'79 GS850 off idle "sputter"

[Guest]

'79 GS 850.
-Stock airbox, stock carbs, 4-1 Jardine Exhaust (they claim no re-jetting required.....I guess I'll find out)
-Synched perfectly at 10 inch pounds per carb at 3,000 rpm on vacuum gauges.
-New O-rings, boots, carb o-rings and perfectly clean carb, floats as good as I can get them.
-Bike accelerates and pulls very hard just off idle to whatever. Perfect acceleration. Awesome.
-2 and 3 plugs very light tan on 1/8 throttle chop test. 1 and 4 are a little darker (what I would prefer I guess).
-new Dyna S ignition and coils
-air screws 1and 3/4 turns out.


Here's the deal,

I have an off idle "sputter". Also when reving the engine freely, or when the engine doesn't have a load on it, at a steady rpm the bike sort of has a steady sputter as well. Once in a while between first and second gear it pops....but rarely. Doesn't miss really, just....sputters and doesn't run real clean. On the rev its perfect. But at a steady rpm the sputtering begins.

A spec sheet I had for my VM carbs showed a pilot screw setting at 7/8ths turn. Bike backfired like crazy at that setting in between gears. Re-set to 1 and 1/4 turn. From there, I've been slowly working the pilot screws out, and the backfiring is gone and the flat spot has greatly improved...but it isn't where I would like it to be yet.

My plan is to just keep on easing out the pilot screws until it gets better or worse. Its just that I've gotten to the point where its not getting much better.

Is there a better way to proceed?

Am I on the right track for solving this problem? Any other ideas?

Thanks.

[chef1366]

Try richening your needle by raising it one notch(going down one notch which will raise the neede) You might have to go up one on your pilot jet.

[chef1366]

Shoot Mark, you're probably more in a position to help me! But you can also use a shim to raise the needle half notches because you sound real close.

[bwringer]

You might also consider raising float height a smidgen (1 smidgen = 0.5 - 1.0 mm or so).

[mark]

Shoot Mark, you're probably more in a position to help me! But you can also use a shim to raise the needle half notches because you sound real close.

I don't have the "shim" system on VM carbs, I have the lock-nut deal. I tried to "raise" them by getting the synch right and then raising them all the same by turning the screw but all hell broke loose. If somebody knows how to do that, I'd try it.

I can try the floats. They're set at 24mm, I've got a mm to play withm according to my book anyway.

[KEITH KRAUSE]

It's not the float level you set. I'd leave that if you know you did it right.
Everything you say points to a lean mixture.
I assume you actually set the side air screws for best rpm, not just set them to 1 3/4 to "be close"? Set them correctly, if you didn't, and test.
Your "10 inch pounds" seems like a low vacuum read to me...at 3,000 rpm's. What vacuum tool are you using and how many lines up were the readings? Sounds like you may have had a little trouble with the vacuum synch??
You DO have adjustable e-clips on your jet needles, if it comes to that. The jet needles effect kicks in very soon on your carbs.
The pilot fuel screws, on a stock bike with pipe (air box lid must be on too) are generally 1 to 1 1/2 turns out. Don't try turning them much more than 1 1/2 out, that would be compensating for the real problem. Any chance you have broken screw tips in the carb bodies(s)? Did you verify that fuel/cleaner shoots through the tiny pilot fuel screw holes in the throat?
My son needs the computer so I gotta go. I have some more ideas, but let us know if you've made progress.

[mark]

I don't have a mercury type synchronizer, its a set of 4 actual vacuum guages that measure inch pounds like for a car. 10 inch pounds is what they read when they are all even, and their wasn't that much varience in vacuum from idle to 3,000 rpm. Pilot screws were good when I put new o-rings on them recently, and although they don't drip, sometimes they seem a little "weepy" or wet from gas. Air screws were adusted for best idle but the process wasn't all that exact and is probably ball park at best. All are "around" 1 and 3/4. I didn't make sure that fuel/cleaner shoots through the holes in the throat as I recall, but I cleaned and soaked and hit them with compressed air good when they were rebuilt.

I'll mess around some more and post again tomorrow.

[bobl200]

Spluttering

Hi

I have just rebuilt an old 1978 GS850 and I put on 4 air filters one on each carb. (The old air box was a real mess). The bike was a dog to get going and when it ran it spit back through carbs (is this what you mean by splutter) and wouldn't it rev freely. I stripped the carbs and cleaned them out. Checked all float levels... set them to maximum level. Put it all back together and it was still the same. No change. I changed the ignition unit thinking it was that causing the problem with no change.

On GS resources people had been telling others with problems to make sure the air box is well sealed so I decided to clean up the old air box and fit it back on the bike. Fully sealed. Doing this cured all the spluttering and the bike now runs fine.

The main thing that I found when cleaning the old air box. There is an oil breather pipe going in the top of the box with a gap round it. This is where rain water can get into the top of the air box and rot down through to main camber (where the carb inlets are). My box had holes in that partition. The outside of the box was fine but the hidden rot inside would have caused air leakage (even if the side doors are sealed ok). I cleaned out all rot inside the box and put silicon sealant in the gap round the oil breather pipe to fully seal the carb inlet chamber.

