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Author Topic: cb 750 sohc engine problems.  (Read 1872 times)

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Offline meech313

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cb 750 sohc engine problems.
« on: May 04, 2014, 04:19:46 AM »
Hi there. I have just got a cb 750 and found the engine won't turn over with the kick start but when I remove the points cover and rotate back half a turn I can turn the engine with the kick 5/6 times then it sticks again.
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Offline Airborne 82nd

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Re: cb 750 sohc engine problems.
« Reply #1 on: May 04, 2014, 06:40:45 AM »
Quote from: meech313 on May 04, 2014, 04:19:46 AM
Hi there. I have just got a cb 750 and found the engine won't turn over with the kick start but when I remove the points cover and rotate back half a turn I can turn the engine with the kick 5/6 times then it sticks again.

I think it is a big no no to turn it backwards with that big nut by the points. Have you seen how small that bolt is very easy to bend even with the plugs out.
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Offline meech313

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Re: cb 750 sohc engine problems.
« Reply #2 on: May 04, 2014, 07:21:34 AM »
I have all the plugs out and the engine turns backwards with minimal effort
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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: cb 750 sohc engine problems.
« Reply #3 on: May 04, 2014, 07:33:53 AM »
You're better off using the alternator bolt to turn the engine. If the points shaft snaps that WILL be a #$%* to get out. 

Does it turn with the electric starter? If the kick start mechanism is fukd then it's an engine removal and tear down. Sounds like you may have gotten a good price.
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Offline meech313

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Re: cb 750 sohc engine problems.
« Reply #4 on: May 04, 2014, 02:17:12 PM »
The starter motor is removed at the moment. I don't think it's the kick start as I can get it to turn over with it 5/6 times before it sticks.
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Offline meech313

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Re: cb 750 sohc engine problems.
« Reply #5 on: May 05, 2014, 05:16:57 AM »
Is there anyone that can help me out??
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Offline meech313

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cb 750 sohc engine problems.
« Reply #6 on: May 05, 2014, 07:56:40 AM »
Just got a cb 750 and found that the engine won't turn over with the kick start but when I turn the engine back half a turn then try to turn it over the engine will turn over 5-6 times before it sticks again need help!

If there is anyone that can help get back to me on this or my email
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Offline iron_worker

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Re: cb 750 sohc engine problems.
« Reply #7 on: May 05, 2014, 09:07:25 AM »
If you turn the engine over by hand can you feel a "sticky" spot?

IW
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Offline Airborne 82nd

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Re: cb 750 sohc engine problems.
« Reply #8 on: May 05, 2014, 10:03:17 AM »
I see you reworded this a little. When you say turn over 5-6 times do you mean 5 or 6 rotations of the crank? or 5 or 6 kicks on the kicker? Usually if the crank turns over 360 deg. it will keep on turning
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Offline meech313

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Re: cb 750 sohc engine problems.
« Reply #9 on: May 05, 2014, 11:12:22 AM »
No. No sticky spots. Yes I can turn the engine with the kick 5-6 full rotations of the engine then it sticks
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Offline meech313

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Re: cb 750 sohc engine problems.
« Reply #10 on: May 05, 2014, 11:17:52 AM »
Also I have all the spark plugs out and starter motor removed. When I turn the engine over the cam rotates and all valves move as I said around 5-6 full rotations then it sticks.
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Offline iron_worker

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Re: cb 750 sohc engine problems.
« Reply #11 on: May 05, 2014, 12:09:17 PM »
So it only sticks when you roll it over with the kicker but not when you turn the crank by hand? Your post is not super clear.

IW
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KayOne

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Re: cb 750 sohc engine problems.
« Reply #12 on: May 05, 2014, 02:21:59 PM »
A shot in the dark, but... Cam sprocket could be detached on one of the two bolts, the other backed off. Chain piles up until it is reversed and released only to go into misalignment and jam again. Unless you have a frame kit it looks like the engine will be coming out for disassembly.
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Offline rb550four

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Re: cb 750 sohc engine problems.
« Reply #13 on: May 05, 2014, 03:48:41 PM »
Take off your points cover  and see if it gets stuck in the same place every time....since you have already tried to kick it a few times..... I'm thinking stuck valve ... but it's probably already bent now.
second thought is look around the oil pan for broken parts that may be getting hung up in the primary chain or cam chain ... like maybe cam chain tentioner parts that may have failed.
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Offline meech313

