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How to tell if I need to rejet?

  • Thread starter Thread starter ccs
  • Start date Start date
C

ccs

Guest
Hello All;

I am going to put a Mac 4-1 header on my 82' GS850G.

How do I tell if I need to rejet or not?

If I need to re-jet, how do I determine what size jets to use?

BTW, I have a colortune and vacuum gauges to synch the carbs and presume I need to do this after replacing the exhaust, correct?

TIA.

Charles...
 
You will most likely need to rejet your mains, and depending on how it runs in the midrange, you may have to raise your jet needles. Are you running the stock air box or pods? The best way to tell is to install the exhaust and test the bike. If it pulls hard through all ranges from just off idle to WOT, the next step will be to run some plug chops and check your color at various stages of throttle position. If it runs flat in the WOT position, and your plug reads are white, you'll be looking at jumping up a size or two on the mains. There are "stage" kits that come pre packed with the jet sizes for your various mods. Stage two and three are the most common.

Below is a link that will explain the process if you choose to go at it without the kits...it also explains what you are looking for through the various stages of your retune. Good luck and keep us posted.

http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html
 
Dave8338 said:
You will most likely need to rejet your mains, and depending on how it runs in the midrange you may have to raise your jet needles.
Click to expand...

Hmmm... just *what* is the midrange? And... what would you be looking for?

Dave8338 said:
Are you running the stock air box or pods?
Click to expand...

Yes -- I've got the stock 2 into 2 exhaust and air box.


Dave8338 said:
The best way to tell is to install the exhaust and test the bike. If it pulls hard through all ranges from just off idle to WOT, the next step will be to run some plug chops and check your color at various stages of throttle position.
Click to expand...

By pulling hard do you mean that it runs GOOD or BAD?

Dave8338 said:
If it runs flat in the WOT position, and your plug reads are white, you'll be looking at jumping up a size or two on the mains. There are "stage" kits that come pre packed with the jet sizes for your various mods. Stage two and three are the most common.
Click to expand...

Well... the guy @ crc2online said, that if I really do need to rejet, he could sell me just the jets I needed -- which according to him would be *only* the main jet. He didn't say a thing about raising the jet needles! (He told me there is a lot of mis-conceptions about re-jetting and you don't really need to do it unless you put some sort of short-drag racing style of exhaust that is only about 2 feet long.)

Dave8338 said:
Below is a link that will explain the process if you choose to go at it without the kits...it also explains what you are looking for through the various stages of your retune. Good luck and keep us posted.

http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html
Click to expand...

OK, I'll take a peek at this.

Thanks.

Charles...
 
Oh -- I forgot to include.

The guy at crc2online said that older bikes (like my 82') don't need to be re-jetted like the newer bikes.

Charles...
 
The mid-range is you throttle position from 2500-6000 or just off of idle through roughly 2/3 throttle roll. By flat, I mean if the bike bogs/hesitates without smooth acceleration under moderate load, that is a good indication of a lean mixture, which you will verify through your plug chops.

(A plug chop is done at a set throttle position, where as you run the RPMs at 3000 for example, and hold them there for 5-7 seconds and then hit the kill switch and pull in the clutch...coast to a stop and pull your plugs and have a look. White plug tips signify a lean burn condition, light brown is correct and black is a rich mixture condition).

With the stock air box combo, you may get away without a rejet as pods increase airflow (leaning out the mixture) and usually require a rejet.

When I say "pulls hard"...I'm refering to the engines ability to lug or "pull", without stumbling or hesitating. You should be able to run the bike in 5th gear at 30mph and roll the throttle and it should steadily climb without any signs of a hesitation. (GOOD) If in this situation, your bike hesitates or bogs flat, you are almost for sure, lean.

At wide open throttle, your main jets should over compensate for a lean midrange, though you will notice a hesitation until the mixture richens up.

As far as exhaust length is concerned, it has less to do with the equation than does back pressure. The more back pressure you have in an exhaust system, typically, the less fuel is required to maintain a consistant throttle response. Kerker...V&H...and others, along with their tube length (equal tube length is always desired to maximize flow and lessen the pressure difference within the tubes themselves), run less back pressure than do stock systems and as a result, allow the engine to breathe easier and therefor, allow for more fuel in and in effect, more power out.

If you need to rejet...follow the process in the link to the letter and you will have a much easier time getting things right.

Hope I didn't bore you with details.
 
I ran a 4 into 1 Mac on my 79 GS850 for about 14 years and I never needed any re-jetting. The only time I re-jetted was when I added pods to my carbs and then I just went up 1 size.
 
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