New Member DOHC cb750k

[Colin]

Hello everyone i am a new user to these forums and i am also relatively new to the CB's. I just recently picked up a 1979 CB 750k from a guy in texas, a very far drive for me but when i saw it i had to have it. I got it on the cheap cuz they seller said that the engine was seized up, i was prepared for this as i work on cars/trucks for a living. Dont have pictures right now but will update when i get home. My question to the fine people on this forum is the following: I just recently tore the valve cover off and found that nothing was seized it would turn over by hand and the only thing i found wrong was that it must have jumped timing. After fixing the timing issue, throwing new plugs, oil, filter, fuel etc into it, after a quick 45 mins i got her running. However my issue is that After about a minute of running, while i was trying to get the carbs synced etc i noticed oil was pouring out of the spark plug! the spark plug was tightened down to spec and it is definately not coming from the head/valve cover. My fear is that the piston rings are shot, the jugs are cracked, or the piston itself is finished. the bike came with a parts engine so some of the parts i can steal from that. But i am curious to find out if anyone else has run into this issue ever, what their fix was, and what my best course of action is from here on out.

Pictures to come soon,
Thanks in advance
Colin

 

[Colin]

Here are the pictures of the bike and the engine during and after tear down. After finally getting the jugs off, there was so much gunk in the stud galleries, the only thing that was remarkable was the fact that as i started pulling the rings off they started breaking into 2-5 pieces. The only ones that didnt break were the 3 oil rings at the bottom. Im going to go ahead and replace all 4 sets of rings, and the valve seals while i have everything torn apart. Ill let everyone know what my results are after the parts come in. Discussion time: on the DOHC 79 K models how do you guys feel about a dual mikuni vm 34MM setup. The guy i bought it from replaced the stock ones with this setup from cyclex and when i had it running before oil cam out of the spark plug they seemed pretty good, aside from a stuck float, fixed that tho . All in all so far cheap bike b/c the guy said it was seized, through ~200$ at it in seal kit piston rings and new oil etc, so far im pretty happy hopefully the rings fix the leak issue.


Thanks for input
Colin

 

[dirtdigger]

Those carbs are 10x better than the stock carbs. Much more tunable. Should have the cylinders deglazed since you are putting new rings in. How do the bores look? If the rings are broke that bad I am wonder how bad the scoring is in the cylinders. If they are scored you should have them bored or you will end up with lower compression and possible bad oil usage.

 

[cyclebuster]

I would think a standard bore DOHC cylinder should be easy to come by

 

[Colin]

a standard bore cylinder is pretty cheap on ebay etc, however to both of you amazingly enough the cylinder walls look perfectly fine, no obvious scoring/scratches etc. Digger i am unfamiliar with deglazing and how to tell if i need the process done. The rings were breaking as i took them off not that they were broken inside of the cylinders or anything. My friend and I think that the previous owner jumped timing, and in the process burnt some rings up, probably at wide open throttle or throwing it open so fast that the chain skipped, and burnt up the rings in the process. The whole motor looks good inside, the valve body look completely redone, minor carbon buildup on the actual valve heads etc, the cylinders look unremarkable in terms of damage, and the same goes for the pistons and everything else. new rings wont be here until tomorrow or friday, but if i absolutely need some work done that i dont know about let me know.

Thanks again for all the input,
Colin

 

[dirtdigger]

You need to deglazed also sometimes called honing. You need to do this to prep the cylinder for the new rings or you will run the risk of oil usage and compression leaking by the rings. Deglazing puts a rougher finish on the cylinders, it holds more oil on the wall during break in and causes the rings and cylinder to wear into each other and make a good seal. This can be done at a machine shop or you can do it at home if you have a deglazing or honing tool. I do it myself because I have the tools but any machine shop can do it for you You will have people that will tell you that they never deglazed when they put new rings in and never had a problem. I completely disagree and have seen the effects of just throwing new rings in old bores. I guarantee there will be some compression leakage and can have oil usage issues if you don't deglaze the cylinders. It really is worth doing a search on the internet about the process and purpose of deglazing to make your motor happy. I found this from a ring manufacturer. http://www.hastingsmfg.com/ServiceTips/cylinder_deglazing.htm Something for you to think about. Beings it is apart and you are getting new rings I would not want to take the chance and not deglaze.

