I found a GT750 yesterday on Craigslist

[The Dark Knight]

Hello, I am new to the forum and GT750s. I'm trying to research and learn as much as possible.

I just bought a 1975 GT750 yesterday.

The wiring is toast, I mean really, burnt. I assume a redneck decided a bigger fuse would fix it, but it just burnt the wires. I will be getting a new harness and reading the wiring section here to diagnose the problem and fix it.

I would really appreciate if you could give me a link or make a list of things to check on the motorcycle. Is there anyway to check the crank case seals?

Thanks for any help and sorry to be the new guy asking all the same questions again, but it is always best to learn from others.

[Madbuffalo]

Welcome to our little neck of the woods. You will definitely find all you need to know here. Where abouts are you located? Maybe there's a member nearby who would be willing to give your new purchase a good looking over with you. For checking the crank seals some other members have done extensive posting on setting up a leak-down test otherwise there are usually some pretty strong indicators. Usually bad seals will either pressurize the transmission blowing oil out the overflow (behind the kick starter) or depressurize the transmission sucking it into the crankcase and burning it (black smoke as opposed to the normal blue haze). Injection lines are a big thing to check and make sure they're flowing properly. I've got some used harnesses off '75 buffalos if you ever need some 'pieces' let me know. Good luck with your new project.

Josh

[The Dark Knight]

Welcome to our little neck of the woods. You will definitely find all you need to know here. Where abouts are you located? Maybe there's a member nearby who would be willing to give your new purchase a good looking over with you. For checking the crank seals some other members have done extensive posting on setting up a leak-down test otherwise there are usually some pretty strong indicators. Usually bad seals will either pressurize the transmission blowing oil out the overflow (behind the kick starter) or depressurize the transmission sucking it into the crankcase and burning it (black smoke as opposed to the normal blue haze). Injection lines are a big thing to check and make sure they're flowing properly. I've got some used harnesses off '75 buffalos if you ever need some 'pieces' let me know. Good luck with your new project.

Josh

Hey Josh,

I updated my location, so if anyone is close I will buy them lunch for the visit. I'm in Fairmont WV 20 minutes from Morgantown WV and an hour and half from Pittsburgh.

I do machining for living, I'm a gunsmith and I specialize in AKs. I have a press, mill and lathe, so I was thinking it might be possible for me to replace the crank seals. Looks like this motorcycle sat for a couple years, so from everything I read that means the seals will be bad.

The main problem right now is the wiring, I need what I think is called the "main Harness", runs from the carbs to the handlebars, basically most of the wiring haha. I'm going to meter the coils, but I think it's safe to say they're bad.

I would really appreciate any help.

[Madbuffalo]

Unfortunately nowhere near me.

The rule of thumb is if a GT sits for 10 years or so the crank seals will be bad, although it really varies by bike. If you're not planning any other engine work you might wait and see if the symptoms present themselves. There's quite a lot of set up in order to rebuild the crank so it can be done but most think it's worth having it done by someone who's already got the jig set up. The going rate's about $300.

[The Dark Knight]

Unfortunately nowhere near me.

The rule of thumb is if a GT sits for 10 years or so the crank seals will be bad, although it really varies by bike. If you're not planning any other engine work you might wait and see if the symptoms present themselves. There's quite a lot of set up in order to rebuild the crank so it can be done but most think it's worth having it done by someone who's already got the jig set up. The going rate's about $300.

Ok, well maybe there is hope, I will pray hard, I just assumed the worst.

Here is my plan so far

1. Fix the wiring and coils
2. clean gas tank
3. clean carbs and see if they need rebuilt.
4. Oil the engine (I still need to read more about priming)
5. check oil injection pump operation
6. ?
7. attempt to start.

I know I missing something, I really don't want to blow this thing up, so any help is welcome. I have wanted a two stroke street bike for years.

[Madbuffalo]

Looks like you're really thinking everything out as much as you can which is good.

You might rig up a temporary fuel tank initially rather than cleaning the proper one. Goal being to get it running and pinpoint the issues before spending a lot of time in one area. I use a spare oil tank bungeed to the handlebars with a fuel line running down to the carb. That would also take the petcock out of the equation for you temporarily.

[The Dark Knight]

Looks like you're really thinking everything out as much as you can which is good.

