1981 GS550e Mikuni Carb tuning progress.

[Ira]

Hi, I'm starting this thread to talk about the process of tuning the carbs on my project.

First, I'm not using stock configuration, and I know this is going to make what I am doing harder. That is a given. However, I want to make this bike run as well as I can with what I have.

I'm using the stock Mikuni carbs, with the dual-intake K&N pod filters (pictured). I'm using the stock 4-2 headers with Emgo reverse megaphone exhausts.

The changes I have made to the carbs so far:
- Increasing the main jet size (I don't remember the size of the jet, I'll report back).
- Raising the needle height. I did this by moving the plastic donut from above the clip to below the clip.

What the bike is doing now:

- Idles a little high, but not bad.
- Good throttle response off the line. I can whack it open in 1st and get good acceleration.
- The bike begins to bog at 1/2 to 3/4 throttle. The engine stumbles and surges, and there's no more power.
- If I baby the throttle and open it slow and gradually, the revs will sustain higher, but it eventually bogs around 3/4.

From what I know, this is a symptom of running rich, which I'm assuming is the result of a main jet that is too large. I'll get the measurement next time I take the carbs off.

 

[Steve]

From your listed symptoms, your bogging is happening before the mains.

You moved the plastic spacer to the other side of the clip, you may have raised the needle too much. Try adding a couple of washers above the clip.

Stock mains for your bike are 92.5. With your setup, you will probably end up in the neighborhood of 105, +/- a size or two.

Have you fine-tuned the mixture screws for your idle circuit?

.

 

[Ira]

From your listed symptoms, your bogging is happening before the mains.

You moved the plastic spacer to the other side of the clip, you may have raised the needle too much. Try adding a couple of washers above the clip.

Stock mains for your bike are 92.5. With your setup, you will probably end up in the neighborhood of 105, +/- a size or two.

Have you fine-tuned the mixture screws for your idle circuit?

.

You're saying leave the plastic spacer where I put it, but add washers above the clip?

I removed the plugs over the mixture screws, but I haven't touched them yet.

 

[Steve]

You're saying leave the plastic spacer where I put it, but add washers above the clip?

Correct. The position of the needle is determined by the stack of whatever above the clip. Removing the nylon spacer allowed the needle to be raised two or three clip positions, adding a few washers (two or three, depending on how thick they are) will be about half the thickness of the nylon spacer, making the total difference about 1 or 1 1/2 clip positions.

Personally, I would also suggest that you do some fine-tuning on the mixture screws. They won't help much above half-throttle, but they might ease the transition into it.

Start by gently turning the screws IN until they seat lightly, counting the turns on the way. Let us know what you find, so we can make some better suggestions based on proper information.

.

 

[mrl1000l]

Hey Ira, I think Steve is right about the mixture screws. Did you get a chance to set up your idle mixture screws properly? If not I'd start here, it's a really great guide:

http://www.thegsresources.com/_forum/showthread.php?t=199096

 

[Ira]

Since you guys both mentioned the mixture screws, I need to ask: it's always been my understanding that the mixture screws control the idle circuit, but don't have much effect once the throttle is open. Am I missing something here?

My idle is a little high, which needs to be fixed, of course, but the symptoms I'm seeing are at half to 3/4 throttle. Wouldn't this be related to needle height or main jet size?

Unrelated to that question, I noticed a few threads suggesting the '82 katana 550 dynojet kit is a direct fit for this engine/carb combination.

 

[hampshirehog]

Unrelated to that question, I noticed a few threads suggesting the '82 katana 550 dynojet kit is a direct fit for this engine/carb combination.

It fits your bike - same carbs.

 

[mrl1000l]

Jetting is an additive process. Idle mixture screws still supplies fuel and air through the entire throttle position, albeit a very small % at 3/4 throttle, it's still there.

This seems to be a popular image for jetting overlap, although I've seen it drawn different for direct lift carbs, I think this is close to accurate for CV

 

[Ira]

Crappy news - one of my air screws was completely frozen and I broke the carb body trying to free it. Actually, my #2 and #3 carb bodies are cracked by the air screws.

So does anyone have a rack of Mikuni BS32 carbs they want to sell, or at least those two carb bodies?

