfire. Have you put the plugs on the header to check for spark? You may be getting good readings on an ohm meter but unless you can see a spark I wouldn't place any bets.
Do all of your float bowls drain a (seemingly) equal amount of gas? When you cleaned the carbs did you do a bench sync afterwards?
I can't think of any more questions right now
Did you rebuild the top end when you put the bike together? If so, perhaps you got the cam chain off by one link. It's very easy to do, and will completely throw off your timing (and perhaps leave an intake valve open, causing the backblow).
The backfiring is probably because your plugs are only firing intermittently. Raw gas vapor is exhuasted into the header and ignites, causing the backfire. Have you put the plugs on the header to check for spark? You may be getting good readings on an ohm meter but unless you can see a spark I wouldn't place any bets.
Do all of your float bowls drain a (seemingly) equal amount of gas? When you cleaned the carbs did you do a bench sync afterwards?
I can't think of any more questions right now
Re: 650custom will not run
DammitDan wrote:Hmm... might be overly simple, but are you sure you didn't cross the plug wires? The left coil controls cylinders 1 and 4, and the right controls 2 and 3.
I agree with that point, if you mix up the coils/plugs and have a lot of fuel in the cylinders, it will ignite into the exhaust. If the exhaust is a bit rusted, it may even break from that . Don't ask me how I know about that .
The left coil for 1/4 and right one for 2/3 may also not be true if you mixed up the plugs at the spark units.
I understand you did already many timing checks, but are you sure about your timing marks? I would recommend to use a straw to feel the OT of an outer cylinder and check if the position of the "T1.4" timining mark is correct then.
Did you ever try new plugs? If they only fire at low pressure, that also may perfectly explain all your symptoms. Possibly cleaning out the black stuff from inside with a thin, stiff wire or a needle is enough. This way I "repaired" a chain saw last week.
cb650custom wont run
yes i have checked the wires goign to each coil and the way the wires are set up of the harness if i wanted to swqap coil wires a coulndt. the length isnt there. uhm about the polug wires both sets still have the factory lables on each wire. have replaced the coils. yes we have used the straw method to check the tdc and posistions of cylinder. all floats did what should have equaly ............i belive. the fireing out the back of the carbs i jsut think its the way the choke is set up those goofy buterflys lol . ...............my friend wants to pain the biek this weekend but if i cant get it running whats the point. im bout ready to pull this wiring harness and put my other wireing harness on. have not tryed new plug thinking bout getting a new set tonite if i go into town. they didnt look to bad lil dark nothing to bad or to terible out of gap. rack those brains lil more for my guys i pareciate it ALOT
thnx
Shorty
thnx
Shorty
Re: cb650custom wont run
Shorty24 wrote: have not tryed new plug thinking bout getting a new set tonite if i go into town. they didnt look to bad lil dark nothing to bad or to terible out of gap. rack those brains lil more for my guys i pareciate it ALOT
thnx
Shorty
Don't give anything on the optics. Plugs looking allright and firing perfectly outside are often misleading as firing under pressure is something really different (and may not work). At least you should have a try with new ones before you consider changing the wiring or do anything else which requires much effort.
Good luck!
Re: 650custom will not run
cb650 wrote:Put in a set of new plugs on the recently resurected 80 and #4 wouldnt fire. Swaped a old one from the bottom of the tool box and all 4 are firing.
NGK plugs I suppose ?
Re: 650custom will not run
I heard that some years ago NGK sold a larger series of plugs with a high failure rate. There were many complaints, however personally I never had bad experiences. Meanwhile they should be allright again. Maybe they fell back into that quality problems or Terry's plugs may still be from that series.
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