GS 425 Stutters Through acceleration

[BrianH]

Hey everybody it's my first time posting here. I've searched a lot on this forum before and everyones opinions seem helpful.

I've got a 79 GS425 that I've been restoring with some cafe emphasis.

I had taken the bike out for a ride one day and rode for about two hours with everything operating fine. After doing some high speed riding on the highway the bike began to bog and stutter at all speeds through out all gears.

I got it home and began performing some tests on the bike to see what could be causing the stuttering. Its also interesting to note that when the bike is in neutral and revved it revs fines throughout the entire powerband.

First thing I did was pull the plugs, they looked a bit rich. This is probably because of the re-jetting I did on the bike in order to compensate for the new air filters and less restrictive exhaust I had put on the bike.

Next thing I checked was ignition timing. This was perfect.

Then I checked the charging system to find that it was not operating correctly due to a improperly connected rectifier wire. I fixed the connection put a new battery on and verified that it was charging properly and the stuttering still remained.

I've gone through the carbs 12618618 times everything is good in there float heights are set right all the jets are clear.

So at this point I'm stumped. Anyone have any ideas?

Mods on the bike are pod filters and a new exhaust and rejetted carbs. And a modified wiring harness because i took of some gauge lights.

[BassCliff]

Hi Mr. BrianH,

Welcome. Did you follow the GS-recommended carb clean/rebuild procedure? You'll find it below. Is your petcock functioning properly? Is there gunk in your tank clogging up the petcock screen? Are you using an inline fuel filter? Now I'll just say "HOoooowwDY!"

Here is your very own magical, mystical, mythical, mind-expanding "mega-welcome". Please take notice of the "Top 10 Common Issues", the Carb Rebuild Series, and the Stator Papers. Now let me roll out the welcome mat for you...

Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike!

Thanks for joining us. Keep us informed.

Thank you for your indulgence,

BassCliff

[Ian W]

I take it you checked the points? Did you check the carb Dia thingys? You know the rubber things sitting in the top of the carbs? The word I canna spell .

They, or one of 'em could have developed a hole in it. It would be worth checking them to rule them out.

Let us know how you get on .

[BrianH]

Yeah the diaphragms are good no holes.

The petcock works well, just rebuilt it. The screen that leads to it is clear as well.

The inline fuel filter is clean too, no blockages.

The bike revs fine when its not in gear, only while its in gear and accelerating does it miss or stutter.

I've got new o-rings o the manifold. Theres no vacuum leaks anywhere in the system.

The exhaust is modified, but I've re-jetted to compensate for that.

Do you think new points are necessary? The stock ones are old but theyre functioning properly.

The problem started during a ride. The bike was running well before that. I think this is something important to keep in mind.

Thanks for the responses guys.

[Ian W]

Only if the points are pitted really. Sounds electrical to me though.

Have you noticed the lights flashing/ dimming, or anything when the bike starts to stutter?

Is it just when the bike gets warm?

Does the bike 'bog down'?

Do the bike miss-fire?

Is it the 'L' model you've got? (just curiuos )

[79 sh850]

Yeah the diaphragms are good no holes.

The petcock works well, just rebuilt it. The screen that leads to it is clear as well.

The inline fuel filter is clean too, no blockages.

The bike revs fine when its not in gear, only while its in gear and accelerating does it miss or stutter.

I've got new o-rings o the manifold. Theres no vacuum leaks anywhere in the system.

The exhaust is modified, but I've re-jetted to compensate for that.

Do you think new points are necessary? The stock ones are old but theyre functioning properly.

The problem started during a ride. The bike was running well before that. I think this is something important to keep in mind.

Thanks for the responses guys.

How are your condensers? Black plugs can also be caused by a weak spark which might not be evident when the engine isn't under load. You shouldn't see too much arcing on the points with the timing cover off and the engine running. I would check for capacitance and leakage using a multimeter.

I would also check that the timing advance isn't stuck and that the ignition is advancing correctly. Double check the clearances on the points since it is possible for them to slip if not torqued down correctly. If you have a strobe, you can sometimes tell if the spark on one cylinder is intermittent by connecting to the lead for that cylinder. This can give you clues if you are certain that the problem is not carburetion.

Andy

[Nessism]

Most likely a jetting issue. Do a search on "plug chop"; you need to check the mixture in the various carb circuit ranges to verify the jetting is correct. I'm not the sort that messes around with jetting but I'd guess you would have to go up at least 4 main jet sizes depending on what pods and exhaust you are running.

