Author Topic: cb750 aluminum rods stretch help!!  (Read 2669 times)

0 Members and 1 Guest are viewing this topic.

Offline 73nancy

  • Enthusiast
  • **
  • Posts: 149
cb750 aluminum rods stretch help!!
« on: September 11, 2012, 04:38:12 PM »
ok guys first post in the racing forum!! i just recently picked up a k3 750 race bike from a older gentleman that built and raced the bike a few times. the specs on the motor are big bore 911 race cam don't know what it is. 13:1 compression with dyna ignition, aluminum rods. other parts in engine not sure on. So the bike was road raced a few times before he found it was to heavy for him and built a stock class cb350. i was told that the motor ran great but he said but near the end of the races the rods would stretch and close the plug gap. he said no damage was done it would do it just enough to make the motor almost quit. my plans for this bike is i want to convert it to a drag bike i have a extended swing arm and a air shifter from the same guy. the bike has been sitting since 03 and i have yet to mess with it yet. so my question is what should i do about the plug problem. i was thinking shimming the plugs out but i don't know how far since it has good plugs in it? any help from you guys would be great would love to get this bike to the track at some point down the road. never drag raced before always wanted to. thanks ryan
k1 cb750 cafe racer (aka Helter Skelter)
k6 cb750 stock daily rider project
k3 cb750 race bike 911 big bore, alum rods, 13:1, race cam other goodies

Offline Retro Rocket

  • Eggs are hard due too a
  • Really Old Timer ...
  • *******
  • Posts: 19,279
  • ROCK & ROLL
Re: cb750 aluminum rods stretch help!!
« Reply #1 on: September 11, 2012, 07:33:22 PM »
Change the rods, its not the spark plug you should be worried about, its the VALVES, hit one of those and its all over. CycleX has super rods for around 600 USD

http://www.cyclexchange.net/Cranshfts%20and%20Connecting%20Rods%20Page.htm

It will save you a lot of grief further down the track...
750 K2 1000cc
750 F1 970cc
750 Bitsa 900cc
If You can't fix it with a hammer, You've got an electrical problem.

Offline Big Jay

  • Expert
  • ****
  • Posts: 1,252
    • CBRzone
Re: cb750 aluminum rods stretch help!!
« Reply #2 on: September 11, 2012, 11:26:24 PM »
If you are not running nitro, you don't need aluminum rods.

Offline turboguzzi

  • Old Timer
  • ******
  • Posts: 4,084
Re: cb750 aluminum rods stretch help!!
« Reply #3 on: September 12, 2012, 01:07:53 AM »
aluminum natural fatigue limit + unknown engine history = steel rods.

a no brainer IMHO

Offline 73nancy

  • Enthusiast
  • **
  • Posts: 149
Re: cb750 aluminum rods stretch help!!
« Reply #4 on: September 12, 2012, 04:34:52 AM »
The guy I bought it from owned a honda dealership been working on bikes a long time. The told me they were $800 rods about 15 years ago. Hate to just take them out. The work he did to it is not backyard so I trust that it was built bad. For a drag bike would it stretch like that compared to a road racer which is run a lot longer? Is there anything I can do or should I drop compression down
k1 cb750 cafe racer (aka Helter Skelter)
k6 cb750 stock daily rider project
k3 cb750 race bike 911 big bore, alum rods, 13:1, race cam other goodies

Offline 754

  • Really Old Timer ...
  • *******
  • Posts: 29,058
Re: cb750 aluminum rods stretch help!!
« Reply #5 on: September 12, 2012, 08:07:32 AM »
At one point in the late 70,s or early eighties NGK came out with a new plug, may have been the U tip.
 I put them in my 900cc single cam Honda, and the pistons hit the plugs closing the gap, put normal plugs back in, and it was fine..
 the ?900? turned out to be an 836 using cut down CB 350 pistons, they do have high domes.
 Just mentioning it ..if clearance is quite close (due to parts used) shimming or changing plugs might make a difference..
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline Sam Green Racing

  • Moderator
  • Really Old Timer ...
  • *****
  • Posts: 16,068
  • I REALLY? hate black rims.
Re: cb750 aluminum rods stretch help!!
« Reply #6 on: September 12, 2012, 08:33:38 AM »
Ryan, if it happens near the end of a race when the motor will be stinking hot, could it be that the rods are expanding due to the very high temprature and not actually stretching. I would have thought that if the rods stretched they would stay stretched and not return to normal size and ready to race again.
Thoughts anyone ???

