GT750J over heating.....or is...it?

[chance johns]

DEAR DIARY: i am actually over heating. i took her out on a test ride last night. zoomed around for a bit. gauge stayed between center(2nd) and third line. the small unnoticeable dot that is the hot side of normal operating temperature is just before the third line. i seemed to be doing good. i said ***** it lemme go a little farther. i hit the interstate. for about 5-10 minutes I'm good. I'm going about 60-70 about 7-8k rpms (i think?) then the line on the gauge does creep slightly onto the third line, and creeps slowly and slightly just past it. boom 70mph dies instantly. doesn't bog down, no, "oh crap whats going on lemme flutter the throttle and finesse this bitch to the next exit, it was done. dead. i coast to the shoulder and I'm racking my brain. i test the top end with the thermo laser everything seems to be around 200-210f give or take a little. top of headers and top of radiator a little more. my fan does not cut on when hot. i have rewired it into the circuit and tested it, as the previous owner had it hooked through a manual switch. i look around, theres some coolant that has leaked. i assume from the overflow. i checked the fluid before i left. i was barely in the overflow reservoir so i added coolant to be at the line or about 1/2" from bottom. bike cools down a bit after 15 minutes or so and I'm able to start it up again and limp her home.

what happens when a bike over heats and shuts off? pressure can build and blow top end gaskets? but what else? i know crap expands when hot but when gets hot cause me to instantly shut down, then when cool i can ride again? besides seals and seizures.

i did realize it appears i have a replacement car type radiator cap and not an oem gt cap.

ok so i guess I'm gonna start looking deeper into my coolants system. first check the plastic pump gear, then the coolant pump and impeller

[Alan H]

60-70 at 7-8k rpm? What gear was you in?

Note specs of the early GT750J
GT 750 J 1972
Overall Length: 2,215 mm (87.2 in)
Overall Width: 865 mm (34.0 in)
Overall Height: 1,125mm (44.3 in)
Wheelbase: 1,460 mm (57.5 in)
Ground Clearance: 140 mm (5.5 in)
Dry Weight: 214 kg (470 lbs)
Engine type: Water-cooled 738 cc inline-3, 2-stroke. 67 hp/ 6,500 rpm, 7.7 kg-m/ 5,500 rpm.

Info from HERE

Peak power is 6500 rpm. If you are running at 8000 then you will knack the engine in quick time.
These are a Tourer type bike/engine, and work on torque not peak horsepower.
Unless you go to TR750 spec they should never see 8000 rpm.
WATCH THIS AND SEE
I suggest you either use higher gears or check the gearing.
6500rpm at about 110 top end gives 17mph/1000 rpm (near enough) so in top gear 5000rpm should be about 85mph.

[chance johns]

well. i might have been wrong about that hence my question mark. i am used to the idea of two strokes preferring to be run high in the rpm range. not in the red of course. ok i just looked at a rpm gauge. i stayed around 6k. typically riding between 5-7k.

[tz375]

At 80mph the motor should have been less that 5,000 rpm on normal gearing. I run mine past 8,000 at the drag strip but road motors make about 2hp at such high revs. The drop off past 6500 is significant.

The temperature numbers you quoted don't sound extreme and if it did not seize but acted as if the ignition switch was thrown, then it suggests a coil breakdown or all three condensors failing simultaneously from heat and that would be odd. Could be fuel bubbling and boiling but that usually causes misfires. I would suggest checking the filler cap too but lack of fuel usually manifests as a slowing before it dies. Check the cap for air flow though just to be sure.

I would tend to think electrical starting with the coils. I have a truck with blown head gasket/s and it runs fine but overheats and sends out clouds of white smoke but it does not just die.

It would be interesting to know what the sender unit resistances are compared to Mike's data. It might indicate if teh sender or gauge are faulty.

[Alan H]

well. i might have been wrong about that hence my question mark. i am used to the idea of two strokes preferring to be run high in the rpm range. not in the red of course. ok i just looked at a rpm gauge. i stayed around 6k. typically riding between 5-7k.

