View Full Version : 78 cb750 help please

03-11-2014, 8:00 PM
Well just scored a descent sohc cb, mainly stock just needing love. So I started tinkering today and got it to fire up for the first time, then the problem showed up. When you rev it up it smokes and oil blows from the exhaust. Im guessing rings, but could it be something else? Could it be valves, maybe a head leak, or cylinder not firing? I know that seems optimistic but I've only had hd's and xs's.
My goal for tomorrow is do spark test on all cylinders, and do a wet and dry compression test. That sound like a good idea? Thanks guys:cheersmate:

03-11-2014, 8:50 PM
Did you drain the old shitty sitting on it's thumbs oil out? Prime the oil pump then fire it up? There's two O-rings on top of the head that may be at fault or fucked up exhaust valve guides... before getting that deep I'd get new oil and the right amount in if you haven't make sure the pump is primed, adjust the valves let some oil resoak the gaskets, retorque the head bolts and see what happens if it's still jizzing oil pull the head replace gaskets check the valves, valve guides and cylinders.

You can do a compression and or leakdown test to see what the condition is before going in there... but a bike that's been sitting is going to have some cobwebs to blow out when it comes back to life and sometimes this includes oil. I've had this occur on new project fire ups and magically stop the next day...

03-12-2014, 12:50 AM
Jets on the right track, good news is its a cb750 so it will be rebuildable. The things are damn near bulletproof.

I had a strange problem with mine after a head rebuild, started smoking and fouling the #1 plug and I thought the machine shop had fucked a valve seal or something but when I went to pull it down I found a puddle of oil in the #1 intake boot :confused:

I pulled and striped the head and found a tiny casting defect under the #1 intake valve seat. It was a void in the casting that had opened up into a almost unnoticeable pin hole right into the intake chamber. It was constantly sucking in oil through that tiny hole and when grab a bunch of throttle it would pull even more in and start smoking like an old 2stroke. I cleaned the fuck out of it and packed that void full of JB weld and I've been running it for 2 years now.

Whats my point? Keep your eyes peeled, because different problems can have the same symptoms.

03-12-2014, 1:19 PM
I've seen the bottom end full of gas looks like oil smell it .

03-12-2014, 5:12 PM
thanks guys those are all things I didnt keep in mind. I had to get a new compression tester so I wont know the numbers till tomorrow. I did get all the stuff to flush the oil, previous owner said he did but I aint taking his word on it. I'll let yall know how it goes, hopefully it all pans out so we can start chopping it up

03-12-2014, 5:15 PM
before getting that deep I'd get new oil and the right amount in if you haven't make sure the pump is primed,

Ive read about getting the pump in oil, spinning it till the bubbles stop the rotate the motor till you see oil squirting in the valve cover adjusters. Is that how you recommend it?

03-13-2014, 3:31 AM
Here ya go from in house http://www.chopcult.com/forum/showthread.php?t=27763