Tuning an '81 GS1000 - Jet Needle Woes

[Guest]

So, I've been chipping away at this project for months. In the process of fine tuning the carbs. Recent work includes: valve clearance adjustment, carb sync.

Mods:
- Dynojet Stage 3 Racing Jet Kit (stock needles)
- Raised pilot jet from 40 to 45 for better idle/low end response
- 4 to 1 Pro Sport Exhaust
- K&N High Flow Pods
- 530 chain/sprocket conversion, ratio'd for racing
- replaced leaky petcock with a welded-in vacuum valve.

Bike rides like a bat out of hell. Low end is very responsive, solid. At ~4k rpms, 1/4-1/2 throttle, the bike hesitates, lurches a bit. Opening the throttle to 3/4 alleviates the lurch and she flies, however 3/4-full eats so much gas that the engine seems to starve.

Seems like 2 separate issues:
1) Lurching @4k rpms / 1/4-1/2 throttle
(jet needle and/or needle jet)
2) Starvation at 3/4-full throttle
(main jet and/or fuel filter/petcock)

I've attempted to adjust the jet needle both richer and leaner. It now rests 4 notches down from the top, the sweet spot. Richer and she starts to idle poorly, leaner and the hesitation comes down to ~3k rpms. My stage 3 kit came with untapered racing jet needles which are **** if you want to ride on the street... However, I'm starting to think that the stock taper isn't appropriate for my setup... I just can't seem to iron out the hesitation.

I've spent countless hours trying to tune this thing, but I'm starting to reach the limit of my understanding/ability to identify and resolve symptoms. Any pointers/advice would be much appreciated.

Thanks!

[Steve]

Just a few questions to clear some confusion:

1. Where are you that you have a chain-driven '81 GS1000?
Here, in the US of A, the last chain-driven 1000 was in '80, but there was a 1000 shafty in '81.

2. What's the point of using a "Dynojet Stage 3 Racing Jet Kit" with stock needles?
The whole purpose of the Dynojet kit is to get the needles with a better taper.
OK, I just re-read your post and noticed the emphasis on "Racing" kit. Maybe should have gotten a "Street" kit.

3. Why did you go up TWO sizes on the pilot jet?
Even with pods and a pipe, you seldom need to increase it at all. Those that do increase, go one size.
Even with pods and a pipe, there is no extra air at idle, so no extra gas is needed.

4. I hope your petcock lasts a LONG time with no problems. Why would you possibly want to weld it in place?

5. You briefly mention a fuel filter and possible fuel starvation. What fuel filter did you install?
If it is one from the auto parts store, that is likely your starvation problem. Automotive-type fuel filters rely on pumps to get the fuel through the filter. We have gravity-fed systems with almost no pressure.

6. You mention stock needles and "four notches down from the top", giving further proof that you do not have a US-spec bike.

What are your pilot screw settings? With your super-rich jets, they are probably just barely open. I would suggest going back to the stock pilot jets and turning them out three full turns from lightly seated. This might require lifting the needles one notch.

What size main jets are you using?

.

[Phaedrus]

Lots of good questions!

1) I miss-typed. It's a 1980.

2) This is the kit I purchased: http://www.z1enterprises.com/ItemDet...px?item=DJ3103
- You'll notice that there's basically no taper on the jet needles. I was having a lot of hesitation issues with them (still at 4k), so I swapped back to the stock needles hoping to clear it up. Doesn't seem to have done the trick...

3) Called a local dyno center, duder gave me some pointers and suggested raising the pilot 2 sizes. I read in some mikuni tuning guide that the pilot jet affects 4k rpm, so I decided to give it a shot.
- No idea if it helped because in I swapped them in, adjusted the valves, and synced the carbs all at once. (I know... I know... one step at a time...)
- Does the pilot jet affect 4k rpm operation / is it in use at 1/4-1/2 throttle, or is it's circuit closed via vacuum pressure?

4) PO did it. Whole project is a rat bike. Tank is in good condition otherwise.

5) Again, PO installed the filter, but I'm fairly certain it's from an automotive store. I took it out and did a blow test to see which direction is less restrictive. Both directions are very restrictive...
- What fuel filter do you recommend?

6) They're Mikunk BS34SS carbs.

Pilot screws are 2 1/2 out all around. Idle seems happy there.

Sounds like I should replace the fuel filter, swap stock pilot jets back in, and perhaps try the dynojet jet needle again, 4-5 notches down from top?

Just a few questions to clear some confusion:

1. Where are you that you have a chain-driven '81 GS1000?
Here, in the US of A, the last chain-driven 1000 was in '80, but there was a 1000 shafty in '81.

2. What's the point of using a "Dynojet Stage 3 Racing Jet Kit" with stock needles?
The whole purpose of the Dynojet kit is to get the needles with a better taper.
OK, I just re-read your post and noticed the emphasis on "Racing" kit. Maybe should have gotten a "Street" kit.

