Coolant Fuckery

[Falco]

Bike (CB500s) overheated badly during Friday morning filtering and had to pull over to let it cool off (didn't even have to do that in the height of summer). Was fine when out of traffic but ran hotter than usual rest of the way in and back out.

Fan checked, reservoir looked about right, I haven't found any obvious leaks. On opening up the coolant in the system was so low that I could open thermostat housing without having to drain it (thermostat also fine).

Easy fix right? Well, after topping up and running for 5 minutes to let air make it's way out, the reservoir was almost overflowing. Removed almost as much as I refilled by.

10 miles of slow (20-30 mph) riding around town this evening and it *seems* to be behaving itself, but the commute will be the acid test of this.

What is going on here? If the coolant is going into the reservoir when the bike heats up, why was the system empty when cold? and why did running it push an equal amount of coolant back into the reservoir?
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I tell you what, mathematically, I'm having it

[P.]

Sure the thermostat is ok?

[doggone]

Is the fan definitely on when it gets hot.

[Islander]

[stinkwheel]

Could also be a blown head gasket pressurising the water jacket with exhaust gas and forcing the coolant out through the overflow.

You can sometimes feel this happening if you squeeze the coolant hose and rev the engine. If it gets firmer when you rev the engine or they are rock-hard anyway, it's probably not good news.

It can also be a lot more subtle and only happens at certain rev speeds.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[cb1rocket]

ride around for at least 20 mins and check coolant level from the rad cap filler neck, if its disappearing the headgasket has gone. Equally saying that if it doing was stinkwheel said then it's also likely to be HG failure.

However do check the obvious that the fan is working.

Source: I have had many bikes die this way.
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Honda H100S2 Rebuild - https://www.bikechatforums.com/viewtopic.php?t=253852

[RhynoCZ]

Check the radiator cap, it should seal the system properly. I'm not sure what pressure the CB500 engine runs the cooling system at, but it sure is higher than the atmospheric pressure, to move the boiling point of the coolant further above 100°C.

This brings me to air in the system, bleed it.

Also, if what Stinkwheel says is happening, you might going to see some bubbles forming, with distinctive smell, under the radiator cap and/or the expansion tank. I've seen this on a car engine, motorcycle engines should be the same.

This kind of stuff: https://www.youtube.com/watch?v=qkrsyaMDuxM
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[ADSrox0r]

Lad that sits next to me at work had the very same thing, same bike. He replaced all manner of things till it turned out it was a blown head gasket.
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Current bikes: '08 VFR 800 VTEC(yo) , '07 ZZR1400 Winter hack: '95 Aprilia 650 Pegaso Currently lusting after: RC30

[Chris45]

[Falco]

Cheers for the replies.

[P.]

It's pretty significant. I'd not chance rising it and risk overheating and lunching the head.

I have "fixed" a car that was consuming water with one of those dodgy head gasket repair in a bottle things which worked for another 2 years.

Replacing it is more than what you've done but unsure if it can be done in the frame on a CB, doesn't look too tight in terms.of pics online so probably possible. Quicker and easier on a bench though.

[Falco]

[Chris45]

[Falco]

[Falco]

A thought has just occurred to me before I start the faff of this repair.

What are the causes of head gasket failure? I have read that overheating can do it, but the bike has never overheated before and is a cool running bike.
I was not under the impression that head gaskets were "consumables". Could it just be old?

My main question is: is there some other problem I should be hunting for, before I end up doing this again 200 miles down the road?
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I tell you what, mathematically, I'm having it

[RhynoCZ]

[sickpup]

Before stripping the engine I would recommend getting one of the test kits.

It will waste some time waiting for it to arrive but will confirm if its the head gasket.

[Polarbear]

[redeem ouzzer]

Slinging another engine in would probably work out cheaper and quicker than doing a head gaskets. Rebuilding a 4t engine is a fools game unless it’s rare or special.
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Be a REAL MAN!

[stinkwheel]

That said, it's a twin.

Moreover, it's a honda twin from the era where they were making bikes that were designed to be taken apart. Toss of a coin if changing out the head gasket or engine swap is the bigger job.

I'd say it is a moderately busy Sunday for a reasonably competant amateur mechanic to swap out the head gasket on a CB500. If I was doing it on the wifes bike I'd expect to start after breakfast and be road testing it before dinner.

Biggest iffy bit will be getting the exhaust studs off.

Tools wise, you don't need anything too fancy. A 3/8 drive socket set and some half decent screwdrivers from memory (use the bike toolkit screwdriver, it looks flimsy but fits the screw heads) Other than a torque wrench for the head bolts you'll need to fabricate a little key-type tool to lock the camchain tensioner back while you refit it (I cut one out of a tuna can lid with tip-snips). There's a template in the workshop manual.

My top tip would be to make sure you measure the tappet clearances BEFORE taking anything off so you know if you'll need any shims when you reassemble and be meticuous about keeping the buckets and shims for each valve so you can put them back on the same one. I have a little tray with compartments in it but a big egg box is also good. If you need tappets, you'll have to get those in before you can fully reassembe (but you can do most of the job).

Also take digital photos from all angles at each stage so you know where things go back (like cable routing).
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Chris45]

[Falco]

Thanks guys.

[Paulf]

I think I have the same problem. From what I understand check coolant for air bubbles when running is one of the signs of a failed head gasket. Beginning to think I have a blown head gasket. Honda 125 pantheon with identical problem to op.
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Past bikes simsom mofa 1s, suzuki hustler 250 (£35 in 1973), suzuki ts125k, mz150, yamaha yb 100, honda pc50, honda vision 50, all 2 stroke - then cars - honda pcx 125 (sold) own honda fes 125 and Mrs 125 spares or repairs. Started 1972 all accidents walked away no insurAnce claims

[cb1rocket]

[stinkwheel]