can't tach over 4500

[Anonymous]

I'm new to the motorcycle world, so bear with me. I did a search here and found a few issues similar to mine but it seems that I may have a different issue or multiple issues here.

Here are the facts.
It's a '77 GS750 with electronic ignition and a 4into1 header. I bought it from my brother-in-law the beginning of the year. He says it was running last summer. It wasn't running when I bought it, but I replaced an ignition coil and the battery and it'll start now. I also added about 2.5 qts of oil to fill it up. I don't know what other info is necessary to diagnose this so I'll relay the symptoms now.

It starts only if I give it a little gas, even with the choke on full. If I let off of the throtle completely, it dies. Until it warms up, I have to be really gradual with the throtle or it dies. Once it warms up, I can't get it to rev above 4500 rpms.

My brother-in-law says he cleaned the carbs out a year or two ago, but the bike's been sitting for at least a year. The registration and inspection are dated 06/01. I've never dismantled a carbeurator before and am a little squeamish about doing so. Budget's tight so I'll do what I have to, to save the $50/hr labor by doing it myself. The Clymer manual doesn't tell me much, so I'm coming to the experts.

Let me know what other info is necessary to diagnose. It's my first bike, so I'm anxious to get her on the road.

Thanks in advance.

 

[KrazyKarl02]

might be spark plugs

might be spark plugs

Try changing your sparl plugs, I know I used to have a 1000 that would foul plugs left and right, particularly if left to set for a while

 

[Anonymous]

also run some gas treatment in the tank and try to ride it a bit if possible-sometimes it will help clear a gummy situation if it is not too serious. But dont get cheap, go for those plugs. Keep the gang informed. There are sa lot of folks aroung here with 750 experienc.

 

[bybowtie]

plugs are cheap.. I paid 2.00 a piece... also check your ariflow.. seems as if you have som resriction.. I had a turbo car do that after we custom fabbed a ne intake hose.. turned out that as we cranked up the boost it was sucking the hose closed thus preventing it from revving.also flush the tank, the gas may have spoiled....

 

[Anonymous]

Unfortunately, I'm not blessed with a whole lot of free time to mess around with the bike trying to figure out what's going on. I was able to spend about 30 minutes with it last Sunday and was able to determine that even with the replacement coil, It's still only running on cylinders 2 & 3. I don't yet have the right size spark plug wrench to check the plugs. The funny thing is that last Saturday, it was running perfectly for a few minutes. I was reving the engine a bit and was just about ready to take it for a spin when all of a sudden, 1 & 4 quit on me. Any thoughts?

 

[Anonymous]

try turning your idle screw to increase RPM's, maybe it's just too far back

Dave W

 

[Dark Jedi]

Unfortunately, I'm not blessed with a whole lot of free time to mess around with the bike trying to figure out what's going on. I was able to spend about 30 minutes with it last Sunday and was able to determine that even with the replacement coil, It's still only running on cylinders 2 & 3. I don't yet have the right size spark plug wrench to check the plugs. The funny thing is that last Saturday, it was running perfectly for a few minutes. I was reving the engine a bit and was just about ready to take it for a spin when all of a sudden, 1 & 4 quit on me. Any thoughts?

Could be that the coil wires are loose. You said electronic ignition, so not condensers to worry with.

 

[earlfor]

Usually, abrupt failure is an electrical problem. That it quit upon reving the engine could be nothing more than a loose or dirty connector and the increased vibration caused a loss of continuity. I would also take a multimeter and check the coils for resistance between the + and - terminals and between the plug caps.

Earl

Unfortunately, I'm not blessed with a whole lot of free time to mess around with the bike trying to figure out what's going on. I was able to spend about 30 minutes with it last Sunday and was able to determine that even with the replacement coil, It's still only running on cylinders 2 & 3. I don't yet have the right size spark plug wrench to check the plugs. The funny thing is that last Saturday, it was running perfectly for a few minutes. I was reving the engine a bit and was just about ready to take it for a spin when all of a sudden, 1 & 4 quit on me. Any thoughts?

