GT750 Running Hot

[cl4yd0h]

At highway speeds it runs on the last white mark on the gauge, just before hot.

Here are the troubleshooting steps I've taken:

Tested Gauge and sender with a pot of boiling water and secondary thermometer, In the middle of the gauge I am getting readings of 185, which seems accurate

Boiled thermostat, Starts opening at 180 and full open at 200, correct per the manual

Tried a different thermostat

Checked and increased oil output

Checked oil lines for blockage

Ensured bike is getting adequate fuel

Ran with choke to richen mixture

Changed plugs

Checked transmission fluid for coolant

Resurfaced heads and replaced head gasket (compression is equal at 125-130 per cyl

Soaked radiator in acid solution, no damage, free flowing, very clean

Checked all radiator hoses for blockage

Flushed cooling system what seems to be a million times

Replaced radiator cap

It will never overheat, it never looses coolant out of the overflow and only runs warm at steady speeds of 60+

Verified water pump by pouring water into lower rad hose while running, and it is definitely sucking water

Does anybody have any ideas? I am running out! Thank you.

[Alan H]

If the rad is clean inside and out and the cooling system is well flushed, you should check the electrical connections as a poor connection can give a wrong reading - but usually a tad low, also make sure that the voltage regulator isn't allowing the voltage to go high, as that will give you a higher temperature reading when the engine is revving. Next time your gauge is running hot, flick on the high beam and just touch the brake to allow the brake light to come on. This will drop the battery voltage a bit and the gauge may also drop a small amount. If it does, then the reg needs setting to give no more than about 13.5 - 13.8 volts. This is more likely to happen with the Denso alternator as it has a higher charge rate than the Kokusan.
Just a thought - has the battery got water in to the correct levels?

[cl4yd0h]

Thanks for the reply, I haven't thought about output voltage having an effect on the gauge reading. When I originally restored the bike I did check voltage values, but it was a few years ago and I can't remember. I do know that the temp will continue to show high when the engine is not running and has been recently shut off. The battery is a maintenance free sealed battery that load tests wells

I will have to check when I get it back together. Currently I am ultrasonically cleaning and flushing my oil lines. I will have it back together tomorrow and check voltage when the reading is high.

[cl4yd0h]

Wiring is good and Voltage doesn't go above 13.8 while revving.

Runs a little cooler with strait water. Tried a cooling additive in coolant.. no help there.

pulling my hair out!

[Cliff]

Does it only run hot on the highway? What RPM are you running at on the highway? Do you have a fairing or windshield? Any mods to the bike/engine? What sprockets are you running?

[cl4yd0h]

Everything is stock... it didn't do this before and nothing has changed. It just started doing it at speeds of excess 60

[GT750Battleship]

Hi,sorry to read of your cooling problems!,this maybe a long shot....but,how is the very thing the cooling system is trying to cool,the Engine !!,in good tune,done many miles,is the engine block as clean on the inside as your now pristine radiator ? I've experienced an early Mini-Minor with an over heating radiator,it took weeks to find the problem...a broken engine mount...motor sitting slightly side ways upset the cooling dynamics of the car,strange but true,fixed the mount,car returned to normal operating
temperatures !
Cheers ,
Roger

[Cliff]

Running a bit lean at the higher RPM? Check the strainer in the petcock? May she's not getting enough fuel?

[jeff kushner]

If nothing changes, nothing changes....obviously, SOMETHING has changed...now you've got to find it!

What are your plugs telling you? Remember, to a large degree, they are the "windows" to the engine on the inside at its running. I would check them first and if you have the means, look at the crowns of your pistons while you've got them removed!

From what I remember, the pump moves a lot of water so if your positive that it hasn't lost any vanes, then you may have to check for leaks and possibly do a cylinder leak test to make sure that you aren't running lean because of an air leak. This would involve a pumpdown kit but a spray can of WD40 can be used while the engine is running.....its messy but it works, as you spray the base of the block, the the boots of your carbs it the engine picks up speed, you've found your leak. I'm sure there are others ways too that won't cover your machine in WD40 so many someone else can pipe up?

good luck..the weather is to nice not to be riding!!

jeff

[Suzukidave]

It sure sounds as though the bypass is staying open but you say you changed out the thermostat ?

[cl4yd0h]

I have used an cleaning acidic additive on the whole cooling system, and when I changed out the head gasket all the passages I could see were clean.

The bike has adequate fuel flow from the petcock, the vacuum system works well and I removed the fuel filter to rule out any issues. I did freshly rebuild the petcock thats on it and swap out the old one.

Also if it was a lean condition you would think running at highway speeds with the choke on would resolve the issue?

I pulled the jug to replace the base gasket and piston and bores are in good visual condition. Plugs look well.. A little on the rich side if anything (where I like it!)

Timing is dead on with a light & has new points.

I have tried checking for leaks with carb cleaner sprayed around boots and base and it has no effect on RPM.

I did change out the thermostat with a NOS one.. inspected it on the stove and it starts to open at 180 and is full open at 200. correct for the manual.

When it runs there is a very faint "plasticky" noise from behind the left side engine cover. I thought maybe the nylon water pump gear had gone but then I remembered the tach would not function if this gear had an issue which is not the case.

Tried a "water wetter" additive in the system and it has no effect. It runs coolest on strait water..
The heat is definetely making it to the radiator

[Suzukidave]

I still feel its in the water bypass somehow as this is where the bypass portion of the thermostat should be closed and if its not will allow enough of the coolant to bypass the radiator to make the engine run hot . You said when you tested the thermostat it opened at the correct temp but did the bypass valve also close at the same time ?

[GT750Battleship]

Dave could please explain about the by pass valve..are you talking about the hole it the filler neck that allows coolant to flow to the reservoir bottle under the tank
Cheers,
Roger

[jeff kushner]

"I pulled the jug to replace the base gasket and piston and bores are in good visual condition. Plugs look well.. A little on the rich side if anything (where I like it!)"


Well you have certainly dug plenty deep enough into the engine and it's easy to see why you are at the hair pulling stage!! BTW- I like my plugs dark brown too...it just makes me feel "safe"<LOL>

Following up on what Dave said....
Did you by chance blow out the passages with compressed air in case something got lodged in one of the passages? I can't imagine one of the alum impeller blades coming apart but a quick removal of the water pump cover might not be a bad idea. There is also the bypass union that may have become blocked. Looks like this:
http://www.ebay.com/itm/Suzuki-GT750-Cr ... d7&vxp=mtr" onclick="window.open(this.href);return false;


I think if it were my bike and the engine assembled and ready to run, I would get two large containers, 15 or 20 gallons each. The pump is supposed to move 16.2-19.2 gallons per minute at 6K RPM. I would want to see how much is moved in 30 seconds. Just start with one container higher then the inlet and it should provide enough head pressure (.434lbs per vertical foot) to keep it at full flow via siphon action.

Last questions.....I know that you've said the gauge is reading a bit higher than it used to...does it "FEEL" hotter? If you've owned it for while and ride it often, you know your bike....does it really seem that something is wrong or is it just the gauge? Lastly...a gauge is 20 bucks...maybe swap it to see?

Good luck,
jeff

[Suzsmokeyallan]

Did you try using an infrared hand held thermometer on the components to verify surface temperatures when running?