Topic: Advice on buying a CB750

[Stev-o]

Sounds good, idle might be a little high...

[the technological J]

i cant really tell you should stand about 20 feet behind and redo the rev vid.... do you have any exaust leaks? and the idle sounds a little high... take it almost as low as it can go...

[NP3]

Alright guys, here are some better videos.

The first video is taken just as I started the bike.  The engine is cold, choke lever all the way up, and the engine is idling at about 1,500 RPM.  If you apply steady throttle, the RPMs climb gradually.  Abrupt throttle results in the motor flattening out and almost shutting off.  The exhaust system on there is wide open, no baffle or muffler, and remember that the carbs only have those stacks and no filter.  I'm not sure that the PO paid any attention to the jets, so I think the stacks, open exhaust, and stock jets are not the greatest combo.  You can hear the bike struggling at the end of the first video and in fact it shuts off just as I stopped recording.

In the second video, the engine is warm, choke lever is all the way down, and the engine idles at about 1,200 RPM.  I lowered the idle to about 1,000 RPM, and it sounds like anything lower would cause the bike to just shut off.  Motor is still flattening out at throttle, but not as bad as when the engine is cold.

Also, there is a crack in the exhaust.  I attached a pic, you can see the crack right in front of the weld.

Video 1: Cold Idle

1978 Honda CB750K, cold idle, full choke

Video 2: Warm Idle

1978 Honda CB750K, warm idle, no choke

[Stev-o]

Warm idle sounds good, actually surprising good since you have stacks and no baffle (no back pressure). 
I wouldn't be concerned about the old idle, they don't always like to run cold, but do warm up quick.
I would get the crack welded and get a baffle in there. And you have an air box, right?
That bike is gonna be a good runner!

[NP3]

No, Stev-o, I don't have an airbox, yet.  Actually, I'm missing quite a few things that I'll have to pick up.  I started taking some of the bike apart.  I focused on the front brakes to start.  I'm having to drill out every 2nd bolt.  But, I guess that's part of the fun.

[the technological J]

use a number 3 head on the phillips screws.... if i feel its gonna strip i crack it real good with a hammer and try again

[NP3]

Hey guys, I need a little bit of help with my brakes.  I've loaded some shots of one side of my caliper for reference.  In short, my piston is shot, and I wanted to change it as well as the seal.  I've been looking for a piston for a '78 CB750K, but all I find are pistons that fit K models up to 1976.  Are the 77-78 K model's brakes the same as the previous years? The calipers on my '78, look just like those on prior year models.  The same goes for the master cylinder rebuild kit; I find that they fit 69-76 K models, and then I can't find anything for 77-78 K models.

One more thing; any advice on how to remove the C-clip holding my master cylinder plunger in place, if the C-clip is rusted and one of the holes to fit the C-clip pliers is broken off?

[the technological J]

i had a pick set that  I used... i put the point down in the one hole and kinda wrenched it outta there

[Cheffish]

Looks like the pistons are different. Here they are side by Side
http://charlies-place.com/PAGES/Store/store_brake_pistons.html

[jaguar]

best advice is to buy the best bike that you can afford.
If you are searching for the cheapest bike then you get what you pay for.  After time and knowledge you will be able to weed through tons of listings and find good deals but for now the goal should be to get a bike and ride.  Its winter soon so something that needs some work is fine....but locked up, seized, not turning over is more then just alittle work.  Sure there are stories of people getting lucky and with a can of WD40 they rode it cross country.....and others bought sandcasts for $100.....yes it happens but lets be realistic.

An old bike will give you plenty of things to mend or chances to learn how to work on a bike.  If you have to ask how bad a seized engine is fix chances are you are not up to that....its not a bad place as everyone starts some place but if you get in over your head then you will be spending tons of money and having no fun.....this should be fun above all.

What is your realistic budget for everything?
ill tell you this.....buying a $500 bike and then putting a grand into is will normally net a bike that is not as good as if you had just spent the whole $1500 on the bike in the first place.  this is because the guy selling the $1500 is $2500 into it.....

you are in north NJ?
Phil has a 78K bike that runs and is cheap.  needs some work but might be be a good place to start for you.
Im on long island and can help if you need it

[NP3]

King and the tech... - thanks for the advice about getting the beat up screws and bolts out. I'll definitely use that technique.

Cheffish - great link, that's exactly what i was looking for!

Jaguar - thank you, these are all solid points. I did end up scrapping the idea of buying a seized bike and I ended up getting a 78k that I cleaned up a bit and got it running. Still needs some work, but its been fun and I look forward to continue the work on it.