1979 GS750E repair advice needed

[fgrobe]

I have a 1979 GS750E. It does not like to run. I have had it to 3 places for repair and still am not satisfied with how it runs. The last place told me it was just cold blooded. I can work on cars but this is my first motorcycle and I have little knowledge about repairing one. Any ideas as to what the problem is would be greatly appreciated. I have hardly ridden it the past 3 years I have owned it since I am afraid I will get stranded somewhere. Prior to my ownership it was driven a lot during the summer. I am tired of dumping money into the bike. The symptoms, etc. I will list have been the same for the past 3 years. The bike has 20,000 miles on it.

Symptoms At Startup:
*Will not start if it sits for about a week. If it sits I have to constantly choke it, running the battery dead, charge the battery, run it half dead, then it starts.
*After it starts, you can not rev the engine for about 5 minutes or it will die.
*If I let it set and run with the choke on it will run at about 1500RPM for a while and then backfire and smoke for a while, usually black, then it will rev up to 3000RPM. This takes 15+ minutes.

Drivability symptoms:
*If I do not let it "warm up" until it goes to the 3000RPM and drive it, it wants to die when giving it throttle.
*If I can get it going down the road it will either have no power, running smoothly, or will be backfiring and jerky to ride.
*It seems like it fouls the plugs or is running on 2 cylinders because when it has no power, if I just about redline it for about 2 blocks it will backfire a few times and then I have twice the power. This seems to happen about every time I ride it.
*At highway speeds on flat road I can not keep it at steady speed, with steady throttle rpm's drop sometimes

Other symptoms/things I have noticed:
*Even after warmed up and running "good", if I give it quick throttle it will instantly die.
*The plugs get fouled a lot. They are extremely black, almost like a carbon buildup. Sometimes they are flooded with gas. After the plugs are replaced it runs better for about 3 times of riding.
*One side of the exhaust is missing about 4 inches of baffle because the muffler rusted out in the back. Both mufflers are black where the exhaust comes out.
*If air filter is removed completely it seems to run a little better
*I hooked a timing light to the wires to check for spark and the two inside must be linked and the two outside must be linked. I can not remember which, but I think the inside cylinders sparked a lot faster than the outside ones at startup. If I remember correctly the outside cylinders sparked a lot more intermittently.
*I also noticed that I have vacuum type lines that look like they come from the carbs just hanging down over the starter cover, prossibly for overflow of gas?
Repairs done to engine and what I have been told by the mechanics:
2 Years Ago:
*Carbs cleaned and kits put in because two were leaking
*Carbs synchronized
*New Air Filter - old one was flaking apart
*New plugs
*Check points and condenser - They are fine
*Oil and filter change
Last year:
*Carbs cleaned and synchronized -everything looked good
*Air filter looked good
*Not getting enough air - took some rubber thing off the air filter box to allow more air flow - was told it was to keep dust out but not needed
*New valve cover gasket
*Valves adjusted
*checked timing of engine - fine
*Points and condenser - good condition
*Choke sticks on carb - cleaned and have to manually push it down sometimes, have to keep an eye on it
*New plugs
*Oil and filter change

I either run premium unleaded or regular unleaded gasoline because I was told ethanol was not good for the seals in the carburetors. I have tried seafoam and it did not help. I always put fuel stabilizer in at each fill because I was told to do so by a mechanic. I do not know much about points, condenser, or the coils on a motorcycle, but I was thinking that one of these may be my problem. Any help would be greatly appreciated. Thank you. Fred.

 

[almarconi]

Hey Fred,
Sounds like your bike has multiple issues. Don't despair the bike can be fixed. First thing I would do is purchase a repair manual for your bike. Trying to diagnose and correct a multitude of problems at once can confusing. Sounds like you have some electrical issues and carburetor/fuel issues. Begin your diagnosis by checking each system to make sure it is functioning properly. The repair manual will have step by step procedures that you can follow. If you can work on cars then you can apply your mechanical skills to bikes, they are not that much different to work on.

From what you described your choke circuit in the carbs is probably clogged. The carbs have lots of tiny orifices in them that can get clogged. Take the carbs off, disassemble and clean them. There is a step by step article on this site that explains how to do it. Its also a good idea to replace the o-rings inside of the carbs. If your plugs are fouled chances are your spark is weak. Check the voltage to the coils and check the plug wires. Make sure your air filter is clean, that the carb holders are tight (with no cracks) and that the airbox is sealed and intact. Running a bike without an air filter or air box can cause all kinds of driveability issues. Also make sure your exhaust is installed and does not leak. You should also check your valves to make sure they are adjusted properly. Try doing the basic maintenance on your bike and then see how it runs. Don't get discouraged, just look at it as a learning experience. This forum is an excellent resource with alot of members that are willing to help you out.
Good luck and have fun!

 

[earlfor]

I used to have a 79 750E that I put together as a restoration project.
The bike has 20k miles on it and there is no telling what the previous owner did to it. I would start at step one and not assume he did anything correctly.

These old bikes are older than most of the mech's in the bike shops, and few know/understand them well, if at all. I've had shops screw up a bike as bad as any back yard wannabe mech.

20K miles, and I will bet the bike has never had a valve adjustment. I would check and adjust valves, then do a compression check and also (while I was there with the valves, check the valve timing. It would not be the first time a PO had a bike apart and put it back together with the cam chain a tooth off.

Unless you have the bike rejetted for pod filters, you must use the airbox in stock configuration. It will not run well otherwise.

Considering the symptoms, I think your carb bodies need dipping and the spray bars cleaned out, also the choke emulsion tubes. I will also guess that it still has the original intake "O" rings and those are hard a rock by now instead of pliable and sealing as they should. You're losing vacuum with old intake "O" rings and fuel draw will be for crap. That also destroys mixture levels, so it aint a gonna run worth a diddy poo poo.

Next, I would check the voltage levels to the coils. If the wiring harness electrical connectors are clean, voltage drop compared to battery voltage should be minimal. If you're getting less than 12v going to the coil terminals, you have a electrical problem.

Also, when you're checking timing, are you checking it on the #1 or #4 cylinder (ignition timing wise they are the same) at 3K rpm and aligning the
|F4 ? The marks for the 1,4 cylinders you will see on the centrifugal advance are |T |F1 |F4. T is top dead center.
F1 is idle, 1000 rpm minimal advance timing. |F4 is full advance timing mark at 3k rpm.

There is a half breaker plate for setting the timing for the 2,3 cylinders. For full advance timing on it, you need to set the right side points to break on the | F3. Basically, you must time 1,4 by rotating the breaker plate and then time 2,3 by moving the independent half breaker plate which does not cause the main breaker plate to change setting on the 1.4 side.

We can fix this.

Earl

 

[fgrobe]

Thank You for your help

Thank You for your help

I will try your suggestions and see how I turn out. I did find a wire missing some insulation inside the points case leading to the points today. I am going to work on this bike more when I have time. Thank you for the advice on what to check. I will let you know how I turn out.
Thanks,
Fred

 

[earlfor]

That could be a possible fault also. The white lead (to the left pointset cyl 1,4) and the black lead (to the right pointset cyl 2,3) are both positive. Worn insulation could result in intermittent grounding of their respective pointsets which would play hell with the bike running properly.

Where are you?

Earl

I will try your suggestions and see how I turn out. I did find a wire missing some insulation inside the points case leading to the points today. I am going to work on this bike more when I have time. Thank you for the advice on what to check. I will let you know how I turn out.
Thanks,
Fred