stumbles at WOT, 5.5k to 7k

[Guest]

I've been working through sorting out fueling issues with my 1979 GS750E and am getting a little stuck on this.

Carbs have been dipped with all new orings 1 year ago
I synched the carbs a month ago
points have been checked
timing has been set to factory specs. Mechanical advance works fine
Factory airbox w/o snorkel (I never had the snorkel for it, so I can't put it back in.)
Kerker exhaust
stock pilots (15)
122.5 mains
needles in the stock position

The bike is really strong up to 4500 revs, where the power drops off noticeably. If you are in 4th or 5th, it will only go past 6k if you are less than about half throttle. More throttle won't do anything, but it will eventually stutter and start to cut out. If you are in lower gears and make it past 6500 revs, it suddenly has tons of power from 7k to redline. The powerband is like a CR125 with an MX pipe it's that sudden. If you're on the highway at 5500-6000 revs, it's fine. Roll on acceleration seems to be okay.

Any ideas? I'm stumped, especially with everything else doing so well. My first thought was that in that midrange there wasn't enough vacuum to open the petcock, but leaving it on prime doesn't make a major difference. Reserve doesn't change anything either. Tested the petcock; it works fine and flows great. Pulled it out and the filter is clean. I don't have an inline filter. Pulling the vent tubes makes no difference at all.

If I get a chance tomorrow I'm going to try to do some plug chops as it's stuttering and dying and see if that tells me anything.

fwiw the bike had pods on it for awhile before I went back to the airbox due to riding in the rain and an annoying throttle tip-in issue that I never could track down. I don't remember it having this issue, but I really only rode it around town then and wasn't over 5k revs very often. Now that I'm riding on the highway more for work, this is starting to feel a bit dangerous.

[chuck hahn]

Did you set the slide stop at 1MM per the service manual instructions? The slide is set with the screw at the back of the rack.

The reason the slides are stopped at no more than 1mm above the top rim of the throats is that any further and the venturis loose their negative pressure effect..thus they can stumble and not act right.

Not saying this IS the cause..just that it can be ONE of them!!!


Bottom pilot screws are where? 3/4 to 7/8 is the suggested setting. Side mixture screw is suggested to be at twice what the pilots are...or approximately 1 3/4 to 2 out as a baseline setup/

[OldVet66]

You are running too rich on the main jets. I can't answer for the lack of a snorkel (?), but stock main jets are 100. The 122.5 seem large for a 4-1 pipe and stock air box. Raise your main jet needle clips one notch and see if that helps lean it out a bit. When it gets really hot and muggy in the dog days you will start fowling plugs trying to break through the mid range. Ask me how I know. As ender22 said you will most likely have to go to smaller mains. Enlighten me on the snorkel, I couldn't identify it on the Air box diagram. Does this add a lot of air flow?

[ender22]

...
122.5 mains
...

I'm working through tuning my 79 GS850 and with my Pods + exhaust my main jets are 112.5, and I'm thinking of kicking them up to 115 tomorrow to see if they go too rich.

So with this said, I think your mains are too rich. the stock mains on the GS750 is 100. So going 22.5 points up from stock with only an exhaust upgrade it seems really high to me.

I'd recommend ordering 112.5, 115, 117.5, and 120's. Then put the 115's in and go for a ride.

Also try the road test plug pull at WOT to see how the plugs look

[chuck hahn]

The function of the snorkel is to keep the box from sucking in air right from in front of the tire where the road grime and stuff is heaviest. Other than that with or without a snorkel doesn't make a difference.

[gearheadE30]

Did you set the slide stop at 1MM per the service manual instructions? The slide is set with the screw at the back of the rack.

The reason the slides are stopped at no more than 1mm above the top rim of the throats is that any further and the venturis loose their negative pressure effect..thus they can stumble and not act right.

Not saying this IS the cause..just that it can be ONE of them!!!


Bottom pilot screws are where? 3/4 to 7/8 is the suggested setting. Side mixture screw is suggested to be at twice what the pilots are...or approximately 1 3/4 to 2 out as a baseline setup/

Hmm, I don't remember ever reading that, but I will definitely check it. I know I fiddled with the stop at some point because the slides weren't opening all the way, so you might be on the money there.

Fuel screws are 1 turn out, airs I started at 2 turns and then adjusted to highest idle.

You are running too rich on the main jets. I can't answer for the lack of a snorkel (?), but stock main jets are 100. The 122.5 seem large for a 4-1 pipe and stock air box. Raise your main jet needle clips one notch and see if that helps lean it out a bit. When it gets really hot and muggy in the dog days you will start fowling plugs trying to break through the mid range. Ask me how I know. As ender22 said you will most likely have to go to smaller mains. Enlighten me on the snorkel, I couldn't identify it on the Air box diagram. Does this add a lot of air flow?

The snorkel is the rubber bit that runs from the hole in the bottom of the airbox upwards somewhat. My bike suffered an airbox fire in its previous life, which melted the snorkel, so it's gone. As for the jets, yeah, I think I might be a little rich but according to the plugs it seems pretty close in the 7k+ rpm range.

