« previous next »
Pages: [1]   Go Down

Author Topic: how to tell if motor work has been done  (Read 2539 times)

0 Members and 2 Guests are viewing this topic.

Offline fatkid

  • you cant undo whats already been done. so just ask
  • Enthusiast
  • **
  • Posts: 107
how to tell if motor work has been done
« on: January 20, 2012, 11:52:18 AM »
OK guys i got a 75 750f of a guy that told me the guy before him used to drag race the bike and had a lot of money in the motor . I don't hold my breath on this but it was cheap and it fired up and spit and sputtered so 300 isn't to bad . but I noticed the rear motor mount bolt is not factory and the front mount are changed. also the sprocket cover is off and came with 5 differant sprockets . so i am wondering  how is the best way to tell or is there any way to tell if its not factory without just strait ripping it down
Logged

Offline CycleRanger

  • No comment about being an
  • Really Old Timer ...
  • *******
  • Posts: 5,477
  • Central Texas Shop Manual Advocate
Re: how to tell if motor work has been done
« Reply #1 on: January 20, 2012, 11:56:23 AM »
$300 for a running '75 with or without engine work is still a good deal.

Post some photos of the engine. I'm sure there's someone here who may be able to spot something.
Logged
Do you have a copy of the Honda Shop Manual or Parts List for your bike? Get one here:
https://www.honda4fun.com/materiale/documentazione-tecnica
CB750K5        '79 XL250s     CL350K3
CB750K3        '76 XS650      '76 CJ360T

Offline KJ790

  • Enthusiast
  • **
  • Posts: 248
Re: how to tell if motor work has been done
« Reply #2 on: January 20, 2012, 12:31:47 PM »
Pull off the carbs and look into one of the intake ports. If it has not had any porting done then there probably hasn't been too much money put into it to make it faster (or at least well used money). If there has been porting done then you will probably have to tear it down to know what else is inside.
Logged
The most dangerous part of a motorcycle is the nut that connects the handlebars to the footpegs.

Offline fatkid

  • you cant undo whats already been done. so just ask
  • Enthusiast
  • **
  • Posts: 107
Re: how to tell if motor work has been done
« Reply #3 on: January 20, 2012, 12:47:22 PM »
like i said guys this may be all bull but i will lok into it . but there sure is allot of stuff changed to say its not been worked on
Logged

Offline Bankerdanny

  • Eventually I will be old enough in reality to be
  • Really Old Timer ...
  • *******
  • Posts: 7,626
  • Endeavor to persevere
Re: how to tell if motor work has been done
« Reply #4 on: January 20, 2012, 12:48:47 PM »
Off the top of my head intake and exaust porting would be the only work that you could ID without opening up the engine (and even they require removal of the carbs and exhaust. 

Upgraded cam, bigger valves, new pistons etc just aren't externally obvious. There is also the question of carb jetting too.

Drag friendly mods are not necessarily street friendly.

Cycleranger is right though, $300 for a complete and running bike is a good price. I would start with the carbs. Major jetting changes could indicate a hotter cam and/or higher compression.
Logged
"The problem with quotes on the Internet is that you never know if they're true" - Abraham Lincoln

Current: '76 CB750F. Previous:  '75 CB550F, 2007 Yamaha Vino 125 Scooter, '75 Harley FXE Superglide, '77 GL1000, '77 CB550k, '68 Suzuki K10 80, '68 Yamaha YR2, '69 BMW R69S, '71 Honda SL175, '02 Royal Enfield Bullet 500, '89 Yamaha FJ1200

Offline seanbarney41

  • not really that much younger than an
  • Really Old Timer ...
  • *******
  • Posts: 10,898
Re: how to tell if motor work has been done
« Reply #5 on: January 20, 2012, 01:18:15 PM »
although spurious at best...non-original cylinder paint may be another external clue of at least an engine rebuild
Logged
If it works good, it looks good...