I later found out that the spit back (splutter) is due to a weak petrol to air mixture igniting in the inlet ports. The sealed air box gives a negative air pressure in the carb inlets causing fuel to be drawn in via the carb ports and air via the filter giving the correct fuel to air ratio.

For a quick fix ...just try putting some sealant round the oil breather pipe on the top of the box and then try the bike.

I hope this helps you and may be others that are having problems.

Cheers Bobl

[KEITH KRAUSE]

Bob, sounds like you didn't re-jet for the pods. Glad you got your bike running better.

[KEITH KRAUSE]

Mark, reading back, it sounds like your pilot fuel screws ARE operating right, so forget my babbling about possible broken tips in the carb bodies.
As usual, I'm in a hurry to get out the door. But you said the stutter will also sustain itself at a certain rpm?? Rpm doesn't really matter, throttle position does.
What throttle position is it at when this stutter happens?
As chef said, the pilot jet or jet needles may have to be richened, but it depends on what throttle position the problem is at. There's some overlap of the pilot/cut-away/jet needle at "lower" throttle positions, but if you're sure everything else has been tuned well, then your problem should be fixed by richening the pilot or needle position.
Need plug reads at solid 1/3 throttle. The jet needle adjustment is the most work on the VM carbs and it also requires a re-bench/vacuum synch. So it's best to try to reason which circuit is lean before jumping in. Pilot jet changes are simple and quick.

[mark]

Mark, reading back, it sounds like your pilot fuel screws ARE operating right, so forget my babbling about possible broken tips in the carb bodies.
As usual, I'm in a hurry to get out the door. But you said the stutter will also sustain itself at a certain rpm?? Rpm doesn't really matter, throttle position does.
What throttle position is it at when this stutter happens?
As chef said, the pilot jet or jet needles may have to be richened, but it depends on what throttle position the problem is at. There's some overlap of the pilot/cut-away/jet needle at "lower" throttle positions, but if you're sure everything else has been tuned well, then your problem should be fixed by richening the pilot or needle position.
Need plug reads at solid 1/3 throttle. The jet needle adjustment is the most work on the VM carbs and it also requires a re-bench/vacuum synch. So it's best to try to reason which circuit is lean before jumping in. Pilot jet changes are simple and quick.

Its definitely a throttle setting issue. Bike has been "sputtering" at all rpms if reving with no load, and has a flat spot with the same "sputter" just off idle. If you let the clutch out without giving it any throttle, it runs fine until you give it any gas.

I turned all the pilot screws to 1 and 1/2 turns out and did the 1/3 throttle plug read. A couple of the screws weren't that far out yet after all my tinkering so I just set them all the same as a place to start over.
Bike ran better (still great acceleration) and no load rev was much better, but still felt flat just off idle. Very little "sputter".

Plug reads: 1 and 2 (light grey) 3 was greyish with very light tan spots and 4 was a little lighter grey than 1 and 2.

[KEITH KRAUSE]

Quick reply, off to work.
I need to go back here. You said you synched the carbs with a tool that I'm not used to using. One cylinder at a time is how you did this? Any chance you can use a regular tool and do them all at once? Accurate plug reads depend on this synch.
But from what you've said, it DOES sound like you're lean.
I would try raising the jet needles. More work, but your 1/3 throttle test suggests it needs to be done.
One position or a half position (with a .022" spacer) is the question. I can't be sure without seeing the plugs, but I'd have to say a full position. That should be position 4, from the top. Then the carbs must be re-bench synched.
Gotta go. Will try to help later.

[mark]

Quick reply, off to work.
I need to go back here. You said you synched the carbs with a tool that I'm not used to using. One cylinder at a time is how you did this? Any chance you can use a regular tool and do them all at once? Accurate plug reads depend on this synch.
But from what you've said, it DOES sound like you're lean.
I would try raising the jet needles. More work, but your 1/3 throttle test suggests it needs to be done.
One position or a half position (with a .022" spacer) is the question. I can't be sure without seeing the plugs, but I'd have to say a full position. That should be position 4, from the top. Then the carbs must be re-bench synched.
Gotta go. Will try to help later.

I agree, I would prefer that the plugs were a little darker grey or tan.

Carb synch tool I have is a set of 4 vacuum guages (one for each carb of course) attached to a metal backing that is hung between the mirrors on the bike, not just one guage. I've just preferred it over the mercury testers I've used before. I'm pretty sure that the synch is very accurate.

I'd be happy to play around with this (messing with it is what I like to do anyway) and try stuff. I've had the carbs totally separated and back together a few times, so that's not a big deal, but I don't know how to raise the needles or about "spacers" (I wrongly thought that was only for CV carbs).

I don't know where to buy spacers or jets either.

[chef1366]

Radio Shack

I bought an assortment of small washers from Radio Shack. There are some that fit your needle exactly. To raise your needle half a notch put the washer under your needle clip. To raise your needle one notch, lower the needle clip one notch.

[mark]

Thanks guys for the info. really appreciate it. Unless someone chimes in otherwise I think I'll try raising the needles a notch and see what happens. Won't be able to do it for a couple of weeks though because I'm heading out of town early next week and have a busy weekend.