Author Topic: 5k rpm cutting out  (Read 383 times)

0 Members and 1 Guest are viewing this topic.

Offline antleo

  • Full Member
  • *
  • Posts: 59
5k rpm cutting out
« on: July 23, 2016, 08:59:04 PM »
Bike is new to me. I purchased it less than 2 days ago. i knew it would need work, but this stumps me. The bike starts and runs great, when its on the stand in neutral it revs straight through the range. but when under load the bike pulls hard out of the hole but will not rev past 5k. It sputters and practically dies. I have the carbs off as this was my first guess. It has 120 main jets. I have a set of 97.5s I think I'm gonna throw in it as i did a plug chop and the plugs (which where brand new) are black. meaning rich. stock jetting i believe is 90? so i if that goes the right direction and helps get me up in the revs ill check that out.   I also found that one of the coil packs(if sitting on the bike looking forward, the one on the left) is cracked straight down the center.

I purchased the Bike from a local used bike dealer who has a plethora of used parts as well. It did not have a side cover or airbox cover, but if i  can find one in the plethora of parts(i found a side cover already, need the airbox cover) it'll be tossed in. 

So what else should I be looking at if these don't solve the issue?


Offline Scott S

  • Global Moderator
  • Really Old Timer ...
  • *****
  • Posts: 8,234
Re: 5k rpm cutting out
« Reply #1 on: July 23, 2016, 09:20:17 PM »
 What bike? You didn't state the year or model. Any mods to the exhaust? What part of the air box are you missing, exactly?
 Also, 120 mains are pretty huge for any of the SOHC4's.
'71 CB500 K0
'17 Triumph Street Scrambler
'81 Yamaha XS650

Offline mrbreeze

  • Not your average
  • Really Old Timer ...
  • *******
  • Posts: 5,902
  • Shut up when you're talkin' to me!!
Re: 5k rpm cutting out
« Reply #2 on: July 23, 2016, 11:14:03 PM »
Spark advance?
MEMBER # 257
Fool me once..shame on you. Fool me twice..I'm kickin' your a$$......

Online PeWe

  • Really Old Timer ...
  • *******
  • Posts: 15,800
  • Bike almost back to the 70's 2015
Re: 5k rpm cutting out
« Reply #3 on: July 24, 2016, 01:25:00 AM »
It can be the ignition coil that get a spike to ground. I have had similar with bad wires/caps in wet condition.
Try other coils with wires and caps. The seller has loads of parts, right?
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline antleo

  • Full Member
  • *
  • Posts: 59
Re: 5k rpm cutting out
« Reply #4 on: July 24, 2016, 06:04:07 AM »
that is my bad, the bike is a 1980 cb650. The airbox lid is the part that is missing. it is a stock bike. exhaust and all. stock jetting as i was told was a 90 main. the rest are pressed in and i do not think have been changed. the 97.5 i have are manufacturer jets i have from another bike.   valves are in spec, oil was changed as soon as i got it, chain lubed and adjusted, plugs done, i plan to do wires and apparently a coil soon here.