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Inconsistent Miss on 80 GS750L

  • Thread starter Thread starter 80gs750forme
  • Start date Start date
8

80gs750forme

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I have an 80GS750L that has a miss in the 2500 to 300 RPM range. Before and after that it runs great. Carbs are clean, have good compression, new battery and voltage regulator as well. Pretty sure it is something electrical. The miss varies between cylinders as well. I am about to give up on her....

If this is a repeat thread, I apologize...have mercy.
 
"I am about to give up on her...."

might be electrical- have you checked charging output? More likely, carbs transitioning into mid range- how did you clean them?

How do know the miss varies between cylinders?
 
Thanks for the reply. Bike is charging good. Two and three were missing and I got a new coil. They are working. Now number four isn't firing all the time. Pipe coming off the number four is cooler than the others. Could it be the ignition module? How do I test it?

Previous owner cleaned the carbs. I had them off to fix a cracked diaphragm on number one cylinder. Carbs are super clean and numbers 1 and 4 were firing prior to the changed 2 and 3 coil change.
 
Understand that this is the lost spark (or wasted spark) system. One ignition coil fires two plugs like 1 and 4 together - so if 1 plug is firing its mate #4 should be too, unless you have bad spark plug i.e., not leaping gap. A faulty ignitor would fire neither.

Carbs can appear super clean, but you can't see the tiny passages that aid the transition into mid range. Once stuff gets sucked up into these passages , you got problems.
 
How was the diaphragm repaired? Ive never heard of that before.

I recently had a spark plug cap go bad- resistor in cap was bad, plus all 4 wire connections were corroded.
 
Should've said I replaced the diaphragm. I wish you could fix them. This is why I am at loss guru. I am sure it is getting gas because I get a raw gas smell (definitely not leaking anywhere) when it is running. I pulled the plugs and checked for spark and there is spark, but it seems weak. It seems to be jumping back and fourth between the 1 and 4 cylinder not firing correctly.
 
I know with my old van, it would hesitate when I accelerated. Turned out to be a bad coil pack. Replaced it and worked fine. The bike does the same thing. Misses under load when accelerating and always in the same range (2500-3000). I will pull the carbs off and blow some carb cleaner and air through them and see if that helps.
 
80gs750forme said:
I know with my old van, it would hesitate when I accelerated. Turned out to be a bad coil pack. Replaced it and worked fine. The bike does the same thing. Misses under load when accelerating and always in the same range (2500-3000). I will pull the carbs off and blow some carb cleaner and air through them and see if that helps.
Click to expand...

If you are going to pull the carbs off, go ahead a rebuild them right (soaked. new orings etc. just because the PO had them cleaned doesn't mean it was done right) Blowing some some carb cleaner and air through just ain't gonna cut it. Why take them off for a second time?
 
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As you can tell, I am really procrastinating cleaning and rebuilding carbs. I can do it, don't like it, but I can do it. When I had them off the it looked like it had new float needles and jets. Here's another stupid question... The petcock on this thing is vacuum operated if I am not mistaken. Could it be not getting enough gas? This is why I like working on v-twins. They don't have four carbs...
 
"When I had them off the it looked like it had new float needles and jets. "

Someone might have substituted inferior aftermarket carb parts to give the shiny clean look- best to find out . Cleaning them properly yourself is only way to be sure that you have mikuni jets and unclogged passages.
Singles are even easier than twins!

petcock clogged problem would be real noticeable at high loading.
 
80gs750forme said:
As you can tell, I am really procrastinating cleaning and rebuilding carbs. I can do it, don't like it, but I can do it. When I had them off the it looked like it had new float needles and jets. Here's another stupid question... The petcock on this thing is vacuum operated if I am not mistaken. Could it be not getting enough gas? This is why I like working on v-twins. They don't have four carbs...
Click to expand...

It doesn't have a in-line fuel filter on it does it?
 
No MrBill, it does not. No in line filter. I have not pulled the petcock to see if it is dirty either. It seems worse when you really get on it. If I accelerate at a lower pace it seems better. But then again...who wants to do it that way.
 
I cleaned my spark plugs and a little missing went missing.

I've fixed a lot of carb problems with new plugs too.
 
have you checked how much voltage you are getting to the coils at the orange/white wire connection? could be you are getting the dreaded voltage drop. if everything is good, you should see about .3-.5vdc less than what the battery is reading when you turn the key on. and it could also be bad plug caps on cylinders 1 & 4.
 
If the plugs are rusty and old it might be time for a new set.
How about the lead wires from the coils? I put new ones on and it no longer stumbles in the rain.
 
New plugs, carbs have been cleaned (by me) and a new coil. I am checking all of the connections as well. To add insult to injury I went out to warm it up and came out to find a puddle of oil from the head gasket. Waiting on the new gasket. Top end off so my miss is taking a back seat.
 
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