Hello fellow people!

[cdaiscool]

Hello! I've been stalking this forum for approximately 2 weeks now, preparing for my first motorcycle purchase. After watching several bikes sell out from underneath me, I found a few that I could check out. I've ridden a dirt bike a few times about 4 years ago, and that's the full extent of my riding experience.

My dad, a veteran of riding for a total of 30 years probably, has taken up the task of teaching me how to ride. Picking up a helmet tomorrow, and other gear as well. So far I've just been riding up and down my small street, just so I can practice riding and turning at low speeds.

Checked out a 1982 GS550 Touring, 6 speed and it was in awesome condition. But I want a bike that I can hit the highway, do 80 or so, and not have an issue with it. My dad goes to DC during Memorial Day for Rolling Thunder. I've never served, as the military said no thanks due to scoliosis, but I do appreciate what gets done by the boys.

So, we settled on a 1981 CB750K. 11,000 miles, in awesome condition! Well, so I thought. Paid $1,025 for it after we picked it up, my dad drove it for a little while to make sure all was good. Didn't want to idle well, but he rode it, sat somewhere a minute or 2, then rode like a bat outta hell on the way back. He said everything was perfect.

On the way back, we were hit by a MAJOR downpour, and the bike stalled out. Even before that, we got to a gas station and my dad couldn't get it to idle well. So after we got home, we brought it to a local dealer that works on older bikes, and they did a carb sync, spark plug swap, and fresh oil change/filter. Turned out it was only pulling 4 in at idle - should be 7 in at idle per dealer. Did a compression test, and it's at 75, 65, 70, 70. Oil got 'em up to 120, 95, 105, 100. So rings are gone. At 11k miles. :( Dealer's notes include "compression is low and head gasket is leaking, the engine needs top end repairs.

So plan is to ride it locally and up to 30, 40 miles away to Grand Rapids (live in Holland, MI) and then later, pick up an 823cc kit and do some work.

Cliffs: Purchased first motorcycle, and compression sucks. Needs rings. So a rebuild it should get once I've ridden it in and gotten used to the bike. Now, I am off to get my roommate, come home, and get you pictures!

[cdaiscool]

As promised...













Sorry they're bad and dark, as I just took the shots and I took 'em on my cell phone.

[shinyribs]

Welcome to the party and welcome to Honda's! You are gonna love that bike. That sucks about the low compression. The numbers after the oil change don't look too awful bad. If it ain't smoking too bad I'd probably ride it this summer and overhaul it when it gets cold. But thats just me. Good luck!

[cdaiscool]

Smokes a bit on start-up, but that's it. Doesn't at all after it's warmed up, at least from what I can see. And that smoke that I DID see could have been oil from the compression test - I never saw oil burning any other time, nor smelled it.

Do you think it would probably be okay to make the trip to DC as-is? Obviously, if I ride it and it loses more and more power, then I'll know the compression's getting worse and it wouldn't.

[shinyribs]

Unless the rings are in seriously bad shape.So bad that they would eat in to the cylinder walls I dont see what it would hurt. If the rings were that bad it would probably smoke severely,I would think. Had it sat for a long time? Sometimes sitting will cause the rings to stick in the piston and after riding it some they will loosen up. Also,the cylinder walls can surface rust and do the same thing. Not the best case scenario,but after the motor runs a good bit it will sort of work the rust off and seal better.A good reason to do a few fairly low mileage oil changes after you get a previously sitting engine up and running

[cdaiscool]

Grr. I had an entire message written up, but then hit 'tab-enter' and it killed this.

It runs, though backfires a fair amount. I can't rev it above 6k, though there's a decent chance that's user error. I quite likely didn't have it full open, and I'm just running up and down a small street, so I don't want to go too fast on the road. Which this bike will likely do.

I will get a video of it starting up, and running a bit so this site can hear it run and tell me what's going on. I'll put a fair amount of mileage on it - hopefully over 2,000 miles - and if it's still running awesome afterwards plan to hit Rolling Thunder with my dad.

[cdaiscool]

Oh ya, and the previous owner claimed he put between 3500 and 4000 miles on it last year, and 50 - 100 this year. If he's to be believed.

[TOOLS1]

It sounds like the Air-cut valve on the carbs might be bad. Also I am with Shiny on the rings might need to re-seat. Warm the bike up, and get it rolling in high gear. Then let off the throttle, and coast in gear until the engine starts to lug. Then give it full throttle until it is back at speed, and the engine is revving freely. Do this 5 times, and the rings should be seated. If this does not improve the compression, I would still ride it. You are not going to hurt anything.
TOOLS

[cdaiscool]

Sounds like I have a project for my dad. Thanks!

[LukeM]

Enjoy it.  My son got an old CB550 a couple of years back, and he and I enjoyed resuscitating it.  So much so, that my family got me back into motorcycles with a CB750.  It needed some work too, before it went *boom*.  Have fun working with your dad. Remember to put his tools back when you're done.

Ride it for a little while, get to know its noises and stuff, and +1 to TOOLS's recommendations.  I'm thinking the carbs may need a cleaning or two. Good luck, and let us know how we can help.

Luke M

[cdaiscool]

Oh they're my own tools, at my own place. I've got a few other projects I'm working on- one being a turbo Integra, so I've got a fair amount of tools. I'm 26, etc etc. 

My dad owned a CB750K in his old days, though the more we talk about it, the more I suspect it was a SOHC. I should ask him. I'm going to keep riding it up and down my street and working on my low-speed turns while my dad's away (he's in Florida visiting family until the 29th) and then we're going to be off on the streets to practice riding at speed. I'll first ask him to do what TOOLS recommended though. 

