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Outboard cylinders need different jetting than inboard?

  • Thread starter Thread starter MechMessiah
  • Start date Start date
M

MechMessiah

Guest
I know that the carb jetting on these bikes are supposed to be the same across all 4 carbs but is that ever not the case? I'm starting to wonder if I may need to mix and match different jets for the outer (1 & 4) and inner (2 & 3) cylinders.

The issue is that 2 & 3 seem to run much richer and cooler than 1 & 4. For example, I have the air mixture screws for 1 & 4 at nearly 3 turns from bottom, anything less and the pipes smoke. Meanwhile, 2 & 3 are only at 3/4 turns from bottom and anymore makes the engine stumble. From my understanding, those setting indicate I could use smaller pilot jets on 1 & 4 while 2 & 3 could use larger jets.

Does any of this make sense? or am I losing it?
 
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I'm not sure if Suzuki does it, but some inline fours and engine tuners use larger main jets on 2-3 to help cool them as they don't get the air flow the outer two cylinders get. I know that the case on many import V-Twins, the rear cyclinder will have a larger jet size.
 
I think you may have carburetor issues. Most of us have them jetted the same across all the cylinders and it doesn't cause the symptoms that you describe.
 
Sounds like someone has either drilled out some jets or replaced with some non-stock jets.

On the '83-and-newer 550s (the 16-valve models) and the 1150, Suzuki had different MAIN jets, but the pilots were all the same.

It would also help to know that you do not have "air mixture screws". Those mixture screws (Suzuki calls them "pilot" screws) actually control a mixture, not just air. Turning them out will richen the overall pilot mixture, turning them in will lean it out.

.
 
Steve said:
Sounds like someone has either drilled out some jets or replaced with some non-stock jets.

On the '83-and-newer 550s (the 16-valve models) and the 1150, Suzuki had different MAIN jets, but the pilots were all the same.

It would also help to know that you do not have "air mixture screws". Those mixture screws (Suzuki calls them "pilot" screws) actually control a mixture, not just air. Turning them out will richen the overall pilot mixture, turning them in will lean it out.

.
Click to expand...

It's very possible that the jets have been drilled out or met with some other tom-foolery. The caps over the pilot screws (thanks for the proper term) were already removed when I got the bike. Add to that, the person I bought it from wanted to hack the frame, give it pods, and kept complaining about how much new intake boots cost. I wouldn't be surprised if some of the jets are knock offs.

When I rebuilt the top end last summer the carbs got a thorough cleaning. Full break down, soaking in dip, small wire cleaners through all the holes, all rubber replaced. Then carbs have also been synced with a Morgan carb tune pro.
 
Take a look at, and through, all the pilot jets and main jets to compare them. If necessary, get new jets from Jets R Us, they carry OEM Mikuni jets.

If your bike has stock intake and exhaust, stock jetting will be just fine. If you have pods and/or a header, you will need to adjust your jetting accordingly.

The few dollars for new jets will at least ensure that you have proper-sized jets, and can make adjustments based on that.

.
 
I'm starting to wonder how well a set of Mikuni RS flat slides would run with this engine.
 
GS1150Pilot said:
Do you have an aftermarket pipe? Are the pods the cheapo junk versions or K&N/APE?
Click to expand...

There are no pods. That was the guy I bought the bike from talking about pods. He's was trying to do a quick cafe racer job to flip the bike but seems it was more than he bargained for. I kept the stock air box in and have been trying to get it running smoothly. I did have to get an aftermarket 4-1 pipe due to the stock being rusted out.
 
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These carbs might be FUBARed. They just seem way to touchy. They'll idle differently at the bottom of a hill vs the top of a hill, they'll run better on a cool shady back road than on a sunny hot city street, sometimes the engine stalls when I slow down other times it starts to race. I'm getting the sense that each carb will have to be individually jetted and tuned or at least the outer vs inner carbs.

So Mikuni RS Flat slides or a used ebay set?
 
MechMessiah said:
These carbs might be FUBARed. They just seem way to touchy. They'll idle differently at the bottom of a hill vs the top of a hill, they'll run better on a cool shady back road than on a sunny hot city street, sometimes the engine stalls when I slow down other times it starts to race. I'm getting the sense that each carb will have to be individually jetted and tuned or at least the outer vs inner carbs.

So Mikuni RS Flat slides or a used ebay set?
Click to expand...
everything you have stated here points to not properly cleaned and tuned carbs as well as most likely running lean due to a air leak possibly intake boots or O-rings and improperly tuned,
check to see if your pilot screws are all in one piece and not missing a tip broken in the carb body. Tuning is a trial and error process make 1 adjustment at a time test it and then repeat for each circuit do not to multiple circuits and changes at the same time you will get nowhere fast and just get frustrated if everything is stock then get stock jets and make sure everything is sealed up
then sync the carbs
now tune reading your plugs. Your plugs will tell you everything going on in the combustion chamber and help you to make changes in the right direction
 
TxGSrider said:
You might also at least bench synch the carbs to be sure they are at least close to start with.
Click to expand...

Spyderman said:
everything you have stated here points to not properly cleaned and tuned carbs as well as most likely running lean due to a air leak possibly intake boots or O-rings and improperly tuned,
check to see if your pilot screws are all in one piece and not missing a tip broken in the carb body. Tuning is a trial and error process make 1 adjustment at a time test it and then repeat for each circuit do not to multiple circuits and changes at the same time you will get nowhere fast and just get frustrated if everything is stock then get stock jets and make sure everything is sealed up
then sync the carbs
now tune reading your plugs. Your plugs will tell you everything going on in the combustion chamber and help you to make changes in the right direction
Click to expand...

In post #5 I said "When I rebuilt the top end last summer the carbs got a thorough cleaning. Full break down, soaking in dip, small wire cleaners through all the holes, all rubber replaced. The carbs have also been synced with a Morgan carb tune pro."

Also I'm not talking about changes to multiple circuits at one time. The only circuit that's really being discussed here is the low speed/idol circuit. Mainly the pilot jet and pilot mixture screw relation.

I've taken Steve's advise and ordered new genuine Mikuni jets for the pilot jet and pilot air jet.​
 
MechMessiah said:
I've ... ordered new genuine Mikuni jets for the pilot jet and pilot air jet.
Click to expand...
For just a few more dollars, I would have ordered new main jets, too. :-k

And even if all the new jets don't fix the problem, at least you will know what is NOT your problem. :encouragement:

.
 
Steve said:
For just a few more dollars, I would have ordered new main jets, too. :-k
Click to expand...

I just installed new main jets that are genuine Mikuni. They're a step up from stock though from 110 to 112.5. The top end is much better than it was, the mid range seems good as well. It's just been the low speed circuit that's really been giving me trouble. I've been trying to read up on the connection between the pilot jet, air pilot jet, and pilot mixture screw. It's all starting to make more sense.
 
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