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  • Thread starter Thread starter '79 550
  • Start date Start date
7

'79 550

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Getting tired of messing with filters, pods and jetting. I have a 79 550L that had 4-1 (unknown brand) pipes installed, K&N replacement filter for the box, and some really interesting jetting done to it before I bought the bike. There are 2 different sizes of main jets installed, and the bigger sizes are causing some blueing of the pipes, and may also be causing some white smoke out the back.

So, I want to go back to the way it was (the 550 is no speed demon, so really don' t need the added "performance"). Do you see any issues in going back to a stock air filter, stock jetting, but keeping the 4-1 pipes?

Thanks.
 
I have a set of GS550M Pipes for sale in good condition if you are interested. Its mostlikely not the airfilter and its the backpressure issue with the 4-1 pipes.
 
I would prefer to keep the 4-1 pipes, but not sure if there is special jetting or filters needed. Could you put 4-1 pipes on a stock motor and have no adverse issues with performance. I don't need better performance, just the same as it would be stock with 4-2 pipes.
 
What specifically is the 4-1 pipe? V&H? MAC? JCW? If it's a high flow pipe you will most likely need to richen the mix. But maybe not. Try it and make sure you check plug readings. Make sure it's not running hot.
 
I will check on the pipes. I have looked before but have not been able to find any markings indicating a brand, and I would imagine that if they were V&H the markings would be proudly displayed. The plugs that align with the smaller main jet seem normal, with the larger main jet, they are wet. Will start by matching all the mains and see if that works.

Is there any reason for the main jets to be different sizes?
 
Mains should be identical. PO(s) may have butchered things and tried to compensate the jetting. Hard to say what needs work. Lower compression in a cylinder(s)...wrong float level...etc, could make someone make odd jetting changes.
The blue header is most LIKELY a lean jet needle setting because you spend the most time riding on the jet needle circuit.
Your known mod's are just the pipe and replacement filter? You still have the air box lid on, correct?
Stock mains for your model are Mikuni #80. Stock jet needle e-clip position is #3 (middle position on stock jet needle), stock pilot jet is #15 I believe.
Mains need to be increased, jet needles raised, and, at least, pilot fuel screw (underneath) adjustments for the pilot circuit. Then side air screws adjusted for best rpm and float bowl vent lines removed (remove only if air box lid is gone.) Check ignition timing/spark/points, if you still have points. Then vacuum synch the carbs.
Let us know if the lid's on or off. We'll make some jetting suggestions if interested. But be aware the rest of the bike must be in decent tune to accurately re-jet. Good compression, spark/timing, no intake leaks, good fuel flow, float levels, etc.
 
Thanks for the help.

You are right on the jet sizes for the bike. What I found is that the two outside carbs have 82.5 for the mains, and the inner carbs are using 85s. The inner carbs are the ones blueing the pipes. The outer pipes have a golden color to them.

The mods are the 4-1 pipe (still can not find a brand) and the filter. I have a set of pods but would rather not add complexity to the current issues. The air box lid is on. The floats are within specs, and the fuel flow is good. Have not checked the compression yet, but there are no intake leaks.

Any jetting suggestions that you have will be very much appreciated.
 
For just a pipe (unknown flow ability) and slightly better flowing filter replacement, I'd try 2 1/2 sizes up. With Mikuni jets, a full size up is 5 and a step up is 2.5....so that would be 92.5 on ALL mains. That's just my guess, especially not knowing what pipe you have. Some pipes are junk and will not breath any easier than a stock exhaust and some flow much better. So that's my guess on the mains. Whatever you try, make them all the same on your model. Don't blame the pipes being different colors on the mains. If you don't nick up the new mains while installing, many shops will swap you if you want to swap mains for smaller/larger ones. Without a Dyno, only way to test/get plug reads for correct mains is a full throttle and chop off test. Do what the plugs/performance tell you.
As for the very important jet needle position, which regulates jetting from just above idle up to 3/4 throttle, again, you must test...FIRST. Test at a solid 1/3 throttle position and chop off. Do what the plugs say. If that pipe really does flow better, there's a good chance you'll have to raise the jet needles one position. This means lowering the jet needle e-clip one notch. You may have to install a jetting spacer if you need only a 1/2 position change. A jetting spacer is about .023" thick. EVERY TIME you disturb the needles in your VM carbs, you must re-bench synch the carb slides followed by a vacuum synch. Getting accurate plug reads/performance depends on correctly synched carbs. Without this, you're just guessing and jetting will be more difficult and more trial/error.
As for the pilot circuit, simple pilot fuel screw (underneath) adjustments will fix you up, followed by adjusting the side air screws for best idle. Probably an additional 1/4 turn out or so will be enough on the pilot fuel screws unless they're way off now. Use a good fitting tool and be careful of tight screws for both pilot fuel and side air screws. If you seat them for any reason, seat them LIGHTLY. Fine tuning always needed if you're a perfectionist.
Before any re-jet, be sure the bike is in decent tune such as ignition timing, valve clearances, clean carbs/air filter, etc.
 
Thanks for the information. I think that there is a lot to do on this one.

Much appreciation to those that responded.
 
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