Good luck and don't forget to post photos of your bike. Love those 400/425's.

[BrianH]

Hey guys thanks for the responses once again.

Ianw- The bike bogs and misfires when under load, but doesn't misfire when revving not under load.. This only occurs when its under load though. When i'm on the throttle the bike kind of jumps and bogs and jumps and bogs.

I'm gonna take the points off today and get a better look at them, and clean the rank sahft off because it has some rust deposits on it.

I'm not sure if its the "L" or not, first time bike for me.

SH- The plugs are fouled, so maybe it is a weak spark and a combination of being rich. I know the bike is jetted a little rich but not to the extent where it would be causing it to not fire. The weird thing is it was running great for a week before this occurred with the same jetting.

I also suspected the points plates slipped so i went through there and doble checked as well.

[SqDancerLynn1]

I would start with a new set of plugs, if it runs better you need to work on the jetting. Also make sure you have 12 volts on the coils. you should block the points open to get correct reading

[BrianH]

Alright i'll try the new plugs.

[BrianH]

Also where is the timing advance located exactly?

[BrianH]

Put in the new plugs today, they helped the top end a bit but there is still some stuttering through the lower end.

I think I'm going to mess with the carb jetting some more and see what happens.

[BrianH]

I finally got around to some serious carb tuning work today. I put the needles back to their stock position to see how this would affect performance and it seem to take car of the stuttering in the mid to high rpm range. I previously had them set in the most rich setting. I also put in the stock pilots. I changed the pilots initially to compensate for a backfiring problem i thought i had due to the lean mixture. Changing the pilots did solve the problem, but now with the stock pilots the problem is gone too.

I have aftermarket main jets in they are 165's.

I still have the stuttering and stumbling in the low rpm range, when the bike transitions from the pilot system to the needle system.

I pulled the plugs and they look like they're on the lean side. Light gray coloration.

The air screws or mixture screws are set at about five turns out. I kept turning them out from the original two turns out position and it seemed like it was helping the problem.

The stuttering problem is not bad when the bike is colder and I accelerate for the first time during the ride. As the ride goes on it gets progressively worse.

Anyone have any ideas if i should continue going leaner to solve the low end stuttering problem?

[Steve]

... First thing I did was pull the plugs, they looked a bit rich. This is probably because of the re-jetting I did on the bike in order to compensate for the new air filters and less restrictive exhaust I had put on the bike. ...

I finally got around to some serious carb tuning work today. I put the needles back to their stock position to see how this would affect performance and it seem to take car of the stuttering in the mid to high rpm range. I previously had them set in the most rich setting. I also put in the stock pilots. I changed the pilots initially to compensate for a backfiring problem i thought i had due to the lean mixture. Changing the pilots did solve the problem, but now with the stock pilots the problem is gone too.

I have aftermarket main jets in they are 165's.

I still have the stuttering and stumbling in the low rpm range, when the bike transitions from the pilot system to the needle system.

I pulled the plugs and they look like they're on the lean side. Light gray coloration.

The air screws or mixture screws are set at about five turns out. I kept turning them out from the original two turns out position and it seemed like it was helping the problem.

The stuttering problem is not bad when the bike is colder and I accelerate for the first time during the ride. As the ride goes on it gets progressively worse.

Anyone have any ideas if i should continue going leaner to solve the low end stuttering problem?

I am seeing a series of carb tuning issues here. If it runs better cold and gets worse when warm, it's running too rich. I could not find specs for a 425, but the 400s had 112.5 mains, the 450s had 115 or 120, depending on the year. Your 165 sounds WAY too rich, even if they were DJ numbers.

To read your plugs, you can't just run it for a while then stop and read them. You have to hold the throttle at a set position, then hit the kill switch and pull the clutch, then roll to a stop and read the plugs. You need to do this for every circuit in the carbs, so you need to do this four times. Running at low throttle or idle while slowing down to find someplace to pull the plugs can discolor the plugs enough to affect your reading.

With your pods and pipes, I'm guessing your main should probably be in the area of 125-130, the pilot jets close to stock, raise the needles a notch or two, the mixture screws about 3 turns out.

Do some proper plug chops, and when you make changes, make them one at a time, otherwise you don't know which one made the difference.

.