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Blew it up! Time to rebuild.

Hippie459MN

CB750 Addict
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Saint Francis, MN
Sad sad day about 2 weeks ago. I was out riding and the bike (1979 CB750) was running fantastic and about an hour and a half into my ride, that consisted of all back roads, I hopped on the highway and got going then after about a mile it felt like the bike was starting to slow down and loose power so I rolled off the throttle just a little and pulled the clutch in and BAM! Motor locked up tight with no hesitation what so ever. So I got it home and 3 hours later, the exhaust was cold too the touch but the valve cover and upper portion of the cylinders were still very hot to the touch. I tried turning it over by hand at the crank/Alt rotor the next morning and I couldn't even get it to budge what so ever. I have been wanting to rebuild it anyways cause I didn't really know the history of the motor and its been showing signs of some blow by that's been slowly getting worse but I didn't want it to be like this. 832 big bore kit, here I come I think. LoL Looks like my riding season is officially over with. This sucks! And to think I had just topped of the gas tank too. Later on after a closer inspection, it looks the I started loosing oil at the bottom of the oil filter housing while I was riding and didn't know it. Sadly this bike, due to it being very bare bones, has no low oil light or oil pressure gauge (It will be getting a gauge for sure) so I didn't know at all and it ran out of oil.

I was looking at maybe getting some performance cams and they look to cost as much as a motorcycle. Cheapest I could find was like $600 for a set of cams so im looking at putting a set of stock CB900 cams in it. I havent pulled the motor yet to see what all has happened just yet, im just hoping the damage isnt too bad. As long as the case is good I can rebuild. Thankfully I know some damn good machinist that will be able to get me taken care of.

It will be a month or two before I get started on this but I will be sure to take lots of pictures and post my progress.

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Sorry to hear the bad news bro! :( Hope the engine is restorable. Now I know to look into the oil level light.
 
Pressure gauge is far better, it would have likely shown the OP the problem long before the engine blew.

I ran one on my F for many years and they show things no light on the planet can ever. Additionally, once the light has popped on the damage is likely already in place and happened.

OP don't be surprised if that lockup at the end included either a broken rod or seizure so severe the main cases and/or crank are no longer useable..................the head cam journals often tear up well into that too.
 
Yeah, im hoping the damage isnt too bad that the case is junk. I can get a crank and rods and stuff, I just dont want to have to find a whole new motor if I dont have too. I will for sure be putting an oil pressure gauge back on it. Either way, it will be back on the road by spring weather this one is rebuilt or I get a new (different) motor.
 
Well, I have yet to mess with the bike do to lack of time but I may have found a motor. It hasnt run in about 10 years but its on a full custom built chopper that was built, taken to sturgis in 2004 or so, then brought back, put away very shortly after that and has sat ever since pretty much. Sadly some good parts have been stripped off of it but that is ok, the motor and all of the electronics are there and its a 1979 motor as well and other than a lot of dust, the motor looks super clean. The bike has been for sale semi-localy here for a year or so but due to the missing parts and being such a wild custom I think its a tough sell so hopefully he still has it in about 4-6 weeks and if so, im going to go pick it up, so long as the compression checks out and we can strike up a deal. I will keep everyone posted.

Oh yeah, im not giving up on the motor thats in the bike now. It will be rebuilt eventually but if this motor checks out, it will get swapped so I can get the bike going long before spring.
 
Nice update, hope it all works out. When I was browsing this thread, I thought to myself, "a complete motor swap would be so much easier". :thumbsup:
 
That it would but for some reason CB750 DOHC in the 1979 vintage are very hard to come buy and the CB750s in general seem to be made of gold when it comes to asking prices. I would show pics of what im looking at but with them being a hot commodity around here, I dont want anyone else swooping in on my deal. lol
 
ANY 750 DOHC from '79-83 will work there, a few like the Nighthawk ones will have slightly different covers but they still interchange.
 
I have been keeping my eye out but would like to keep it all 1979 if I can. Thought about finding a CB900 motor but them are like a folklore around here. LoL

I might do a CB900 cam swap on the 750 though if it all pans out. I have heard of people getting good results from putting a set of 900 cams in the 750.
 
It works great but the bigger the OEM cams you go the more they are likely worn when you get them and nobody ever tells you that part, the 1100s are almost impossible to find good cams for. The 750s much more likely as they have much less lift to wear parts out. The hardcoat is not real good on any of them.

900 and bigger will be another animal as they are rubber mounted and the mounts are fairly complicated. Easy to crack cases by the uninformed if modding that.
 
Well, I know its been a while but life got crazy busy and the thought of rebuilding my current motor scared me but if its not crazy bad damage, something some machine work can easily fix, I will rebuild it better than ever, but in the mean time, I scored a 1981 Honda CB750C that someone was going to rebuild, got in over there head, and then was loosing his work space/his girlfirends garage, so it had to go. I got the whole bike, and motor for $100(USD). The motor is still in the bike, and most of the parts are in two pretty good size boxes. But the motor has not been tore apart. It has compression, turns over great, and the oil in it look great so I took it home. Got it home, hooked up some power, jimmy'ed some wires directly to the starter to check the compression, and all 4 cylinders were all within a pound or two of each other. Thumbs up! Now to get both motors out of both bikes, take the "new" one to the car wash as its dirty as heck from sitting for like 10 years, thankfully, not from oil or anything. Just lots of dust and junk, then decide if im going to paint it or not. I just might but im not 100% sure yet.

Anyways, just wanted to give a long over due update to this and I will certainly be posting pictures along the way. :)
 
Current situation. Have the old blowed up motor out and the "new" replacement just about ready to go in the bike. Hoping tomorrow maybe or by the weekend. I had to swap a few things as my wire connections were different then the stock connections and did some cleaning up of the new motor. Wish I had time to clean it even more but a very sudden last minute move in the next few weeks has forced me to put it together as it is. This winter maybe I will pull it out and really clean it up but for now, at least get the motor in the bike and hopefully running with little effort.

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Progress has been super slow. The motor is 100% ready to go in but an unexpected move has put the bike on hold for a tiny bit until I can get this move finished with. Good news is, I now have a garage to work on my bike and not have to work in my basement anymore. LoL

Hopefully in about 2 weeks I should have this all done so I can get in a tiny bit of riding before winter.
 
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