TOPIC:

I have a two into one pipe on my 79 750 twin ,, The stock main jet is a 126 ,, should I go to 130 or higher ? it is such a pain to get these carbs off I want it to be real close the first time around ?? Thanks

Here's a test to diagnose main jet performance which might possibly indicate the exiting main jets to be okay, or at least some idea as to the degree of additional richness needed. A tan color on the plug ceramic is the desirable target.


Checking the Main Jet– Warm up the engine and go for a short ride letting the engine come up to its normal operating temperature. Install brand new properly gapped plugs. With the new plugs installed aggressively accelerate through the gears until you reach 4th or 5th gear. For best results you should accelerate up a slight up hill section to place additional load on the engine. Continue to run the engine at WOT for 20 to 30 seconds longer if there is not fear the engine is running lean. If you suspect the engine is running lean 15 to 20 seconds to give you an indication. At the end of your full throttle run simultaneously push the kill button, chop the throttle and pull in the clutch. This procedure is often refereed to as a " plug chop". It is important to perform a plug chop exactly as described. If you allow the engine to run or leave the throttle open for even a few seconds after the plug chop the plug readings will be invalid. Now remove the spark plugs and carefully look at the color.

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Post edited by: Patton, at: 2007/01/29 17:17

Patton wrote:

Here's a test to diagnose main jet performance which might possibly indicate the exiting main jets to be okay,.... At the end of your full throttle run simultaneously push the kill button, chop the throttle and pull in the clutch. This procedure is often refereed to as a " plug chop". It is important to perform a plug chop exactly as described. If you allow the engine to run or leave the throttle open for even a few seconds after the plug chop the plug readings will be invalid. Now remove the spark plugs and carefully look at the color.....

Let's try this again. I already worked up a lengthy reply only to have it fly off into the twilight zone, never to be seen nor heard from again.
I am dealing with Mikuni carb issues now, and this sounds quite useful.
When you chop the throttle, I assume you hold the clutch in 'till you come to a dead stop, take out the plugs on the side of the road or wherever you are, and then put different plugs in to ride back home, or analyze, on the spot, the ones you pulled out, then put them back in.
I hope to get my scoot running before 2008 gets here.
As it stands, I can't ride off without worrying I won't make it back before fowling out all the plugs thereby putting me on the side of the road.

I understand what you are saying ,, I am just looking for a starting point

I have what looks to be a Mac 2-1 on my 79 twin, and stock jets. Once I got the carbs set right (running with the starter plungers "up" is not a Good Thing!), I got dry pale grey deposits, and what seemed to be decent performance. Keep in mind, this comes from a newbie, running his first bike ever.

From everything I have read here, I do not think just the 2-1 exhaust makes enough difference to change the jetting. Then again, maybe that is just what I *want* to believe!

Since you have the pipe on the bike I will use stock as a starting point ,, Thanks