Suzuki 2 Strokes

I have a question about thermostats used on the GT750.

Some years ago thermostats used in automobiles were changed so that failure meant - "wide open". They changed them to prevent over-heating in the event of failure since earlier designes failed closed and refused to circulate water through the radiator - causing engines to overheat. Does the GT series fail in the open or closed position? Was there ever any change? Has anyone seen/heard of a failure?

I only ask because I went and purchased me an NOS one on eBay that is 30+ years old, now I'm asking myself if that was a smart thing to do. Just maybe the new replacements have some better engineering.

Inputs/thoughts please.

Thanks,
Gordon

Thermostats either work or they don't. There is no in between or beginning to fail. Take your thermostat and put it in a pot of water on the stove. Use tongs to remove and observe. With a candy thermometer, begin to heat. Should be partially open in the mid 180's and fully open in the mid 190's. They work on some sort of wax pellet design that I have never truly understood.
All I can verify is that I tested and used the OEM thermostat in my 30 year old barn relic Buffalo and had no problems whatsoever.

Don't thermostats always fail to the closed position? There's a spring and bellows causing that.

The GT Thermostat is as bad as any car one. Try to find one that opens early and has a bleed hole or drill a bleed hole in it.

Fit an over temp warning light if you suspect it will be a an issue, but I haven't had too many thermostats failing over the last 40 years. Maybe I got lucky.

Plus the GT750 thermo is double acting , when cold the bypass is open then as the coolent warms up the bypass closes off and the main opens . http://www.3cyl.com/~mraxl/gt/manuals/gt750man/sr57.htm

I have posted this before and several of you think I am nuts, but when I rebuilt the engine on my Buffalo in 2007, it kept puking water out the overflow after running only a couple of minutes. The water being puked out onto the floor still felt cool. I have a perfectly clean radiator, new radiator hoses, new thermostat, new water pump, etc.
I took everything apart (except for splitting the cases again) to verify that everything was installed correctly and found no problem anywhere.
The bypass hose from the starter cavity to the thermostat housing was not the correct Suzuki one, so I bought a correct one and changed it just because the one I had was not formed quite like the original and I was afraid it might be collapsing, so I changed it.
I finally ended up removing the thermostat altogether and it has been running quite well ever since, it just takes it a little longer to get up to normal operating temp when you first start it up.

Wayne

I remember that and it is strange as the engine should over heat at highway speeds with part of the coolent bypassing the radiator and going right back to the pump . I wonder if the bypass could be blocked some how , i know you wrote you replaced the hose and theres not a whole lot between the head and the pump to get clogged ??

Everything was perfectly clean and clear. I removed the new water pump to see if anything obvious showed up, pulled the radiator off the bike and swapped it for another one that was also perfectly clean, the tranny is not getting water in it, so the o-ring between the cases is sealing. I put three different thermostats in it, the old one that I took out and two new ones.

I could not figure out anything else to do, so I just removed the thermostat and it quit puking. I have ridden it about 2000 miles since that time with not a single problem, it just takes a while for it to warm up when started cold.

Wayne

Wayne, that was an unusual problem that none of us were able to explain away.

When the thermostat is closed, water circulates back through the by-pass hose and none goes through the radiator. The overflow is at the filler cap neck and is only "in circuit" when the thermostat is open, Q.E.D. it cannot leak when the motor is cold.

Except that your bike didn't know that and leaked anyway.

The usual cause of overflow is not excessive pressure but is the radiator cap not sealing. Sometimes that's because the rubber sealing ring is shot, but I have found many cases where the filler neck is corroded or distorted and cannot maintain a seal.

In your case there was enough pressure with the thermostat in the closed position to cause it to leak, but it was OK when open.

That implies that the radiator was being pressurized more when cold than when the thermostat was open. The "hot" condition is easier to understand. With a large volume of water flowing freely under very low pressure, doesn't leak.

With the Thermostat closed no water is passing between the radiator and the block, so that implies that the radiator is being pressurized from the bottom hose. I'd have to look at the pump and crankcase design again to work out how that might be happening.

One way to bleed off that pressure for the moment would be to drill a 3mm bleed hole through the thermostat. It should be enough to allow the motor to warm up fast enough and still avoid the puking.

I used three different radiator caps as well. One of them was new and the other two were used, one being the one I was using prior to the rebuild.
I need to see if I can find a schematic showing the coolant flow. Based on how the bypass hose connects to the thermostat housing, it looks to me like water would still flow to the radiator with the thermostat closed, just in a smaller volume because the bypass hose just cannot flow as much due to its size. I guess I just assumed that the reason for the bypass in the first place was to help the motor come up to temp faster and to keep you from overheating the motor if the thermostat stuck closed. Then when the thermostat opens, it sorta plugs the bypass orifice and redirects the water through the larger orifice for proper temp control.

So far, I have not had any problems, but I really like things to work as they should and this one has me stumped.

Wayne

http://www.3cyl.com/~mraxl/gt/manuals/gt750man/sr53.htm The only think i can think of is somehow the bypass circuit is blocked .

Back to Gordon's question; just test it in a pan of hot water. That will ensure that it's OK, and if it does fail in the future, just pull it out until you can get home.



Lane

Can I use a normal car thermostat on my GT or is it a special Suzuki size/design?

Hey Wayne, here is a thought.... I had a '86 Toyota Van/wagon. They have a known problem of leaking head gaskets when you get a lot of miles on them. Any way, when this happened to me, it would pressurize the water jacket with exhaust gases and 'boil' out coolant. As it cooled overnight, one cylinder would suck in coolant. In the morning, it would run on three cylinders until the fourth one(?) pumped out all the water than it ran on all four. Leaving the radiator cap loose virtually stopped all that, but once a month would have to top off the water. Finally pulled the head, put new gaskets, lifters, fuel. water, vacuum hoses on it , and torqued the head down like the book said. Ran better and held the water. Any way, my point is, you might have an exhause leak into the water jacket that is pressurizing the system, making it puke. I have seen all kinds of corrosion, pin holes, etc. in aluminum to give this some credence. Unfortunatly this means pulling the head and cylinders to just look for anything....... Might however head off (no pun) a major problem down (on) the road.

Doing a cooling system pressure test sounds like a good test .

rbond, that is certainly possible, however, since I removed the thermostat, I have ridden the bike about 2000 miles and have not had to top off the coolant, and it has never overheated. Since I don't take this bike on road trips, I think I will just live with it until something happens that gives me an indication of what is wrong, or I ahve to remove the top end for some other reason. I'd rather go bear hunting with a switch than pull the cylinder block off this mutha again!

Wayne