Author Topic: 74 cb360 Engine Compression Question (Read 4686 times)

jude0007 · « **on:** March 12, 2014, 04:58:29 PM »

Hi, I'm working on a 74 CB360 that's in decent shape but I'm a little worried about the motor. I did a quick compression test on the motor and both cylinders were showing about 75 psi. I poured some oil into each cylinder through spark plug hole and let it sit over night. Next day, I got about 120 in one and 75 still in the other. I'm worried that I'm going to have to dig into the motor. Any suggestions? Could a valve be stuck? I can't get it running right now because ignition coils are failed but would higher temps possibly free a carbon built-up valve?

Appreciate any comments.

Johnie · « **Reply #1 on:** March 12, 2014, 05:13:28 PM »

Had the same issue on a 360 a couple years ago. It was a piece of carbon on one of the valves. Lucky for me I could see it through the port and got it off. If it was me I would get it running as sometimes you get a stuck ring which will free once you get a few miles and oil on it. Give it a go and let us know how you come out.

seanbarney41 · « **Reply #2 on:** March 12, 2014, 05:15:39 PM »

yup, rise in compression when oil is added indicates problem with rings. Run it for a while and see if it improves.

Cevan · « **Reply #3 on:** March 12, 2014, 06:37:08 PM »

I have a '74 360. I had low compression on one cylinder. I did a leak down test on it by unplugging the compression tester gauge and connecting my air hose to it with the cylinder at TDC. It was clear air was getting past the rings. I honed and re-ringed it and it's been great ever since.

jude0007 · « **Reply #4 on:** March 12, 2014, 06:40:21 PM »

I was trying to figure out if there was a weird noise coming from the starter motor (which there is) not realizing I still had the compression tester hooked up to low compression cylinder. I glanced at it and it was reading 135. I think I'm going to be ok, once up and running and warm.

My next problem is I have no spark at either spark plug when grounded out. Normally on my 4's I'd swap yellow and blue wires to see if the ignition coil has failed. Not sure what to do or how to test them when both are not working. Should I be checking resistance across ignition coil? I will be ordering a manual as all mine are for 350-750 4's. Thanks guys!

Oh and by the way, this little project is for my best friend's 30th birthday. His fiancé wanted me to find a good bike on craigslist and get it running and surprise him for his 30th, how awesome is she?!

Johnie · « **Reply #5 on:** March 12, 2014, 08:11:49 PM »

She sounds like a keeper there for sure.

grcamna2 · « **Reply #6 on:** March 12, 2014, 08:28:13 PM »

A good amount of that Marvel Mystery oil type thing soaked in there would soften up the carbon a bit in a heated room,just my 2 cents...

joesmotos · « **Reply #7 on:** March 12, 2014, 08:46:19 PM »

Make certain the points are opening,CLEAN, and gapped correctly,also if the condenser goes bad it will not fire.

If you have a spark plug in the spark plug cap laying on the engine,and you turn on the ignition,and break the points open with a insulated screwdriver and it sparks...you should be good.

Check for 12v at the coils too..you could run a 12v wire straight into the coil + positive lead also and see if it fires,might have something goofy in the harness not allowing coils to get voltage..

jude0007 · « **Reply #8 on:** March 14, 2014, 02:26:36 PM »

So I just really quickly ran 12v power to ignition coil lead and got both spark plugs to show a spark so I now feel the ignition coils are just fine. Is there still a chance that the condenser is bad or does that prove that it's ok?

I haven't tried manually opening points with a (insulated) screw driver yet or checked the gaps yet. That will be next. I'm just glad that both ignition coils seem to be ok.

grcamna2 · « **Reply #9 on:** March 14, 2014, 03:37:47 PM »

I would have a look at the points,if you can remove them and open the contacts to see the condition of those contact points,or you could do that w/ your camera while they're still in the engine.The condition of the contact surfaces are what we want to inspect.
Have you taken any more compression tests or tried starting it yet ?
If you have all the wires connected to the original coil leads install a spark plug into each coil plug cap(remove the plugs from the engine so none are in there)and then lay the hex part of each spark plug on the bare metal surface of the cylinder head which will ground the plug so you get a solid spark,make sure they stay grounded(don't change the position of the handlebars or bump the bike while you're doing this) to the bare metal of the engine.Turn the kill switch(red knob)to the "run" position and spin the motor over like you're trying to start it but w/ the spark plugs pulled out of the threaded holes so the engine spins freely.Take care not to have any open gas around when they spark

& while you're spinning it over for about 4 seconds watch the tips of the spark plugs to see if they spark on both plugs.
We'll go from there...

joesmotos · « **Reply #10 on:** March 14, 2014, 04:14:16 PM »

I would say the condenser is ok if you had spark...my experience with this sort of stuff is that if you have all the basics...compression..fuel..spark...the bike should run....also...once you get it running...and idling...as the rings re-seat,and the valves start knocking all the bits of carbon off themselves as the bike is running,the idle starts going up and so does compression(assuming its not all rusty in the cylinder...im not saying it will totally reseal,but you might be suprised at the compression increase...

