Topic: Should I rebuild the engine?

[MTNman]

Hello;

I recently purchased a 1980 CB650C, and from what I can tell it hasn't run since 2008. As I was beginning the disassembly tonight, I've been more and more impressed with the overall condition.

The bike is super clean, just some surface rust on the frame, which I'm getting powder coated. All the chrome pieces are vey good condition, and the fork looks amazing.

It has 46,000 KM (28,0000 miles), and I'll be performing a compression test in the coming week. So my question to the group is, if the pressure is still at 150-170 psi, would you bother with a rebuild?

I started the project with the intention to do one, but I'm curious what experienced people have to say.

Thank in advance! 

[wolf550]

Remember another 650 thread on here I heard that one of the gaskets is hard to come by or it was the whole gasket kit itself. Not sure which. But you should check and see availability first on the appropriate parts for the rebuild first.

[MTNman]

Remember another 650 thread on here I heard that one of the gaskets is hard to come by or it was the whole gasket kit itself. Not sure which. But you should check and see availability first on the appropriate parts for the rebuild first.

Yes of course, I literally started tonight, haven't even pulled the engine out yet lol.

[Alan F.]

I'm sure you'll get replies from those more experienced than I, but I'd say if there are no leaks or noises just run it. Compression tests on engines that have been sitting can be misleading.

[HondaMan]

You can get gaskets at PartsNmore. I just bought some there.

[newday777]

Hello;

I recently purchased a 1980 CB650C, and from what I can tell it hasn't run since 2008. As I was beginning the disassembly tonight, I've been more and more impressed with the overall condition.

The bike is super clean, just some surface rust on the frame, which I'm getting powder coated. All the chrome pieces are vey good condition, and the fork looks amazing.

It has 46,000 KM (28,0000 miles), and I'll be performing a compression test in the coming week. So my question to the group is, if the pressure is still at 150-170 psi, would you bother with a rebuild?

I started the project with the intention to do one, but I'm curious what experienced people have to say.

Thank in advance! 

Welcome aboard
Have you squirted a little oil in the cylinders?
Checked to see it will turn over(motor spins)?
No rust in the tank, old gas left in it? If so do you have a small auxiliary test tank? Then clean the tank of rust and old gas.
Clean the carbs of old hardened gas, clean the jets in ultrasonic  cleaner then new gaskets.
Change the oil & filter and see how it looks, chunks in it, debris?
Electrics are in good condition?
Then fire it up and see how it runs first before deciding if it will need rebuilding. Running it a bit will free up sticking rings and it will probably be fine at that mileage and if it was taken care of.

[ekpent]

Whenever I buy a new project which most of the time I have not heard run the first thing I do is concentrate on getting the engine running before tearing anything apart so I can further assess the direction the bike needs to go. The old saying "If it ain't broke ------------"

[MauiK3]

+1 on get it running first, if possible.

[MTNman]

Thanks for the input everyone!!! I was leaning that way but I wanted other opinions. The engine does turn over with the starter and it still has healthy oil.

Special thanks to Newday777, I'll do all of that. Guess we'll see how the compression test goes.

[Alan F.]

Beware of healthy looking oil, the contaminants can sink to the bottom. Recently I drained the oil from a CB160 that had sat for 30 years, the oil looked clean on the stick but it was dark in the drain pan.

[MTNman]

Beware of healthy looking oil, the contaminants can sink to the bottom. Recently I drained the oil from a CB160 that had sat for 30 years, the oil looked clean on the stick but it was dark in the drain pan.

Good call, I was planning on changing the oil up eventually but I’ll take a look earlier.

[tourmax]

If you get it running and test in the 150-170 range, the rings and valves are good. Make sure your valves are properly adjusted before doing a compression check. Be aware that putting oil in the cylinder and testing it gives you higher readings than a "dry" compression test. Oil in the cylinder is used as a testing method to determine if low compression is related to the valves or the rings. the oil seals the rings and if compression goes up dramatically, that means the problem is in the ring/cylinder and not the valves.

However, if this rig has been sitting for 20 years, I'd oil the cylinders before turning it over just to be sure I wasn't dragging dry rings against dry liners. Then run it and do a comp test after the oil had burned off.

[Stev-o]

Guess we'll see how the compression test goes.

What type of compression tester will you use?  Automotive?  As mentioned, results can be misleading.  More important that the max number is that they are all within 10% of each other.  And low compression numbers have been known to rise after the motor is back running.  Good luck!

[ekpent]

I was reading some reviews on the Harbor Freight compression tester and there were more then several people who were 'pissed off' about the fact they rebuilt a small engine because they thought the numbers were low. Sounds like with those it all about a good schrader valve. ??

[HondaMan]

If you get it running and test in the 150-170 range, the rings and valves are good. Make sure your valves are properly adjusted before doing a compression check. Be aware that putting oil in the cylinder and testing it gives you higher readings than a "dry" compression test. Oil in the cylinder is used as a testing method to determine if low compression is related to the valves or the rings. the oil seals the rings and if compression goes up dramatically, that means the problem is in the ring/cylinder and not the valves.

However, if this rig has been sitting for 20 years, I'd oil the cylinders before turning it over just to be sure I wasn't dragging dry rings against dry liners. Then run it and do a comp test after the oil had burned off.

Like he said...oil the cylinders, wait a while (at least 24 hours) and then I'd just add: put some oil into the first few tanks of gas, too. Like 4-6 ounces (any oil will do, 2-stroke synthetic type won't smoke much) to help release the rings' corrosion in their grooves. It takes a while, but works.

[MTNman]

Hey everyone;

Thanks for the amazing amount of input to my question, I did read through it all and its been very helpful.

As an update, I decided to rebuild the top end of the engine, as well as the carbs. The compression test checked out but it was on the lower side, so I'm playing it safe because its an old bike. It also gives me the opportunity to repaint it.

I'll be starting work as soon as my gasket kit comes in.

Thanks again, sorry for the hiatus

[fizzlebottom]

What were the numbers? Were they all the same or about the same? If so, don't go rushing to tearing it open yet. Do the book adjustments, change oil and spark plugs, make sure the air filter is clean, and give some fresh gas. Run it for a while and see how it goes. Really no need to go rebuilding yet until there's an actual reason.

[Airborne 82nd]

Do not take it apart just yet

When I first bought my \bike I put 50-50 mix of acetone and ATF. It gets in all those really tight spots the acetone is the carrier that the ATF rides on. In my case it took 2 tries after that I still had to put 80-100 miles for one of  cyl. to seat and stop smoking

[HondaMan]

If you take it apart, PM me for some O-rings (or at least for the sizes you need to get) that you will need in order to prevent oil leaks. The ones in the kits do not take into account the need for them to be thicker now, as the head gasket and cylinder base gaskets are now both thicker than OEM units were. This is due to the old ones being made with asbestos fibers, now replaced with other materials, that must be thicker to make things seal well. These 4 O-rings must be similarly thicker, and if you just put it together with the typical gasket kit's O-rings, it WILL leak oil. I just finished re-doing the top end on a CB650 (yesterday, in fact) that was suffering this very problem. It is common to all of these SOHC4 bikes today.