 

[Anonymous]

Okay I checked the resistance on the coils between the + and - and both coils were about 4.7 ohms. Could you explain again how to check the resistance at the caps?

I found some time to mess with it last evening and found that 1 and 4 are sparking when the starter button is initially pressed and when the kill switch is hit. As I understand it, only the orange and white wire goes from the starter button and kill switch to the coils. The fact that one coil is fireing consistently but not the other has me a bit baffled. And why would the 1-4 coil fire when I hit the kill switch? Could there be a short somewhere else?

My only consolation is that south eastern Pennsylvania has been having a very wet spring. There have been only about half a dozen days this month where there wasn't at least threat of some serious rain.

I'm hoping that I'll have this thing running right by the time the weather decides to cooperate.

 

[Anonymous]

Okay, I just read something about wax coming out of the coil signifying that the coil is overheating. I'm guessing that this is a bad thing. The used replacement coil that I put on had wax dripping out of it last night. So do I need yet another replacement coil? Should I go with a pair of after market coils? Any suggestions as to what kind/brand (Dyna vs Accel)? What is causing this coil to overheat? What do I do to prevent another coil from frying?

 

[bybowtie]

you have a voltage problem

 

[Anonymous]

Since I'm a newbie, could you explain a little more? How do I find the problem and how do I correct it?

 

[bybowtie]

What kind a voltage are you having at the battery? what is it at the coil? Have you checked all the grounds? rev the bike up a bit and see waht the voltage does... Earl is the man and if he get s to read this he can elaborate more... but check these things... voltage at idle should be 12.3-13.3 +/- upper rpms 13.8 -14.5 +/- Any higher and you have a problem
Coil voltage at ignition on should be the same as the batter +/- a tenth or so..

 

[Anonymous]

So having higher voltage at the coil than I do at the battery would cause the coil to overheat and drip wax all over the place? What would be causing the voltage at the coil to be higher? My understanding is that the orange wire at the coil comes directly from the ignition switch without passing "Go" or collecting $200.

 

[bybowtie]

what it sounds like is that your coils are overheating. And since there are no real thermal causes that leads to overchatging due to high electrical current.....Like I said though you may want to pm earl on this... He is the man when it comes to these bikes..

 

[earlfor]

Hmmm. 4.7 ohms on the coils between the + and - terminals is fine. Anything between 3 and 5 is fine. Its commonly accepted that 3 ohm coils are used with the factory electronic ignitions and 5 ohm coils are used with points ignitions, but my 1150 has a factory electronic ignition and the factory service manual specs 2 to 5 ohm coils as being within usable range, so I think it would be the same for your bike. A 1977 750 would have been manufactured with a points ignition system, so I am guessing you probably have a Dyna S electronic system. The Dyna can use 3 to 5 ohm coils.

You can check resistance between the spark plug boots/caps on the plug wires. Set the multi meter to the 200K setting on the ohm side. Place one lead of the tester on the metal point inside the spark plug cap. Place the other lead on the metal point inside the other spark plug cap. (it doesnt matter which is which) Resistance should be between 30K-40K.
Just for the heck of it, would you check both coils this way. I am curious if they have the same values. (another piece of the puzzle maybe)

On your ignition. It doesnt make sense to me that 2 and 3 are sparking and working with the ignition on and 1 and 4 only work when the starter is engaged and then only work with the kill switch. Are we describing this right? The kill switch when switched to on means the ignition circuit is in the on/continuity state. Switching the kill switch to off means continuity on the ignition circuit is lost.