Offline chewbacca5000

  • I polish covers!
  • Old Timer
  • ******
  • Posts: 3,536
Re: how to tell if motor work has been done
« Reply #6 on: January 20, 2012, 02:31:00 PM »
I say get her cleaned up and running good and put her on a dyno.  If it's posting 70+ HP they maybe there is some truth to what the PO has told you.
Logged

72500john

  • Guest
Re: how to tell if motor work has been done
« Reply #7 on: January 20, 2012, 04:52:17 PM »
"if" the bike "was" used for drag racing. and "if" alot of money was put into the motor. most likley someone has been in there. how well it was done is the question? for 300 bucks pick it up and make the best of it!
Logged

Offline Retro Rocket

  • Eggs are hard due too a
  • Really Old Timer ...
  • *******
  • Posts: 19,279
  • ROCK & ROLL
Re: how to tell if motor work has been done
« Reply #8 on: January 20, 2012, 05:13:01 PM »
What carbs are on it.?
Logged
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 wildcatmahone

  • Poseur
  • Hot Shot
  • ***
  • Posts: 320
Re: how to tell if motor work has been done
« Reply #9 on: January 20, 2012, 06:31:26 PM »
Go rent a bore scope, stick it in the the spark plug holes and check the tops of the pistons you'll know then.
Logged

Offline 736cc

  • Master
  • *****
  • Posts: 1,724
Re: how to tell if motor work has been done
« Reply #10 on: January 20, 2012, 07:30:49 PM »
Ride it back-to-back w/ a CB750 stocker and get a seat-of-the-pants dyno run. And if you don't have an extra CB750 lying around, well, I'm sorry to hear that.

Logged

Offline RustyStuff

  • If it aint broke, Fix it till it Is.
  • Enthusiast
  • **
  • Posts: 120
Re: how to tell if motor work has been done
« Reply #11 on: January 20, 2012, 08:22:08 PM »
Quote from: 736cc on January 20, 2012, 07:30:49 PM
Ride it back-to-back w/ a CB750 stocker and get a seat-of-the-pants dyno run. And if you don't have an extra CB750 lying around, well, I'm sorry to hear that.


You sir are a hoarder, and what time should I stop buy for your intervention.....
I'll take 2 of those Piles-o-junk off your hands for free...

:D
Logged
'80 CB650

Offline ekpent

  • Really Old Timer ...
  • *******
  • Posts: 13,512
  • To many bikes-but lookin' for more
Re: how to tell if motor work has been done
« Reply #12 on: January 21, 2012, 07:00:30 AM »
A few other 'clues' might be if it has some quality aftermarket coils and ignition,allen bolt motor kit,clutch feels stiffer than normal with HD springs. As mentioned earlier large main jets.You may be able to get a peek of a partial piston top with the right kind of flashlight,at least a peek at the valve cut out on it  ;)
Logged

Offline 754

  • Really Old Timer ...
  • *******
  • Posts: 29,058
Re: how to tell if motor work has been done
« Reply #13 on: January 21, 2012, 09:43:49 AM »
 ditto on looking thru the plug hole.... start it up and ride it,  the seat of your pants will tell you..
Logged
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 Tews19

  • I am no
  • Really Old Timer ...
  • *******
  • Posts: 6,465
Re: how to tell if motor work has been done
« Reply #14 on: January 21, 2012, 04:41:26 PM »
Quote from: RustyStuff on January 20, 2012, 08:22:08 PM
Quote from: 736cc on January 20, 2012, 07:30:49 PM
Ride it back-to-back w/ a CB750 stocker and get a seat-of-the-pants dyno run. And if you don't have an extra CB750 lying around, well, I'm sorry to hear that.


You sir are a hoarder, and what time should I stop buy for your intervention.....
I'll take 2 of those Piles-o-junk off your hands for free...

:D


736- you have one in every color? God you are greedy!!!!!!!!!!!!!!!!!     AND MEAN
Logged
1969 Honda CB750... Basket case
1970 Honda CB750 survivor.

