Stuttering at 5k rpms

[Guest]

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

[wymple]

How old and how good are the coils and the wiring that feeds them?

[drivera84]

Hi.

30+ years old.

They appear to have good spark though.

[OldVet66]

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.

[Weapon]

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).

[John Park]

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.

[Weapon]

good call! John I have seen that myself.

[drivera84]

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

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.

[drivera84]

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.

[OldVet66]

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.

[drivera84]

How do i know if it is rich?

How do I adjust the carburator?

[tkent02]

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/

[OldVet66]

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.

[JTGS850GL]

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.

[drivera84]

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.