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Author Topic: 1978 CB750F PROJECT (NEED HELP!!)  (Read 1673 times)

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Offline Lakermatt

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1978 CB750F PROJECT (NEED HELP!!)
« on: July 23, 2015, 12:43:57 PM »
Hi All, I am a new member here. I just recently picked up a 1978 CB750F from a local shop/recycler. the owner had stated that the motor was frozen, and that they hadn't had time to check any further. I seen that the plugs were already out and figured that previous owner had either already tried, or was an idiot and left it stored without plugs... I fell in love with it and had to have it.... I have 0 experience in these bikes and am here to get your guys advice on how to proceed. I REALLY want to get this bike going, but I don't want to dump a huge amount of money into it if it is not worth it... but I do want the fight if it possible to resurrect this machine... I need someone here to steer me in the right direction so I don't just spin my wheels... thanks in advance guys...
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Offline calj737

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #1 on: July 23, 2015, 02:34:32 PM »
The first thing to do is to verify if the motor is stuck. With bike in neutral, will it roll? If not, then there's several options to try.

Fill the cylinders with a mixture of ATF and Acetone. This can erode the rust and allow the cylinders to rotate through the motor.

Another option is to use a grease gun, make an adapter with an old spark plug, and use the force of the grease to force the pistons free.

Make sense?
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline Lakermatt

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #2 on: July 23, 2015, 02:43:58 PM »
ok. I have heard of the atf/acetone mix. I was told just earlier to use marvel mystery oil? is the atf/acetone mix better?
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Offline Desert-SOHC

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #3 on: July 23, 2015, 04:07:41 PM »
ATF/Acetone is about the best there is
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Offline calj737

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #4 on: July 23, 2015, 04:47:46 PM »
Marvel is good in conjunction with other components. Kroil, MMO, ATF mixed with Acetone is pretty good. Honestly, the grease gun method is about the best, but you'll have to tear the motor down afterwards to fully clean it

Is it for sure stuck?
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline Lakermatt

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #5 on: July 23, 2015, 05:21:06 PM »
When the bike is in neutral it rolls. Put it in 1st, nothing. Kick start nothing. So do i mix the acetone and atf in a glass cup or what theheck do i do?
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Offline calj737

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #6 on: July 23, 2015, 05:22:49 PM »
What does "kick start, nothing" mean? Does the kick starter cycle? With the bike in gear, raise the rear wheel and rotate it. Does that work? In "No" to both of these, then the pistons are stuck.
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline Lakermatt

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #7 on: July 23, 2015, 07:13:11 PM »
Craziest thing... i came home and found a half bottle of wd40. Sprayed it in all 4 holes. Waited about 1 minute and thought let me see what happens? Took off points cap and put a wrench to it and it turned freely!! Then i tried kick start and it worked a little and the got jammed again. But that was with my hand, then put wrench to it again, and it turned freely.... i didnt want to keep turning it till i get your guys advice!!!
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Offline calj737

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #8 on: July 23, 2015, 07:42:29 PM »
STOP using the bolt under the points cover! It will shear under the stress of wrenching a seized, or partially seized motor. If you must wrench on a bolt, use the one under the stator cover.

Good news about the motor freeing up. Go ahead and soak it with the ATF/Acetone, or use MMO now that it will turn.
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline Lakermatt

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #9 on: July 23, 2015, 08:11:29 PM »
I am using the bolt on the RIGHT side of the motor. Went back out there and turned it fully around 2 times with no force. It turns just fine... now what? I can hear the pistons going up and down and sounds out of the plug holes.
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Offline SOHC4 Cafe Racer Fan

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #10 on: July 23, 2015, 08:56:25 PM »
It is free. That should make your rebuild less extensive and time consuming.
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1975 CB550F1 frame/CB650 engine hybrid "The Hot Mess" (http://forums.sohc4.net/index.php/topic,150220.0.html)
2008 Triumph Thruxton (http://forums.sohc4.net/index.php/topic,190956.0.html)
2014 MV Agusta Brutale Dragster 800
2015 Yamaha FZ-09 (http://forums.sohc4.net/index.php/topic,186861.0.html)