Thanks guys! If anyone is in/around Holland/Grand Rapids, stop on over - let's ride!

-Craig

On Wed, Mar 21, 2012 at 8:54 AM, LukeM [via Honda CB750'S] <[hidden email]> wrote:

Enjoy it.  My son got an old CB550 a couple of years back, and he and I enjoyed resuscitating it.  So much so, that my family got me back into motorcycles with a CB750.  It needed some work too, before it went *boom*.  Have fun working with your dad. Remember to put his tools back when you're done.

Ride it for a little while, get to know its noises and stuff, and +1 to TOOLS's recommendations.  I'm thinking the carbs may need a cleaning or two. Good luck, and let us know how we can help.

Luke M

Back on 2 wheels after a 30+ year break. Used to have a 1979 CB750L, sold it as a parts bike, now riding a 1984 VT700C. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.

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NAML

[settles]

Just a thought, I wonder if the valves have ever been adjusted.  Might check and see if that is needed on that year CB. If I remember right, I was told the first adjustment was needed at 4k and then again at 10k.

[cdaiscool]

Not a bad idea. I've done it on my Integra, so not a big deal. Just a different machine.

Where do I get the tool and shims from? Would my local Yamaha dealer that also works on old bikes like this, and my dad's Goldwing, have 'em you think?

Also, what gaskets need to be replaced when doing this? And how do you do 'em so they don't leak? Currently this motor leaks no oil, so I don't particularly want to introduce leakage.

[LukeM]

Oops...
What we should have told you first is to go to the home page of this forum, and download the Factory Service Manual for your bike.  It's a big PDF file, and is very good.

Then, go to the Maintenance section, read through it once or twice, then do a full tune-up on your bike.  It seems mr. dealerman may have done some of that.  Easy enough to do, though.  It should cover carb adjustments, cam chain adjustments, all that stuff.

Then after all that is done, we can dig into the specifics of how to make your bike run better.
Luke M

[cdaiscool]

Did that 2 weeks ago when I started looking for a bike, lol.

I just don't have it here with me at the moment. I'll check it out and go from there.

I am planning, depending on weather, to do the full tune-up to the bike this weekend. Supposed to rain the next few days and I don't have a bike cover nor somewhere to park it :(.

I'm going to plan for chain check, rebuild master cylinder (one side has a screw broken in it ) and check/adjust valves. That's probably it. Dealer was supposed to check air filter, did plugs, sync'd the carbs.

Man, those plugs were black at the end. I'm not sure if they were just old, or the bike's running uber-rich. They gave me my old plugs back (that's a first for me!) and can upload pics if someone wants.

[seestheday]

I have the same bike as you, just a different paint colour.  I'd definitely recommend doing a full tune-up and a carb clean.  After you do that you'll need to re-sync the carbs.  There are lots of resources here on how to do a full carb clean.  Note that you probably don't have to split the bank when you do it (I didn't), but if you really want to clean everything then that is an option.

Backfiring and idle problems could be caused by a lot of things.  It could be as simple as it running lean.  It looks like you have an aftermarket 4-1 exhaust on the bike.  If it is more free flowing than the factory exhaust (it probably is) then your air/fuel mix probably needs to be adjusted differently than factory spec.

[cdaiscool]

Awesome. I don't have a 4-1 exhaust. It has 4-4 exhaust; there are slash-cut pipes instead of mufflers. I'll get a better shot. Dealer told my dad before I got there that it was stock, except for the mufflers. I'll get better pics of it tonight. In fact, I'm going to shoot some video of how it's running and everything. 

On Wed, Mar 21, 2012 at 12:40 PM, seestheday [via Honda CB750'S] <[hidden email]> wrote:

I have the same bike as you, just a different paint colour.  I'd definitely recommend doing a full tune-up and a carb clean.  After you do that you'll need to re-sync the carbs.  There are lots of resources here on how to do a full carb clean.  Note that you probably don't have to split the bank when you do it (I didn't), but if you really want to clean everything then that is an option.

Backfiring and idle problems could be caused by a lot of things.  It could be as simple as it running lean.  It looks like you have an aftermarket 4-1 exhaust on the bike.  If it is more free flowing than the factory exhaust (it probably is) then your air/fuel mix probably needs to be adjusted differently than factory spec.

1981 CB750K

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NAML

[Piute]

      SO Did ya ever have to do more then check valve's ,Was going to pull my covers n such this winter ,
  As well as cut frame to remove rocker cover with-out pulling engine completely out(may be a rush job now).May try to change screws and check how bad then 1st,NEEDING SIZES though.
      After A few wk's I'll be able to tel how much time I'll have (best time to get away from here)if I can go for a winter vaction,I need it tell ya that much.Let see charging prob.,new batt. rear tire,dog sitter,.....hhmmmmmmmmm....

[cdaiscool]

So,  just found this post so I can properly reply...

I had another shop (the one I'm going to currently) run a full battery of tests on it. They said the charging system is weak (though I suspect they didn't check it @ 3k rpm since there's other nuances they don't know about these bikes), the compression test was good, and leakdown test was excellent. Ultimately, they said its in great condition, and its healthy.

However, it does cut out while driving - you can feel it suddenly drop in power, then kick up where it throws you back and then cut out again. He suspects the coils, which are still OEM, are going bad. So I'm replacing those (ordering a new set today - $70 for both) and then going to also do iridium plugs after those are installed. Then just need to do the wiring for the cigarette lighter, and maybe the heated grips, and the bike is ready (except for maintenance stuff) for the trip.

How's your work coming along? Have you decided if you want to join me on my trip?

[shinyribs]

I'd be interested in what you end up finding as far as heated grips. The ones I see local are $100+.