90...75...90...115...

[Go Mifune]

One set of plugs isn't working properly, in my opinion...I do not know. No one's told me that my situation is unusual as of yet.

You have no better idea than I do that the valves aren't closing. Maybe they are...maybe they're not. I've had the bike less than 24 hours.

The carburetors are flowing just fine. If I cover a bellmouth it blows fuel all over the place. The engine isn't providing enough suction to pull the fuel through the jets.

I've listed the reasons why it won't run: low compression and a possible ignition glitch. I never asked "why won't it run".

It's a 30 year old machine. I'm looking at rebuilding the top end because:

1...it may be necessary.

2...it can't hurt anything.

Check your points and set the timing properly.

[rustybronco]

It's a 30 year old machine. I'm looking at rebuilding the top end because:

1...it may be necessary.

2...it can't hurt anything.

then overhaul it.
why?

3. it's yours
4. you can learn from it.

[Steve]

You have no better idea than I do that the valves aren't closing. Maybe they are...maybe they're not. I've had the bike less than 24 hours. ...
I've listed the reasons why it won't run: low compression and a possible ignition glitch. I never asked "why won't it run".

OK so you have had this bike for 24 hours. What is your experience wrenching on other bikes?

Yes, you have listed a couple of items that might make it not run, includind low compression. You know what one of the primary causes of low compression is? Mis-adjusted valves. But...since that can't possibly be the problem, go ahead and rebuild the engine. Don't forget to adjust the valves on the new rebuild.

Oh, and the ignition "problem"? As mentioned by KiwiGS, there is power at all times (when the key is on) to both coils. The points will interrupt the ground, causing the coil to fire. Apparently you have been lucky enough to have the engine stop with the same set of points open any time you have checked it. By connecting or disconnecting a plug, you are completing the circuit and yes, it will fire. There is no problem there, but you may as well replace the points and coil, along with the (probably unnecessary) rebuild.

Sorry if any of this sounds harsh, but we have offered suggestions and you blast them down, then admit you haven't even checked them, so we, as a group, will be moderately offended.

Since you don't know the history of the bike, does it not make sense to check everything? Compression check is a good thing to do. Valve check is supposed to be done every 3000 miles, according to the service manual. When do you suppose they were last done? Points should also be checked every 3000 miles. Again, when were they last done? Have you checked the battery condition and the ability of the bike to charge the battery? Sorry, I guess you can only do so much in 24 hours.

Don't forget that the carbs need cleaning, so you had better buy some new ones.

One other thing:

It's a 30 year old machine. I'm looking at rebuilding the top end because:
1...it may be necessary.
2...it can't hurt anything.

1...it probably is NOT necessary
2...the only thing it will hurt is your wallet.

Checking the valves will cost about $10-15 for a gasket, less than $10 for feeler gauges and about an hour of your time. If you need to change any shims, they are less than $5 at Z1 Enterprises.

Rebuilding the top end will cost several hundred dollars for the parts, about $100 for a gasket kit and you will still need feeler gauges, and it will take considerably longer than an hour.

Why not just check the easy stuff and do the harder stuff if, and when, it becomes necessary?

.

[mcloud]

then overhaul it.
why?

3. it's yours
4. you can learn from it.

5. it's fun
6. if you don't, your wife will list chores to do

[flyingace]

One set of plugs isn't working properly, in my opinion...I do not know. No one's told me that my situation is unusual as of yet.

You have no better idea than I do that the valves aren't closing. Maybe they are...maybe they're not. I've had the bike less than 24 hours.

The carburetors are flowing just fine. If I cover a bellmouth it blows fuel all over the place. The engine isn't providing enough suction to pull the fuel through the jets.

I've listed the reasons why it won't run: low compression and a possible ignition glitch. I never asked "why won't it run".

It's a 30 year old machine. I'm looking at rebuilding the top end because:

1...it may be necessary.

2...it can't hurt anything.

1. That compression won't keep the bike from running. That's pretty much agreed on.
2. Sounds like a short in the ignition system.

My advice is to resolve the electrical problem first and see how the bike runs. You can't determine compression with a cold engine, so you can't assume it needs a rebuild until it runs. Remember, these rebuild kits probably cost more than your bike is worth. And if money is no object, just buy a new bike.

Also remember to be patient with people that are trying to help you.

[Griffin]

Last year I picked up a GS750 that had been sitting for about eight years. It had electrical problems, but the previous owner (who was giving me the bike for free, so had no reason to lie) said the engine ran great, but the battery wouldn't charge and he'd lose spark occasionally, so he parked it. The first compression check I did showed 90-100 psi across the board in all cylinders. The engine was cold, the rings were rusty, and it'd been a long time since the crank had seen anything over 10 rpms. It didn't worry me a bit. Once I got the bike running, I didn't worry about compression, because everything was running fine.

