GS550 with 4-1 exhaust and single filters Jetting Question

[tpeeples]

I usually just read the posts here looking for information and suggestions, which I usually find without much difficulty. This place has helped me so many times.

However, I am a point where I need some direct help.

I am rebuilding a 1980 GS550E that was missing the carb intake boots, carbs and airbox, but does have a 4 into 1 Kerker exhaust. I bought a set of Mikuni BS32's for a 550 off E-Bay, a cheap set of foam air filters and a new set of intake boots and o-rings.

From posts here I knew that the Main Jet and Pilot (aka Idle) jet should be changed since the exhaust and filters are not stock. So I read Jetting Advice from Denny Zander at http://www.motorcyclecarbs.com/Default_W43.cfm.

Mr. Zander stated that for a 4-1 Exhaust and Single Air Filters, the Main Jet should be increased by 3 steps (2.5) and the pilot should be increase by 1 step. I found somewhere the GS550 BS32 stock Main Jet is 92.5 and the Pilot Jet is 40, I bought Mikuni Main Jets at 100 and Mikuni Pilot jets at 42.5.

I cleaned, rebuilt with the new jets, checked float level and bench sycn'ed the carbs. I put the carbs on the bike, set the 'idle' screws around 2 1/2 turns out, charged the battery, choked it and it started (much to my amazement). Adjusted the idle, checked for leaks around the intake boots and found none.

The problem is the idle will not return down to 1100 RPMs. I read in several threads that this is a lean condition. I attempted the max idle method described several places with no luck. Even at 3 to 4 turns out the idle still will not return. As mentioned I checked for leaks around the intake boots and this did not have a change in the idle.

I believe this is related to the Jets, so the question is what should the Main and Pilot Jets be with this setup? Or any other advice you would have.

Thanks in advance.

[tpeeples]

Just for Informational purposes

Final results:

Pilot: 42.5
Main: 127.5
Needle: Shimmed with two washers

Still needs to be tweeked, but it runs fairly well.

[ddaum]

Does this bike have Pod fair filters installed?

I'm interested in why you went up so high on the Main jets.
I have a 1980 GS 550L with a Mac 4-1 exhaust, a K&N replacement filter (inside the airbox), 92.5 mains, and 40 pilots (stock jets).

My bike starts every time (with choke) and once it's warm it will idle nicely at 1100 RPM and has good throttle response. My problem is while the engine is warming up. I don't let my bike sit and idle to warm up; I leave the choke partially on and ride away. Then I usually turn the choke all the way off after a few blocks down the road and she is fine. But during those first few minutes of riding the bike has flat spots in the throttle roughly between 2000-3000 RPM and 4500-5500 RPM. These flat spots go completely away when the engine gets good and warm, which I suspect is a lean condition from the new air filter and pipe. Trust me when I say this: everything else with the carbs/sync/fuel delivery/spark/compression is good. The bike is running noticeably warmer now and I have a slight gold/yellowing of the header right near the exhaust ports on each cylinder.
With my setup, I was thinking about going up to 97.5 Mains (+2 sizes), and 42.5 Pilots (+1 size), but I don't want to order the jets until I understand why you went up so high on your mains. Do you have the airbox with cover installed, or do you have pod filters? What is your current gas mileage?
Thanks,
Dean

[tpeeples]

Dean,

Yes, it has very cheap pod filters from JCWhitney.



The reason I went so large was that a friend, who worked on these things professionally years ago, said that was close to what I needed and it so happened he had an extra set he gave me.

Currently, I think it is running a little rich, but it runs pretty well. I have not check the MPG as of yet. This is a custom and I am still working on getting it assembled. I am hoping to do a better job of jetting later in the year.



My friend also has told me that the 550's were naturally cold blooded, so I would expect what you are experiencing normal (I could be wrong since I know not much about jetting). If it runs OK, after warmed up, I wouldn't mess with it. I have three 550's and all of them behave how you describe.

Of course, you could do the 'plug chop' testing that is described in other threads to help you out with the jetting. But that is a lot of trial and error and a few jets.

Sorry I am not much more help than that, but that is the reason why the main is so large.