Advice is great, you can take it or leave it.
Now think. What revs did you use? Even 6K gives 102mph in top. If you don't give accurate info, how can we give accurate answers to a problem?
Even 7K is pasting the motor more than it needs or can take for any length of time.
If you like revs, get something modern and 4 stroke that likes 12K+.

The GT750J is 70s technology, but actually 60s design. That means low revs, high torque, and (relatively) low power.
You don't need (in fact can't) rev these engines to achieve more power unless you redesign the engine - like the TR750, which was not the same engine as the GT750, except externally. It could do 180mph+ at 8K. A bit fast for street methinx. Watch the video in the earlier link.

They are meant to be run at 2500 - 5000 revs and used as a GT - or Grand Tourer - that's what GT stands for. Not FAF, or Fast As ....'Anything'.
I would suggest you flush the entire cooling system, fill it with the correct cooling fluid - not just water - run it as designed with a correct rad cap set to the correct pressure and enjoy it for what it is - a 1970s two stroke touring bike.
The fan was deleted on models after the J as it wasn't needed due to the engine being VERY understressed and overcooled in any case.
If you want a 750 2 stroke and scare yourself, buy a H2, or a TR750 if you need liquid cooling in the garage.
Notice it says 'Liquid cooled' on the later barrels - not 'Water cooled'.

In case you think I'm talking out of my arse, I have run two strokes for over 40 years and know what I'm talking about.
I have a 'few' GT550s, one running, one almost resto'ed and more to do - as well as a V4 130HP reed induction Yamaha outboard.
Look at viewtopic.php?f=2&t=11247" onclick="window.open(this.href);return false; if you'd like a read.

[chance johns]

no,no. listen I'm not trying to do anything crazy or go really fast.
i'm not some dumb young kid thats stupid and trying to be reckless. although inexperienced with this machine(besides all the research and work I've done to it the last 4 years), i am taking my time, trying to be very careful, and trying to get this bike on the road and do the necessary repairs and adjustments the right way according to factory specifications.

this forum has been a great help thus far and i respect each and every one of you for giving a crap to share your knowledge and experiences and want to help another machine get back to where it was and be on the road.

the guy i bought this from is an old guy who half assed everything ,cut harnesses apart and hack jobbed the whole bike. but it was a good running bike. it worked. a younger guy(me) has been undoing his mess and redoing the wiring and electrical and mechanical for 4 years, have spent 3500$ ON TOP of what i paid for the bike and i have an un rideable bike. i have felt defeated, over-whelmed and literally depressed at times, over this bike on and off for years.

it is to my previous knowledge that two strokes typically like to be run higher in the rpm range. to burn all the oil up and not foul plugs.
now i know this isn't always the case. or probably is more frequently the case in premixed two strokes?

i am using antifreeze. never did i suggest i was using water. although i am using cheap antifreeze, it hasn't been suggested that i do otherwise.

are you suggesting i over heated because i ran it too high in the rpms?

just when i think i take a step forward it seems i take two steps back.

i love and respect this machine, i am trying my best to do everything the right way and not take shortcuts.
I've never dealt with cooling issues.
i didn't add the wrong radiator cap, the previous owner did and to tell you the truth i just noticed that it was different other day.

what benefits does an oem cap give?

[tz375]

I think Alan was reacting to to your comment about RPMs. I am guessing that teh revs are not that high, but I could be wrong.

Why don't we clear that up for once and for all. Do you know what the front and rear sprockets are? That will determine if your bike is so undergeared that it really is working too hard or not.

I don't use antifreeze in mine - just water wetter because we are not allowed to have antifreeze at teh race track, so don't worry too much about that.

For sure a radiator flush is worthwhile. I have removed large lumps of crystalized anti freeze from radiators and it's important to flush them out from time to time. Good quality antifreeze is always a good idea to minimize corrosion and to improve heat transfer.

Factory OEM cap may have a different blow off pressure, but I usually replace old OEM caps with automotive caps because they seal better and are cheap.

Interestingly enough this stage of a project is always longer than we expect as we work through niggling issues. I tell people that when they think they are 95% done, it means they have as much work to do as they have already done......