When i buy old bikes like yours,i usually end up cleaning the carbs,checking the ignition timing,draining and relacing the oil,fresh battery...once i get em running i let it run a bit and listen to the rpms going up and up...sometimes i stick a carpet fan(one of those squirrel cage thingies) in front of the motor so it stays relatively cool...and let it run for a bit..i usually end up dumping the oil back out after a few go arounds with that,and let the engine cool all the way back down,then go check/readjust the valve lash/clearance,re check the timing,then synchronize the carburetors..

grcamna2 · « **Reply #11 on:** March 14, 2014, 05:33:05 PM »

That sounds like a good way to get the most out of that present top end if it'll fire up joe,I hope the compression comes up some,some Marvel Mystery or Seafoam in the cyls. wouldn't hurt either.

joesmotos · « **Reply #12 on:** March 14, 2014, 06:21:41 PM »

Yup...once it runs a bit the valves usually need readjusting...we have all seen the ole rings semi-stuck in the ring lands when we tear engines open that have been sitting a while...im just suggesting things based on past experiences..plus i have had several CB360's...hoping to get some pistons made up for my latest project,took a 360 cylinder and had a friend bore it till it broke thru to find out where the thin spots are...got some purty big sleeves stuck in it now ,waiting on me to cough up the dough to get pistons made..should be interesting lol..

But back to this thread...

If it were me,just for now,i would put the plugs back in the bike,hook up some fuel to the carbs,stick a battery in it,and run a 12v source right to the coils and hit the starter button(or kicker if you like) and see what happens...might here it go vroom vroom..

jude0007 · « **Reply #13 on:** March 22, 2014, 06:10:55 PM »

I put new points in and gapped correctly. The old ones were a bit rough, I'm sure they would have worked if gapped correctly (which they weren't) but cheap enough to replace. After the swap, we've got good spark at each plug. I'm going to drain the old gas out of the tank and try to see if she'll start.

grcamna2 · « **Reply #14 on:** March 22, 2014, 06:27:56 PM »

Looking forward to hearing how it runs,I hope you installed quality contact points.

jude0007 · « **Reply #15 on:** March 24, 2014, 01:47:15 PM »

I installed NOS points, should be good. After thoroughly cleaning carbs yesterday, reinstalled them and she fired right up. The exhaust on this bike is real bad so didn't sound good exhaust-wise but the throttle was responsive. I've got new Mac exhaust coming so that should take care of that.

I wasn't able to take for a test drive because I'm currently derusting the tank, had to use a different tank to see how she ran which prevented me from being able to close the seat.

Thanks everybody for the help! I'll post some pics once she gets fully put back together and scrubbed. And some post revealing birthday party pics.

jude0007 · « **Reply #16 on:** March 28, 2014, 04:36:18 PM »

I got MAC exhaust in yesterday and installed them pretty quickly. The old exhaust was rusted out. I have to say they look really good and sound good as well.

Also got a master cylinder rebuild kit in which was kind of fun repairing as I haven't done it before.

Bike fires right up, went for a little ride. All in all, was pretty decent for first ride. Mid range is a little weak, cuts out some. Also I can tell that the front brake is a little stuck and after a short ride was hotter than it should be. My plan is to take that apart and check everything out. The Clymer manual talks about specs but is there anything I should be looking for specifically?

grcamna2 · « **Reply #17 on:** March 28, 2014, 04:46:00 PM »

Quote from: jude0007 on March 28, 2014, 04:36:18 PM

I got MAC exhaust in yesterday and installed them pretty quickly. The old exhaust was rusted out. I have to say they look really good and sound good as well.

Also got a master cylinder rebuild kit in which was kind of fun repairing as I haven't done it before.

Bike fires right up, went for a little ride. All in all, was pretty decent for first ride. Mid range is a little weak, cuts out some. Also I can tell that the front brake is a little stuck and after a short ride was hotter than it should be. My plan is to take that apart and check everything out. The Clymer manual talks about specs but is there anything I should be looking for specifically?

The caliper piston needs cleaned up real good,do you think the master cylinder is cleaned out good and has smooth operation ?

jude0007 · « **Reply #18 on:** March 28, 2014, 04:55:14 PM »

Quote from: grcamna2 on March 28, 2014, 04:46:00 PM

Quote from: jude0007 on March 28, 2014, 04:36:18 PM
I got MAC exhaust in yesterday and installed them pretty quickly. The old exhaust was rusted out. I have to say they look really good and sound good as well.

Also got a master cylinder rebuild kit in which was kind of fun repairing as I haven't done it before.

Bike fires right up, went for a little ride. All in all, was pretty decent for first ride. Mid range is a little weak, cuts out some. Also I can tell that the front brake is a little stuck and after a short ride was hotter than it should be. My plan is to take that apart and check everything out. The Clymer manual talks about specs but is there anything I should be looking for specifically?

The caliper piston needs cleaned up real good,do you think the master cylinder is cleaned out good and has smooth operation ?

The MC is spotless, completely went through that all new parts. I broke as many connections as I could and cleaned them up as best I could. Used guitar string to make sure every things free of passage. I'm sure with the condition of some of the lines, they are perfect. I'm thinking the brake needs to be torn apart, inspected and cleaned.

grcamna2 · « **Reply #19 on:** March 28, 2014, 05:37:39 PM »

That brake caliper is sticking and dragging the brake,that's why the disc gets hot while you're driving.

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Author Topic: 74 cb360 Engine Compression Question (Read 4686 times)