New CB900C Owner

On a roll Joined: Oct 04, 2021 Posts: 222 Location: NW NJ

I'm a lifetime motorcyclist (been riding since 1973 at the age of 10) who has owned over 30 motorcycles....but only one Honda (CT70H...when I was 10). My most recent motorcycles have been custom cafe racer/scrambler builds that I have built from unloved wrecks. I currently have three bikes in the garage and had no need for a 4th...but we all know what happens. A local retired gentleman was selling his '81 CB900C. I saw it...and it spoke to me.

The bike only has 35,000 miles. On inspection I saw a bike that had lived an easy life, but maintenance wasn't the owners forte'. The front master cylinder is frozen. The rear tire is the wrong size (140/90 square profile) and is rubbing on the right side swingarm/driveshaft cover. The bike starts and runs great - even on old fuel. I didn't note any smoke on start-up, but I did hear a cam chain that was in need of adjustment. The owner said the engine smokes, but I didn't see any smoke on the cold start or after warm-up. He also said that the suspension wont hold air for long.

I got the bike for a very low price and decided I could ride it home safely using the rear brake. The three roads I would need to ride on are sparsely traveled with 35 mph limits. The rear tire was rubbing so badly that brakes weren't necessary. Every time I let off the throttle the bike came to an intimidate stop. After a mile of riding I could smell smoke. I looked in the mirror and I could see smoke. I thought it might be from the rear tire rubbing. I had to give this (smooth running) engine quite a bit of throttle to keep it moving. Just a mile from home I turned on to my street. A quick check of the mirror showed a cloud of smoke like I had never seen before. Holy smokes, this thing smokes. I made it up my driveway and pulled right into the garage. Oil was pouring onto my garage floor...not dripping. The head was dry, but the #1 cylinder below the head was drenched in oil. Clearly, this bike needs a head gasket. The rear tire was in fact smoking too. Also smoking was the rear brake - which was also locked-up. And I'm ok with all of it - these are all simple (but involved) fixes. For the 10 seconds this bike was running in my garage, the fog was thick and breathing was not possible. I actually had to go take a shower.

I'm sure that these engines smoke for the same reason as any other engine; stuck valve or bad valve seal, a head gasket leak or stuck rings. Since there was absolutely no smoke on start-up, that eliminated a valve seal. Also, the engine runs so smoothly that I can't believe that one or more of the cylinders isn't firing. I'd rather swap a head gasket then rebuild a head...so I'm thinking it's "just" a head gasket. Here's my theory on the head gasket leak. The rear tire should be a round profile 130/90. This bike has a square profile 140/90 (Engager by Kenda). It has obviously rubbed on the swingarm since the day it was installed. This tire looks well used, so I'm thinking that under light loads, the engine is (was) fine. But the way this tire rubs, it has to overcome significant drag - imagine if the bike was pulling a 3,000 pound trailer (I'm not kidding it's that much drag) - requiring 1/2 throttle just to go 30 mph. That's what blew this head gasket.

And that's why I joined the forum. I'm gonna need technical and moral support. In the coming days I will disassemble the body parts, degrease and powerwash the engine and frame, and then begin tear down of the engine. My Clymer manual will be here tomorrow.

So that's my winter project.

Posted: Mon Oct 04, 2021 4:37 pm

Welcome to here wave

Posted: Tue Oct 05, 2021 8:08 am

Welcome to the site Gary_in_NJ ... Smile

If you haven't read the following page , now is the time .
http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&p=80601#80601

If you run into problems with any of that , Please let us know . Thanks . Smile

You'll do better downloading the factory service manual from our Technical Documents section . Clymers don't have the best reputation .

On a roll Joined: Oct 04, 2021 Posts: 222 Location: NW NJ

Thanks for the welcome. I did download the FSM yesterday...I didn't discover it until my order for the Clymer was placed. Yes, I've never like their manuals but the photos are usually good. It's good to have a FSM on the computer to study prior to doing the work. Then I can make notes in the Clymer book and bring that out to the garage.

I've been using the search function, but I'm having trouble finding specific results. For example, I know that I need to remove my engine to replace the head and base gaskets. So I've been looking for ideas on how to build an "engine cradle" or "engine platform" - these are my two searches. I've gotten what appears to be thousands of results. I will start a topic on engine removal that isn't (hopefully) redundant, but I'm having trouble stringing search words together. If I search "engine AND cradle", every thread with the word "and" is returned...as you can imagine - that's a useless result.

Posted: Tue Oct 05, 2021 12:38 pm

There are some search tips in the links in my sig that might help.

Posted: Tue Oct 05, 2021 9:26 pm

I have an engine change fitting that makes removing the engine simple.

$25 plus the ride, man.

Posted: Wed Oct 06, 2021 1:15 am

Yeah, that seems to be a popular design.

On a roll Joined: Oct 04, 2021 Posts: 222 Location: NW NJ

Thanks for the offer Cliff. I should know in a day or two if the engine needs to come apart.

Posted: Wed Oct 06, 2021 10:31 am

Head gasket will not leak oil........ Valve cover will.

On a roll Joined: Oct 04, 2021 Posts: 222 Location: NW NJ

cliffiec wrote:

I have an engine change fitting that makes removing the engine simple.

$25 plus the ride, man.

@cliffiec

If the offer to sell that engine stand/holder is still on the table, I’d like to take you up on the offer and buy it from you.

Posted: Tue Oct 26, 2021 8:33 pm

PM sent, man.