Tim

[ddaum]

Great job on that custom 550. Man, that's sweet! Did you make forward controls for it? Oh, thanks for the reply. I think I'm gunna order the new jets and see what happens. I'm enjoying my gas mileage right now, but it may be running too lean. We'll see...

[KEITH KRAUSE]

I'm surprised by Denny Zander's main jet suggestions. A 7.5 increase for a quality pipe and cheaper pods? Not even close.
If your stock mains are 92.5, I'd try something closer to 117.5 or 120's.
Jet needle raised about .070"
Pilot jet 42.5 or 45's.
Air jet...not sure what yours is stock but you can look up DJ/K&N jet kit info (?) and use their suggestions. Lighter throttle springs may help too if you have any throttle response issues.
Set mixture screws for best rpm, using 1,000 rpm as a base/setting point.
If you have float bowl vent lines, remove them and leave the ports open.
These are just guesses. But I think good ones.
You still need to take full throttle and 1/3 throttle reads to see what the main jet and jet needle are doing. Only way to know. Do what the plugs/performance say.

[tpeeples]

Great job on that custom 550. Man, that's sweet! Did you make forward controls for it? Oh, thanks for the reply. I think I'm gunna order the new jets and see what happens. I'm enjoying my gas mileage right now, but it may be running too lean. We'll see...

Thanks for comment on the custom. About the forward controls, I was watching someone on the GSR talking about building some forward controls and selling them...so I thought I would give it a try. Unfortunately they didn't work quite as well as I thought. Besides, I am upright sitter and having my feet out in front of me is uncomfortable. So, I just left the pegs up there and put back the stock brake and shifter.

I would take Keith's suggestions on the jets, from what I have seen, he knows what he is talking about.

Tim

[tpeeples]

I'm surprised by Denny Zander's main jet suggestions. A 7.5 increase for a quality pipe and cheaper pods? Not even close.
If your stock mains are 92.5, I'd try something closer to 117.5 or 120's.
Jet needle raised about .070"
Pilot jet 42.5 or 45's.
Air jet...not sure what yours is stock but you can look up DJ/K&N jet kit info (?) and use their suggestions. Lighter throttle springs may help too if you have any throttle response issues.
Set mixture screws for best rpm, using 1,000 rpm as a base/setting point.
If you have float bowl vent lines, remove them and leave the ports open.
These are just guesses. But I think good ones.
You still need to take full throttle and 1/3 throttle reads to see what the main jet and jet needle are doing. Only way to know. Do what the plugs/performance say.

I forgot where I found the Denny Zander article on jetting, but it seemed to be a good place to start. Although it didn't work very well.

Again, not knowing much about jetting, I think your advice is very good. Later in the year, I am planning to look into the jet kit and do the plug chop read.

Thanks

[ddaum]

I would take Keith's suggestions on the jets, from what I have seen, he knows what he is talking about.

I've used Keith's good advice more than a few times, but in this case I think he is referencing YOUR bike...lol.

[tpeeples]

I've used Keith's good advice more than a few times, but in this case I think he is referencing YOUR bike...lol.

Oh yeah, I forgot I was the one that started this thread!!!

Later I am going to back the main down to 117.5 or 120, shim the needle as suggested (currently around 1 mm) and maybe the pilot at 45.0 while do the plug chop. We will see when I find the time. Other things are getting in the way since the weather is good now.

Keith - Here are the spec I have been using for all my 550's:

GS550 (80-82) the specs are for the CV's

idle r/min 1100-+/-100r/min
carb mik bs32ss (4 seprate carbs)
id no 47160 (us) 47170 (can)
bore size 32 (1.26)
float height 22.4.+/-1.0 (0.88 +/-0.04)
fuel level 5.0+/-0.5 (0.20 +/-0.04)
main jet #92.5
main air jet 1.6
jet needle (4bel2 us model) (5f42-3rd notch canadian adjusable needle)
needle jet x-6
pilot jet #40
throttle valve #135
by pass (0.9, 0.7, 0.7 us model) (1.0,0.8,0.8 canadian)
pilot outlet 0.7
valve seat 2.0
starter jet #35
pilot screw 3.5 turns back (mixture screw)
pilot air jet (us model #150) (#120 canadian)

When I get it worked out, I will report what I end up with.

