Author Topic: Deal too good to be true. Need advice (Read 3628 times)

Jon T · « **on:** March 06, 2016, 11:30:00 AM »

Yesterday I purchased a garage kept 1973 CB750k with minimal rust and only 7k miles for $500. Found the engine seems to have a cracked cylinder head

. A rebuild doesn't scare me too much, but this bike must have been really hard to crack a head in 7k miles. Am I better off buying a completely rebuilt engine and trans or should I take it apart and see if I can just replace the head? If it is just the head that'a cracked and it didn't throw a rod or anything, do you know how much or where I can buy a cylinder head? Is there any other problems that are associated with a cracked head? Thanks guys, im a little disappointed but I knew it was a gamble.

alacrity · « **Reply #1 on:** March 06, 2016, 01:55:43 PM »

Where is it located?

Jon T · « **Reply #2 on:** March 06, 2016, 02:11:25 PM »

I bought the bike in Newark NJ, I live a half hour away in Bergen county NJ.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #3 on:** March 06, 2016, 02:19:30 PM »

And how do you know it's a crack and not a casting flaw?

Retro Rocket · « **Reply #4 on:** March 06, 2016, 02:26:05 PM »

Before you do anything, post detailed pics showing the "crack", I've never seen a cracked head on a 750/4...

Jon T · « **Reply #5 on:** March 06, 2016, 02:29:54 PM »

I poured mystery oil in the spark plug holes cause it's been sitting and seized. I was going to let it set and turn it a little every day with the kick starter just to loosen it up (advice from my father). Cylinder #2 just pours oil out of the casing in front of the cylinder, towards the front of the bike. It's not slow, it pours out very quick when you pour oil in so I'd image it's not a blown head gasket. That cylinders spark plus was rusted unlike the others and I assume that's why it's seized.

Stev-o · « **Reply #6 on:** March 06, 2016, 02:31:03 PM »

Quote from: Retro Rocket on March 06, 2016, 02:26:05 PM

Before you do anything, post detailed pics showing the "crack", I've never seen a cracked head on a 750/4...

...or a thrown rod.

Dont be disappointed, did you expect a perfect bike for $500?

Jon T · « **Reply #7 on:** March 06, 2016, 02:34:05 PM »

It is coming out In between the fins. It does not look like it's coming out of an exhaust manifold. I can't see because it's coming from Inbetween the fins and the front tire is in the way. Do you just think it could be something else?

TomsK8resto · « **Reply #8 on:** March 06, 2016, 02:34:50 PM »

Jon T, welcome to the forum! There is a whole bunch of great technical expertise on this forum. The better you can describe the issue/problem and if possible, post pics, the easier it is for the technical giants to provide guidance to you. Knowing the little that I do about these SOHC bikes, if I found a garage stored '73 CB750 that was all there with minimal rust, I'd be dancing in the streets, even with a "possible" issue with the motor. Take one step at a time.

Good luck!

Tom

Johnie · « **Reply #9 on:** March 06, 2016, 02:46:46 PM »

Agreed...never seen a cracked one. Might be just a casting seam. Post up some pics and welcome to the board.

Jon T · « **Reply #10 on:** March 06, 2016, 04:42:32 PM »

Thanks Guys, you are the experts I may be wrong. It is only happening on cylinder 3. When I got home a put some more oil in to show the leak and took the best pictures I could. Here they are. What do you guys mean by casting seam?

70CB750 · « **Reply #11 on:** March 07, 2016, 05:27:21 AM »

The die they use to cast aluminum into is assembled from steel elements, where the steel pieces meet it shows on the aluminum product. Fairly easy to mistake it for a crack.

70CB750 · « **Reply #12 on:** March 07, 2016, 05:29:20 AM »

Welcome to the forum, btw.

Your engine may need new head gasket and few other things to go with it - like pucks - but the price is pretty good for your area, IMHO.