3. Why did you go up TWO sizes on the pilot jet?
Even with pods and a pipe, you seldom need to increase it at all. Those that do increase, go one size.
Even with pods and a pipe, there is no extra air at idle, so no extra gas is needed.

4. I hope your petcock lasts a LONG time with no problems. Why would you possibly want to weld it in place?

5. You briefly mention a fuel filter and possible fuel starvation. What fuel filter did you install?
If it is one from the auto parts store, that is likely your starvation problem. Automotive-type fuel filters rely on pumps to get the fuel through the filter. We have gravity-fed systems with almost no pressure.

6. You mention stock needles and "four notches down from the top", giving further proof that you do not have a US-spec bike.

What are your pilot screw settings? With your super-rich jets, they are probably just barely open. I would suggest going back to the stock pilot jets and turning them out three full turns from lightly seated. This might require lifting the needles one notch.

What size main jets are you using?

.

[GSX1000E]

Did you also make the mistake of following the Dynajet instructions and drill the carb slides? Can you post a picture of the inside of one of those pod filters? Is the shelf inside blocking all air the the ports at 4 and 8 O'clock on the back of those carbs?
(I see that you stated that they are K & N's. Are they?) If you have not drilled those slides, try running a 125 Mikuni main jet, the stock pilot jets and adding (4) 3mm flat washers to the stock needles in place of the plastic spacer and report back.

[Phaedrus]

Mistake was made. Carb slides are drilled.

Pods are K&N's, air ports are all free (tis why I bought the K&N's).

Sooo, since I have drilled the slides, what's the game plan?

Did you also make the mistake of following the Dynajet instructions and drill the carb slides? Can you post a picture of the inside of one of those pod filters? Is the shelf inside blocking all air the the ports at 4 and 8 O'clock on the back of those carbs?
(I see that you stated that they are K & N's. Are they?) If you have not drilled those slides, try running a 125 Mikuni main jet, the stock pilot jets and adding (4) 3mm flat washers to the stock needles in place of the plastic spacer and report back.

[GSX1000E]

Mistake was made. Carb slides are drilled.

Pods are K&N's, air ports are all free (tis why I bought the K&N's).

Sooo, since I have drilled the slides, what's the game plan?

As that mod while most likely allow the slides to lift faster (just a guess as I do not use kits in my bikes), I would try 120-122.5 Mikuni mains and the other suggestions?
If that does not show enough improvement, re-install the factory plastic spacer to those stock needles?
Perhaps some of the forum members that swear by those kits could help you?

[Phaedrus]

So it seems like I may have found my culprit...

"Don't drill the slides

If you have a Dynojet kit, it will tell you to drill the vacuum hole ("the slides") of each needle-holder diaphragm with their included drill bit. However, it is HIGHLY RECOMMENDED that you don't do this. This is another reason why Factory Pro kits are recommended.
Dynojet kits are made for race applications. That larger hole they want you to drill means there needs to be a larger pressure difference between the top of the diaphragms and the carb body before the slide raises. That higher pressure difference means the slides don't start to move until there's more air passing through the carbs (this contributes to a 'bog' feeling at low rpm) and when they do go up they tend to snap up. You loose the CV part (Constant Velocity) that makes this kind of carb so desirable for street bikes. On the other hand, it's ideal for race bikes, where you're wide open most of the time and you just want the slide to be there to make it 'stupid-proof', so you don't bog down the motor when you snap the throttle open.
" http://faq.ninja250.org/wiki/How_do_...l_a_jet_kit%3F

This is exactly the symptom I am experiencing... At low rpms, there doesn't seem to be enough air flow to get the slides to raise, flick the throttle, the slides fly up, and I'm off like a rocket ship. If I pretend like I'm racing from 0mph, the bike flies through all of the gears with ease and would continue up to 140mph if I don't let go of the throttle.

Sounds like I might need my old slides back... Problem is, they're expensive. Is there anyway to plug the drilled hole (JB weld or something?) and redrill to factory spec? Or am I just asking for (more of) a headache?

As that mod while most likely allow the slides to lift faster (just a guess as I do not use kits in my bikes), I would try 120-122.5 Mikuni mains and the other suggestions?
If that does not show enough improvement, re-install the factory plastic spacer to those stock needles?
Perhaps some of the forum members that swear by those kits could help you?

[GSX1000E]

Where are all of those folks on here that profess and recommend using those kits now? How wonderfully do THEIR bikes run??? Perhaps a search might find some of those members in the archives?

[Steve]

I have suspected that the only reason to drill the slides was for quicker reaction, but did not suspect that it also delayed it.

I have plugged the holes with JB Weld before, and it seemed to work for the short time I was in touch with the owner.

