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Stuttering at 5k rpms

  • Thread starter Thread starter drivera84
  • Start date Start date
D

drivera84

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Hi guys,

My 78 gs 750 usualy runs pretty well. Every now and then she beggins to stutter at around 5k rpms. It keeps running bad for a couple of days and then with nothing done fixes herself. I usually add gas at the same gas station.

Carb orings have been replaced, boots too. Airbox seems tight. Sometimes it solis sparkplug on cylinder 1.
valve clearances have also been checked. It has a newer model petcock.

What do you think?

Thanks
 
When mine did that it was because it was too rich. It sounds like you are fowling plugs and they are eventually clearing themselves as long as you don't try to push it through the stutter barrier. If you haven't done a complete strip down and 24hr. soak in carburetor dip it might be time to do so, replace O-rings gaskets, check float height and carburetor boots and replace the boot O-rings.
 
what's your air filter situation? pods? factory air box with single filter? any chance it or they are getting wet? I know if I put too much filter oil on mine it will splutter at around 5k, same if I go to the car wash and get the pods wet ( I do bag them but it happens sometimes).
 
It's possible that the ignition advance mechanism is partially seized or getting sticky. I've seen it on late 70's GS and had a 78 myself that did that. Usually, you can just remove the whole points plate, remove the bolt [10mm] and the whole advance comes off and the points will be close enough when you put it back. It's nice to disassemble it and clean the surfaces and all that - plus a dab of grease, but you can also just spray it with WD40 or whatever and twist it around until it seems good. If it's free and snappy you're fine.

The pointless ones are perhaps less prone to rust, but maybe I just haven't seen one stuck yet. It's a three screws operation to see if it's happy in there or not, so worth a look. And about 4500 is where the stuttering starts if it's sticking.

I won't do it, but I've seen motorcycles bought for larcenous prices because they just wouldn't run right past about 4 or 5, and that was all that was wrong.
 
Hi John,

I havnt checked this.

Could you be a little more explicit in the process, If you have pictures it would be awesome. Im not very knowledgable in this, but I am learning.

thanks


John Park said:
It's possible that the ignition advance mechanism is partially seized or getting sticky. I've seen it on late 70's GS and had a 78 myself that did that. Usually, you can just remove the whole points plate, remove the bolt [10mm] and the whole advance comes off and the points will be close enough when you put it back. It's nice to disassemble it and clean the surfaces and all that - plus a dab of grease, but you can also just spray it with WD40 or whatever and twist it around until it seems good. If it's free and snappy you're fine.

The pointless ones are perhaps less prone to rust, but maybe I just haven't seen one stuck yet. It's a three screws operation to see if it's happy in there or not, so worth a look. And about 4500 is where the stuttering starts if it's sticking.

I won't do it, but I've seen motorcycles bought for larcenous prices because they just wouldn't run right past about 4 or 5, and that was all that was wrong.
Click to expand...
 
I checked the airbox, its stock airbox with single filter. it was a little oily I cleaned it up, filter looked clean so I just put it back in. Same problem.

Yesterday the sparkplug on cylinder 1 was very dirty barely any spark in the plug. I changed it and the bike ran like a beast. I am beginning to thing it is the coils and cables like someone mentioned above. any ideas?

Just a side note: I had changed the sparkplugs no more than 6 months ago so they should all still be good.
 
Try adjusting that carburetor so it isn't so rich before guessing what else might be wrong with it. If it continues to stutter run some other tests.
 
Most carburetion problems are caused by the ignition. Before you go changing mixtures and such, at least make sure your points are adjusted and working properly, the condensers are good, and that all the connections in the ignition system are clean.

There's a ton of info about it here:
http://members.dslextreme.com/users/bikecliff/
 
The carburetor with the dirty plug that you changed out should be the same color as the other three. It sounds like that one carburetor is rich and fowling the plug with too much fuel when you try to go passed 5,000 rpm. What you described as running badly for a few days and fixing itself is typical of a fowled plug that eventually burns itself clean enough to spark again. It could be other things but running better after changing the plug would make me want to find out why and seems to point to that cylinder. I would change all the plugs run the heck out of it hit the kill switch and coast to a stop somewhere where you could pull all the plugs and compare the color. That will tell you something about how each cylinder is firing.
 
Agreed with OldVet66. Something is causing that one cylinder to foul the plug. The question is what? Is it fuel fouled or oil fouled. Besides carb problems, it could be weak spark as well. Fuel may be good but insufficient spark to ignite it will cause similar issues. Off hand I'd bet on a stuck float as the source of the problem but you need to verify what's happening.

As stated above, I'd also recommend doing some plug chops with new plugs to see what's going on.
 
Got some homework then. Thanks to all of you.

Funny signature. Where is eric bang anyway? Had some good advice sometimes, sold me some good parts too.
 
hi guys,

kinda reviving this thread. Ive been doing alot of work on the bike. Ive changed the points plates, got new oem points and condensers. The carbs have been pulled, checked the float levels, they appear to be fine. Valve clearences were done again, checked the compression all 4 cylinders at 120.

The bike starts easily no choke, even on cold days. Idles excellent. But still stutters after 4000 rpm. Someone told me to change the needles, does this sound reasonable?

Carb #1 float always seem to be humid with fuel, no leaks, just humid.

Could a faulty petcock cause this?
 
I am surprised that nobody has suggested trimming the spark plug wires.

Remove the wire from the spark plug. Unscrew the cap (the angled rubber piece that clips onto the plug) from the wire. Cut about 1/4" (6mm) from the end of the wire. Screw the cap back on the wire. Put the wire back on the plug.

Try this on your #1 plug, see if it helps. If it does, do the other three plug wires as a preventive measure. If it doesn't help, go ahead and do the other three anyway, but you will have to keep trying things to see why it's stuttering.
 
If you can start it without the "choke" on a cold day, you are running WAY too rich. When you do start it up, does it smoke? If so, what color is the smoke? When you are sitting at a light for a minute or 2, does it puff some smoke when you take off again? When you pull the #1 plug out is it just black, or is it really goopy and black?
 
Thanks for the help.

The plug wires have no more space left in them to cut some more. They have been cut in the past. Plugs are just black. Somewhat clean. I'll post a pic later.

Left sitting for two days had to use choke to start it. The battery is almost dead so I always kick start it. Can having a dead battery affect how the bike runs?

When it starts it blows some black smoke then runs clean. When sitting in neutral I can pull the throttle and notice some black smoke not too much. I can Rev it all the way up to the redline and it doesn't stutter, sometimes it backfires though.

Any ideas?
 
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