I'm working through tuning my 79 GS850 and with my Pods + exhaust my main jets are 112.5, and I'm thinking of kicking them up to 115 tomorrow to see if they go too rich.

So with this said, I think your mains are too rich. the stock mains on the GS750 is 100. So going 22.5 points up from stock with only an exhaust upgrade it seems really high to me.

I'd recommend ordering 112.5, 115, 117.5, and 120's. Then put the 115's in and go for a ride.

Also try the road test plug pull at WOT to see how the plugs look

Yeah, the plug test is on the list, along with adjusting the WOT stop. When I had pods (K&N), 125s ended up being perfect to slightly on the rich side. After these tests I'll go from there on ordering stuff.

The function of the snorkel is to keep the box from sucking in air right from in front of the tire where the road grime and stuff is heaviest. Other than that with or without a snorkel doesn't make a difference.

Yeah, I'm keeping an eye on how much crap actually gets in the airbox. So far so good. 2k miles of daily-ing, including rain, and nothing has gotten in there. the fender extends all the way down the front of the back wheel, and there is a rubber bit between the fender and the swingarm that seems to do a good job of keeping crap out of there.

Thanks for the help guys! I'll check back tomorrow and see how things get on.

[OldVet66]

Thanks Chuck. He is running rich then. I had the same problem with my first set of carburetors trying to jet for pipes and pods. A rich main will kill the mid range and make it rough to go WOT. Mine finally started fouling plugs and gave me a clue as to what was going on. Those were the so called professionally built carburetors built by Wired George that I ended up consigning to the scrap bin this winter. Such is life, I build my own now. How people like that live with themselves is beyond me.

[chef1366]

Even with the rich 122.5s it should go to redline. May be rich and slow but it should. I'd lean the needle first and see if it helps. If not try to richen it.
This is a needle issue. (Even though I would go down on the main)
Unless your points are not set up right.

[chuck hahn]

I agree with Bills assessment. I would reset the stop and then see what it does first. This may remove a great deal of it.

One thing for sure, it WILL remove the potential problem of a venture spoilage. Sounds like he has the basic settings right too.

[chuck hahn]

Yes, the little rubber mudflap will keep a lot out, but the air is swirling all around from the tires rotation and from it wrapping around the fender itself.

I would look into an add in the parts wanted section for a replacement snorkel.

[gearheadE30]

Even with the rich 122.5s it should go to redline. May be rich and slow but it should. I'd lean the needle first and see if it helps. If not try to richen it.
This is a needle issue. (Even though I would go down on the main)
Unless your points are not set up right.

It does go to redline; it's just hard to get it through that midrange zone unless you are in a fairly low gear or are really patient with the throttle. That said, it running rich does make sense. Also, this problem isn't going to be with the needle, as it is at 70% throttle to WOT and gets worse with more throttle.

I will look into getting a snorkel for it as well, though I've got a pretty good list of other semi-critical things I need to do first. The nicer it gets the nicer I want it to be and the more things I want to fix. It's a vicious (motor)cycle, haha.

[OldVet66]

Also, this problem isn't going to be with the needle, as it is at 70% throttle to WOT and gets worse with more throttle.
.

And why not? Both jet size and needle position affect fuel flowing through the main jet. Raise the clips a notch to drop the needles deeper in the jet. It might not be the total answer but it will be enough to tell which direction you need to go in. As I see it you basically tried to jet for pipes and pods and then stuck a stock air box back on without changing your jetting. I have experienced the same thing jetting for 4-1 pipes and K&N Pods. In my case dropping the needles got it right in the ball park. Personally I think you are going to have go to smaller main jets but you have a known quantity now and raising the clips one notch will make a change for better or worse giving you the direction to go in.

[gearheadE30]

Well, just as you guys predicted, it's running stupid rich. I waited until it had pretty much totally cut out when I killed it and pulled the plugs - they were totally black and wet. For kicks and giggles I pulled off an airbox cover for a quick run through the trouble RPM area and the stumble went away. Time to buy some jets.

[OldVet66]

You could probably go back to the stock 100's and adjust with the needles if you need it a little richer. I believe radio shack has washers you could put under the clips for a half notch raise if dropping the clips a notch turns out to be too rich.

[gearheadE30]

You could probably go back to the stock 100's and adjust with the needles if you need it a little richer. I believe radio shack has washers you could put under the clips for a half notch raise if dropping the clips a notch turns out to be too rich.

yeah, I think it's going to end up being a combination of the two. At one point I had the stock mains and just the exhaust on it and it was dangerously lean, so I got a few jet sizes between stock and the 122.5s that are in it now. I'd prefer to be a little rich than a little lean just for a little bit of a safety margin since it's so abnormally hot and humid here right now. I'm not sure how much of a difference it makes, but it's typically around 90 here right now, and I ride as long as it's above about 35 degrees out.

I would guess that I will need to run 115 mains to keep my AFR near redline close because the volumetric efficiency is so much better up there and then possibly drop the needles down a slot or a half a slot to try to smooth the transition into the mains at 3/4 ish throttle. I'm going to do it all one change at a time to make sure that I can separate the effects of different changes. I'll try to remember to post back here with the results.