When you drilled the holes in the slides, did you drill straight up or did you follow the original angle? Yeah, the original holes were at an angle

Stock main jets for your bike were 107.5, according to the factory manual. Depending on how restrictive (loud) your pipe is, add 3 to 5 sizes for the pipe. Add 4 to 6 sizes for your pods. Adding 7 to 11 sizes to the 107.5 will put you in the range of 125 to 135. Those are Mikuni sizes. The same jets in Dynojet sizing would be 134 to 144.

Stock pilot fuel jets are #40. Stock pilot air jets are #170.

Did you also install the "correctors" for the main air jet? They might affect the jetting requirements a bit, but I'm not sure which way or how much.

You have already kicked yourself for doing multiple changes at one time. Learn from that and now do only one step at a time, so you KNOW whether that change worked or not.

EDIT: I just looked up the kit that you installed. It includes main jets of 114, 118, 165 and 170. The DJ114 is the same as the MIK107, the 118 is the same as a 110. The larger jets are really surprising, they are equivalent to MIK 155 and 160. Seems you don't get anything that is "middle of the road", it's either basically stock or REALLY rich.

.

[trippivot]

don't drill the slides? what a joke. I am an expert . I DO USE the dynojet 3133 -- know for a fact I bought 6 of them in 2014. and installed them tuned them .. I also had proof the carbs were Suzuki GS - I get paid to make bikes run faster / better. you are doing something wrong - maybe you are turning the throttle too quickly- making you own problem when none exists.

KZ carbs fit the GS- and look a lot alike -parts interchange also = and you may need the dynojet kit 2310 maybe the 2301 or the 2134 all these are for mikuni CV carbs of 80~85 era for the KZ 1000/1100 ---- god help you if some one split the rack and installed a single carburetor that does not match. I see a lot of carb problems here / I see a lot of mixing of carb banks here too.

go back and check the simple things -- that fuel tap if restricted // pilot plug shrunken // bypass circuit plugged .... etc...

[Phaedrus]

Alright, well, I spent the morning tuning until my throttle cable snapped--thing was old and falling apart anyways.

Quick history on the carbs. I freshly rebuilt them with all new hardware before starting this tuning journey. They're all the same, though I don't honestly know if they are stock 1980GS1000 carbs. They look identical to what's in the shop manual for the 1980 GS1000ET/ST. Is there any way to verify what model carbs I'm running?

This morning's tuning attempt:

- Tried using the stock 40 pilot jet, swapped back in the Dynojet jet needles as well. Bike couldn't hold idle, no matter how rich the mixture screw.
- Swapped back for the 45 pilot jet. Bike idles strong at 2 turns out from lightly seated (started at 3 out). Lots of hesitation at 3k-4k rpms, full throttle was starved.
- Moved the jet needle up a notch (richer). Hesitation moved to ~4k-5k. Full throttle still starved.
- Moved the jet needle up a notch. Hesitation moved to ~5k-7k. Full throttle better, but still starved.
- Moved the jet needle up to final notch WITH 1 washer. No hesitation up to 7k. Throttle cable snapped before I could test full throttle.

So, I'm not sure why I'm having to veer so far off path with these carbs in order to get them to kinda run right. I think trippivot might be on to something... Perhaps I mistakenly ordered the wrong Dynojet kit and the jet needle length/taper is wrong...

Have to wait for a new throttle cable to arrive before I can get back to tuning :/

don't drill the slides? what a joke. I am an expert . I DO USE the dynojet 3133 -- know for a fact I bought 6 of them in 2014. and installed them tuned them .. I also had proof the carbs were Suzuki GS - I get paid to make bikes run faster / better. you are doing something wrong - maybe you are turning the throttle too quickly- making you own problem when none exists.

KZ carbs fit the GS- and look a lot alike -parts interchange also = and you may need the dynojet kit 2310 maybe the 2301 or the 2134 all these are for mikuni CV carbs of 80~85 era for the KZ 1000/1100 ---- god help you if some one split the rack and installed a single carburetor that does not match. I see a lot of carb problems here / I see a lot of mixing of carb banks here too.

go back and check the simple things -- that fuel tap if restricted // pilot plug shrunken // bypass circuit plugged .... etc...

[Phaedrus]

Ya know... I never took out my needle jets when I did the rebuild.... Maybe they're gunked and causing all of this mayhem. I'll inspect them for ****s and report back. Thanks for the epiphany.

You seem to have a pretty good handle on re-jetting, here is some reinforcement data...[ATTACH=CONFIG]37289[/ATTACH]

[Joe Guilbeau]

OEM was a #115 MAIN
OEM was a # 40 Pilot Jet

I am guessing off the top of my head from doing my carbs a bunch when moving from sea level to 4-6,000 feet and tooling back and forth 3-5 times per year would be the following...