You might rig up a temporary fuel tank initially rather than cleaning the proper one. Goal being to get it running and pinpoint the issues before spending a lot of time in one area. I use a spare oil tank bungeed to the handlebars with a fuel line running down to the carb. That would also take the petcock out of the equation for you temporarily.

Yeah, I might just do that.

[RING_DING]

Madbuffalo mentioned it already but I would add "check oil injection pump operation" to your list. There's a ton of info on this forum about how to do it. These motors are not designed to run on premix so your oil pump is your friend.

Just to echo what others have said, and having been through this twice myself, I would recommend you get it running first before diving in to major overhaul.

Good luck!

[The Dark Knight]

Madbuffalo mentioned it already but I would add "check oil injection pump operation" to your list. There's a ton of info on this forum about how to do it. These motors are not designed to run on premix so your oil pump is your friend.

Just to echo what others have said, and having been through this twice myself, I would recommend you get it running first before diving in to major overhaul.

Good luck!

Yup, I will add that to the list, thank you.

I don't plan on a restoration, just making it run good. Solid brakes, motor and recover the seat, somebody did it with what looks like a trash bag haha.

[The Dark Knight]

So, what if I ultra sonic clean the oil pump lines? Would that be a bad idea?

[RING_DING]

Haven't tried that myself, but what I did was flush them out using medicinal alcohol. It is important NOT to use compressed air because there is a check valve assembly at the banjo ends that will be damaged by compressed air. After you have flushed the lines you should check the operation of the check valves. If they are not working correctly they will let oil leak into your motor when not running (not a good thing). Search for posts on cleaning oil lines/check valves. There is one by OldJapaneseBikes that shows a picture of the ball & spring check valve assembly.

[The Dark Knight]

Haven't tried that myself, but what I did was flush them out using medicinal alcohol. It is important NOT to use compressed air because there is a check valve assembly at the banjo ends that will be damaged by compressed air. After you have flushed the lines you should check the operation of the check valves. If they are not working correctly they will let oil leak into your motor when not running (not a good thing). Search for posts on cleaning oil lines/check valves. There is one by OldJapaneseBikes that shows a picture of the ball & spring check valve assembly.

Yeah, I was reading that at 3:00 am last night, maybe I will understand it better now haha. I saw where you guys where saying no air.

I need to read some more on checking the valves.

I'm still try to find a harness, I guess a 74 will work for a 75 because they had the gear indicator too.

[RING_DING]

Can't help you with a wiring harness but I do have a pile of various leftover parts for the 1975 M models which I am keen to move. If you are needing anything else send me a Private Message and I'll see if I can help you out.

[Madbuffalo]

I'm still try to find a harness, I guess a 74 will work for a 75 because they had the gear indicator too.

The '74 harness is no different. I recommend if you are going with a whole wiring harness you search for new. I think there was somebody on Ebay offering them. If used is your only option I could probably spare one, just let me know and I'll have a look in my bin.

Josh

[Coyote]

First off, don't automatically assume the crank seals are bad. I bought a 77 GT750 that sat idle in a chicken coop for 25 years. The crank seals were fine.
For the wiring, there are 2 harnesses. Forward and rear. Paul Miller generally has both. They are pricy, but worth every penny. I bought mine from him. Supple, soft and correct.
For flushing the oil lines, buy a cheap pump type oil can. Remove the pump from the bike. Remove the six bolts that retain the banjo fittings from the block. Now fill your cheapo oil can with plain old rubbing alcohol. If necessary, grind the tip of the oil can so it fits in the tiny hole in the oil line manifold NO COMPRESSED AIR! It will destroy the check valves. Just pump alcohol through the lines from the manifold end several times. Now stick all six banjo fittings (one at a time) in your mouth and try to blow back through them.. No air should pass in that direction. If it does, the check valve is stuck open and needs further attention.
As a final test, dump the alcohol out of the can and fill it with 2 stroke injection oil. Pump all six lines full of the oil and hang the manifold on a nail. Place a clean rag on the floor beneath it. Wait 24 hours and check. All lines should still be full and none has leaked out onto the rag. If one or more has drained out, that check valve is still stuck open.

2 questions. How many miles on the clock? Was the bike stored indoors or out?