Popping /bogging at low throttle

[Almonus]

Hey there, first post on the forum but I figure its about time I ask for some insight.
I've got a 1982 GS750t that I'm having some trouble with the whole "rejetting" process (which seems to be typical with rejetting carbs). I've been through all the stickys and searched through older threads but cant quite find what I'm looking for. So first off, what has been done to the bike so far:

- Checked valve clearance (all seated within tolerances)
- New valve cover gasket
- New clutch friction plates and springs
- K&N stock air filter replacement
- Changed stock exhaust (With loose and rusted out baffles) to a Vance & Hines 4-1
- re-painted the engine (PO had painted it silver and it was all flaking off)
- New exhaust gaskets were put in
- Vacuum synced the carbs

I've got everything running well in the driveway but when I took it out for a test drive today, its running fairly poorly. Idle sits comfortably around 1100 rpms both cold and warm, but on takeoff from a stop the engine seems bogged down and the exhaust "pops". Once the bike gets moving everything seems to run fine, although there is some minor popping between gears. When I come to a stop again though, or leave the clutch pulled in and let off on the throttle it stalls out.

I've checked the system for intake leaks using starting fluid and cant find any exhaust leaks on either the header or the header/canister "slip-on" area. Both areas seem to be good.

I'm running #122.5 Main Jets and #42.5 Pilots as suggested by Vance & Hines, but I'm thinking maybe the idle jets are too high? or maybe its the air screws, any ideas on what could be the issue or how to start on fixing this would be great. I've got an inkling of whats wrong but I don't want to mess around too much and get myself into a pickle.

I'm lucky enough to have stock pilots and mains (112.5), as well as 130 and 135 Mains already and really don't want to use a DJ kit. Any help would be great!

 

[Guest]

It's the jet needle that needs shimmed or just get a jet kit with adjustable needles. I like Dynojet.

 

[JTGS850GL]

Using the stock air box with the K&N filter should be pretty close to stock. The V&H exhaust will make a little difference but not as much as you might think. The stock mains were indeed 112.5. I wouldn't think that going anything above 120 mains would be needed. The 122.5's should still work. Certainly not 130 or 135. Stick with the stock 42.5 pilots. Get you main jets dialed in first. No pilot jet changes are usually needed. Might need to raise the needle a little but that would be more fine tuning if you have some surging at light throttle application.

Have you adjusted the fuel enrichment screws using the highest idle method. If not, then what did you set them to. Since it will rev and run fine but die if you drop the throttle too quickly, I'd bet you're too fat (rich) in the bottom end instead of being too lean.

Do some plug chops and see where you are. Start at full throttle and work you way down from there.

 

[Almonus]

Thanks for the replies! I have the pilot air screws out two turns but I've since dialed them down to one and a half turns from seated. Its idling on its own, but I've got a feeling I'm still a little rich in the low end. Just did another test ride and there's a bit of mid/low range jumping or chugging and its still dying out on me after a good run.
Haven't tried using the Highest Idle method yet, but I'll give it a shot next.

 

[tom203]

Try 2 1/2 turns out , and set your idle stop screw when bike is fully warmed up. The stalling stuff sounds like you have too little mixture to keep revs up with the clutch dragging.

 

[Almonus]

Tried the Highest Idle method at 2 turns and less and it made a little bit of a difference but still stalls out after high revs, even in neutral. I'll try opening them up to 2 1/2 next. If that doesn't work I'm going to look at maybe lowering the mains to 120, see if that changes anything. I'm considering shimming the needle like chef said, but I've got to look into how exactly that works. Still a newbie when it comes to carb mods.

 

[Steve]

I have the pilot air screws out two turns but I've since dialed them down to one and a half turns from seated. ...

Also need to get some terminology straight, too. You do not have "pilot air screws". :-k

What you do have are officially known by Suzuki as "pilot screws", but many of us call them "mixture screws" because they control the amount of a pre-mixed mixture that is added to the air stream. Because that mixture is primarily fuel, turning the screws out will richen the overall mixture, turning them in will lean it out. If they were "air" screws, it would be the opposite.

Now, I did not see anything in your first post about the history of the bike. Did you just get the bike? Do you know anything about its (recent) history? Has it been ridden regularly or has it been parked for a few years? I ask this because there is no mention of cleaning the carbs or intalling new o-rings. Dirty carb passages can cause the same symptoms you are describing, so I would suggest making sure the carbs are CLEAN before playing with any jetting changes.

.

 

[Almonus]

Haha thank you! I only really saw "pilot screws" but I didn't know what to really call them. I didn't do a full clean of the carbs this year as I've had the bike for 3 years now and they had a full cleaning last year. Stored it in my basement over the winter and ran it with the tailpipe out the back door every so often just to keep it moving . Before the tear-down the bike was running well in stock condition, aside from the problems I fixed (Leaking oil out the top cover, worn clutch, rusted out exhaust). I know the past two owners of the bike and before I got my hands on it it was running, but barely. I purchased it with a seat of duck tape and two of the mains plugged. Never sure about what POs say but apparently its spent most of its life out in a farmers field.

 

[Almonus]

Hey all, Its been a while but I finally fixed the problems. Found that the airbox was warped somehow and wasn't connecting properly to the carbs. Threw it out and got some APE pod filters. Messed with the jetting a bit more but still couldnt get it perfect so I gave up and bought a Dynojet kit. The 3315 kit worked for the bike and, well if it wasn't just that easy!

Bike runs better than before with no issues. Only been out twice since putting the kit on but it's looking like I'm gonna get a decent riding season this year. Wahoo, thanks for the help guys!

 

[JTGS850GL]

Yep, chef got it right as usual.