"There are some things nobody needs in this world, and a bright-red, hunch-back, warp-speed 900cc cafe racer is one of them — but I want one anyway, and on some days I actually believe I need one.... Being shot out of a cannon will always be better than being squeezed out of a tube. That is why God made fast motorcycles, Bubba." Hunter S. Thompson, Song of the Sausage Creature, Cycle World, March 1995.  (http://www.latexnet.org/~csmith/sausage.html and https://magazine.cycleworld.com/article/1995/3/1/song-of-the-sausage-creature)

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1973 CB750K2 "Bionic Mongrel" (http://forums.sohc4.net/index.php?topic=132734.0) - Sold
1977 CB750K7 "Nine Lives" Restomod (http://forums.sohc4.net/index.php?topic=50490.0) - Sold
2005 RVT1000RR RC51-SP2 "El Diablo" - Sold
2016+ Triumph Thruxton 1200 R (http://forums.sohc4.net/index.php/topic,170198.0.html) - Sold

Offline Lakermatt

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #11 on: July 23, 2015, 09:27:08 PM »
Should i open the top end now? Or? The kick start turns, but gets stuck by hand, then i have to put te wrench on it again.
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Offline calj737

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #12 on: July 24, 2015, 04:42:03 AM »
Quote from: Lakermatt on July 23, 2015, 08:11:29 PM
I am using the bolt on the RIGHT side of the motor. Went back out there and turned it fully around 2 times with no force. It turns just fine... now what? I can hear the pistons going up and down and sounds out of the plug holes.
That is the wrong side. Use the bolt on the left side (while seated) which is #1 cylinder, and under the stator cover. It sounds as though you're using the bolt under the points plate. That is the weak bolt.
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline Lakermatt

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #13 on: July 24, 2015, 07:47:32 AM »
Good Morning All, I went out there this morning and kicked it over, and it is free. tried it a few times actually. shot all the damn wd40 out of the holes!! now I guess I need to have it compression tested right?
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Offline SOHC4 Cafe Racer Fan

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #14 on: July 24, 2015, 10:34:53 AM »
That's a good idea.  If you compression is good, and all you have is gasket leaks, then your work and cost will be reduced even more (no rebore or hone really needed).
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1975 CB550K1 "Blue" Stockish Restomod (http://forums.sohc4.net/index.php?topic=135005.0)
1975 CB550F1 frame/CB650 engine hybrid "The Hot Mess" (http://forums.sohc4.net/index.php/topic,150220.0.html)
2008 Triumph Thruxton (http://forums.sohc4.net/index.php/topic,190956.0.html)
2014 MV Agusta Brutale Dragster 800
2015 Yamaha FZ-09 (http://forums.sohc4.net/index.php/topic,186861.0.html)

"There are some things nobody needs in this world, and a bright-red, hunch-back, warp-speed 900cc cafe racer is one of them — but I want one anyway, and on some days I actually believe I need one.... Being shot out of a cannon will always be better than being squeezed out of a tube. That is why God made fast motorcycles, Bubba." Hunter S. Thompson, Song of the Sausage Creature, Cycle World, March 1995.  (http://www.latexnet.org/~csmith/sausage.html and https://magazine.cycleworld.com/article/1995/3/1/song-of-the-sausage-creature)

Sold/Emeritus
1973 CB750K2 "Bionic Mongrel" (http://forums.sohc4.net/index.php?topic=132734.0) - Sold
1977 CB750K7 "Nine Lives" Restomod (http://forums.sohc4.net/index.php?topic=50490.0) - Sold
2005 RVT1000RR RC51-SP2 "El Diablo" - Sold
2016+ Triumph Thruxton 1200 R (http://forums.sohc4.net/index.php/topic,170198.0.html) - Sold

Offline calj737

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Re: 1978 CB750F PROJECT (NEED HELP!!)
« Reply #15 on: July 24, 2015, 02:36:47 PM »
Squirt a little oil into cylinders while open
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis
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