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Re: cb 750 sohc engine problems.
« Reply #14 on: May 05, 2014, 11:43:03 PM »
Yes I can turn it as many times as I want  with the crank. So no bent valves.its only when I try to kick it over a few times it sticks then I can't turn it over with the crank until I take it back half a turn then I can turn it with the crank again.
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Offline meech313

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Re: cb 750 sohc engine problems.
« Reply #15 on: May 06, 2014, 09:42:57 AM »
Any ideas?
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Offline madmtnmotors

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Re: cb 750 sohc engine problems.
« Reply #16 on: May 06, 2014, 10:05:55 AM »
Quote from: rb550four on May 05, 2014, 03:48:41 PM
Take off your points cover  and see if it gets stuck in the same place every time....since you have already tried to kick it a few times 

second thought is look around the oil pan for broken parts that may be getting hung up in the primary chain or cam chain ... like maybe cam chain tentioner parts that may have failed.

Have you tried either of these suggestions yet? More information is needed.


Regardless of the problem, I would expect that some disassembly is going to be required for an accurate diagnosis and repair. Can't imagine a problem like this going away without deep intervention. Obstruction of the primary driveline could include any (or all!) of the following:

>Tach drive gear binding on cam (loose? broken?)
>Valve obstructing piston (broken valve spring or missing retaining collet?)
>Foreign object bouncing around in one or more cylinders (bolt? washer? broken piston? broken piston ring? gremlin?)
>Cam chain binding (broken cam sprocket? loose parts getting wedged between chain and sprocket?)
>Primary drive chain (between crank and trans) in a bind (primary chain tensioner failing? loose parts getting wedged between chain and sprocket?)
>Transmission input shaft binding (bearing failure? bent? loose parts getting wedged between clutch basket and crankcase?)
>Kickstarter ratchet mechanism failing?
>Electric starter ratchet mechanism failing?

Along with rb550four's suggestions, I would recommend removal of the valve cover and make sure all the valve springs, spring retainers, cam sprocket, tach drive gear, etc are all intact...
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Offline meech313

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Re: cb 750 sohc engine problems.
« Reply #17 on: May 06, 2014, 01:23:07 PM »
Just an update to where I am now with the engine. I have all the plugs out cam cover off starter motor removed. Turn the kick starter and all valves move up and down no problem camshaft turning. I can feel compression with my finger over the spark plug holes on every cylinder after 5-6 full rotations of the engine the kick start sticks solid and I cannot turn the engine at all. If I turn the crank back half a turn using the bolt it frees the kick start and I can kick it over again 5-6 times. And it's not the same position the engine sticks every time.
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Offline rb550four

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Re: cb 750 sohc engine problems.
« Reply #18 on: May 06, 2014, 08:09:30 PM »
When the kick start gets stuck after 5 or so kicks....when you turn the motor back 1/4 turn it frees up, do you hear the spring release the gear on the kick start shaft at that time?
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A few Honda 500's, a few Honda 550's, a few Honda 650's, '72 cb 450, a couple 500/550/650 hybrids, and 2001 750. 
  550 Snowbike -Somebody had to do it.
  http://forums.sohc4.net/index.php/topic,101678.0.html             
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Offline meech313

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Re: cb 750 sohc engine problems.
« Reply #19 on: May 06, 2014, 11:35:58 PM »
 I hear a click when I turn it back. Not sure where the nose comes from though
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Offline iron_worker

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Re: cb 750 sohc engine problems.
« Reply #20 on: May 07, 2014, 02:46:45 PM »
Kind of sounds like kick start ratchet (or whatever it's called) isn't working properly. There is a little mechanism in place to only allow the kick start pedal to drive the motor and not the other way around. I don't think there would be any way of fixing it without splitting the cases.

IW
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Online Don R

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Re: cb 750 sohc engine problems.
« Reply #21 on: May 07, 2014, 09:19:58 PM »
 I had a similar problem on my  stroker motor. I did split the cases and replace the kick start mechanism. I haven't worked on it since due to other projects but there is a pin that can only come out with the cases split. There is a spring that winds around the kicker shaft, it could be out of place or broken. I'm no expert, just what I observed.
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