I'm bummed

 

[Guest]

Crappy news - one of my air screws was completely frozen and I broke the carb body trying to free it. Actually, my #2 and #3 carb bodies are cracked by the air screws.

So does anyone have a rack of Mikuni BS32 carbs they want to sell, or at least those two carb bodies?

I'm bummed

I had same issue an had to "borrow" a body from a set. I just happen to have 2 & 3 available. They were off an 80' 550E and they are the same as yours I do believe

 

[Ira]

thanks, pm sent.

 

[Ira]

I got the replacement carb bodies in the mail, and my dynojet kit arrived as well. Hoping to put it all together this weekend.

 

[Ira]

Removing these bolts has been brutal. This is as far as I got yesterday.

Tried heat, impact driver, pb blaster. I'm using JIS drivers.

 

[Ira]

By the way, the replacement carb bodies are nice and clean! Looking forward to installing them.

 

[Ira]

A bunch of updates:

The rack came apart and I replaced bodies #2 and #3.

- Removing the bolts for the rails was awful. They were frozen to the carb bodies and required maximum ugliness to remove. All but one eventually broke free, and that one I drilled and retapped. I replaced all of the mounting hardware.

- I regret not replacing the o-rings on the pipes between each carb body. I'm going to do that next time I pull them off.

- I really should have taken before and after pictures. I momentarily confused myself by putting the throttle and choke cable stops on the wrong carbs. (Edit - after looking at the fiche, I realized my fuel line T is in the wrong place too! I have it between 2 and 3. I don't understand why there are two plastic Ts in the first place - can the line be attached to either, or do I need to tack the rack apart again to switch their positions?)

- Next, I installed the stage 3 dynojet kit (550 katana). It was easy to do, but mine had no instruction sheet at all! I tried to find the correct pdf on their website, but it doesn't seem to exist.

- Drilling the slides was obvious because there's one drill bit and one hole.
- Clip height was a guess. I picked 4 down from the top, because it's close to the middle. Used the original washer and plastic donut in stock positions.
- Used the smaller of the two main jets (dynojet 150) rather than 155.
- Did not add the mixture jet adapters. I don't know what they do, could someone explain?

- I set my mixture screws at about 3 turns out. this was hard because one of them is sticky(not stuck, just turns hard/loose depending on the position) and i'm getting bad feedback as to the "seated position". Any tips on how to chase those threads or unstick the screw?

- When reassembled, my idle is complete crap. bike will not idle at all without a bit of throttle.

- Once i'm on the throttle, it pulls well through all positions, which is fantastic. I didn't check the plugs for rich/lean, but it feels good.

- One of the carbs (at least) leaks. I need to take them off and see what's going on. Hard to tell at night with them on the bike.

 

[Ira]

I emailed dynojet to see if they have instructions for my kit.

Any ideas about the sticky mixture screw though? If I wanted to chase the threads, what tap would I use? Should I try replacing the screw as well?

 

[Ira]

dynojet just told me there are no instructions for the gs550 katana kit!

The stock settings are:

-Needle 3 positions from the top
-150 jet
-Mixture screw at 2.5 turns

 

[Ira]

my carbs were dumping fuel whenever the bike would run. it turned out the connecting pipe o-rings had turned to brittle plastic. i replaced those and have no more drips.

when i put the carbs on, the bike wouldn't start unless i covered the intakes with my hand. i had omitted the little dynojet parts that press into the air jets.

after i put them it, the bike fired right up. it now runs strong and is responsive through the throttle positions.

the carbs are only bench-synced, so there's some work to be done, but it's completely ridable.

 

[Ira]

I went out for a longish ride in city traffic. the stop-and-go really brought out some problems in my idle and early throttle response. Lean characteristics.

I have size 40 pilots right now, and the mixture screws don't seem to do anything for me.

I ordered 42.5 jets, and my supplier didn't have 45 jets in stock, but I'm going to try those too.

 

[tigerfan474]

Did the dynojet kit work? I have the same filters and at wits end trying to jet this thing. I started with stock and I'm up to 110's. Never had a stock air box. Can't even put one in thanks to PO cutting crap off.