Offline 736cc

  • Master
  • *****
  • Posts: 1,724
Re: how to tell if motor work has been done
« Reply #15 on: January 21, 2012, 07:48:36 PM »
They did look nice all lined up. Each had a good story behind them, I like to think they were pretty unique restorations. But that was a few years ago, I just have 1 nowadays.


Logged

Offline Danno

  • Hot Shot
  • ***
  • Posts: 715
  • 74 CB550,75 GL1000,76 KZ400,77 GL1000,73 CB750
Re: how to tell if motor work has been done
« Reply #16 on: January 21, 2012, 09:49:41 PM »
you can also buy one of those small ryobi camera probes and look inside the plug holes at the bore top of the pistons and valves and if you take the pan off you can see lots of fun stuff
Logged
when you own a motorcycle the wife does not have to find you handy she just has to find you

Offline fatkid

  • you cant undo whats already been done. so just ask
  • Enthusiast
  • **
  • Posts: 107
Re: how to tell if motor work has been done
« Reply #17 on: January 30, 2012, 10:17:42 AM »
well guys it has stock carbs not sure whats inside i did see it had these coils . i am not to familiar with what coils are what . anyone have any insight on these and should like look for anything else when it comes to the ignition
Logged

Offline CycleRanger

  • No comment about being an
  • Really Old Timer ...
  • *******
  • Posts: 5,477
  • Central Texas Shop Manual Advocate
Re: how to tell if motor work has been done
« Reply #18 on: January 30, 2012, 10:27:54 AM »
Quote from: fatkid on January 30, 2012, 10:17:42 AM
well guys it has stock carbs not sure whats inside i did see it had these coils . i am not to familiar with what coils are what . anyone have any insight on these and should like look for anything else when it comes to the ignition

Those are Dyna 3 ohm coils. You can pull the points cover and see it it's got a Dyna ignition as well.
Although none of that is necessarily indicative of a "hot" motor.
Logged
Do you have a copy of the Honda Shop Manual or Parts List for your bike? Get one here:
https://www.honda4fun.com/materiale/documentazione-tecnica
CB750K5        '79 XL250s     CL350K3
CB750K3        '76 XS650      '76 CJ360T

Offline fatkid

  • you cant undo whats already been done. so just ask
  • Enthusiast
  • **
  • Posts: 107
Re: how to tell if motor work has been done
« Reply #19 on: January 30, 2012, 10:29:22 AM »
why 3 ohm isnt there differant ohm ciols what the purpose
Logged

Offline Tews19

  • I am no
  • Really Old Timer ...
  • *******
  • Posts: 6,465
Re: how to tell if motor work has been done
« Reply #20 on: January 30, 2012, 10:55:24 AM »
I would buy those if you are looking to sell them
Logged
1969 Honda CB750... Basket case
1970 Honda CB750 survivor.

bollingball

  • Guest
Re: how to tell if motor work has been done
« Reply #21 on: January 30, 2012, 12:12:27 PM »
The purpose is to drain your battery ;D
Logged

Offline CycleRanger

  • No comment about being an
  • Really Old Timer ...
  • *******
  • Posts: 5,477
  • Central Texas Shop Manual Advocate
Re: how to tell if motor work has been done
« Reply #22 on: January 30, 2012, 01:02:25 PM »
Quote from: fatkid on January 30, 2012, 10:29:22 AM
why 3 ohm isn't there differant ohm ciols what the purpose

Stock coils are 5 ohm.
Three ohm coils should provide more voltage to the plugs hence a hotter spark.
They also pull more power from the electrical system with arguable benefit for a stock machine.
Logged
Do you have a copy of the Honda Shop Manual or Parts List for your bike? Get one here:
https://www.honda4fun.com/materiale/documentazione-tecnica
CB750K5        '79 XL250s     CL350K3
CB750K3        '76 XS650      '76 CJ360T
Pages: [1]   Go Up
« previous next »