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MAC Exhaust Question

  • Thread starter Thread starter 427cobra
  • Start date Start date
4

427cobra

Guest
I just put a NOS MAC 4 into1 exhaust on my 79' GS850G.

What jetting changes if any do you guys recommend?

Do I need to drop the needles or install larger mains?

Any suggestions would be great!!

Thanks
 
Do you still have stock airbox or have you installed pods?

For just the pipe, increase the mains about three or four sizes.

Don't drop the needles, RAISE them.
Don't forget, though, to raise the needles you put the clip in a lower notch.
Moving one notch should be sufficient.

Turn the pilot fuel screw (they are the ones on the bottom of the carbs) settings to 7/8 to 1 full turn out from lightly seated.

Turn the pilot air screw (they are the ones on the sides of the carbs) settings to double the fuel screws.

If you also have pods, add another three or four sizes to the main jets and raise the needles another notch.

.
 
Steve said:
Do you still have stock airbox or have you installed pods?

For just the pipe, increase the mains about three or four sizes.

Don't drop the needles, RAISE them.
Don't forget, though, to raise the needles you put the clip in a lower notch.
Moving one notch should be sufficient.

Turn the pilot fuel screw (they are the ones on the bottom of the carbs) settings to 7/8 to 1 full turn out from lightly seated.

Turn the pilot air screw (they are the ones on the sides of the carbs) settings to double the fuel screws.

If you also have pods, add another three or four sizes to the main jets and raise the needles another notch.

.
Click to expand...
three or four sizes is a little overboard by my findings when installing a pipe only. Most manufacturers recommend 2 sizes at most. This is assuming the 4-1 flows more freely than the stock exhaust system, which in most cases, isn't the case. With VM carbs, the needle will play more of a part than the main in this situation. You may try the pipe without any adjustments FIRST, and see how things go. If it stumbles, breaks up or acts up, note where and which THROTTLE position (NOT RPM, as that has little to do with what circuit in the carbs is being used) is the problem(s) and then decide if you need a bump in the mains, or need to move the needle clip down a notch or two ( as Steve said, moving the clip DOWN on the needle raises the needle in the needle jet, allowing more fuel to pass through the circuit SOONER than before, thereby richening the system)

IMO, I wouldnt mess with the fuel screws just yet. The air screws may need adjusting, but unlike the CV (vacuum carbs on later models) the pilot circuit on your carbs is its own entity, not fed by the main, and generally, the installation of a pipe doesn't generally effect the pilot circuit much. You may find, in fact, that you need to actually need to slightly richen the circuit by closing the air screw down just a touch. MAC pipes are not known for their amazing performance gains. Its looks, and weight savings at best honestly.

Of course, if you've also installed pod filters (and please, for the love of God, dont be a cheap ass and install those piece of crap EMGO cone junkers. They are worthless, dont filter a damn thing, and actually DECREASE the port diameter on the inside, causing a bottle neck, and poor air flow) then you can throw all of this out the window, and prepare yourself for alot of trial and error tuning....
 
I am running the stock air box with filter.

Its not running quite right with stock jetting. SO I am planning on trying the larger main.

Have to see what happens!!

Thanks for the help....:D
 
427 Cobra - could you post a couple of pics of that exhaust on the bike? Does the kickstarter clear?
 
TheCafeKid said:
three or four sizes is a little overboard by my findings when installing a pipe only. Most manufacturers recommend 2 sizes at most. This is assuming the 4-1 flows more freely than the stock exhaust system...
Click to expand...


Agreed. :o


Of course, if you've also installed pod filters (and please, for the love of God, dont be a cheap ass and install those piece of crap EMGO cone junkers. They are worthless, dont filter a damn thing, and actually DECREASE the port diameter on the inside, causing a bottle neck, and poor air flow) then you can throw all of this out the window, and prepare yourself for alot of trial and error tuning....
Click to expand...
Click to expand...

So Josh, I can get you a GREAT deal on a case of them there Emgos! :p
 
Yep kickstand clears!

I will post some pics when I get the bike back!

Thanks!
 
Here it is...

Everything clears and fits great! Increased the main jet size which made it run great!

Its ready to ride now!
 
Your bike looks great!
Can you post a closer pic of the Mac. It is a lot different looking than the one i had years ago. The collector is in a different spot and the upswept muffler looks somewhat like a Kerker.
 
Hey cobra, what size main jet have you ended up with and did you change the needle height at all? What were the symptoms of not running right? Cheers.
 
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