Sam. ;)
C95 sprint bike.
CB95 hybrid race bike
CB95 race bike
CB92
RS 175. sprint/land speed bike
JMR Racing CB750A street ET drag bike

Offline mec

  • Hot Shot
  • ***
  • Posts: 503
Re: cb750 aluminum rods stretch help!!
« Reply #7 on: September 12, 2012, 08:41:42 AM »
they return to normal size only if they are made from rubber.

mec
Honda Monkey
Takeuchi TB 070
Massey Ferguson

Offline Sam Green Racing

  • Moderator
  • Really Old Timer ...
  • *****
  • Posts: 16,068
  • I REALLY? hate black rims.
Re: cb750 aluminum rods stretch help!!
« Reply #8 on: September 12, 2012, 08:57:51 AM »
What about ELASTIC then. ;D ;D ;D ;D
C95 sprint bike.
CB95 hybrid race bike
CB95 race bike
CB92
RS 175. sprint/land speed bike
JMR Racing CB750A street ET drag bike

Offline 73nancy

  • Enthusiast
  • **
  • Posts: 149
Re: cb750 aluminum rods stretch help!!
« Reply #9 on: September 12, 2012, 09:35:41 AM »
I will call the guy to see what details I can get from him.  I don't want to blow it up. He did tell me he bought big bores for all his bikes since it way just as cheap as nos pistions back then. I might try to shim the plugs up a bit to start the bike see what the motor sounds like. Do most big bore pistions have valve reliefs? What would be the draw back of moving the plug up? Is there a plug that might work that is one of those weird center fire ones. Thanks for the help guys
k1 cb750 cafe racer (aka Helter Skelter)
k6 cb750 stock daily rider project
k3 cb750 race bike 911 big bore, alum rods, 13:1, race cam other goodies

Offline turboguzzi

  • Old Timer
  • ******
  • Posts: 4,084
Re: cb750 aluminum rods stretch help!!
« Reply #10 on: September 12, 2012, 10:54:46 AM »
nobody is saying that the guy didnt do a good job.... but as you are stating yourself, they are giving already "some problems". so maybe space the plugs just to hear it running, but seriously, with 13:1, would lift the head and have a good look at piston to head and piston to valve clearance with a plasticine check.

taking an engine apart for no apparent need, specially on a 750 sure sucks. welding broken cranckases sure sucks more ::)

Offline 73nancy

  • Enthusiast
  • **
  • Posts: 149
Re: cb750 aluminum rods stretch help!!
« Reply #11 on: September 12, 2012, 04:20:16 PM »
nobody is saying that the guy didnt do a good job.... but as you are stating yourself, they are giving already "some problems". so maybe space the plugs just to hear it running, but seriously, with 13:1, would lift the head and have a good look at piston to head and piston to valve clearance with a plasticine check.

taking an engine apart for no apparent need, specially on a 750 sure sucks. welding broken cranckases sure sucks more ::)
yeah i know what your saying it does suck to have to do it though. so i think im going to shim the plugs start the bike let it idle see if there is anything else it might need while i will have it apart. then pull the engine. So my question would be to you guys plasticine how does it work? can you check it with the engine out turning it over by hand?  Also 13:1 is that common for a lot of 750 race motors? thanks a bunch guys really helpful.
k1 cb750 cafe racer (aka Helter Skelter)
k6 cb750 stock daily rider project
k3 cb750 race bike 911 big bore, alum rods, 13:1, race cam other goodies