 

[Colin]

Good to know after you stated it i started researching it lol. I probably will go ahead and take it to be done, i dont want to have to buy or rent more tools that i already have. And the friends i have at shops around here have more experience in the field than i ever will. So as long as it doesnt cost me an arm and a leg i will have it done for sure. Now that i have your attention digger, my original question that i still cant find an answer to anywhere is why there was oil getting pushed out of my spark plug slot in the first place, im going to be putting new rings, valve seals, and gaskets on the entire motor, but curiosity has gotten the best of me and makes me need to know why lol. The amount of oil it was led me to piston rings but since im replacing everything i will never know the exact fix unless i did everything one by one, and that would take too long its riding season soon.

 

[dirtdigger]

So the oil was coming out of the spark plug hole, by the threads?? If so then you really need to take a very close look at the threads in the head. Either there is a crack or the spark plug wasn't seated all the way down. If there is carbon build up at the bottom of the threads...you can tighten the spark plug down and it will get tight but the carbon in the threads keeps the spark plug from threading all the way down and crushing the spark plug washer. Did the previous owner maybe have a thread insert put into that hole? I have seen inserts leak by because they didn't use a sealer on the insert threads.

 

[Colin]

the spark plug was seated, brand new, and the threads had no cracks or any indication that they were messed up in anyway, and by the amount of oil coming out of there, i could be wrong here, but i didnt think there was supposed to be that much oil that got ontop of the piston/combustion chamber. There was probably a shot glass of oil that came out of there in the 5 seconds before i shut her down.

 

[dirtdigger]

There should be no oil at all in the combustion chamber. I am sure that is the result of the piston rings. I just dont see how oil can come out of the spark plug hole if it is tight and the gasket was crushed. If it is leaking oil by then it should be leaking compression by also. Which cylinder was doing this? More than one?

 

[Colin]

the only cylinder that was doing it was the cylinder furthest on the right if you are sitting on the bike, #4 if i remember correctly. Thats a good point about compression. the only thing that i can think of is that there was so much oil built up in there as a result of the rings and the oil couldnt all escape out of the exhaust valves while they were open, mind you i had been running it for approximately 1 minute as i was adjusting the idle screw on the carbs etc. I suppose i could put some thread tape or similar product on that spark plug when i go to reinstall just to make sure it is not going to be an issue as far as compression goes down the road, other than that im not sure

 

[dirtdigger]

No need to put anything on the threads because the gasket does the sealing not the threads. If you put something other than some antisieze on the threads you risk getting the plugs stuck and then ruin the threads trying to get them out in the future. If you are 100% certain the plugs are seating all the way and crushing the gasket and there is no crack in the head then I am stumped right now. Its really hard to try and diagnose over a computer too. I will keep thinking and see if I can think of any thing else but I dont know what else it could be.

 

[Colin]

Very true I'll have to take a closer look at the plug and the threads tomorrow to be sure I'll make sure to take some pictures and keep you updated on the progress and everything after I get the new parts on etc

 

[Colin]

Updated pictures of the bike. We got the engine completely put back together new seals rings etc all the way around, aside from the transmission Got the bike running finally, however now the problem is the carbs, predominately the left hand carb as the bike will not idle well if at all, and the only way it idles steadily is with the choke on the left carb on. I am going to take the carbs somewhere to have them synced up, and probably have them check into the not idling problem while i have it there, as carbs are the only thing on an engine that i have very little experience with. If anyone has any input as to what may be the problem however i would be more than happy to check it out.
Colin