Hang in there. You are close.

[yeadon_m]

TZ <<I tell people that when they think they are 95% done, it means they have as much work to do as they have already done...... >>

True, but rather than risk someone feeling like they're only half way there, where they usually are is into the 'snagging and fettling' phase. My first time around I ended up getting pretty irked about it. These days I treat it all as interesting learning, and it helps to have the confidence that, provided its built basically OK, the fettling takes time and thought rather than big chunks of $$. They're already gone for good

My GT750A has been shiny and operative for 5 years, but it only this year that I finally feel I have it running just as I would want (endless carb tweaks) and boy it feels good as I ride along to think 'I did this'.

Cheers,
Mike

[Alan H]

If you PM me your email addy, I'll send a copy of a GT550 service manual from 1972.
There's a lot of info about the early GT750 in there and it's a worthwhile read.

Tz was right - I was horrified to see how you appeared to rev the engine, so check the sprockets and run the engine at lower revs through the gears, you'll find it just 'wafts' along rather than has to be pushed like a modern bike. But surprisingly quickly and easy riding. You won't feel like you've run a marathon after a ride on a kettle, just happy.
Something like a H1 or H2 Kawasaki or RD/YPVS Yams like higher revs as they are more highly tuned and run colder plugs which can oil up easier if run slow for lengthy periods. These old girls run cooler, so need hotter plugs which self clean better.
The Suzuki CCI is a very good system for oiling these bikes and if you have the correct plugs in, and ride 'normally' (whatever that is!) you shouldn't have any problems with oiled plugs or overheating at all. They are very under stressed and great for touring. Keep the revs below 5k and you'll see a very different kind of biking. The extra 1.5k can be used whenever, but really, they aren't that sort of bike to run at max revs all the time. Watch the video in my earlier post to see the difference with yours and a racing version.
The correct cap allows the cooling system to run at the correct pressure as commented, for racing, usually a slightly different pressure cap will allow the temperature to run slightly hotter - or cooler, depending on what that circuit needs.

[chance johns]

how can i be sure my pump is pumping? sure i can remove it and look for damage but is there anyway to tell while its on the bike?

[Coyote]

I don't know the answer to that for sure but I think something else is driven by the pump - like the tach maybe? Not sure.
I'm still trying to figure out why she cut out flat on you. An overheated motor will gradually slow as things get tighter and tighter and eventually stop or seize. I seriously doubt what happened had anything to do with the temp, even if it was a little high. That might take a while to nail down or you will have to wait till it happens again which it may never do.
I feel your pain. Like me having to tear down my fresh 550 before I even got to ride it. Sometimes you just need to walk away and regroup. Have a beer and watch TV -- although there are so many gay shows on now it's enough to make you puke.

[chance johns]

so i want to to do another test run but keeping my rpms in a decent range. assuming my gauge and sensor works properly when should i pull over and cool down? and also this bike has a hard time starting when warm or hot. but usualy starts first or second kick cold. if my petcock is partially clogged in the on position how do i test that? how do i put vacuum on the line? should i just do test rides in the reserve and prime positions to determine a partially clogged petcock? ive relined the tank and cleant the petcock with new seals.

[chance johns]

also, since the incident, i checked to see if my points were arcing incase i fried the condensors and one does not arc one does just ever so slightly and one does just a little but more. but it is to my educated assumption all are ok to run and should not be causing an issue. unless the condensors failed when hot but work fine when cooled.but from what i know thats not a characteristic of condensors. the fry or dont. im reluctant to get deep and pull the pump and soak my radiator and do all this work if this is all just an issue of riding style

[tz375]

Start with a sprocket count at both ends and let's go form there one step at a time.

[yeadon_m]

If your tachometer runs when the engine does, the water pump drive is also turning, since they are driven in common.

Its faintly possible the pump is turning but the impeller isn't - so feel the top hose just as it comes up to temp from cold and, when the T'stat opens, if all of a rush over a few seconds, the hose and rad gets hot, your pump is fine

Mike