Thanks

Tim

[KEITH KRAUSE]

Reading back...what is this "slow to return to idle" symptom? Can you be more specific as to what it does? Only when hot, etc??
I still suggest plug reads before taking any exact jetting suggestions. The plugs will tell you a lot.

[tpeeples]

Reading back...what is this "slow to return to idle" symptom? Can you be more specific as to what it does? Only when hot, etc??
I still suggest plug reads before taking any exact jetting suggestions. The plugs will tell you a lot.

Yes, that statement was rather vauge. I had the pilot at 42.5, the main at 100.0, no shims on main needles. This was sitting, after blipping the throttle, the idle would take a few minutes to return back to 1100 RPM from 2500 or so. This was even after it had idled enough to be 'warmed up'. Didn't take it out on the road at that time.

I even went to 105.0 on the main and two shims on the main needles and that seemed to help the return to idle problem I saw (not sure why). But just taking it around the neighborhood it wouldn't pull, just stumbled over itself.

That is when my friend gave me the 127.5 main jets more as a science experiment. It idles, returns to idle and pulls fairly well (with some flat spots through the RPMs).

As mentioned earlier, I haven't done the plug chop read as of yet. But, just riding it several days and looking at the plugs they are a tan color (I understand that doesn't really tell you much). Again, I am planning to do the plug chop reading soon to get a better handle on it.

Thanks much for the help and I might bring this thread back to life after I do the plug read.

Tim

[KEITH KRAUSE]

OK. Now it sounds like you may have an intake leak. You can't attempt to tune or re-jet until you're positive there are no intake leaks.
You say the carbs are clean and I can only assume the throttle plates are moving smoothly. Sticking throttle plates can sometimes be the cause of a slow to return idle, though not as common as intake leaks. If you're sure of the plates, then be sure the throttle cable is routed correctly and operating smoothly with the correct amount of slack. If so, and the rpm's are still slow to return, you have an intake leak somewhere.

[KEITH KRAUSE]

Oh. One other thing.
After any intake leaks are fixed, or hanging rpm's due to mechanical reasons, I suggest synching the carbs with a vacuum tool. Sure there's a small chance your bench synch is close enough, but I doubt it. If the synch is off enough, it can contribute to the hanging rpm's too.
Also, there's not much sense in taking plug reads without KNOWING the vacuum levels are the same. The plugs reads may not be uniform. If you see varying reads/colors between plugs, how do you know what plug is the "base" color and what you'll be basing jetting changes on?
Performance will be off somewhat too. You could end up making jet changes because of something synch related. Uneven/unsteady cruising performance is often taken as a lean condition.
I know a vacuum synch may be adding work not planned for, but it saves time in the long run. Before any re-jet, the bike must be tuned as best as possible. Otherwise, you're mixing jetting with troubleshooting.
A good vacuum synch is a part of jetting as important as the jets.

[tpeeples]

Oh. One other thing.
After any intake leaks are fixed, or hanging rpm's due to mechanical reasons, I suggest synching the carbs with a vacuum tool. Sure there's a small chance your bench synch is close enough, but I doubt it. If the synch is off enough, it can contribute to the hanging rpm's too.
Also, there's not much sense in taking plug reads without KNOWING the vacuum levels are the same. The plugs reads may not be uniform. If you see varying reads/colors between plugs, how do you know what plug is the "base" color and what you'll be basing jetting changes on?
Performance will be off somewhat too. You could end up making jet changes because of something synch related. Uneven/unsteady cruising performance is often taken as a lean condition.
I know a vacuum synch may be adding work not planned for, but it saves time in the long run. Before any re-jet, the bike must be tuned as best as possible. Otherwise, you're mixing jetting with troubleshooting.
A good vacuum synch is a part of jetting as important as the jets.

When I first asked this question, I had done only the bench sync. I checked for intake leaks and I later did the vaccum sync seeing if that would help, but it didn't.

After I put the 127.5 mains in it started to come back to idle good. The idle problem first mentioned is gone and it runs good now, although most likely rich.

This question for when I get back to fine tuning the jets...Should a vaccum sync be done after each jet change?

Thanks for help.