Stev-o · « **Reply #13 on:** March 07, 2016, 06:01:28 AM »

Cal gave you the good news but the bad news is you will have to pull the motor to change the gaskets, unless you have a frame kit

Johnie · « **Reply #14 on:** March 07, 2016, 06:02:38 AM »

7,000 miles and appears to be the common head gasket leak. Might want to clean the oil off the head and get it running. Then see how bad the leak is. General rule of thumb here is not to mess with a minor head gasket leak until it gets bad. Which may be down the line a while. At least get it running so you can diagnose any running issues. However, if you have your heart set on the rebuild then have at it. Just don't forget to post up those pics for us. We like pics here

Jon T · « **Reply #15 on:** March 07, 2016, 06:44:40 AM »

Thanks for the warm welcome guys. Should've explained what I did earlier in the post, I was not looking at a crack I assumed. But it could be a blown head gasket. I am very happy with the bike regardless, really looking forward to this rebuild. I'm sure I'll be on here with questions. I'll let you guys know when I drop the engine and what I find. I'll drain the oil today if i have time after school and before work, or tomorrow. My first question is about the tank. It has some dents and a deep scratch that dented it on the top and the gas cover barely works. It also looks like he patched the bottom. Should I try and salvage it or am I better off getting a new one since this one has a hole in it?

danyo · « **Reply #16 on:** March 07, 2016, 06:47:23 AM »

Are you sure it isn't overflowing through spark plug hole?? If pistons #2 and #3 are at TDC, there is not that much space for a lot of fluid.

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Stev-o · « **Reply #17 on:** March 07, 2016, 06:49:32 AM »

Quote from: Jon T on March 07, 2016, 06:44:40 AM

Should I try and salvage it or am I better off getting a new one since this one has a hole in it?

All depends on what you want the final result of your bike to look like.

Mad MaX? - salvage it!

Clean restored bike? - look for a replacement

RevDoc · « **Reply #18 on:** March 07, 2016, 07:13:40 AM »

Welcome to the asylum, Jon!

Some of the real wizards have already given you excellent advice; don't be hasty in jumping to conclusions about the status of your new baby.

I would add that, if you have not already done so, go to the download page and get the manual for your bike. Read the 3K checklist and become very familiar with the steps. Free up the cylinders and get the bike running before you pull the engine. You need to confirm just what problems there may be before getting into major work that may not even be needed. "If it ain't broke, don't fix it!" You would certainly need to run a compression test to verify cylinder status.

There are some members over there in NJ who may be able to help you with basics.

Johnie · « **Reply #19 on:** March 07, 2016, 07:20:36 AM »

One last thought...get yourself the Hondaman CB750 rebuild book. Hondaman (Mark Paris) is on this board and is considered the guru of these bikes. He wrote a book for us on the subject of rebuilding all facets of this bike. It will be so helpful to you on your project. Consider getting it...

grcamna2 · « **Reply #20 on:** March 07, 2016, 07:21:54 AM »

Jon,
I'd like to see a few good pics of the entire bike

Jon T · « **Reply #21 on:** March 07, 2016, 08:10:55 AM »

Took some quick pics. Still no luck trying to turn it over. I think I'm better off doing a top end on it before I try and get it running. I'll drain the oil tonight and let it drain all night, hopefully no metal in it. She's rough around the edges but I'll get her beautiful again. Going for a stock look restoration

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70CB750 · « **Reply #22 on:** March 07, 2016, 08:19:20 AM »

For $500, that's a great deal.

ekpent · « **Reply #23 on:** March 07, 2016, 08:32:31 AM »

Might agree that the oil you see could be overflow from the plug hole. Just a few observations. The color of the tank/sidecovers and the fact the petcock is on the left indicates they are either a 1975 or 76. The fact the engine has been painted black would indicate to me that it has been pulled at some point. Swingarm has been plated. Lots of swapping on some bikes over the years. Be a good idea to post up the frame and engine numbers just for kicks to verify what years you may be working with for starters. Lotta help here,hopefully another gets saved.

Jon T · « **Reply #24 on:** March 07, 2016, 08:40:24 AM »

Title says 73 and numbers match the frame. Maybe different engine? Frame and engine numbers should match?

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Author Topic: Deal too good to be true. Need advice (Read 3628 times)