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Motorcycle Repair/1985 honda shadow 750 cc - coolant problem


Hi Bill, I have an issue with the coolant system on my 1985 shadow 750, and I noticed someone else had posted the exact same question, and you had responded (in 2005) but I'm no further ahead. When I look at the Hi and Low marks on the reserve tank, I see nothing - there's no coolant in there whatsoever. When I add coolant, it runs out on the ground immediately through a small hose sticking down under the bike. There are 2 small hoses, the other is capped, this one is not. I thought maybe this one was supposed to be capped and it fell off, so I tried capping it and adding coolant again. This time not a drop would go in, like it was air locked or something. So I took the cap off and once again every drop that goes in comes straight out. And the levels show nothing - there's not a drop of coolant in there.

I find the guide unhelpful as it shows how to disassemble/reassemble the entire coolant system, but I don't think that's the cure. Any advice or suggestions? Here's the question from the other guys previous post... it's word for word the exact way I would describe the problem as well:

I recently purchased a 1985 Honda Shadow VT700. I noticed that the reserve coolant tank is empty and, upon trying to fill it up, the coolant runs out of a hose hanging down underneath the bike. There are two hoses of the same size side-by-side hanging down under the bike, one is capped, one is not. The hose that the coolant runs out of is the uncapped one.

I consulted the repair manuel for the bike but it does not talk about this uncapped hose at all. However, it does talk about its capped neighbor as being the drain tube for the crankcase breather. In the picture in the manuel which shows the drain tube for the crankcase breather it shows the neighboring hose in question as uncapped. Is this hose the cause of my empty coolant reserve tank or could something else be going on? Does this tube need to be capped? Also, does this mean that there is no coolant in my bike at all?

I'm at  and if you have any words of wisdom that would be much appreciated. Cheers,


Mike, my 89 Hawk GT650 had some cooling issues this summer, primarily caused by a defective fan switch which caused the engine to overheat in traffic and spit out 1/2 of its coolant. Apparently the fan switches are a problem for bikes of this age, so I would do some troubleshooting first to see if your fan is working, then plan on replacing the fan switch. You can just put a grounded jumper wire to the fan switch terminal wire to activate the fan when the ignition switch is ON. At least that is how my bike system was designed. Looking at the link below, you might have a 2 pin cooling fan switch, so you would just want to run the jumper from one pin to the other one with a cotter pin or paper clip.

When the coolant level is too low, the temperature sensor for the cooling system isn't sensing the coolant at that level and thus gives a false reading. My bike never showed an overheating condition even though it was spitting coolant on the ground at the time. Once the cooling system is fully filled with no air bubbles, then the sensors can do their job correctly.

Your bike is no different than a car. You can fill up the coolant overflow reservoir all you want, but if the radiator level is too low, it won't draw the coolant back into the cooling system. You have to ensure that the radiator is full by going to the radiator filler neck and opening the radiator cap to fill the system.

The uncapped hose is the overflow, which will drain excess pressure in the system to the ground. The other hose goes back to the cooling system line to the radiator. Once the radiator is completely full and the reservoir tank is full, the fluid will transfer back and forth from the radiator to the tank when hot, then draw fluid back to the radiator as the system cools down, which ensures that the radiator is full at all times.

On my bike, the instructions in the shop manual say to start the engine when the radiator is filled and rev it up to about 4-5k rpms quickly a couple of times and then shut it off. This will help "burp" the system and purge it of any air pockets. You want only coolant in the system and don't use anything except motorcycle-rated coolant. Some automotive coolants have harmful additives that will damage the cooling system of a motorcycle.

You may have to remove the fuel tank to access the radiator cap on your bike and/or remove a plastic shroud cover on one side to find the cap and be able to fill the radiator from there. There should be details in the "maintenance" section of the repair manual, rather than where it describes the cooling system dis-assembly and repairs.

Bill Silver

Motorcycle Repair

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Bill Silver


Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.


I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

VJMC (Vintage Japanese Motorcycle Club) of North America

VJMC newsletter, as editor for two years and as contributing editor currently.

3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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