The orange and white wires that attach to the coil + terminals are 12 volt + input power to the coils. Both coils orange and white wire plug into the same double connector for power supply. This single wire with the double connector is fed from the Kill switch. Turning the kill switch to the off position can only result in killing the ignition to all four cylinders (normal), or to none of them ( if not functional). I would check to see the the + lead supplying the coils is actually coming from the single orange and white wire from the kill switch as it should. The plug/connector you will be looking for will be found inside the headlight shell and measures aprox 1/2 x 1/2 x 1". It will have three leads. An orange and white (power supply to coils), a green/yellow (goes to starter relay) and an orange, comes from ignition switch.
(Let me know what you find on your wiring....if its correct or not)

What is the voltage at your coil + terminals? (Ground the neg lead of the multimeter on the engine case). What is the voltage at the battery terminals? (both test leads on the battery)

I dont have a conclusion yet as to why you would be melting a coil
Usually, running coils of lower resistance results in the coils operating at higher temepratures, but youre not doing that, so I'm puzzled for the moment.

Considering the symptoms, I am still convinced you have an electrical problem rather than a carb problem.

Earl

Okay I checked the resistance on the coils between the + and - and both coils were about 4.7 ohms. Could you explain again how to check the resistance at the caps?

I found some time to mess with it last evening and found that 1 and 4 are sparking when the starter button is initially pressed and when the kill switch is hit. As I understand it, only the orange and white wire goes from the starter button and kill switch to the coils. The fact that one coil is fireing consistently but not the other has me a bit baffled. And why would the 1-4 coil fire when I hit the kill switch? Could there be a short somewhere else?

My only consolation is that south eastern Pennsylvania has been having a very wet spring. There have been only about half a dozen days this month where there wasn't at least threat of some serious rain.

I'm hoping that I'll have this thing running right by the time the weather decides to cooperate.

 

[Anonymous]

Thanks for the input, Earl. I just got two more used coils on eBay and I want to get this issue worked out before I put them on so I don't end up ruining another coil. I'll check the resistance at the end caps and then check the wiring in the headlight shell.

It doesn't make any sense to me either why 1 & 4 would be firing when the starter is engaged or when the kill switch is hit. When they do fire, they only fire once. I had the plugs out on 1 & 4 when I experienced this and they had an orange spark.

Another funny thing happened Saturday. I pulled the cap off of the plug on 3 while the engine was running and it started arcing accross the engine block. You'd think that if 1 & 4 aren't firing, pulling the cap off of 3 would cause the engine to stall out completely. Instead, it sounded like 1 & 4 were firing along with 2. I got quite a jolt out of the whole situation 8O and am even more confused now then I was before :? . Anyone have any ideas?

I'm with Earl being convinced it's electrical. When I pull the plugs out of 1 & 4, they're nice and wet. I'm getting juice to those cylinders, just no spark.

 

[bybowtie]

grounding is the problem... try to add a ground to the mount on the coil....where the screw mounts to the frame.. make sure you are resting on the coil part not the frame side run it back to the battery

 

[bybowtie]

grounding is the problem... try to add a ground to the mount on the coil....where the screw mounts to the frame.. make sure you are resting on the coil part not the frame side run it back to the battery on other thing check to see if the coils are hooked up right I had an incident when I pulled them off and it was quite some time before I goot back to them.. well I had moved the wires all around and were no longer anywhere near where they started.. well to make a long story short I happened to hook 2,3 to 1,4 coil and vice versa maybe you have a similar issue ????? I dont know should probably just listen to EARL!!! :lol:

 

[Anonymous]

:?:
Okay, my understanding is that the coils are grounded thus completing the circuit and igniting the spark. If the coil for 1 & 4 is not connected properly to the electronic ignition system, the electronic ignition will not complete the circuit and no spark will happen. My question is, what is happening when the starter is engaged and when the kill switch is hit that completes the circuit at those times that doesn't do it any other time?

And why did having the cap off of cylinder 3, and grounded to the engine block, make 1 & 4 fire? Also, where do I start to look for the bad connection between the coil's ground and the ignition circuitry?

... Okay, wait...

I don't have the wiring diagram in front of me but if I remember correctly, the kill switch breaks the flow of current from the battery to the positive terminals of the ignition coils by grounding it? ... I don't know where my mind was going there, but perhaps someone else does and can complete my thought.