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Author Topic: how to tell if motor work has been done  (Read 2538 times)

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Offline fatkid

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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
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Online CycleRanger

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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.
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Offline KJ790

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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.
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Offline fatkid

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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
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Offline Bankerdanny

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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.
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Offline seanbarney41

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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
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Offline chewbacca5000

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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.
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72500john

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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!
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Offline Retro Rocket

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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.?
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Offline wildcatmahone

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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.
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Offline 736cc

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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.

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Offline RustyStuff

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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
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Offline ekpent

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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  ;)
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Offline 754

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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..
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Offline Tews19

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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
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Offline 736cc

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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.


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Offline Danno

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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
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Offline fatkid

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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
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Online CycleRanger

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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.
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Offline fatkid

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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
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Offline Tews19

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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
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bollingball

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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
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Online CycleRanger

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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.
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