After seeing this thread, I checked compression on cylinders 1 and 4 last night for giggles. Both were at 140-145 cold. That reading would probably be 10-15 lbs higher hot.

I know that everybody reads the 128 psi minimum compression then rebuild thing in the manual, and then they flip out when a new bike doesn't make it there. Truth is it may never get there, but usually after running for a while, breaking the rings free and getting them reseated into the cylinders, they usually do.

Your bike has a lot of other issues that are easier and much cheaper to address than a top end rebuild to raise compression. That may or may not fix itself once you get the bike running, but the bike won't run without sorting out carbs and ignition first.

There's no point sending the dog into the foxhole if the fox ain't there.

[SqDancerLynn1]

Getting shocked ? First thing - Check the engine ground cables, Check the wiring for the points, Replace the point condensers, (check the little insulator washers on the points) Replace the points if needed.
Then go on to the carbs

[makenzie71]

But...since that can't possibly be the problem...

I never said the valves couldn't be the problem. I said it isn't likely because a wet test wouldn't have shown such a considerable jump in compression. And don't assume I wont be checking the valves. I'm more concerned with the constant power to the plugs.

[makenzie71]

...and I'll point out that reasons #3~6 are also valid. I'll likely end up rebuilding this engine because I want to and little else.

[TheCafeKid]

I never said the valves couldn't be the problem. I said it isn't likely because a wet test wouldn't have shown such a considerable jump in compression. And don't assume I wont be checking the valves. I'm more concerned with the constant power to the plugs.

Ok...so start there! Find out WHY its not getting power to 1and4. There's a reason for it, and none of us can tell you what it is since none of us are there. Everyone here has responded with S.O.P of getting one of these GSs running CORRECTLY, especially one that has been sitting forever. 30 year old machine...SO? I have MANY 30 or nearly 30 year old bikes sitting around here, not one of them NEEDED the top end rebuilt. (ok i admit i have a 750 8v motor as a spare thats burning a lil oil, but it still ran like a bat out of hell when it was in the bike i had it in, and I have a 16v 750 motor that needs a top end, but it was a POS when it left the factory in 80 :P ) Find your spark issue, fix it. And while you're at it, check your charging system. At LEAST check the valves, you're a fool not to. They need checked every 3k miles, anyway. If the motor turns over and you see NO BLUE SMOKE, and the valves arent burnt from neglect, and are adjusted, you do NOT need a top end rebuild. After you check your valves, clean your carbs, assure there are NO air leaks in the airbox and intake boots. Check that the head bolts are torqued down per spec. Then you should be golden. Refusing to do these things, you might as well park it back out in the yard with a for sale sign on it, or maybe it DOES make a nice lawn ornament...

[Griffin]

I never said the valves couldn't be the problem. I said it isn't likely because a wet test wouldn't have shown such a considerable jump in compression. And don't assume I wont be checking the valves. I'm more concerned with the constant power to the plugs.

If the rings are bad, a wet test will increase compression 50 psi or more, unless the cylinder is so badly scored all the oil immediately flows down around the rings. That certainly isn't the case, because you have around 100 psi or so compression to begin with.

10-20 psi isn't jack. You'll find cold compression tests will vary by 10 psi from day to day on the same cylinder just because of different ambient conditions.

[Steve]

Ok...so start there! Find out WHY its not getting power to 1and4.

Ahhh...but it is getting power to 1&4...

it's getting strong spark on all four plugs in correct time.

The constant sparking is puzzling, though.
The coil only produces a spark when the magnetic field collapses, and that happens when current is interrupted (the points open). Once the field is collapsed, it has to build up again with current flow, then get inerrupted before it will spark again.

[TheCafeKid]

Ahhh...but it is getting power to 1&4...



The constant sparking is puzzling, though.
The coil only produces a spark when the magnetic field collapses, and that happens when current is interrupted (the points open). Once the field is collapsed, it has to build up again with current flow, then get inerrupted before it will spark again.

Maybe the points and condensor system is poop? maybe the coil is poop? Maybe the plugs and wires are poop? Wont know untill you TEST..roit boys?

[greg78gs750]

me-ow. testy group, eh? really, though, these guys know their stuff....and want to help. figure out that electrical prob first. then clean/seal those carbs. and check the valves. all the standard stuff. that is, if you want to get it running before you tear into the rebuild. wish i could be more help, but i gotta shop for another bike for my wife....

greg

[rustybronco]

The constant sparking is puzzling, though.

Very rough points or rough points just touching.
or points that are open and a condenser that is grounding intermittently