Stick with the OEM Pilot Jet

Your problem is that your Main Jet OEM is #115, and just with pods you probably want a #118 as this is the closest you have on hand.

Adding in 4-1 exhaust and you are now in the #120 to#130 range, depending on displacement, yet your kit jumps from #118 to #165???

I have a 1981 GS1100EX with a Wiseco 1166 Big Bore Kit, Pods, Vance & Hines 4-1 and my main jets are #138...go figure. This is strange, you have Mikuni BS34SS Right?...Me TOO!!!

My way of thinking is that this is too wide a spread, but hey...try your #118.

The Pilot Air Screw at 2.5 turns out is for the Idle only.

Get those set and then the Needle height is the only thing left for Mid-Range.

Idle can be a bit lean, as the bike generally does not operate in this range much.

From your Z1 Dynotek Kit link
"...STAGE 1,3 Dynojet Kit Fits Suzuki 1980-83 GS1000 GS1100 8-Valve (4) 114 Main Jets, (4) 118 Main Jets, (4) 165 Main Jets, (4) 170 Main Jets, (4) Fuel Needles, (4) Adjusting Washers, (4) E-Clips, (4) Main Air Jets, (1) Slide Drill, (1) Plug Drill, (1) Screw..."

Finally, here is your Technical Service Manual at the end of the .pdf

http://www.mtsac.edu/~cliff/storage/gs/GS1000.pdf

[Steve]

Ya know... I never took out my needle jets when I did the rebuild.... Maybe they're gunked and causing all of this mayhem. I'll inspect them for ****s and report back. Thanks for the epiphany.

What else did you not take off the carbs when you "rebuilt" them?

Did you actually dip them and let them soak for about 24 hours each, or did you simply unload a can of spray on each one?

I have said it before, I'll say it again, ... "Every shortcut you take is just another opportunity to do it over."

OEM was a #115 MAIN
OEM was a # 40 Pilot Jet

Joe, I'm going to have to respectfully disagree with you. He has an '80 chain-drive 1000.
Yes, the 1000 shafties do have 115 main jets, but here is the page from the factory manual.
Note the header "GS1000E/ST", which says this is for a 1980 GS1000E (chain-drive):

Your problem is that your Main Jet OEM is #115, and just with pods you probably want a #118 as this is the closest you have on hand. ...

And your problem is mixing up facts.

First, the stock main jets are Mikuni 107.5. Mikuni jets go up in increments of 2.5, so will never have a number like 118. Those 118 jets are Dynojet jets, and are equivalent to a Mikuni 110, which is only one size larger than stock. For pods, you typically add 4-6 sizes to the mains, not one. Even if they used the same numbering system, the 118 is not that much larger than the 115. You are mixing up references to Mikuni and Dynojet, creating a LOT of confusion in the process.

I made suggestions a few posts back what might be needed. He tested some of them and found that he still needed more work.

The Pilot Air Screw at 2.5 turns out is for the Idle only.

First, these carbs do not have a "Pilot Air Screw". The only adjustable screw related to mixture are simply called (bay Suzuki) "Pilot Screw". They actually control a mixture, not just "air". Some of us have called them "mixture screws", but they are properly called "Pilot Screw".

Idle can be a bit lean, as the bike generally does not operate in this range much.

You might be surprized how much of the time you are using the pilot (idle) circuit. The pilot circuit actually functions pretty much ALL the time the engine is running, but as engine speed increases, other circuits supply far more to the mixture, making the pilot circuit's contribution rather negligible, but it's still there.

.

[John Kat]

I've gone through the same issues you are mentionning with my BS 34 carbs.
My bike is a GS 1000 ET that was sold in France with the CV carbs.
The engine is stock, the exhaust is a Kerker 4 into 1 with std baffle and my pods come from Dynoman.
I had the stage 3 Dynojet kit installed with the restrictors and the slides drilled.
I will not mention all the issues I had as it would be a strict repeat of what you encountered.
Like you I discovered that the Dynojet needle doesn't work at least on a european spec carb.
In the end the setting that worked for me is:
137.5 Mikuni main jet
5D59 needle of the 4th groove ( from the top)
I don't remember if I have a 40 or 47 idle jet

The 5D59 needle came from GS 1100 ( 16 valve) carbs but there is almost no difference to the 5D57's that come std on the the GS 1000 spec'd carbs.

In practice, the transition from idle to 1/8 throttle is very good, lots of power from there on to 1/2 throttle and then it will pull to redline very easily.
The only small issue is that sometimes it will not take full throttle when you are trying to get the max out of the beast.
Reducing the size of the main jet to fix this brings back all the issues you have seen below 1/2 throttle.
I've settled at that as the bike is a real pleasure to ride.
A professional mechanic tried my bike and was amazed at how smooth the throttle response was.
I wish you the same!